POLYCYCLIC AROMATIC HYDROCARBONS WITH STRAIGHT EDGES AND THE 7.6/6.2 AND 8.6/6.2 INTENSITY RATIOS IN REFLECTION NEBULAE

Abstract

We have investigated the mid-infrared spectral characteristics of a series of polycyclic aromatic hydrocarbons (PAHs) with straight edges and containing an even or odd number of carbons using density functional theory (DFT). For several even and odd-carbon PAHs, the 8.6/6.2 and 7.6/6.2 intensity ratios computed in emission after the absorption of a 8 eV photon match the observed ratios obtained for three reflection nebulae (RNe), namely NGC 1333, NGC 7023, and NGC 2023. Odd-carbon PAHs are favored, particularly for NGC 1333. Both cations and anions are present with the cations being predominant. Relevant PAHs span sizes ranging from 46 to 103–113 carbons for NGC 7023 and NGC 2023 and from 38 to 127 carbons for NGC 1333 and have symmetries ranging from D_2h to C_s. Our work suggests that even and odd-carbon PAHs with straight edges are viable candidates for the PAH emission seen towards irradiated Photo-Dissociation Regions (PDRs).

1. INTRODUCTION

The aromatic infrared bands (AIBs) are a series of interstellar emission bands observed between 3 and 20 µm that are generally attributed to polycyclic aromatic hydrocarbons (PAHs; Léger & Puget 1984; Allamandola et al. 1985) and closely related species, such as PAH clusters and very small grains (Allamandola et al. 1989; Van Kerckhoven et al. 2000; Rapacioli et al. 2005; Li & Draine 2012; Roser & Ricca 2015; Yang et al. 2016; Peeters et al. 2017; Yang et al. 2017). The emission process is attributed to stochastic heating of the molecules by gas-phase absorption of a UV photon followed by intersystem crossing to high vibrational levels of the ground electronic state; the observed IR emission arises as the molecules cools to the background temperature (Sellgren 1984). The discovery of bands at 16.4 and 19.0 µm attributed to the C₆₀ and C₇₀ fullerenes (Sellgren et al. 2010; Cami et al. 2010) further supports the presence of PAHs since it is expected that intense UV processing of interstellar PAHs with ~60 carbon atoms will lead to fullerene formation (Zhen et al. 2014; Berné et al. 2015).

Extensive observations have shown that the relative strengths of these emission bands vary depending on the source and within a given source (Hony et al. 2001; Bregman & Temi 2005; Smith et al. 2007; Galliano et al. 2008; Stock et al. 2014; Shannon et al. 2015; Peeters et al. 2017). Their shapes and peak positions depend on object type (Joblin et al. 2000; Peeters et al. 2002; van Diedenhoven et al. 2004; Sloan et al. 2005; Compiègne et al. 2007; Joblin et al. 2008) and, in the case of circumstellar environments, on the effective temperature of the central star (Sloan et al. 1997, 2005, 2007).

Detailed studies also show that the strong features consist of multiple, partially overlapping sub-components (Cohen et al. 1989; Bregman 1989; Beintema et al. 1996; Roelfsema et al. 1996; Moutou et al. 1999; Peeters et al. 2011; Candian et al. 2012; Sloan et al. 2014; Peeters et al. 2017; Stock et al. 2017) that can also vary in strength. These variations in the AIBs are presumed to originate from multiple sub-populations of structurally related emitters whose number densities, chemical structures, and charge, and thus their emission characteristics, are shaped by the local radiation environment of the source. Accounting for these variations is absolutely critical for better constraining the specific emitting molecules.

The 6–9 µm region, in particular, contains three well-known major features: a band at 6.2 µm, a large complex of multiple overlapping bands at approximately 7.7 µm, and a band at 8.6 µm. Decomposition of the 7.7 µm complex using either Gaussians or Lorentzians identified several sub-components with the dominant ones centered around 7.6 and 7.8 µm (Bregman 1989; Cohen et al. 1989; Verstraete et al. 2001; Peeters et al. 2002; Van Kerckhoven 2002). A study of nine sources (reflection nebulae (RNe) and Hii regions) showed that their Spitzer IR continuum-subtracted 7–9 µm regions can be decomposed into four Gaussian components centered approximately around 7.6, 7.8, 8.2, and 8.6 µm and respectively denoted G7.6, G7.8, G8.2, and G8.6 (Peeters et al. 2017; Stock et al. 2017). For the strongly irradiated reflection nebula NGC 2023, the G7.6 and G8.6 components are very tightly correlated (with a correlation coefficient of 0.987) and peak in intensity close to the star, while the G7.8 and G8.2 components are correlated to a lesser degree and peak further away from the star (Peeters et al. 2017). This suggests that at least two classes of molecules, partially segregated from each other, contribute to the 6–9 µm emission. Stock et al. (2017) confirmed the correlation between the G7.6 and G8.6 components towards irradiated Photo-Dissociation Regions (PDRs) but found that towards more quiescent regions, each of the Gaussian components was observed to correlate with its neighbors (when normalized to the 11.2 µm PAH band).

While it is well established that ionized PAHs contribute to the emission at 6.2, 7.7, and 8.6 µm, other factors besides charge, such as shape and size, must drive any spectral and spatial variations (Langhoff 1996). Theoretical calculations of the cation, anion, and neutral spectra of selected PAH structures produced spectra that depended upon the molecules’s size and shape, with a band between 7.6 and 7.8 µm and a band at 8.6 µm that can shift as far as 8.2 µm (Bauschlicher et al. 2008, 2009; Ricca et al. 2012). These calculations showed that the emission at 7.6 µm arises primarily from compact charged PAHs containing between 50 and 100 carbons, the emission at 7.8 µm from large compact charged PAHs containing between 100 and 150 carbons, and the emission at 8.6 µm from large compact charged PAHs with at least 96 carbons. The emission bands centered at 8.6 and at 8.2 µm were assigned to the C–H bending modes along straight edges of the molecule (Bauschlicher et al. 2008), and to the C–H bending modes along bay regions (Peeters et al. 2017), respectively.

A systematic study of the 8.6/6.2 vs 7.6/6.2 intensity ratios using a series of circumscribed poly-acenes (cations, anions, and neutrals) with an even number of carbon atoms and with hexagonal and elongated hexagonal shapes was not able to reproduce the observed 8.6/6.2 vs 7.6/6.2 intensity ratios for NGC 2023 (Peeters et al. 2017). For this series of highly symmetric PAHs we could not obtain a very high correlation between the 7.6 and the 8.6 µm bands due to the fact that the 7.6 µm band shifted to 7.8 µm with size, and only large PAHs with at least 96 carbon atoms had a 8.6 µm band with a sizeable intensity. It was therefore necessary to examine PAH structures with a wider range of symmetries that could better account for the observed 8.6/6.2 and 7.6/6.2 intensity ratios for NGC 2023, as well as for other RNe, such as NGC 1333 and NGC 7023.

Here we focus on compact even-carbon and odd-carbon PAHs containing only straight edges and no bay regions (as bay regions contribute to the emission at 8.2 µm). While even-carbon PAHs have been studied extensively in previous studies, only few data are available for odd-carbon PAHs. However, odd-carbon PAHs are commonly observed in fuel-rich combustion as either closed-shell cations or as neutrals with an additional H atom to tie up the open shell. For species with less than 50 carbons, the PAH cations with an odd number of carbon atoms can be even more numerous in flames than similar-sized species with an even number of carbon atoms (Weilmünster et al. 1999). The presence of odd-carbon PAH cations in fuel-rich flames suggests that they could be formed in PAH-forming outflows from carbon-rich stars (Parker et al. 2011).

For each optimized PAH structure we have computed the infrared emission spectra and compared our computed 8.6/6.2 vs 7.6/6.2 intensity ratios to the observations in order to identify potential carriers of the 7.6 and 8.6 µm bands. We presume that structures with band intensity ratios that are grossly inconsistent with the observations may nevertheless be present in the interstellar medium but are unlikely to be principal carriers of the 6–9 µm region bands.

2. MODEL AND METHODS

2.1. Theory

In this study, we focused on benzenoid (containing only six-membered rings) PAH molecules with straight edges and no bay regions. We selected those closed-shell even-carbon PAHs ranging from C₂₂H₁₂ to C₁₄₈H₃₀ with a color excess, which is the difference in the number of starred and unstarred atoms of their alternant π system (see Cyvin & Gutman (1988)), equal to zero. We also chose open-shell odd-carbon PAH structures ranging from C₃₁H₁₅ to C₁₂₇H₃₁ with a color excess of one. We excluded all even and odd-carbon PAH structures with a triplet or higher spin ground state as they are expected to be reactive and therefore not expected to be relevant carriers. Odd-carbon PAHs have a polyradical character that increases with size (Das et al. 2016), which is reflected by the fact that most large anions have a triplet or higher spin ground state. Those anionic PAHs with a high spin ground state were disregarded, which explains the smaller number of spectra for odd-carbon PAH anions. Our final PAH collection includes even and odd-carbon PAHs already present in the NASA Ames PAH IR Spectroscopic Database (PAHdb; Boersma et al. 2014; Bauschlicher et al. 2010; Mattioda et al. in preparation) as well as new PAHs. All the PAH structures used in this study are shown in Figures 1 and 2.

Figure 1.

The structures of the even-carbon PAH molecules considered in this work and the average binding energies per carbon (E_C, eV) for the neutrals (black), cations (red) and anions (blue).

Figure 2.

The structures of the odd-carbon PAH molecules considered in this work and the average binding energies per carbon (E_C, eV) for the neutrals (black), cations (red) and anions (blue).

Geometry optimizations and harmonic frequency calculations were performed using the B3LYP hybrid (Becke 1993; Stephens et al. 1994) functional in conjunction with the 4–31G basis set (Frisch et al. 1984) using the Gaussian09 suite of programs (Frisch et al. 2009). All the structures are minima, i.e. they have zero imaginary frequencies. The harmonic frequencies were scaled using a single scale factor of 0.958. This approach has been shown to yield PAH spectra that are in very good agreement with matrix-isolation spectra (Bauschlicher et al. 2010) and that agree with scaled spectra computed using larger basis sets (Mattioda et al. 2017). The 4–31G basis set is the current standard for theoretical spectra of pure PAHs listed in the PAHdb. It might initially appear that such a small basis set cannot be used to treat the anions. While this is true for small molecules, we have shown (Ricca et al. 2011) that as the PAH increase in size, the effect of the diffuse functions on the computed anion IR spectra decreases, so that by the coronene anion ${\mathrm{C}}_{24}{\mathrm{H}}_{12}^{-}$, the effect of the diffuse functions is minimal. This is simply a result of the “extra” electron being spread over so many carbon atoms that the difference in charge between the neutral and anion on any individual per carbon atom is very small. Since all of the molecules in this work contain significantly more carbon atoms that coronene, the use of the 4–31G basis set for the anions is justified. The vibrational modes were visualized using the interactive molecular graphics tool Jmol (http://jmol.sourceforge.net).

Previous work has shown that PAHs with the same charge and similar numbers of carbon atoms have similar total intensities (Bauschlicher et al. 2010; Bauschlicher & Ricca 2010). The total intensities of similar-sized PAHs can therefore be used to detect species that are not well described by theory. Similarly, strong bands at unexpected positions can also indicate a computational problem. For the species studied in this work, the harmonic frequencies and intensities were carefully inspected and molecules that produced abnormally large intensities were excluded. Our final compilation of IR spectra consists of a total of 212 spectra, namely 37 spectra for neutral PAHs with an even number of carbons, 37 spectra for their cations and 37 spectra for their anions, 39 spectra for neutral PAHs with an odd number of carbons, 39 spectra for their cations, and 23 spectra for their anions. We also present the average of each broadened spectrum. For simplicity we use equal weights for all the spectra as we don’t perform fittings to obtain ratios.

To aid comparison with astronomical spectra, we broadened the computed band intensities (in km/mol) in wavenumber space because it is linear in energy. The full width at half maximum (FWHM) used in the broadening were: 30 cm⁻¹ for the bands shortward of 9 µm, 10 cm⁻¹ for the bands between 10 and 15 µm, and 5 cm⁻¹ for bands between 15 and 20 µm. For the bands between 9 to 10 µm, the FWHM was scaled in a linear fashion (in wavenumber space) from 30 to 10 cm⁻¹. The reason for picking these values has been discussed previously (Bauschlicher et al. 2009). We do not account for overtones, combination bands, and resonances. Overtones and combination bands are usually much weaker than fundamental bands. Resonances might induce band splittings that could affect the shape but not the overall trends of our spectra, which should be valid.

Typically the astronomical PAHs are observed as the emission from highly vibrationally excited molecules. Following the single-photon emission model (Bauschlicher et al. 2010) for computing emission spectra, we assume that a single PAH molecule is electronically excited by a photon with an energy between 4 and 9 eV and that the subsequent excited state intersystem crosses to the ground electronic state. The temperature reached by the ground state PAH molecule is computed using the heat capacity, which is determined using the B3LYP harmonic vibrational levels. IR emission cools the molecule to the background temperature of 2.73 K. Our emission spectrum represents the total emission that occurs during the IR cooling process. Because of the vibrational temperatures reached, some of the emission comes from higher vibrational levels and is therefore redshifted. Explicitly including this effect is quite difficult, so we approximate this by shifting our computed emission spectra by 15 cm⁻¹ to the red. Intensity ratios are calculated using our computed emission spectra.

2.2. Observations

We use the spectral maps of three RNe, namely NGC 7023, NGC 1333, and NGC 2023, previously presented by Stock et al. (2016), Peeters et al. (2017), and Stock et al. (2017). These spectral maps were taken with the Infrared Spectrograph (IRS, Houck et al. 2004) on board the Spitzer Space Telescope (Werner et al. 2004) using the short-low mode (SL). The SL mode covers approximately 5–15 µm at a spectral resolution ranging from 60 to 128 and has a pixel size of 1.8″. Observational data for NGC 2023, both the north and south, are taken from Peeters et al. (2017) and those for NGC 1333 and NGC 7023 are taken from Stock et al. (2016) and Stock et al. (2017). The Spitzer field-of-view towards NGC 1333 includes both the reflection nebula NGC 1333 and quiescent regions (Stock et al. 2016); in this paper we only consider the reflection nebula. For details on the observations and data reduction, we refer the reader to Stock et al. (2016) and Peeters et al. (2017).

In this study, we focus on emission in the 6–9 µm wavelength range in irradiated PDRs and aim to reproduce the relative intensities of the four Gaussian components (G7.6, G7.8, G8.2, G8.6) as defined by Peeters et al. (2017) for NGC 2023 and slightly modified by Stock et al. (2017) for the entire sample. To this end, we refer to the nominal 6.2 PAH feature and the Gaussian 7.6, 7.8, 8.2 and 8.6 components as the 6.2, 7.6, 7.8, 8.2 and 8.6 PAH band in the remainder of this paper.

3. RESULTS

3.1. PAH stabilities

To compare the stabilities of the even and odd-carbon PAHs studied in this work to those of the PAHs included in the PAHdb we computed the average binding energy per carbon, E_C, defined as

$$E_C=(\mathit{\text{AE}}-N_H{D}_e(C-H))/N_C$$

where AE is the atomization energy computed using the total energies without including zero-point energies, N_H is the number of C-H bonds, D_e is the average C-H bond energy computed at this level of theory for naphthalene and anthracene, its value is 4.78 eV, and N_C is the number of carbons. The advantage of only using E_C values instead of using a mix of IP and EA values is that it allows to compare the stability of the neutral and charged PAHs in a consistent way. The E_C values (in eV) for each PAH molecule considered in this study are given in Figures 1 and 2 and, along with those for the PAHs included in the PAHdb, are plotted as a function of the number of carbons, N_C, in Figure 3. The total energies used to compute the atomization energies are given in Tables 1 and 2. Given the large number of PAHs present in the PAHdb we only plotted the E_C values for N_C ≥ 50. For comparison purposes the data were fitted using an exponential of the type $e^{a+{\mathit{\text{bN}}}_C+{\mathit{\text{cN}}}_C^2}$, their graphs are given in Figure 3 and the fitting parameters are summarized in Table 3. Overall, the trends in stabilities of our neutral and charged even and odd-carbon PAHs are in agreement with those of the most stable PAHs in the PAHdb. We should note that the PAHdb contains a wide range of PAH structures, including PAHs with protruding hexagonal rings and defects, which are significantly less stable than those considered in this study. The most stable PAHs are those with compact, hexagonal or almost hexagonal, shapes, such as C₅₄H₁₈, C₅₉H₁₉, C₆₆H₂₀, and C₇₃H₂₁, as already reported by Ricca et al. (2012) for even-carbon PAHs. Such PAHs are expected to survive the harsh conditions in the ISM (Micelotta et al. 2010). Perusal of the binding energies per carbon given in Figures 1 and 2 reveals that the PAH stability decreases with aspect ratio as PAHs become more elongated. Similar trends were obtained for graphene nanoakes (Wohner et al. 2014).

Figure 3.

The average binding energies per carbon (E_C, eV) for the even-carbon neutral (light green cross), cationic (red cross), anionic (blue cross) PAHs and for the odd-carbon neutral (dark green triangle), cationic (orange triangle), and anionic (purple triangle) PAHs considered in this study with at least 50 carbons. For comparison we also include the E_C values for all the neutral and cationic PAHs included in the PAHdb with at least 50 carbons.

Table 1.

The total energy in Hartrees of the structures shown in Figure 1. The symmetry is given for the neutral; for the point groups with degenerate representations, the ions can break symmetry.

Molecule	Cation	Neutral	Anion
D6h
C24H12	−920.452758	−920.702470	−920.703167
C54H18	−2066.035845	−2066.252989	−2066.292612
C96H24	−3668.408513	−3668.607133	−3668.670216
D2h
C30H14	−1150.043239	−1150.254621	−1150.297016
C32H14	−1226.188860	−1226.413990	−1226.442070
C40H16	−1531.911287	−1532.119087	−1532.167023
C42H16	−1608.049055	−1608.263072	−1608.304243
C48H18	−1837.603890	−1837.791454	−1837.861846
C48H18	−1837.625830	−1837.821162	−1837.883661
C56H20	−2143.335575	−2143.521958	−2143.595169
C64H22	−2449.042262	−2449.222324	−2449.303379
C64H20	−2447.890265	−2448.081246	−2448.149421
C66H20	−2524.032162	−2524.232863	−2524.291064
C78H22	−2982.018966	−2982.207207	−2982.279764
C80H22	−3058.155472	−3058.350349	−3058.415987
C90H24	−3439.985847	−3440.161820	−3440.248123
C90H24	−3439.999885	−3440.178842	−3440.262340
C102H26	−3897.977014	−3898.149457	−3898.240977
C110H26	−4202.529458	−4202.708821	−4202.793026
C112H26	−4278.669966	−4278.856200	−4278.933273
C120H28	−4584.342534	−4584.509127	−4584.607179
C2h
C22H12	−844.320442	−844.549245	−844.571966
C28H14	−1073.905394	−1074.117628	−1074.159408
C34H16	−1303.484244	−1303.684473	−1303.740243
C38H16	−1455.758070	−1455.956239	−1456.014091
C46H18	−1761.467988	−1761.656341	−1761.725931
C52H18	−1989.904093	−1990.104328	−1990.161485
C54H20	−2067.174810	−2067.355951	−2067.434368
C58H20	−2219.445779	−2219.625473	−2219.705417
C62H20	−2371.752728	−2371.941683	−2372.012084
C72H22	−2753.596795	−2753.777328	−2753.857845
C76H22	−2905.871664	−2906.053894	−2906.132491
C82H24	−3135.393283	−3135.562287	−3135.655632
C88H24	−3363.849720	−3364.024620	−3364.111923
C94H24	−3592.276440	−3592.461397	−3592.538587
C108H26	−4126.392929	−4126.569036	−4126.656510
C148H30	−5651.449224	−5651.624087	−5651.715017

Table 2.

The total energy in Hartrees of the structures shown in Figure 2.

Molecule	Cation	Neutral	Anion
D3h
C37H15	−1417.132496	−1417.324320	−1417.387482
C73H21	−2791.103072	−2791.282142	−2791.363235
C2υ
C31H15	−1188.696504	−1188.899564	−1188.952888
C45H17	−1722.830131	−1723.026854	−1723.087856
C47H17	−1798.985548	−1799.174425	−1799.242594
C57H19	−2180.831121	−2181.020426	−2181.090418
C59H19	−2256.974983	−2257.159809	−2257.233701
C67H21	−2562.651839	−2562.845939	−2562.912815
C67H21	−2562.672557	−2562.862867	−2562.934083
C77H23	−2944.511723	−2944.703163	−2944.775271
C85H23	−3249.073314	−3249.256429	−3249.335265
C87H23	−3325.221742	−3325.400480	−3325.483553
C87H25	−3326.349701	−3326.543966
C93H25	−3554.736814	−3554.934861
C111H27	−4241.127927	−4241.319255
C115H27	−4393.433750	−4393.617669
Cs
C35H15	−1340.980789	−1341.180239	−1341.236636
C43H17	−1646.690702	−1646.888636	−1646.948881
C51H19	−1952.397313	−1952.594325	−1952.657626
C55H19	−2104.673841	−2104.869231	−2104.933505
C59H21	−2258.102030	−2258.299164
C65H21	−2486.514476	−2486.709652	−2486.776112
C67H23	−2563.805605	−2564.004764
C71H21	−2714.956285	−2715.141131	−2715.216878
C79H23	−3020.626983	−3020.822300
C83H23	−3172.932974	−3173.119341	−3173.195619
C91H25	−3478.589638	−3478.799532
C95H25	−3630.907113	−3631.095534	−3631.171695
C95H27	−3632.028235	−3632.233747
C99H25	−3783.185636	−3783.369419	−3783.449098
C103H27	−3936.572716	−3936.776859
C107H27	−4088.879958	−4089.072311
C107H27	−4088.845003	−4089.047876
C113H27	−4317.295211	−4317.481962	−4317.560339
C115H29	−4394.542880	−4394.754162
C119H27	−4545.726879	−4545.904691	−4545.991572
C119H29	−4546.852218	−4547.049936
C121H29	−4622.952313	−4623.161181
C127H31	−4852.516127	−4852.722237

Table 3.

Fitting parameters of the exponential function ( $e^{(a+b{N}_C+{\text{cN}}_C^2)}$) used to fit the E_C values, as a function of the number of carbons, for neutral and charged even and odd-carbon PAHs (see Figure 3) and the R² values. The error is given in parenthesis.

	Neutral		Cation		Anion
Parameter	Even C	Odd C	Even C	Odd C	Even C	Odd C
a	1.79224 (±0.01185)	1.78064 (±0.01434)	1.82455 (±0.01)	1.81657 (±0.01432)	1.79107 (±0.01037)	1.77957 (±0.01559)
b (10−3)	1.57 (±0.27)	1.89 (±0.37)	1.23 (±0.23)	1.45 (±0.33)	1.64 (±0.23)	2.0 (±0.38)
c (10−6)	5.0 (±1.43)	−7.42 (±1.88)	−3.92 (±1.21)	−5.7 (±1.85)	−5.45 (±1.25)	−7.84 (±2.27)
R2	0.91502	0.89923	0.90355	0.83212	0.93239	0.94387

The E_C values increase rapidly in the N_C range 50–150. Ricca et al. (2012) reported that a plateau is reached for 400 carbons and the E_C values converge to the computed value of graphite (7.3 eV), which validates the B3LYP approach for treating large PAHs. The cations are more stable than the neutrals and anions whereas the anions are more stable than the neutrals. The ionization potential (IP) of the neutral PAH is much smaller than that of the carbon atom, due to the ability of the PAH molecule to delocalize the charge. Therefore, the difference in energy between the cation and the neutral at the dissociation limit is larger than at the equilibrium geometry (bottom of the well), which makes the atomization energy larger for the cations than the neutrals. Unlike the IP, the electron affinity (EA) of the neutral PAH and that of the carbon atoms are similar and relatively small, making the atomization energy of the anions only slightly larger than the neutrals. The fits show that neutral and cationic even and odd-carbon PAHs have similar stabilities for N_C ≤ 70 but for N_C > 70 they start to differ as the E_C values for odd-carbon PAHs begin to converge whereas those for even-carbon PAHs continue to grow. Anionic even and odd-carbon PAHs have very similar stabilities for N_C ≤ 110. Odd-carbon PAHs with N_C > 110 have elongated structures and their anions tend to have a polyradical character, which reduces their stability.

3.2. Infrared Spectra

3.2.1. The 3 µm region

The band positions and intensities for neutral, cationic, and anionic even-carbon and odd-carbon PAHs are summarized in Tables 9 and 10 and the average absorption spectra are shown in Figure 4. The band position is dependent on charge and shifts to longer wavelengths going from the cations, to the neutrals, and to the anions. The intensities in absorption are proportional to the square of the dipole derivative with respect to the C-H stretching mode. The dipole derivatives of the C-H stretch increase going from the cations to the neutrals, and to the anions, which explains the increase in intensities going from the cations to the anions. These trends are in agreement with previous work (Langhoff 1996; Bauschlicher et al. 2008; Schmidt et al. 2009; Bauschlicher et al. 2009; Pauzat et al. 2010; Ricca et al. 2012; Peeters et al. 2017). Overall the average spectra of the even-carbon and odd-carbon PAHs are in good agreement with each other. For the cations, the peak positions differ by 0.003 µm whereas for the neutrals and anions they are identical. The difference in average intensities between even and odd-carbon PAHs is the largest for the cations (142.8 km/mol) and decrease for the neutrals (63.6 km/mol) and for the anions (57.6 km/mol). The intensities per carbon are very weak for small cations of both even and odd-carbon PAHs and increase with cation size to reach values comparable to those of the neutrals for C₁₅₀. The dipole moment derivative with respect to the C-H stretching coordinate is close to zero for PAH cations containing 20 carbons or so and slowly converge to the values of the neutrals with increasing size (Schmidt et al. 2009). The intensities per carbon for the anions of even and odd-carbon PAHs are the largest (60–70 km/mol) and are insensitive to size whereas those for the neutrals slowly increase with size.

Table 9.

Band positions (µm) and intensities (km/mol) of the even-carbon PAHs in the 3 µm region. A redshift of 15 cm⁻¹ has been applied.

	Cations		Neutrals		Anions
	λ	I	λ	I	λ	I
C22H12	3.251	41.9	3.276	280.4	3.312	782.2
C24H12	3.255	57.6	3.280	296.4	3.314	787.4
C28H14	3.253	84.3	3.275	346.2	3.306	873.4
C30H14	3.255	80.3	3.275	367.9	3.304	903.1
C32H14	3.259	109.7	3.280	375.4	3.308	872.1
C34H16	3.255	139.4	3.275	412.8	3.301	980.5
C38H16	3.258	137.2	3.275	463.2	3.299	1072.9
C40H16	3.261	169.0	3.280	452.6	3.304	973.6
C42H16	3.261	175.9	3.280	473.7	3.303	976.9
C46H18	3.258	222.3	3.275	561.4	3.296	1292.8
C48H18	3.261	218.4	3.275	595.4	3.295	1364.7
C48H18	3.263	235.2	3.280	527.4	3.301	1081.1
C52H18	3.264	250.8	3.280	577.0	3.300	1112.9
C54H18	3.265	285.9	3.280	576.3	3.300	1077.3
C54H20	3.258	357.8	3.275	658.1	3.293	1591.2
C56H20	3.264	309.7	3.280	596.7	3.298	1191.9
C58H20	3.270	377.4	3.275	730.1	3.293	1821.5
C62H20	3.266	340.9	3.208	678.3	3.299	1277.2
C64H20	3.265	340.1	3.281	716.0	3.298	1302.9
C64H22	3.266	397.7	3.280	659.9	3.296	1308.1
C66H20	3.267	364.1	3.281	686.8	3.299	1207.1
C72H22	3.267	452.3	3.279	771.6	3.296	1463.4
C76H22	3.267	464.0	3.281	860.3	3.296	1567.3
C78H22	3.268	451.0	3.281	799.0	3.297	1358.4
C80H22	3.269	471.2	3.282	814.1	3.296	1357.3
C82H24	3.277	497.0	3.275	1073.3	3.290	1811.7
C88H24	3.268	630.8	3.281	992.5	3.296	1894.1
C90H24	3.268	655.5	3.281	1056.9	3.295	2016.2
C90H24	3.269	554.8	3.282	904.3	3.296	1530.7
C94H24	3.270	579.2	3.282	953.1	3.296	1541.8
C96H24	3.272	629.5	3.284	931.3	3.295	1479.9
C102H26	3.271	688.1	3.282	996.6	3.294	1720.8
C108H26	3.271	710.9	3.282	1088.5	3.295	1771.7
C110H26	3.272	729.2	3.281	1131.0	3.294	1810.2
C112H26	3.273	714.5	3.283	1069.6	3.295	1633.7
C120H28	3.272	696.5	3.281	1495.2	3.293	1624.3
C148H30	3.275	996.0	3.284	1394.0	3.293	2066.1
average	3.268	395.0	3.281	739.5	3.297	1364.8

Table 10.

Band positions (µm) and intensities (km/mol) of the odd-carbon PAHs in the 3 µm region. A redshift of 15 cm⁻¹ has been applied.

	Cations		Neutrals		Anions
	λ	I	λ	I	λ	I
C31H15	3.254	107.5	3.276	379.9	3.303	929.9
C35H15	3.261	118.8	3.278	403.4	3.304	924.0
C37H15	3.260	129.7	3.279	425.2	3.305	922.8
C43H17	3.263	184.4	3.279	483.0	3.300	1055.5
C45H17	3.265	184.2	3.279	490.6	3.300	1027.6
C47H17	3.263	203.2	3.281	521.0	3.302	1036.5
C51H19	3.265	269.7	3.279	561.5	3.297	1212.3
C55H19	3.267	258.1	3.279	586.3	3.297	1171.5
C57H19	3.265	288.4	3.281	613.7	3.299	1177.9
C59H19	3.265	290.0	3.281	625.0	3.299	1156.0
C59H21	3.266	379.4	3.279	628.9
C65H21	3.269	356.2	3.280	680.5	3.295	1381.4
C67H21	3.269	339.1	3.280	695.3	3.296	1301.0
C67H21	3.267	396.2	3.281	706.5	3.297	1349.7
C67H23	3.267	507.5	3.280	691.1
C71H21	3.268	384.5	3.281	729.0	3.298	1297.9
C73H21	3.267	393.0	3.281	751.4	3.297	1283.6
C77H23	3.268	541.7	3.282	788.1	3.295	1542.8
C79H23	3.271	448.3	3.281	788.1
C83H23	3.269	500.0	3.282	840.3	3.296	1483.7
C85H23	3.269	480.9	3.282	842.1	3.296	1428.9
C87H23	3.270	503.1	3.283	866.7	3.296	1430.0
C87H25	3.270	720.7	3.281	856.2
C91H25	3.273	602.2	3.282	869.7
C93H25	3.273	566.7	3.282	879.7
C95H25	3.270	659.4	3.282	936.3	3.294	1717.6
C95H27	3.273	823.9	3.281	901.9
C99H25	3.271	593.2	3.283	967.9	3.295	1613.6
C103H27	3.275	797.2	3.282	956.2	3.294	1875.0
C107H27	3.272	880.8	3.282	1011.5
C107H27	3.275	725.4	3.282	973.9
C111H27	3.271	907.5	3.281	1270.2
C113H27	3.272	742.5	3.283	1070.0
C115H27	3.272	707.4	3.283	1083.2
C115H29	3.275	1002.7	3.283	1040.8
C119H27	3.273	748.1	3.284	1118.2	3.294	1746.4
C119H29	3.273	1120.1	3.282	1096.6
C121H29	3.276	924.6	3.283	1070.2
C127H31	3.275	1188.7	3.283	1119.7
average	3.271	537.8	3.281	803.1	3.297	1307.2

Figure 4.

Average absorption spectra in the 3 µm region for all the neutral (solid green), cationic (solid red), and anionic (solid blue) even-carbon PAHs and for the neutral (dashed green), cationic (dashed red) and anionic (dashed blue) odd-carbon PAHs. The total average spectrum (black) has been obtained by using all the neutral and charged species with equal weight. The spectra have been redshifted by 15 cm⁻¹. A vertical dashed line has been added at the observed dominant band at 3.3 µm.

3.2.2. The 6–10 µm region

The spectra of the cations and anions dominate the 6–10 µm region (see Figure 5), as expected. The average C-C stretching band falls at 6.42 µm for even-carbon PAHs and at 6.45 µm for odd-carbon PAHs. These values are in agreement with previous work on large compact and highly symmetric PAHs with straight edges (Bauschlicher et al. 2008; note that our spectra are redshifted by 15 cm⁻¹ to account for emission). The average C-C stretch intensities are very similar for the cations and anions and those of odd-carbon PAHs are significantly larger than those of the even-carbon PAHs.

Figure 5.

Average absorption spectra in the 6–10 µm region for all the neutral (solid green), cationic (solid red), and anionic (solid blue) even-carbon PAHs and for the neutral (dashed green), cationic (dashed red) and anionic (dashed blue) odd-carbon PAHs. The total average spectrum (black) has been obtained by using all the neutral and charged species with equal weight. The spectra have been redshifted by 15 cm⁻¹. Vertical dashed lines have been added at the observed dominant bands at 6.2, 7.7, and 8.6 µm.

Inspection of Tables 11 and 12 shows that the 7.7 µm C-H in plane bending feature shifts to longer wavelengths with PAH size for both even and odd-carbon PAHs, although the increase is slower for odd than even carbon PAHs. The 7.7 µm feature peaks between 7.2 and 7.6 µm for odd-carbon PAHs with C < 55, between 7.6 and 7.7 µm for even-carbon PAHs with 38 ≤ C ≤ 54 and for odd-carbon PAHs with 55 ≤ C ≤ 71, between 7.7 and 7.8 µm for even-carbon PAHs with 54 ≤ C ≤ 86 and for odd-carbon PAHs with 71 ≤ C ≤ 113, between 7.8 and 7.9 µm for even-carbon PAHs with 86 ≤ C ≤ 120 and for odd-carbon PAHs with 113 ≤ C ≤ 127. The 7.7 µm band intensities are very similar for odd-carbon PAH cations and anions and somewhat different for even-carbon PAH cations and anions. The relative intensities of the 7.7 µm and 6.2 µm bands are very different for even and odd-carbons PAHs. The 7.7 µm feature is stronger than the 6.2 µm one for even-carbon PAHs and the opposite is true for odd-carbon PAHs. Interestingly, the ratio of even to odd-carbon PAHs thus affects the 7.7/6.2 µm ratio of the overall spectrum.

Table 11.

Band positions (µm) and intensities (km/mol) of the even-carbon PAHs in the 6–10 µm region. A redshift of 15 cm⁻¹ has been applied.

Neutrals
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
9.426	6.2	8.887	13.0	9.501	4.9	9.766	3.7	9.546	1.7	9.476	4.7
8.542	14.6	8.339	2.0	8.639	13.4	9.118	2.3	8.678	16.1	8.566	25.6
8.080	8.1	7.710	48.1	8.360	6.8	8.546	13.8	8.162	17.5	8.085	10.0
7.629	7.7	7.289	1.7	8.187	8.1	8.157	11.1	7.768	25.5	7.699	8.0
7.294	2.2	6.759	2.7	7.678	8.3	7.857	11.5	7.353	22.8	7.458	1.7
7.041	12.9	6.298	26.2	7.067	43.0	7.249	22.6	6.998	3.7	7.064	82.8
6.844	7.3			6.442	26.9	6.799	13.6	6.643	3.7	6.420	33.6
6.532	10.4			6.231	11.4	6.490	38.3	6.293	35.6	6.248	13.4
6.374	8.3					6.285	9.4
6.253	10.2
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
9.656	1.7	9.979	1.5	9.601	1.6	9.879	1.6	9.504	4.1	9.908	1.6
9.420	1.9	9.178	9.4	8.578	25.3	9.574	3.9	8.909	3.2	9.051	25.3
8.475	26.1	8.787	16.9	8.126	32.5	8.477	26.5	8.508	49.8	8.309	5.0
7.977	17.1	8.256	2.1	7.682	25.3	8.009	28.8	7.668	6.2	7.785	69.6
7.598	11.0	7.852	55.4	7.408	7.0	7.823	31.6	7.268	60.8	7.421	13.1
7.355	9.3	7.232	12.3	7.216	16.2	7.256	48.6	6.819	52.7	7.225	10.9
7.250	11.9	7.046	12.4	6.866	9.4	6.786	66.1	6.561	94.1	6.894	37.0
6.823	41.8	6.739	19.2	6.288	51.1	6.413	79.1			6.648	26.9
6.407	61.5	6.312	47.5							6.275	56.8
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
9.818	2.3	9.938	12.6	9.714	10.3	9.835	1.4	9.199	9.4	9.917	4.5
9.419	4.1	8.672	31.5	8.871	5.9	9.454	1.8	8.955	3.1	9.649	2.1
8.663	28.8	7.868	70.2	8.461	27.5	8.833	26.6	8.486	61.7	9.422	4.3
8.185	40.3	7.317	16.6	7.784	113.0	8.261	6.8	7.875	11.7	9.139	10.0
7.689	10.5	6.967	3.4	7.253	71.7	7.770	115.2	7.511	25.2	8.518	56.2
7.457	37.5	6.749	15.5	6.771	121.8	7.156	16.0	7.237	56.2	8.047	27.2
7.206	24.6	6.283	61.0	6.442	94.7	6.894	61.8	6.979	70.3	7.463	48.4
6.769	16.5					6.646	48.1	6.756	29.0	7.169	52.4
6.303	69.8					6.291	63.7	6.585	77.6	6.790	35.4
								6.470	47.3	6.312	99.7
								6.306	17.6
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
9.847	3.9	9.709	1.3	9.407	7.0	9.754	2.6	9.367	14.9	9.974	2.2
9.633	5.2	8.849	27.2	8.920	6.9	9.447	2.5	9.142	5.0	9.523	2.0
9.136	12.3	8.282	9.9	8.660	8.3	9.117	33.7	8.775	6.2	8.819	32.7
8.585	30.9	7.737	141.4	8.371	52.6	8.604	36.4	8.442	28.1	8.589	38.7
8.047	39.4	7.443	25.3	7.921	34.5	8.127	43.8	8.135	57.3	7.766	68.4
7.643	73.4	6.923	106.5	7.655	5.4	7.798	9.3	7.624	75.9	7.542	27.8
7.309	15.3	6.659	77.3	7.309	41.9	7.468	64.7	7.405	62.7	7.231	31.0
6.974	49.2	6.326	67.3	7.037	1.8	7.171	101.9	6.971	29.6	6.738	51.2
6.312	93.4			6.623	31.9	6.793	64.5	6.799	51.4	6.319	123.0
				6.298	77.8	6.326	130.1	6.314	121.5
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
9.674	13.7	9.613	7.3	9.465	20.5	9.722	19.3	9.969	4.4	9.700	4.5
9.074	9.4	8.994	9.6	9.106	10.7	9.351	17.9	9.768	4.1	9.158	18.3
8.488	71.3	8.642	55.3	8.507	48.1	8.438	86.4	9.319	1.3	8.435	116.7
7.921	42.0	8.388	60.9	7.997	54.7	8.056	62.3	8.788	41.3	7.598	42.1
7.306	52.2	7.776	27.8	7.576	174.7	7.677	76.1	8.392	17.3	7.317	70.8
7.054	17.7	7.509	66.1	7.353	46.8	7.369	92.9	8.169	33.7	7.034	11.6
6.870	5.5	7.255	73.1	6.857	150.2	7.112	84.3	7.769	86.7	6.700	48.6
6.585	32.8	7.001	135.0	6.324	156.7	6.756	26.1	7.298	68.1	6.325	147.1
6.308	97.6	6.812	82.5			6.576	25.3	6.756	121.2
		6.695	152.2			6.306	158.1	6.382	158.7
		6.455	165.3
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
9.772	21.2	8.515	74.3	9.982	3.1	9.476	22.3	9.764	17.4	9.983	12.0
9.475	3.3	8.099	12.7	9.553	5.5	8.839	49.5	9.430	8.3	9.421	75.8
9.080	1.1	7.717	183.8	9.122	18.3	8.503	55.7	8.754	66.8	8.546	103.1
8.703	77.2	7.209	44.4	8.482	121.9	7.798	124.1	8.269	48.9	8.161	77.7
8.386	4.9	6.777	221.3	8.008	28.2	7.437	26.8	7.977	22.0	7.926	82.0
7.940	56.5	6.423	189.4	7.575	47.9	7.105	73.0	7.754	23.6	7.556	241.3
7.678	17.0			7.291	147.4	6.868	13.9	7.564	26.3	7.301	154.3
7.488	13.4			6.684	115.6	6.657	62.2	7.332	33.6	7.007	95.4
7.130	20.6			6.367	177.6	6.299	175.6	7.027	33.5	6.842	56.6
6.895	43.4							6.799	52.2	6.705	43.9
6.536	41.9							6.346	184.5	6.317	310.8
6.326	105.2
C148H30		average
9.988	4.9	9.979	2.6
9.702	7.6	9.741	3.5
9.513	6.1	9.433	6.4
9.220	15.0	8.486	62.1
8.750	78.8	7.736	89.4
8.319	99.5	7.275	45.9
7.732	55.9	6.778	83.5
7.357	129.5	6.321	111.1
6.926	48.7
6.747	96.5
6.399	299.6
Cations
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
9.295	5.8	8.850	54.4	9.989	10.1	9.722	10.8	9.508	8.0	9.394	1.7
8.763	7.7	8.347	90.7	9.415	1.1	9.095	9.0	8.511	90.7	8.354	444.9
8.472	18.7	7.474	440.6	8.821	11.8	8.535	16.0	8.207	182.8	8.076	104.4
8.163	271.3	6.715	56.0	8.233	355.0	8.124	167.7	7.596	329.0	7.568	856.5
7.402	165.6	6.502	460.9	7.528	456.1	7.568	232.7	7.362	82.7	7.106	278.2
7.005	49.5			7.209	82.7	7.224	26.7	7.099	102.4	6.713	17.5
6.801	46.1			6.946	60.0	6.855	83.3	6.551	158.5	6.337	557.2
6.430	261.6			6.588	27.5	6.614	74.8	6.421	260.8
				6.360	358.8	6.404	234.7
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
9.605	8.2	9.914	13.6	9.264	31.9	9.837	5.5	9.512	16.7	9.856	23.2
9.396	1.0	9.108	3.8	8.651	66.7	9.562	11.1	8.903	18.3	8.970	34.4
9.117	2.9	8.125	444.5	8.095	404.0	9.080	3.2	8.514	154.2	8.320	462.7
8.391	164.2	7.617	577.8	7.449	294.8	8.457	353.4	7.639	558.8	8.073	86.8
7.606	554.8	7.220	28.5	6.964	222.8	8.046	149.8	7.232	128.8	7.656	978.5
6.843	140.1	7.086	44.7	6.404	656.6	7.669	867.5	6.799	196.8	7.140	19.1
6.568	201.6	6.846	33.5			7.325	136.5	6.534	490.6	6.962	149.5
6.332	194.3	6.424	337.8			6.811	198.9	6.308	115.5	6.632	10.4
						6.478	501.1			6.406	338.7
						6.296	145.3
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
9.806	3.0	9.916	1.3	9.707	21.4	9.797	29.6	9.440	22.0	9.893	2.8
9.042	26.7	9.316	12.2	9.217	4.3	9.413	35.0	9.183	59.1	9.628	13.8
8.408	318.1	8.511	195.7	8.485	889.5	8.409	663.9	8.508	415.4	9.387	10.5
8.178	106.9	8.070	472.5	7.705	2021.6	8.034	244.9	7.735	1203.1	9.094	26.9
7.670	660.0	7.693	301.7	7.308	189.1	7.687	1263.4	7.260	221.8	8.459	544.2
7.071	131.7	7.464	363.3	6.879	424.5	7.375	271.3	6.996	172.4	8.004	113.9
6.865	51.1	7.062	85.8	6.630	151.3	6.904	309.1	6.781	226.6	7.732	1168.1
6.724	96.1	6.799	357.5	6.467	839.9	6.371	485.2	6.494	1163.3	7.157	104.7
6.366	554.0	6.428	1190.6	6.287	107.2					6.970	120.7
										6.592	293.5
										6.328	412.4
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
9.807	9.2	9.674	89.0	9.365	7.4	9.828	29.4	9.108	29.9	9.481	8.9
9.639	4.8	8.457	1219.2	8.428	364.5	9.445	32.3	8.488	662.0	9.189	5.5
9.094	30.2	8.039	230.1	7.952	322.9	9.056	42.7	8.119	166.0	8.386	369.0
8.475	375.0	7.722	1888.7	7.573	762.0	8.485	1094.3	7.820	1191.1	7.756	1183.7
7.691	824.0	7.368	615.1	7.012	101.2	8.094	181.9	7.309	262.5	7.220	85.6
7.172	109.1	6.898	681.8	6.829	190.6	7.775	1764.4	6.850	193.1	6.828	189.3
6.956	61.4	6.450	378.4	6.641	78.3	7.518	220.9	6.703	331.7	6.328	369.2
6.742	310.8	6.367	386.8	6.402	691.7	7.120	458.9	6.431	772.6
6.408	649.0					6.565	622.0
						6.320	502.6
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
9.779	17.4	9.439	99.6	9.450	18.3	9.737	47.2	9.958	4.0	9.996	4.3
9.431	1.5	8.925	117.2	9.093	57.3	9.347	26.5	9.777	13.1	9.692	2.0
8.530	671.2	8.490	663.1	8.532	1736.9	8.511	687.9	8.973	77.8	9.134	34.0
7.776	1151.0	7.721	3113.8	7.843	2938.5	7.839	1371.1	8.455	554.6	8.493	758.5
7.439	199.1	7.298	120.5	7.272	503.1	7.405	476.6	7.798	1763.4	7.854	1474.0
7.248	173.8	6.960	213.4	6.738	929.9	6.908	260.7	7.212	159.3	7.268	291.5
6.917	239.2	6.770	418.7	6.447	1338.0	6.633	579.6	6.782	260.2	6.864	307.4
6.636	434.5	6.522	1549.6			6.425	672.4	6.290	352.1	6.648	299.7
6.381	1146.5	6.303	185.4							6.371	863.3
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
9.774	28.0	9.364	44.2	9.530	1.3	9.614	22.4	9.764	16.3	9.836	2.7
9.468	48.4	8.939	131.1	9.103	52.4	9.445	21.9	9.423	39.1	9.348	58.6
9.066	53.0	8.568	1418.0	8.562	1358.9	8.894	107.8	8.543	648.6	8.953	81.0
8.516	1005.3	7.835	3079.7	7.880	2220.0	8.530	963.1	7.824	1971.2	8.554	927.0
7.660	2438.6	7.193	364.9	7.355	360.6	7.888	1777.8	7.322	206.8	7.891	2496.5
6.906	970.7	6.955	177.6	6.844	437.7	7.452	345.0	7.023	194.5	7.230	562.8
6.407	2582.0	6.768	369.6	6.640	220.6	7.058	351.7	6.816	495.4	6.744	1479.8
		6.517	514.9	6.460	569.0	6.794	498.1	6.561	481.7	6.459	1331.9
		6.290	406.1	6.300	292.6	6.544	871.9	6.369	755.0	6.330	484.8
						6.389	741.4
C148H30		average
9.989	51.4	9.730	12.1
9.678	17.0	9.418	14.7
9.195	24.1	8.503	639.9
8.645	1554.8	7.771	1629.9
7.943	2625.3	6.804	390.4
7.383	468.1	6.423	886.0
7.191	334.0
6.784	912.8
6.509	902.5
6.313	405.6
Anions
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
9.577	13.9	9.099	71.6	9.488	3.1	9.827	21.7	8.794	41.9	9.460	3.3
8.881	6.5	8.323	130.0	8.921	4.6	8.621	52.9	8.290	30.2	8.445	323.0
8.565	18.7	7.834	411.5	8.297	223.8	7.923	343.4	7.760	708.9	7.994	127.4
8.063	179.6	7.463	118.0	7.986	23.2	7.639	100.4	7.499	168.3	7.683	681.1
7.712	298.1	6.894	152.5	7.676	480.0	7.030	34.8	7.216	36.8	7.383	9.4
7.190	24.3	6.738	24.1	7.164	67.7	6.592	95.7	6.943	62.8	6.980	288.1
7.035	8.4	6.482	268.5	6.981	61.9	6.388	107.4	6.528	128.4	6.518	369.0
6.526	110.3			6.546	154.4			6.403	150.1	6.320	334.1
6.413	167.1			6.341	254.6
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
9.677	2.1	9.995	1.0	9.572	2.3	9.873	5.5	9.604	17.2	9.922	1.9
9.463	2.3	9.182	2.2	9.261	5.1	9.610	19.8	8.494	366.4	9.084	37.7
8.482	189.1	8.829	24.1	8.597	93.1	9.134	27.7	7.865	295.0	8.350	273.1
7.883	275.1	8.353	55.6	8.125	460.4	8.782	83.8	7.669	298.3	7.778	1165.5
7.695	174.9	8.141	72.7	7.722	288.4	8.482	333.6	6.854	207.5	7.385	61.8
7.339	25.0	7.765	928.5	7.538	267.7	8.001	99.1	6.647	93.6	6.987	434.9
7.021	72.0	7.323	61.3	7.272	66.3	7.705	504.7	6.488	118.7	6.430	357.9
6.633	51.4	7.126	126.7	6.961	113.2	7.235	145.3	6.352	241.4
6.481	198.3	6.943	83.6	6.381	452.7	6.964	333.9
6.319	56.4	6.435	265.0			6.512	812.6
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
9.852	10.0	9.984	7.5	9.737	133.3	9.666	2.8	9.525	37.6	9.928	38.7
9.474	21.5	8.587	313.4	9.279	47.1	9.468	22.1	8.987	68.8	9.669	7.4
9.112	8.0	8.000	758.4	8.522	1125.9	8.860	62.3	8.554	998.4	9.449	15.4
8.691	51.3	7.754	265.5	7.949	139.3	8.408	528.3	7.798	597.3	9.147	14.0
8.417	144.4	7.593	620.2	7.705	876.7	7.795	1532.3	7.528	553.2	8.465	405.7
8.247	138.7	7.152	138.2	7.427	221.6	7.415	208.7	7.181	590.7	7.823	1176.1
7.808	954.4	6.809	78.3	7.222	418.2	6.952	629.6	6.814	655.3	7.455	129.7
7.231	52.1	6.430	727.0	6.992	1508.5	6.800	364.3	6.548	1543.0	7.185	253.8
7.038	92.5			6.522	1746.4	6.437	553.3	6.363	201.3	6.972	129.8
6.746	81.7									6.571	323.5
6.576	35.0									6.366	477.8
6.365	384.7
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
9.839	10.4	9.838	27.4	9.405	1.1	9.766	12.6	9.403	100.3	9.973	2.8
9.672	6.6	8.738	884.2	8.961	23.1	9.480	51.0	8.779	83.7	9.524	16.2
9.244	8.1	8.438	667.1	8.485	261.6	9.207	86.2	8.495	542.7	8.876	25.7
8.810	84.7	7.810	2152.1	7.973	708.4	8.549	905.2	8.214	124.4	8.463	127.6
8.452	404.8	7.448	802.6	7.750	669.9	7.844	1587.1	7.870	1082.7	7.827	1501.0
7.955	502.3	6.861	2203.5	7.352	175.7	7.480	413.1	7.392	234.8	7.243	259.6
7.707	354.1	6.466	826.8	7.070	94.1	7.158	942.2	6.875	345.4	6.807	159.1
7.358	89.2			6.826	45.2	6.864	293.3	6.686	127.6	6.683	80.6
7.020	146.9			6.676	59.1	6.578	1037.8	6.400	818.3	6.399	263.9
6.741	51.1			6.405	366.9	6.388	685.5
6.349	461.8
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
9.873	4.0	9.513	98.6	9.500	359.6	9.712	56.6	9.979	27.0	9.690	2.2
9.465	2.1	8.574	1122.7	9.117	71.3	9.377	62.9	8.663	229.1	9.418	8.0
9.091	8.5	7.802	2640.7	8.536	1590.9	8.929	160.3	8.468	230.9	9.176	8.5
8.476	841.8	7.285	97.2	7.864	2441.7	8.499	631.1	8.222	132.1	8.533	740.6
7.872	1407.6	7.038	199.0	7.331	809.2	8.258	272.1	7.876	1928.3	7.938	1569.4
7.452	179.9	6.583	717.1	6.860	1034.9	7.886	924.6	7.249	445.3	7.422	331.1
7.150	71.9	6.311	118.8	6.776	1159.8	7.390	247.0	6.841	638.0	7.036	76.4
6.962	137.0			6.466	1961.6	7.140	536.9	6.560	558.2	6.889	135.5
6.578	143.5					6.579	602.7	6.407	334.2	6.692	105.7
6.390	782.4					6.326	916.0			6.437	735.1
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
9.503	19.0	9.964	12.2	9.977	11.9	9.604	50.0	9.668	20.3	9.853	5.1
8.570	1354.1	9.477	43.2	9.564	15.9	8.607	1157.9	9.453	14.6	9.389	130.4
7.927	920.8	8.747	2528.6	8.579	1226.9	7.962	1678.5	8.613	521.8	8.941	152.4
7.765	1461.4	7.879	1587.1	7.960	2256.8	7.416	351.2	8.294	297.2	8.481	1113.6
7.229	193.5	7.727	2275.7	7.350	772.2	7.092	155.5	7.893	2045.8	7.860	1719.6
6.913	311.9	7.092	2165.0	6.711	1169.4	6.822	120.3	7.328	77.7	7.250	521.1
6.563	296.4	6.910	1387.0	6.471	1150.1	6.388	1291.6	7.102	132.3	7.035	158.6
6.416	1251.3	6.582	1601.3					6.847	144.7	6.835	506.1
		6.413	381.8					6.402	505.3	6.437	1648.0
										6.302	861.2
C148H30		average
9.517	19.5	9.964	2.7
9.237	53.2	9.491	41.5
8.664	1530.7	8.532	707.8
7.986	2522.2	7.838	1529.0
7.520	370.2	7.207	173.8
7.227	394.1	6.863	507.6
6.808	553.4	6.438	890.2
6.519	1210.9

Table 12.

Band positions (µm) and intensities (km/mol) of the odd-carbon PAHs in the 6–10 µm region. A redshift of 15 cm⁻¹ has been applied.

Neutrals
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
9.408	4.8	9.445	2.4	9.535	1.2	9.900	1.4	9.930	2.5	9.843	1.1
8.791	5.0	8.557	17.5	8.561	23.8	9.618	1.2	9.479	4.3	9.083	4.1
8.548	9.6	8.179	10.7	8.269	29.3	9.231	5.1	8.567	30.3	8.647	25.9
8.396	10.2	7.760	22.2	7.696	7.7	8.743	8.8	8.184	20.4	8.120	20.6
7.869	3.6	7.514	11.7	7.289	18.1	8.550	12.7	7.526	20.9	7.834	14.1
7.543	8.0	7.307	10.1	7.086	18.2	7.822	40.3	7.222	24.8	7.645	29.1
7.065	43.1	6.825	7.6	6.354	48.8	7.231	27.0	6.591	38.8	7.201	14.4
6.415	35.5	6.612	20.5			6.799	20.5	6.304	31.4	6.899	16.3
		6.319	26.1			6.566	20.1			6.331	57.8
						6.325	30.9
C51H19		C55H19		C57H19		C59H19		C59H21		C65H21
9.874	1.4	9.544	3.3	9.933	1.1	9.865	6.0	9.745	1.5	9.833	1.5
9.495	2.1	9.335	2.9	9.806	2.6	8.761	18.2	9.557	2.5	9.469	5.6
8.894	13.5	9.070	1.2	8.966	8.2	8.431	23.7	9.098	3.5	9.098	9.5
8.537	15.9	8.642	32.7	8.449	22.5	7.895	55.2	8.790	10.9	8.454	34.2
8.029	8.6	8.137	21.3	8.240	36.2	7.367	26.7	8.549	12.0	8.142	15.3
7.791	29.5	7.690	14.3	7.930	10.5	6.766	32.9	8.015	10.5	7.708	7.8
7.505	10.3	7.476	24.1	7.482	40.3	6.332	70.4	7.763	45.0	7.440	23.0
7.263	27.0	7.210	15.8	7.166	22.2			7.228	27.5	7.190	41.3
6.839	36.3	6.956	11.4	6.807	11.4			6.879	54.8	6.809	35.7
6.555	34.4	6.787	12.8	6.607	5.7			6.715	21.8	6.606	48.3
6.325	33.0	6.588	44.1	6.322	67.1			6.566	25.1	6.320	40.2
		6.314	38.6					6.322	28.3
C67H21		C67H21		C67H23		C71H21		C73H21		C77H23
9.807	2.4	9.930	3.6	9.695	3.4	9.840	2.2	9.532	2.3	9.768	2.8
9.492	8.8	9.412	2.1	9.194	3.2	9.739	1.2	9.093	9.7	9.433	2.5
9.088	11.3	9.084	3.2	8.831	10.8	9.453	5.7	8.430	75.7	9.075	5.7
8.616	19.8	8.613	42.5	8.530	10.8	8.456	50.1	8.014	4.1	8.684	31.5
8.078	30.4	8.098	35.5	8.006	16.8	7.916	42.3	7.315	76.0	8.169	43.2
7.604	40.8	7.515	55.6	7.736	37.7	7.635	26.5	6.974	5.0	7.843	20.0
7.275	13.6	7.168	32.7	7.345	31.2	7.328	24.2	6.642	38.3	7.444	43.1
6.998	21.5	6.800	21.2	6.900	64.4	7.018	10.8	6.349	83.9	7.163	43.2
6.772	30.0	6.576	15.9	6.688	49.8	6.835	16.9			6.813	49.2
6.579	50.4	6.329	78.5	6.324	27.2	6.722	23.8			6.583	28.7
6.336	45.9					6.337	82.9			6.344	81.9
C79H23		C83H23		C85H23		C87H23		C87H25		C91H25
9.929	4.2	9.986	3.7	9.830	2.7	9.760	7.7	9.866	1.6	10.000	1.3
9.341	13.7	9.453	3.3	9.726	1.9	9.438	7.1	9.038	10.6	9.428	6.1
8.423	27.0	8.710	49.0	9.149	10.7	9.045	8.1	8.565	36.1	9.083	4.4
8.075	10.4	8.100	41.7	8.794	11.6	8.610	62.3	8.169	46.3	8.673	25.8
7.615	39.3	7.588	63.5	8.420	73.8	7.899	45.3	7.549	45.5	8.045	22.1
7.358	26.5	7.348	31.8	7.955	11.4	7.651	14.4	7.155	50.2	7.601	44.6
7.031	17.6	6.743	59.3	7.328	68.2	7.271	63.4	6.821	74.1	7.293	24.0
6.778	52.6	6.336	104.6	6.939	31.4	6.905	12.9	6.583	28.5	6.793	79.9
6.596	52.2			6.682	47.4	6.587	47.1	6.331	70.9	6.600	55.5
6.353	39.9			6.366	101.6	6.337	92.9			6.357	38.3
C93H25		C95H25		C95H27		C99H25		C103H27		C107H27
9.783	2.4	9.416	3.4	9.931	1.3	9.657	1.3	9.615	4.6	9.932	2.6
9.546	1.7	8.559	49.9	9.471	4.8	9.424	6.6	9.464	2.7	9.762	2.1
9.368	4.5	8.096	39.8	9.045	9.7	9.002	10.8	9.074	9.5	9.430	1.6
8.934	8.2	7.609	71.5	8.570	30.4	8.485	66.5	8.564	21.5	9.147	4.6
8.410	30.5	7.280	45.3	8.180	25.7	7.933	28.0	8.183	30.1	8.592	44.1
8.033	20.1	6.755	97.3	7.886	8.0	7.655	32.3	7.659	60.6	8.063	43.8
7.329	54.4	6.335	109.4	7.475	35.9	7.303	69.5	7.093	20.4	7.665	64.5
6.988	34.9			7.154	38.7	6.993	28.2	6.775	96.2	7.266	36.7
6.634	105.1			6.821	84.0	6.710	72.4	6.599	57.5	6.775	126.1
6.471	39.1			6.609	56.7	6.364	115.8	6.354	39.9	6.330	101.7
				6.326	37.6
C107H27		C111H27		C113H27		C115H27		C115H29		C119H27
9.998	1.6	9.893	4.9	9.851	4.6	9.452	9.8	9.803	3.9	9.754	9.7
9.435	5.8	9.609	2.9	9.559	6.5	8.949	50.6	9.326	13.1	9.486	9.6
9.220	2.9	9.346	37.4	9.098	11.2	8.104	26.5	8.576	27.9	8.711	68.8
8.909	12.0	8.981	24.3	8.503	43.3	7.764	41.1	8.087	21.0	8.260	35.9
8.437	27.5	8.506	85.3	8.114	46.1	7.502	50.3	7.599	53.9	7.772	65.3
7.951	14.3	8.191	65.6	7.295	112.8	6.909	98.8	7.345	19.7	7.322	49.3
7.309	63.9	7.942	48.5	6.978	33.7	6.701	45.2	6.776	139.5	7.040	55.7
6.961	43.6	7.594	207.7	6.717	89.7	6.589	55.3	6.606	56.3	6.795	35.9
6.706	54.4	7.232	91.2	6.346	108.7	6.409	94.3	6.351	40.4	6.543	73.4
6.623	110.6	7.044	78.9							6.359	128.7
		6.691	70.2
		6.334	149.3
C119H29		C121H29		C127H31		average
9.784	1.3	9.988	1.2	9.420	8.1	9.836	3.4
9.421	2.5	9.605	4.3	9.142	5.2	9.414	5.6
8.651	53.6	9.320	8.5	8.710	25.9	8.538	44.1
8.077	47.0	8.781	8.6	8.107	28.2	8.118	27.8
7.632	68.7	8.439	20.4	7.642	51.4	7.579	45.7
7.290	30.9	8.187	26.2	7.333	23.7	7.288	43.4
6.765	136.1	7.758	9.2	7.066	14.9	6.766	64.0
6.328	105.3	7.292	62.1	6.778	147.9	6.625	26.2
		6.987	36.8	6.609	54.7	6.339	64.0
		6.722	167.1	6.351	41.5
		6.385	22.6
Cations
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
9.425	4.5	9.953	3.5	9.355	11.4	9.893	8.9	9.887	2.2	9.843	1.0
8.484	426.5	9.769	8.1	8.487	414.5	9.616	16.7	9.453	9.1	9.069	31.7
8.057	37.8	9.614	3.8	8.083	179.0	9.030	8.9	8.490	252.2	8.511	497.5
7.554	498.0	9.356	3.3	7.564	432.6	8.487	359.2	8.199	100.9	7.975	124.7
7.282	20.0	8.461	306.9	7.262	20.3	8.005	63.2	7.851	111.8	7.614	704.6
6.927	269.3	8.143	115.2	6.971	141.7	7.601	501.4	7.539	583.4	6.943	196.4
6.364	1491.2	7.515	571.0	6.745	147.0	7.313	206.1	7.069	156.7	6.588	200.7
		7.106	47.0	6.369	839.8	6.991	49.4	6.806	60.3	6.366	1001.5
		6.777	94.2			6.825	64.2	6.424	690.0
		6.381	877.1			6.626	156.4
						6.412	1029.9
C51H19		C55H19		C57H19		C59H19		C59H21		C65H21
9.602	36.5	9.760	1.6	9.796	11.8	9.875	8.7	9.762	20.7	9.479	27.0
8.516	426.3	9.363	17.4	9.542	15.8	9.258	6.5	9.556	26.4	9.106	26.2
8.003	93.5	8.532	292.1	8.526	555.7	8.521	559.5	9.090	3.9	8.527	331.4
7.656	411.4	8.078	54.8	8.189	59.1	7.669	1027.5	8.557	457.4	7.683	410.6
7.304	277.3	7.573	815.3	7.659	900.0	7.313	76.2	7.996	129.4	7.468	409.3
6.876	150.2	6.903	240.7	7.048	288.7	6.859	284.2	7.664	236.1	6.795	441.7
6.625	224.4	6.456	498.1	6.384	1546.6	6.379	1249.0	7.545	259.5	6.496	462.1
6.450	1194.4	6.304	78.5					7.283	308.5	6.288	131.9
								6.863	140.9
								6.488	1424.6
								6.303	140.8
C67H21		C67H21		C67H23		C71H21		C73H21		C77H23
9.344	12.6	9.803	33.4	9.711	36.9	9.980	5.8	9.907	3.6	9.746	25.8
9.070	33.9	9.368	38.7	9.561	35.3	9.421	15.3	9.517	1.3	9.282	98.5
8.477	255.6	9.065	12.7	8.579	487.7	8.534	560.8	9.062	24.9	8.556	579.8
8.113	80.3	8.536	581.6	7.997	231.9	7.697	1204.3	8.558	778.0	8.052	162.1
7.628	901.6	7.992	171.9	7.604	661.7	7.096	60.0	7.712	1209.7	7.792	400.1
7.197	286.2	7.711	560.7	7.290	333.6	6.764	359.6	7.433	110.6	7.498	435.3
6.738	184.0	7.462	208.7	7.060	114.6	6.421	1292.3	7.207	105.8	7.057	341.3
6.405	408.0	7.055	313.1	6.812	161.4			6.816	327.9	6.473	2383.7
		6.442	2029.1	6.520	1424.7			6.376	1635.8
				6.305	205.6
C79H23		C83H23		C85H23		C87H23		C87H25		C91H25
9.957	16.2	9.998	6.3	9.833	18.3	9.755	14.2	9.962	36.5	9.690	5.8
9.334	46.5	9.442	15.5	9.369	3.8	9.416	13.3	9.217	161.2	9.402	47.0
8.540	421.8	8.568	556.4	8.511	514.3	9.007	29.6	8.562	598.0	9.107	91.2
7.649	1374.9	7.756	1143.5	7.736	1180.6	8.584	804.3	7.993	293.5	8.571	457.6
7.004	406.1	6.898	464.6	7.349	119.8	7.785	1457.0	7.620	1034.2	7.760	1683.5
6.745	264.0	6.459	1542.3	7.071	68.0	7.309	218.6	7.075	492.9	7.207	208.2
6.418	530.5			6.615	436.6	6.803	502.1	6.503	2307.1	6.948	565.5
				6.425	833.8	6.418	1776.6	6.279	234.6	6.732	424.5
										6.460	778.3
C93H25		C95H25		C95H27		C99H25		C103H27		C107H27
9.991	11.4	9.766	19.4	9.921	1.5	9.894	13.3	9.900	7.4	9.917	34.7
9.779	70.4	9.380	38.4	9.708	13.7	9.662	11.0	9.357	75.2	9.748	11.3
9.303	44.7	8.581	574.1	9.324	106.2	9.424	10.1	9.056	136.4	9.445	59.0
8.535	623.0	8.035	157.4	9.082	106.9	8.521	495.5	8.551	487.5	8.607	640.7
8.024	247.4	7.754	405.2	8.510	441.5	7.768	1156.3	7.830	1458.8	8.046	378.4
7.729	1497.6	7.590	576.6	7.816	2142.3	7.352	70.3	7.579	817.6	7.698	1332.8
7.410	429.8	6.904	556.1	7.119	614.7	7.053	186.2	7.250	351.8	7.282	204.6
7.073	293.2	6.487	1761.4	6.761	341.5	6.701	261.1	6.770	960.4	6.915	669.0
6.969	333.0			6.552	1041.3	6.430	855.6	6.482	1192.8	6.508	1641.7
6.747	129.9									6.290	178.8
6.564	307.0
6.411	550.1
C107H27		C111H27		C113H27		C115H27		C115H29		C119H27
9.666	47.5	9.909	25.3	9.845	45.1	9.919	15.5	9.981	5.1	9.990	1.4
9.438	48.5	9.638	8.1	9.564	19.3	9.463	36.8	9.795	28.5	9.749	16.8
9.210	58.4	9.339	59.9	9.298	14.9	8.514	446.6	9.466	69.9	9.497	37.5
8.564	930.9	8.964	43.9	8.589	515.3	7.806	1541.5	8.994	163.0	8.602	851.1
7.785	2763.4	8.532	468.0	7.807	638.3	7.118	194.2	8.554	544.2	7.846	1751.3
7.032	895.8	8.165	90.8	7.655	499.9	6.708	473.9	7.910	1893.8	7.364	209.4
6.769	396.1	7.768	1371.3	7.317	306.5	6.402	547.4	7.605	863.7	6.950	187.6
6.552	427.7	7.211	221.7	6.934	347.3			7.240	372.3	6.763	533.8
6.430	504.1	6.912	237.8	6.503	1205.5			7.067	276.2	6.456	1785.5
		6.546	559.9					6.883	325.3
		6.239	496.1					6.754	447.8
								6.487	1412.9
C119H29		C121H29		C127H31		average
9.793	52.5	9.969	95.4	9.852	10.8	9.974	16.1
9.334	46.4	9.748	99.3	9.487	37.5	9.771	15.8
8.634	915.9	9.102	172.9	8.554	785.2	9.330	33.8
7.862	2993.4	8.563	1014.4	7.896	2935.8	8.545	568.6
7.288	529.7	7.860	2772.2	7.236	483.6	7.748	1528.1
6.851	638.5	7.378	498.1	6.867	1036.7	6.913	321.9
6.526	1882.2	6.963	1022.7	6.487	1583.1	6.757	167.9
		6.563	1162.8			6.450	1298.0
		6.445	489.2
Anions
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
9.419	6.5	9.852	2.2	9.579	18.3	9.693	14.0	9.939	12.8	9.880	8.5
8.533	343.9	9.374	6.9	8.737	190.1	9.189	14.2	9.524	7.1	9.234	23.0
7.627	572.1	8.508	264.6	8.163	334.8	8.513	287.1	8.518	246.9	8.554	343.3
7.171	145.3	7.856	184.8	7.692	684.7	7.670	642.7	7.647	852.5	8.258	157.2
6.524	670.9	7.622	499.3	7.256	71.6	7.292	136.9	7.092	29.0	7.754	980.4
6.390	664.7	7.365	62.6	7.062	76.4	6.929	81.9	6.927	40.2	6.989	245.5
		7.155	20.2	6.789	157.8	6.713	93.9	6.427	888.7	6.624	170.5
		6.922	53.5	6.387	982.3	6.445	909.2			6.389	1111.3
		6.635	56.6
		6.405	940.9
C51H19		C55H19		C57H19		C59H19		C59H21		C65H21
9.891	4.0	9.852	1.0	9.500	20.9	9.925	7.1	9.847	1.6	9.825	8.8
9.768	14.9	9.565	18.7	8.588	432.8	9.373	20.7	9.493	14.0	9.508	26.8
9.192	9.9	9.123	8.2	8.297	122.3	8.567	471.3	9.161	18.0	9.115	34.4
8.542	309.9	8.588	201.8	7.759	1016.0	7.748	1311.4	8.564	220.7	8.454	222.9
7.702	586.0	7.678	905.0	7.162	217.7	7.402	110.1	7.757	573.7	7.704	975.2
7.275	194.8	6.801	97.1	6.854	165.2	6.990	242.8	7.465	216.7	6.993	154.0
6.935	118.3	6.445	666.3	6.411	1309.3	6.405	1370.5	7.184	123.1	6.402	775.5
6.489	887.3							7.020	74.4
								6.629	217.1
								6.444	347.1
C67H21		C67H21		C67H23		C71H21		C73H21		C77H23
9.802	19.3	10.000	2.1	9.948	1.1	9.773	4.4	9.471	40.4	9.715	2.9
9.407	11.7	9.855	3.5	9.554	4.1	9.220	43.6	8.590	395.7	9.421	19.7
8.874	69.0	9.588	15.2	9.110	2.0	8.595	418.8	8.150	87.8	9.179	15.2
8.559	432.6	9.398	15.8	8.584	822.4	7.653	980.5	7.812	1233.9	8.598	386.5
7.769	1008.2	8.570	480.2	7.851	1525.8	7.149	365.9	7.249	85.7	8.217	80.0
7.132	299.5	8.214	45.6	7.370	253.7	6.868	168.4	6.976	316.3	7.812	1398.4
6.856	83.6	7.800	1366.4	6.873	291.7	6.488	1026.0	6.465	1105.8	7.154	174.9
6.677	104.4	7.151	140.7	6.409	1699.9	6.327	272.3			6.895	180.1
6.454	1215.1	6.950	227.8							6.438	1039.2
		6.429	1282.1
C87H23		C95H25		C99H25		C103H27		C119H27		average
9.928	2.7	9.788	15.7	9.910	3.1	9.874	23.9	9.765	9.9	9.847	6.9
9.768	3.0	9.421	47.8	9.400	33.1	9.581	24.3	9.456	19.5	9.471	19.3
9.463	16.8	8.598	360.7	8.611	357.6	9.336	16.5	8.629	841.1	8.573	444.6
8.580	851.9	8.255	65.6	7.827	1415.8	8.480	373.3	7.906	1951.4	7.786	1142.8
7.875	1732.6	7.825	601.9	7.073	227.0	7.857	1268.7	7.330	289.6	7.151	141.3
7.357	343.4	7.649	504.8	6.785	175.0	7.447	304.8	6.818	465.5	6.854	184.5
6.824	317.3	7.195	255.6	6.439	798.9	7.024	262.0	6.461	1575.9	6.429	1105.2
6.433	1750.7	6.968	337.6			6.765	270.1
		6.499	957.0			6.440	727.2

The 8.6 µm band, due to concerted C–H in–plane bending motions, peaks around 8.5 µm for even-carbon PAHs and a bit longward, at 8.54 µm for odd-carbon PAHs and its position is almost insensitive to size for C ≥ 50, as shown in Tables 11 and 12. The 8.6 µm intensities increase with PAH size and are also dependent on shape. Figure 5 shows that the average 8.6 µm intensities tend to be similar except for odd-carbon anions, which are weaker. It was previously postulated that the 8.6 µm band originates from compact and highly symmetric even-carbon PAHs with C ≥ 96 (Bauschlicher et al. 2008, 2009). In this work, we have extended the PAH collection studied previously by Bauschlicher et al. (2008, 2009) and have included both even and odd-carbon PAHs with a wide range of structures with symmetries ranging from D_6h to C_s. Our results show that even and odd-carbon PAHs with C ≥ 50 and with straight edges and different shapes can contribute to the 8.6 µm band intensity.

A feature between 6.8 and 6.9 µm is also visible in the spectra and it originates from C–H in–plane bending motions. It peaks at 6.804 and at 6.863 µm for cationic and anionic even-carbon PAHs, respectively, and at 6.913 and 6.854 µm for cationic and anionic odd-carbon PAHs, respectively. It is more pronounced for even-carbon PAHs and it is the strongest for the anions. The average intensities, and their percentage with respect to the 7.7 µm intensity given in parenthesis, are 390.4 (24%) and 321.9 (21%) km/mol for the cations of the even and odd-carbon PAHs, respectively, and 507.6 (33%) and 184.5 (16%) km/mol for the anions of the even and odd-carbon PAHs. Although this band is characteristic of aliphatics, PAHs with straight edges can also contribute to its intensity.

3.2.3. The 10–15 µm region

The predominant features between 11 and 11.5 µm are due to the C–H out–of–plane (CH_oop) bending modes of the solo hydrogens (see Figure 6), consistent with the large number of solo C–H bonds present in our PAH structures. The neutrals contribute to the 11.2 µm band with the average even-carbon PAH feature peaking at 11.17 µm and the average odd-carbon PAH feature peaking longward, at 11.264 µm. The cations contribute to the 11.0 µm band with the average even-carbon feature peaking at 11.01 µm and the average odd-carbon feature peaking at 10.99 µm. The intensity of the 11.0 µm band is larger for even-carbon than for odd-carbon PAHs. The assignment of the 11.2 and 11.0 µm bands to the CH_oop bending modes of neutral and cationic PAHs, respectively, is in line with previous work (Hudgins et al. 1999; Hony et al. 2001; Bauschlicher et al. 2008). The average intensity of the 11.0 µm even-carbon cation band is larger than that of the neutral 11.2 µm band and the opposite is true for the odd-carbon cation and neutral bands. The anion bands peak longward of 11.3 µm, namely at 11.41 µm for even-carbon PAHs and at 11.36 µm for odd-carbon PAHs and the intensities are very similar of even-carbon and odd-carbon PAHs. Small PAHs have their solo CH_oop bending modes peaking longward of the observed range and they do not contribute to the 11.0/11.2 µm emission as reported by Schutte et al. (1993).

Figure 6.

Average absorption spectra in the 10–15 µm region for all the neutral (solid green), cationic (solid red), and anionic (solid blue) even-carbon PAHs and for the neutral (dashed green), cationic (dashed red) and anionic (dashed blue) odd-carbon PAHs. The total average spectrum (black) has been obtained by using all the neutral and charged species with equal weight. The spectra have been redshifted by 15 cm⁻¹. Vertical dashed lines have been added at the observed dominant bands at 11.2, 12.7, 13.5, and 14.2 µm.

3.2.4. The 15–20 µm region

Bands in this spectral region are due to skeletal vibrations, which are characteristic of a given molecule, and provide insights on the emitting species. The average computed spectra have their strongest bands around 16.4, 17.0–17.4, 17.8–17.9, 18.4, and 19.3 µm, and a very weak band at 15.8 µm, as shown in Figure 7. Neutrals and ions contribute to all these bands with the cations having usually the largest intensities. Previous studies (Peeters et al. 2012; Shannon et al. 2015) showed that the 17.4 µm band is dominated by cations, the 15.8 µm by neutrals and the 16.4 and 17.8 µm bands by a mixture of the two. Our computed 17.4 µm band resembles those at 16.4 and 17.8 µm as it is dominated by both neutrals and cations. While the bands at 15.8, 16.4, 17.4, and 17.8 µm are observed in astronomical spectra, those at 18.4 and 19.3 µm are not. The computed 18.4 µm band is strongest for odd-carbon PAH cations and the 19.3 µm band is strongest for even-carbon PAH cations. Perusal of Tables 15 and 16 reveals that the in–plane skeletal band at 19.2–19.3 µm is the strongest for small even and odd-carbon PAH cations with 30 ≤ C ≤ 68. This confims that small PAHs with less than 68 carbons are not contributing to the emission in the 15–20 µm region, consistent with Schutte et al. (1993). The out–of–plane band at 18.4 µm becomes more intense for PAHs with long straight edges and is strong for elongated odd-carbon PAH cations, such as C₁₀₃H₂₇⁺. Such very elongated PAHs are less stable than more compact ones and should be less relevant to the emission. Overall, our results show that compact neutral and charged PAHs with 70 carbons or more can contribute to the emission in the 15–20 µm region, with cations producing larger intensities. Boersma et al. (2010) could not relate the emission characteristics between 15–20 µm to specific PAH structures collected in PAHdb and concluded that the discrete emission features should be attributed to a very limited number of large compact PAHs with 50–200 carbons and the plateau to a larger set of PAH molecules.

Figure 7.

Average absorption spectra in the 15–20 µm region for all the neutral (solid green), cationic (solid red), and anionic (solid blue) even-carbon PAHs and for the neutral (dashed green), cationic (dashed red) and anionic (dashed blue) odd-carbon PAHs. The total average spectrum (black) has been obtained by using all the neutral and charged species with equal weight. The spectra have been redshifted by 15 cm⁻¹. Vertical dashed lines have been added at the observed dominant bands at 15.8, 16.4, 17.4, and 17.8 µm.

Table 15.

Band positions (µm) and intensities (km/mol) of the even-carbon PAHs in the 15–20 µm region. A redshift of 15 cm⁻¹ has been applied.

Neutrals
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
18.939	3.9	18.730	28.1	18.695	3.9	19.062	14.6	18.965	9.7	19.482	8.8
18.505	1.5			17.841	5.3	18.508	1.9	17.809	1.2	18.904	2.4
18.225	1.5			16.753	3.0	16.606	2.3	16.289	19.7	17.841	3.8
17.030	1.7			16.445	1.1					17.370	5.6
										15.006	15.9
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
19.350	7.7	19.283	1.9	19.689	1.5	19.662	1.4	18.005	2.1	19.650	2.6
18.957	1.7	19.044	11.8	18.556	2.1	19.279	9.0	16.998	12.8	19.102	13.6
16.647	10.9	17.947	18.3	17.944	5.3	18.447	9.1	16.562	15.1	17.036	17.4
15.888	5.4			17.042	1.5	17.940	4.8	15.588	9.9
15.652	5.4			16.388	18.4	17.334	7.9
						16.173	8.5
						15.845	3.2
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
19.743	1.2	18.248	23.1	19.964	2.2	19.968	1.9	18.793	16.8	18.129	1.3
18.376	5.1	15.768	8.0	19.286	20.7	19.161	16.3	18.379	1.7	17.615	9.6
17.603	1.4			17.790	10.2	17.643	16.3	17.559	4.1	17.203	2.6
16.938	12.3			17.403	5.8	15.017	2.8	17.277	7.8	16.720	1.7
16.483	8.0			16.335	26.2			16.620	7.7	16.496	16.0
16.082	5.1			15.557	4.3			16.226	6.4	15.072	4.1
								15.356	6.9
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
19.904	5.7	19.205	19.8	19.889	2.7	19.732	1.1	19.535	1.5	19.015	1.8
18.464	9.2	17.307	16.8	18.218	9.1	18.295	2.1	18.225	11.2	18.245	11.5
18.182	4.9	15.918	1.1	17.018	16.0	17.727	3.4	17.963	2.7	17.953	13.9
17.908	1.3	15.024	1.6	16.534	4.2	17.250	10.3	17.144	21.6	16.844	3.8
17.606	17.6			16.210	3.7	16.955	1.4	16.150	1.5	16.581	2.5
17.247	1.6			15.097	3.4	16.548	14.2	15.047	1.1	15.891	1.9
15.022	1.8					16.051	4.5			15.126	1.1
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
19.051	2.4	19.681	2.3	18.674	4.3	19.305	2.5	18.786	1.4	19.666	2.2
17.905	6.7	18.965	10.5	18.349	15.9	18.829	1.9	18.275	13.9	19.350	1.1
17.027	18.2	18.450	10.6	17.516	18.3	17.532	9.7	17.403	8.6	17.999	4.7
16.827	7.1	17.857	5.9	17.044	1.5	17.144	17.7	17.316	7.9	17.630	9.8
16.584	11.0	17.712	4.7	15.997	1.8	16.722	9.7	16.835	4.6	17.346	10.6
		17.215	9.1	15.676	1.2	15.382	4.1	15.620	2.4	17.021	12.6
		16.748	10.2	15.319	1.2			15.225	1.4	16.745	1.2
		16.064	10.9							15.642	3.2
		15.305	16.1							15.035	8.0
		15.138	16.1
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
17.918	20.8	18.947	3.4	18.695	5.5	18.904	4.1	19.272	1.7	18.822	12.3
17.382	29.8	18.305	15.2	17.746	9.8	18.295	1.5	18.146	12.7	18.086	13.2
		17.781	13.6	17.319	7.2	17.947	23.5	17.899	11.5	17.832	5.2
		17.455	2.4	17.091	16.1	17.771	6.7	17.301	13.9	17.693	6.6
		17.053	3.1	16.434	6.3	16.675	2.9	16.972	11.2	17.528	17.5
		16.356	4.4	16.218	8.7	16.106	8.8	16.784	2.1	16.147	1.5
		15.818	2.2	15.576	1.5	15.652	2.1	15.708	1.8	15.610	1.3
		15.373	4.5	15.090	1.5	15.228	1.0
		15.088	3.9
C148H30		average
19.095	1.9	19.120	5.2
18.567	1.2	18.751	2.2
17.797	18.6	18.258	4.6
17.364	19.5	17.918	5.7
17.188	6.4	17.627	2.9
17.010	7.9	17.301	5.5
16.603	3.7	17.030	4.3
15.964	1.7	16.559	3.9
15.704	1.1	16.335	3.8
15.305	2.6	15.610	1.6
15.177	1.2	15.330	1.1
		15.002	4.4
Cations
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
18.512	9.0	18.674	30.5	18.751	1.3	19.194	26.2	19.350	15.9	19.516	16.1
17.182	3.5	15.361	1.0	17.918	10.0	16.606	6.9	16.348	18.9	18.843	4.7
15.033	18.6			16.938	5.1	15.368	2.8	15.420	12.3	17.934	7.0
				15.330	1.4					17.382	4.2
										15.047	22.3
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
19.410	2.8	19.301	17.6	19.704	4.9	19.283	15.6	18.498	1.0	19.616	2.0
19.139	12.4	17.934	24.8	18.936	2.7	18.423	11.8	18.021	2.9	19.264	19.1
17.749	3.8			17.905	8.3	17.908	8.4	16.964	6.2	17.050	20.5
16.669	12.9			16.447	23.6	17.668	1.1	16.529	22.5	15.820	5.0
15.944	4.8					17.331	7.2	15.674	11.7
15.615	8.0					16.200	16.5
						15.850	12.0
						15.610	1.4
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
18.372	5.7	18.454	28.6	19.992	2.6	19.976	1.6	18.755	13.8	17.973	1.1
17.743	3.2	18.225	6.8	19.283	38.7	19.585	2.1	18.560	4.1	17.621	12.0
16.875	11.6			17.809	12.0	19.242	19.8	17.665	7.7	17.227	2.7
16.567	8.8			17.440	8.9	17.680	19.0	17.289	26.7	16.518	21.5
16.064	6.6			16.353	117.8	16.955	3.3	16.844	5.3	16.160	4.2
				15.552	24.7	15.830	2.4	16.584	10.4	15.131	2.5
						15.090	3.7	16.242	4.7
								15.361	10.6
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
19.881	1.2	19.539	4.0	19.853	2.2	19.708	2.5	19.535	2.4	18.990	2.9
18.498	8.0	19.231	23.1	18.362	12.4	18.556	5.5	18.225	17.3	18.349	14.3
18.175	13.3	17.759	2.3	16.984	18.3	18.292	1.4	17.960	3.6	17.905	16.7
17.618	15.9	17.328	19.8	16.576	3.8	17.768	6.5	17.123	23.2	16.812	2.3
		17.033	1.2	16.171	1.1	17.224	10.4	16.155	1.7	16.589	4.1
		15.913	11.1	15.941	1.3	16.929	2.6			15.870	3.4
				15.175	4.1	16.717	7.2
						16.570	18.0
						16.003	17.6
						15.470	1.1
						15.049	2.1
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
19.029	1.2	19.685	10.9	18.875	12.4	19.320	1.2	19.238	7.9	19.662	5.3
17.864	7.3	18.825	3.5	18.688	12.6	18.815	2.3	18.349	18.6	17.976	6.0
17.094	17.0	18.416	12.2	18.389	11.4	18.255	1.4	17.467	6.5	17.655	9.0
16.821	8.8	17.944	9.9	18.106	11.0	17.519	11.8	17.265	12.0	17.416	6.9
16.570	14.2	17.730	1.8	17.724	6.1	17.132	20.6	16.846	8.5	17.200	9.6
		17.182	10.7	17.498	13.9	16.759	17.4	16.098	3.9	17.004	11.8
		16.776	1.4	17.071	3.6	15.392	6.9	15.686	2.5	16.736	2.9
		16.192	10.7	16.010	11.7			15.408	1.2	16.556	1.5
		16.080	3.7	15.669	13.3			15.223	4.1	15.657	2.4
		15.288	10.1	15.291	1.5					15.024	8.3
		15.135	38.4
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
17.982	24.5	19.996	11.6	18.685	10.3	18.896	7.5	19.249	1.4	18.825	2.0
17.379	10.6	19.290	29.2	17.690	11.4	17.944	28.7	17.976	16.6	18.060	11.5
17.209	11.9	18.932	3.7	17.123	31.7	17.709	7.2	17.727	4.6	17.851	15.2
15.122	3.8	18.349	19.0	16.439	7.9	16.681	4.1	17.292	16.4	17.461	13.3
		17.740	16.3	16.200	20.8	16.116	19.4	16.984	12.6	16.145	1.7
		17.446	4.1	15.574	1.3			16.753	1.6	15.605	1.4
		17.062	5.4	15.108	1.7			15.711	1.2
		16.361	32.3
		16.098	1.1
		15.798	9.2
		15.370	3.0
		15.106	4.1
C148H30		average
19.091	3.3	19.264	7.9
17.778	19.2	18.692	3.2
17.385	21.9	18.389	5.5
17.179	26.5	17.924	6.7
16.614	4.0	17.721	3.5
15.954	2.4	17.147	13.3
15.291	1.9	16.548	4.4
15.165	1.7	16.359	5.6
		16.189	1.7
		16.021	2.5
		15.642	2.1
		15.378	1.6
		15.051	6.8
Anions
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
19.131	3.9	19.736	7.7	19.547	1.5	18.947	6.6	19.109	7.8	19.527	15.7
18.563	18.7	19.091	19.1	18.751	9.7	16.600	3.4	16.678	17.8	18.850	8.6
15.145	4.4	15.363	6.9	17.857	2.8	15.853	6.4	15.513	25.6	17.649	2.3
				16.938	2.0	15.244	3.4			17.313	1.7
				16.407	7.1					15.110	8.5
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
19.384	6.1	19.131	9.0	19.539	1.4	19.589	3.2	18.002	5.5	19.732	2.6
17.768	2.4	18.028	35.7	18.801	5.0	19.253	4.8	17.036	35.9	19.161	9.9
16.647	12.0			18.067	4.1	18.423	5.5	16.567	10.2	18.730	2.7
16.100	1.3			17.036	9.0	17.908	7.7	16.464	12.8	17.173	15.1
15.957	4.5			16.731	9.1	17.319	12.0	15.564	8.0	15.843	11.5
15.672	15.7			16.532	13.5	16.231	4.9
				15.004	5.2	15.876	16.8
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
19.778	2.7	18.464	29.8	19.309	24.2	19.194	12.1	19.327	1.3	19.444	4.1
19.604	2.1	16.466	8.1	17.800	26.9	17.621	14.6	18.832	49.7	18.632	1.7
18.699	10.7	15.788	17.4	17.343	2.8	16.966	20.0	18.365	7.7	18.179	2.2
18.409	3.4			16.372	66.7	15.051	2.0	17.507	1.9	17.646	9.6
17.161	14.6			15.555	32.6			17.283	16.1	17.256	2.0
16.981	6.7							16.872	21.0	16.795	1.3
16.554	6.5							16.642	5.3	16.551	18.9
16.033	4.9							16.186	9.7	16.218	4.7
								16.000	4.0	15.415	4.4
								15.387	36.3	15.106	4.2
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
19.956	13.3	19.223	14.9	18.268	7.1	19.716	3.3	19.569	7.2	19.004	6.2
18.529	7.4	18.734	5.7	17.123	14.3	18.292	1.9	19.044	4.7	18.282	8.5
18.268	2.4	17.762	24.2	16.592	5.3	17.832	5.9	18.205	12.1	18.005	12.6
17.876	6.6	17.343	14.8	16.194	14.0	17.337	10.6	17.989	7.1	16.832	8.2
17.658	16.8	15.049	2.1	15.974	6.3	16.955	14.8	17.367	14.9	15.896	11.7
17.286	11.9			15.161	2.6	16.739	6.5	17.114	8.8
15.293	1.4					16.584	11.6	16.129	3.4
15.049	2.2					16.000	4.0	15.893	3.2
						15.463	2.6
						15.063	1.6
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
19.073	3.0	19.704	7.0	18.911	29.0	19.342	24.7	18.847	1.6	19.662	3.1
18.051	6.4	18.975	19.9	18.699	3.9	18.836	3.7	18.312	13.9	18.025	2.9
16.975	37.0	18.379	17.7	18.369	8.1	18.080	2.6	17.458	10.0	17.624	23.0
16.420	21.2	18.070	1.3	18.195	7.1	17.572	7.8	17.310	4.6	17.446	6.7
15.298	1.4	17.699	6.4	17.749	5.4	17.144	15.1	16.804	2.0	17.044	10.2
		17.235	7.3	17.525	15.4	16.750	17.5	16.124	14.8	16.745	5.5
		16.759	18.0	17.062	11.7	15.423	6.4	15.618	2.7	16.548	1.0
		16.200	8.7	16.147	2.0			15.425	3.5	16.051	1.0
		15.326	18.2	16.023	1.7					15.647	4.9
		15.145	7.2	15.721	15.2					15.293	4.4
										15.031	5.4
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
17.989	17.6	19.335	8.7	19.747	1.0	18.911	3.4	19.286	8.7	18.882	1.3
17.373	65.2	18.925	8.6	18.685	4.5	18.325	20.4	18.688	1.7	18.083	16.8
16.036	1.0	18.325	13.8	17.819	10.0	17.908	29.4	18.149	24.7	17.590	19.8
15.149	14.1	17.797	11.6	17.153	22.9	16.694	10.3	17.937	7.3	17.185	1.9
		17.458	4.9	16.228	22.9	16.000	4.8	17.749	2.9	15.972	15.1
		17.050	1.5	15.586	4.3	15.654	1.4	17.331	15.7	15.768	7.7
		16.504	5.0	15.418	2.3	15.235	2.9	17.039	8.8	15.108	2.8
		16.100	5.7	15.103	1.3			16.742	1.8
		15.886	6.4					15.718	5.8
		15.103	3.7
C148H30		average
19.106	4.0	19.279	6.2
17.819	24.0	18.868	5.1
17.397	16.7	18.332	5.1
16.981	13.8	17.986	4.7
16.559	8.5	17.797	6.5
16.316	1.7	17.343	6.8
15.946	3.8	17.042	7.0
15.708	1.4	16.722	2.8
15.518	1.5	16.567	3.2
15.323	1.9	16.391	3.1
15.193	1.1	16.186	2.2
		15.881	3.5
		15.542	2.5
		15.380	2.4
		15.103	3.7

Table 16.

Band positions (µm) and intensities (km/mol) of the odd-carbon PAHs in the 15–20 µm region. A redshift of 15 cm⁻¹ has been applied.

Neutrals
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
19.301	5.5	19.109	9.0	17.809	2.1	19.142	10.9	18.389	1.7	19.841	1.0
17.212	6.0	17.370	3.4	16.428	28.3	18.278	6.2	17.575	2.4	18.713	2.4
15.035	18.3	16.609	10.4			17.759	7.4	17.094	1.6	18.515	4.4
		15.949	1.7			15.047	3.3	16.458	17.9	17.771	10.2
		15.547	1.4					15.170	12.5	16.995	1.3
										16.515	10.7
										15.221	1.0
C51H19		C55H19		C57H19		C59H19		C59H21		C65H21
19.924	2.0	19.168	3.0	18.968	1.3	18.372	9.2	19.932	1.1	17.778	2.5
19.505	1.1	18.467	5.7	17.581	2.4	18.202	6.2	19.227	17.1	17.382	7.9
19.179	12.7	17.316	8.8	17.164	11.5	17.759	2.8	18.012	4.2	16.807	3.1
18.660	1.8	16.784	2.8	16.450	20.3	17.010	9.6	17.655	5.8	16.545	14.6
17.253	12.3	16.532	7.8			16.488	2.2	17.376	3.0	15.808	1.0
15.349	2.0	16.179	4.5			16.239	2.0	15.385	2.2	15.492	6.3
C67H21		C67H21		C67H23		C71H21		C73H21		C77H23
18.464	9.6	18.308	2.6	19.501	6.2	18.965	1.6	17.074	26.2	18.162	1.6
18.031	13.2	17.655	10.4	19.246	13.5	18.315	9.1	16.556	8.9	17.394	13.4
17.400	8.3	17.203	3.7	17.995	1.7	17.883	3.2	15.843	6.8	16.866	2.1
15.523	2.2	16.554	15.4	17.428	12.0	17.712	7.9			16.611	10.4
		15.002	6.3	15.228	2.9	17.071	10.2			16.361	11.2
						16.226	1.5
						15.820	1.2
						15.011	2.1
C79H23		C83H23		C85H23		C87H23		C87H25		C91H25
18.793	2.3	18.325	9.4	19.190	2.5	19.335	1.5	18.152	2.6	19.900	2.3
18.372	9.2	18.126	5.9	17.787	3.1	17.905	14.8	17.662	5.0	18.430	11.2
17.992	4.1	17.749	4.1	17.271	11.0	17.215	14.1	17.397	8.8	17.989	10.3
17.443	7.8	17.346	12.6	17.091	14.9	16.886	11.2	16.889	3.6	17.687	2.8
17.156	9.9	16.526	1.6	16.628	8.0	16.260	1.2	16.586	19.1	17.467	8.6
		15.113	1.1	16.493	2.5			15.316	4.8
C93H25		C95H25		C95H27		C99H25		C103H27		C107H27
18.765	2.5	18.587	3.2	18.954	2.3	19.350	1.2	18.447	13.7	19.220	3.8
17.969	3.2	18.349	12.1	17.986	1.6	18.539	1.5	18.136	5.6	18.379	15.7
17.486	5.5	17.646	8.5	17.677	3.6	18.008	6.6	17.470	15.8	18.077	3.7
17.176	17.8	17.473	9.6	17.434	7.0	17.609	12.7	17.241	3.1	17.674	5.0
16.728	4.8	17.153	2.3	16.966	5.4	17.068	14.4	15.090	2.5	17.467	12.6
15.743	3.2	16.762	1.2	16.653	20.3	16.589	2.9			16.391	2.1
		15.845	1.4	16.103	3.7	15.934	1.1			15.733	1.2
		15.083	1.0	15.456	1.2	15.373	1.9			15.179	2.1
				15.092	4.3
C107H27		C111H27		C113H27		C115H27		C115H29		C119H27
19.912	1.4	19.904	3.5	19.697	1.5	17.937	23.2	19.387	1.0	19.327	1.6
19.298	1.0	19.015	1.1	18.584	1.3	17.409	11.3	19.183	1.8	18.804	2.5
18.073	7.9	18.491	4.8	17.388	20.7	16.972	2.8	18.464	16.2	17.937	17.4
17.699	3.4	18.185	1.1	17.144	13.9			18.041	6.9	17.746	7.2
17.492	9.1	17.944	2.8	16.493	7.4			17.718	8.0	17.322	7.8
17.164	13.2	17.665	12.1					17.464	9.1	17.062	9.7
16.200	1.1	17.449	6.4					15.888	1.1	16.147	2.3
		17.176	14.3					15.060	1.3	15.265	2.0
		16.810	7.4							15.002	5.0
		15.337	5.3
		15.047	1.3
C119H29		C121H29		C127H31		average
19.724	2.4	19.335	1.5	19.279	1.7	19.201	3.1
18.399	19.4	18.741	2.1	18.477	19.2	18.416	5.5
18.136	1.0	17.905	4.4	18.064	5.9	17.966	4.0
17.593	20.2	17.428	15.2	17.683	7.5	17.677	3.7
16.622	2.0	17.203	14.4	17.461	10.6	17.425	6.4
16.155	1.2	16.603	2.4	16.984	1.9	17.161	6.7
15.845	1.1	15.773	2.1	15.198	1.7	16.556	6.8
15.579	1.0	15.024	1.9			15.029	3.6
15.316	1.1
Cations
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
19.320	10.2	19.135	12.3	18.433	1.3	19.183	21.0	19.904	1.1	19.829	2.7
18.734	1.4	17.835	1.2	16.260	31.2	18.282	7.1	18.430	7.2	18.671	3.1
18.460	1.9	17.498	9.4	15.399	8.7	17.902	8.4	17.646	4.3	18.481	4.9
17.434	7.6	17.144	1.6			17.609	3.5	16.450	24.2	17.895	12.8
17.262	5.0	16.548	11.3			16.824	2.6	16.080	2.0	16.445	14.9
15.473	1.9	15.865	2.1			16.260	1.8	15.513	1.4	15.260	1.7
15.170	3.8					16.041	2.8	15.184	11.4
						15.103	3.3
C51H19		C55H19		C57H19		C59H19		C59H21		C65H21
19.829	1.3	19.920	2.0	18.979	3.5	19.904	1.9	19.952	1.3	18.008	1.5
19.231	21.5	19.242	3.6	17.918	1.0	18.349	11.0	19.264	24.1	17.841	1.6
18.653	1.7	18.467	7.4	17.097	11.4	18.139	10.5	18.822	3.5	17.385	15.5
18.152	1.0	17.864	1.6	16.423	26.5	17.828	2.0	18.060	2.6	16.748	3.1
17.768	1.1	17.316	11.6	15.140	1.2	17.247	2.3	17.864	12.5	16.523	20.1
17.532	6.2	16.781	7.9			16.880	9.8	17.388	3.4	16.210	5.2
17.253	13.7	16.534	9.1			16.496	2.6	16.779	5.9	15.815	4.5
16.369	3.3	16.152	7.2					16.488	7.4	15.555	7.2
15.380	6.2	15.623	2.6					15.982	4.2
		15.177	1.1					15.461	2.6
								15.221	5.2
C67H21		C67H21		C67H23		C71H21		C73H21		C77H23
19.106	1.7	19.877	6.8	19.294	26.3	18.965	1.1	19.246	1.2	18.808	2.4
18.488	14.0	18.993	1.2	18.577	3.7	18.302	12.1	18.454	3.9	18.192	1.2
18.067	17.2	18.372	1.2	17.702	5.7	18.028	1.3	18.129	7.7	17.687	1.5
17.857	4.4	17.797	13.3	17.422	21.8	17.775	11.7	17.027	27.7	17.388	14.2
17.400	15.7	17.114	4.2	17.036	9.0	17.056	14.5	16.556	4.4	16.570	13.3
16.504	3.7	16.526	18.2	16.537	1.2	16.223	3.3	15.863	8.9	16.361	14.7
15.557	3.8	15.598	1.6	16.108	5.1	15.949	1.3	15.492	5.2	15.065	14.4
15.307	2.5			15.523	20.7	15.198	2.9
C79H23		C83H23		C85H23		C87H23		C87H25		C91H25
19.861	1.7	19.146	1.6	19.164	5.2	19.728	3.6	19.305	1.9	19.904	9.8
19.482	2.0	18.808	1.1	18.018	1.3	19.350	4.4	19.004	3.3	19.183	8.2
18.808	11.8	18.312	13.9	17.756	6.6	18.975	3.0	18.232	1.1	18.911	4.2
18.322	18.7	18.106	4.8	17.397	1.5	18.678	1.6	17.762	10.1	18.389	17.3
17.960	5.1	17.844	7.4	17.100	26.6	17.918	16.7	17.346	7.2	17.835	15.4
17.569	3.9	17.238	9.5	16.614	13.0	17.044	15.1	16.559	25.4	17.612	2.6
17.364	6.6	16.918	5.5	15.401	1.2	16.827	17.0	16.197	16.9	17.352	29.4
17.153	14.1	16.532	3.2	15.145	2.6			15.803	5.3	16.779	14.6
16.565	15.4	15.913	2.8					15.518	7.1	16.080	6.2
16.431	8.0	15.097	2.6					15.309	5.5	15.853	11.6
16.221	1.1									15.535	4.2
15.775	5.1
15.387	1.3
15.138	12.9
C93H25		C95H25		C95H27		C99H25		C103H27		C107H27
19.478	14.5	19.190	5.3	19.837	4.8	19.342	1.3	19.283	3.7	19.623	4.3
18.861	5.3	18.615	4.5	19.272	6.2	18.549	2.1	19.037	4.8	19.320	7.0
18.349	1.8	18.332	18.3	18.904	2.4	18.359	2.9	18.379	26.3	19.008	3.4
17.947	14.7	17.658	11.6	18.155	7.4	17.986	8.0	18.031	4.1	18.567	3.8
17.400	9.8	17.455	9.6	17.778	10.6	17.615	13.8	17.696	10.6	18.345	19.0
17.156	20.1	17.071	4.2	17.325	26.8	17.056	18.2	17.440	25.9	18.100	5.3
16.795	42.8	16.767	2.2	17.062	2.8	16.592	4.1	16.961	2.9	17.693	12.3
16.507	22.4	16.289	3.3	16.586	32.2	16.213	1.2	16.611	7.5	17.388	8.7
15.785	3.7	16.124	1.6	16.271	7.5	15.954	3.1	15.903	2.6	17.161	2.3
15.499	11.2	16.005	1.7	16.075	6.8	15.640	1.8	15.480	7.8	16.706	4.8
		15.555	1.1	15.833	3.0	15.361	2.2	15.321	3.1	16.393	8.9
		15.385	1.4	15.470	3.5					16.119	6.3
		15.078	3.5	15.088	35.0					15.736	6.0
										15.106	5.5
C107H27		C111H27		C113H27		C115H27		C115H29		C119H27
19.885	1.8	19.932	6.9	19.988	1.1	19.242	3.9	19.771	3.2	19.331	4.6
19.331	5.0	19.264	3.6	18.793	5.5	17.931	31.5	19.212	10.1	18.886	1.0
18.786	27.7	18.450	1.6	18.584	3.1	17.200	16.2	18.692	1.7	18.406	1.4
18.437	8.3	18.188	5.0	18.155	3.5	16.984	4.6	18.389	26.7	17.931	22.0
18.106	9.3	17.734	19.4	17.593	2.8	16.559	1.3	17.976	1.2	17.718	8.8
17.470	39.7	17.403	7.4	17.328	19.9	16.000	5.1	17.743	18.6	17.310	8.7
17.253	41.8	17.088	16.3	17.117	20.1	15.798	1.0	17.235	18.6	17.047	10.6
16.773	10.4	16.807	16.3	16.642	8.5			16.844	1.6	16.415	2.6
16.223	40.5	16.088	1.9	16.545	5.6			16.295	1.1	16.126	4.1
15.946	4.7	15.896	3.6	16.287	1.8			16.155	2.8	15.567	1.3
15.642	24.8	15.387	9.0	15.603	3.7			16.021	1.7	15.235	4.3
15.463	6.2			15.300	1.4			15.858	2.2
15.138	24.2			15.090	6.9			15.279	60.9
								15.006	5.9
C119H29		C121H29		C127H31		average
19.724	20.0	19.889	18.8	19.829	4.7	19.877	2.8
19.301	1.7	19.298	19.8	19.290	6.2	19.257	6.5
18.864	8.7	18.765	6.7	18.692	1.8	18.801	2.7
18.365	28.7	18.546	12.5	18.379	33.6	18.369	8.4
17.618	15.1	18.282	3.4	18.106	8.9	17.895	6.9
17.253	12.3	17.854	8.9	17.658	16.2	17.753	5.2
17.044	9.9	17.391	32.1	17.382	8.9	17.373	10.6
16.776	2.1	17.123	29.4	16.941	5.2	17.106	8.5
16.311	25.3	16.609	15.9	16.661	16.5	16.790	3.6
16.023	3.3	16.369	17.3	16.100	8.7	16.540	9.8
15.858	3.9	16.176	29.9	15.865	27.8	16.228	6.7
15.396	15.1	16.026	11.6	15.691	18.0	15.855	2.8
15.081	2.9	15.584	11.8	15.396	19.8	15.415	6.9
		15.152	39.5	15.011	6.9	15.092	12.6
		15.024	17.8
Anions
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
19.685	1.9	19.497	2.8	17.841	5.3	19.194	9.7	19.712	1.1	19.877	1.5
19.335	31.2	19.161	5.8	16.491	24.8	18.359	5.7	18.362	1.8	18.667	9.8
18.755	5.1	17.507	2.0	15.513	12.2	17.832	2.2	17.559	1.0	18.002	8.4
17.056	3.2	16.824	3.8			17.501	4.0	17.135	8.8	17.627	7.0
15.237	1.3	16.669	6.9			16.231	1.2	16.880	2.7	17.018	7.6
15.053	8.8	16.080	1.4			16.067	1.6	16.534	13.3	16.510	8.0
		15.596	7.8			15.775	2.6	16.095	1.7	15.721	2.0
						15.017	4.7	15.466	1.1	15.458	4.4
								15.092	9.6	15.253	1.9
C51H19		C55H19		C57H19		C59H19		C65H21		C67H21
19.920	1.6	19.654	1.5	19.524	1.7	19.566	7.2	19.124	1.9	19.604	5.6
19.198	9.1	19.354	3.1	18.932	2.3	18.409	18.0	17.928	1.4	19.095	1.2
18.664	4.7	19.179	3.4	17.593	5.1	17.630	2.9	17.382	9.0	18.539	10.7
17.440	4.1	18.567	7.0	17.164	9.7	17.343	3.9	16.978	2.0	17.864	10.3
17.212	5.6	18.155	2.0	16.485	20.5	17.082	9.4	16.849	1.1	17.355	17.6
16.426	2.5	17.298	7.9	15.972	1.3	16.480	3.2	16.576	15.4	16.496	5.1
16.010	8.8	16.759	4.7	15.138	10.6	16.255	7.1	16.247	5.1	15.454	1.7
15.627	5.4	16.606	5.6			15.908	1.8	15.810	2.2
15.375	5.7	16.197	6.8			15.699	1.4	15.454	8.8
								15.288	2.4
								15.149	1.1
C67H21		C71H21		C73H21		C77H23		C83H23		C85H23
18.382	3.6	19.558	3.0	18.467	3.2	18.839	17.4	18.847	1.9	19.829	2.6
17.828	2.6	18.965	3.7	18.109	1.7	18.208	4.1	18.275	18.1	19.186	7.0
17.538	16.9	18.285	13.3	17.077	25.6	17.550	3.4	17.702	4.7	18.376	1.4
17.109	4.1	17.860	4.5	16.559	22.0	17.349	11.7	17.379	15.3	17.800	4.6
16.543	10.4	17.581	5.3	15.815	4.9	16.878	3.4	16.628	2.6	17.349	9.1
16.340	3.3	17.117	8.9	15.193	3.4	16.589	6.5	16.202	2.7	17.114	12.8
15.850	1.1	16.795	6.8			16.434	9.5	16.085	5.9	16.658	8.4
15.581	14.3	16.228	1.7			16.197	11.8	15.790	1.3	15.987	2.6
15.029	4.0	15.823	3.0			15.432	1.5	15.228	1.2	15.451	1.2
		15.040	4.3			15.031	5.9
C87H23		C95H25		C99H25		C103H27		C119H27		average
19.712	1.7	19.554	2.6	19.845	2.2	19.712	8.0	19.335	3.7	19.562	1.9
19.357	2.1	19.205	5.9	19.342	4.2	18.815	2.5	18.822	7.3	19.305	4.2
18.563	3.4	18.639	13.0	18.556	3.9	18.447	1.5	18.142	6.6	18.811	1.8
18.005	6.0	18.332	9.4	18.064	8.8	18.064	5.5	17.905	17.6	18.553	2.5
17.912	8.2	17.784	9.9	17.544	10.6	17.394	18.6	17.532	1.6	18.335	3.2
17.265	23.5	17.449	16.8	17.079	12.0	17.135	14.7	17.343	6.9	18.060	1.8
16.943	9.3	17.161	1.5	16.598	2.2	16.955	3.3	17.044	13.7	17.857	3.0
16.753	2.0	17.027	2.6	16.223	2.6	16.650	8.6	16.404	1.9	17.367	8.8
16.273	2.2	16.770	1.5	15.868	4.8	16.493	4.0	16.160	4.0	17.103	7.9
15.969	1.3	16.548	2.2	15.664	4.1	16.289	7.5	15.853	2.2	16.529	8.5
15.305	1.4	16.049	6.5	15.363	2.4	15.723	1.7	15.569	5.1	16.234	2.8
15.108	3.6	15.569	1.2	15.094	5.9	15.131	1.2	15.270	2.8	16.028	1.1
		15.103	4.0							15.825	1.1
										15.569	3.7
										15.078	4.2

3.3. Band Strengths and Ratios

Figure 8 shows graphs of the intrinsic strength of the 11.2 and 3.3 µm bands as a function of the number of hydrogens. The intensity of both bands depends on size and are insensitive on shape. There are no significant differences between even and odd-carbon PAHs. Power laws have been used to fit the data and the fitting parameters are given in Table 4. The fits are almost linear for the 11.2 µm band of the neutral and charged even and odd-carbon PAHs. The same is true for the 3.3 µm band of neutral and anionic even and odd-carbon PAHs. For the cations of even and odd-carbon PAHs the fits for the 3.3 µm band deviate from linearity. Our fitting parameters are in good agreement with those previously derived for compact PAHs (Ricca et al. 2012), such as coronene, ovalene, and regular very large compact PAHs (Bauschlicher et al. 2008).

Figure 8.

Intrinsic strength of the C-H modes at 3.3 and 11.2 µm as a function of the number of hydrogens.

Table 4.

Power-law functions used to fit the intensities of the C–H modes. The R² value is given in parenthesis.

	Neutral		Cation		Anion
Band	Even C	Odd C	Even C	Odd C	Even C	Odd C
	I(NH)	I(NH)	I(NH)	I(NH)	I(NH)	I(NH)
3.3	$2.52\times N_{\mathrm{H}}^{1.87}(0.956)$	$8.3\times N_{\mathrm{H}}^{1.46}(0.913)$	$0.1\times N_{\mathrm{H}}^{2.71}(0.964)$	$0.035\times N_{\mathrm{H}}^{3.04}(0.972)$	$52.73\times N_{\mathrm{H}}^{1.08}(0.797)$	$36.98\times N_{\mathrm{H}}^{1.18}(0.977)$
11.2	$2.83\times N_{\mathrm{H}}^{1.49}(0.946)$	$4.58\times N_{\mathrm{H}}^{1.33}(0.962)$	$1.39\times N_{\mathrm{H}}^{1.71}(0.932)$	$2.18\times N_{\mathrm{H}}^{1.57}(0.92)$	$6.18\times N_{\mathrm{H}}^{1.24}(0.978)$	$5.06\times N_{\mathrm{H}}^{1.31}(0.969)$

Figure 9 shows graphs of the intrinsic strength of the 6.2, 7.7, and 8.6 µm bands as a function of the number of carbons, and the fitting parameters are given in Table 5. The intrinsic strengths were calculated using the same wavelength ranges as those used for the four Gaussian decomposition, i.e. the 6.2–6.6 µm range for the 6.2 µm band, the 7.365–8.08 µm range for the 7.7 µm band, and the 8.408–8.752 µm range for the 8.6 µm band, and a 15 cm⁻¹ redshift was applied. The intensity of the three bands depends on size and for the 6.2 and the 8.6 µm bands also on PAH shape. For highly symmetric PAHs, with round or almost round shape, the 6.2 µm intensities increase at the fastest rate with a slope of 28.2, followed by compact oval PAHs, such as C₄₂H₁₆, with a slope of 12.1. For elongated oval PAHs, such as C₄₈H₁₈, the 6.2 µm intensity is almost constant, with a slope of only 0.47. For the 8.6 µm bands the trends in intensities are different than those for the 6.2 µm bands, with the fastest rate of increase for the elongated oval PAHs (slope of 23.4) and similar rate of increase for the round and compact oval PAHs (slope between 9 and 10).

Figure 9.

Intrinsic strength of the C-C modes at 6.2, 7.7 and 8.6 µm as a function of the number of carbons.

Table 5.

Fitting parameters of the linear function (σ = a + bN_C) used to fit the intensities of the 6.2, 7.7, and 8.6 µm bands, and the Pearson's correlation coefficient (r). The error is given in parenthesis.

Parametere	6.2 µm			7.7 µm	8.6 µm
	Round	Compact Oval	Elongated Oval	All	Round	Compact Oval	Elongated Oval
a	−285.78225 (±147.69114)	−28.33586 (±160.76706)	319.51573 (±19.12278)	−339.54234 (±160.39973)	5.52502 (±74.39951)	−177.74594 (±148.22882)	−318.71296 (±92.0253)
b	28.20056 (±2.39131)	12.12081 (±2.14815)	0.46772 (±0.28437)	21.46818 (±2.27959)	10.32151 (±1.06845)	8.94646 (±2.03507)	23.40306 (±1.43913)
r	0.98592	0.89397	0.75825	0.91605	0.98945	0.87355	0.99624

The plots of the 11.2/3.3 µm intensity ratios as a function of the number of carbons are useful to trace PAH sizes and their graphs, for excitation energies of 6, 7, 8, and 9 eV, are shown in Figure 10. The ratios depend on size but are insensitive to shape. The plots for even and odd-carbon PAHs follow an exponential increase and their fitting parameters are very similar (see Table 6). At lower excitation energies the 3 µm band has a lower level of excitation than the 11.2 µm band and therefore these plots deviate the most from linearity. As the excitation increases, the excitation of the two bands reaches a more equal footing and the 11.2/3.3 µm ratios follow an almost linear increase for an excitation of 9 eV in agreement with our previous results (Ricca et al. 2012). Using our plots we able to reproduce the PAH sizes as a function of the distance from the star obtained by Croiset et al. (2016) for NGC 7023, namely ≈ 50 carbons at the surface of the PDR, ≈ 70 carbons inside the PDR cavity and ≈ 80 carbons in the molecular cloud.

Figure 10.

Ratios of the 11.2/3.3 µm band strengths as a function of the number of carbons for photon excitation energies of 6,7,8, and 9 eV.

Table 6.

Fitting parameters of the exponential function ( $e^{(a+b{N}_C+{\mathit{\text{cN}}}_C^2)}$) used to fit the 11.2/3.3 µm ratio, as a function of the number of carbons, and for excitation energies of 6, 7, 8, and 9 eV (see Figure 10) and the R² values. The errors are given in parenthesis.

	6 eV		7 eV		8 eV		9 eV
Parameter	Even C	Odd C	Even C	Odd C	Even C	Odd C	Even C	Odd C
a	−1.06958 (±0.15845)	−0.89988 (±0.2138)	−1.30797 (±0.14179)	−1.08101 (±0.20073)	−1.43193 (±0.13752)	−1.23637 (±0.18343)	−1.55546 (±0.12929)	−1.37094 (±0.16564)
b (10−2)	3.892 (±0.297)	3.63 (±0.448)	3.801 (±0.27)	3.409 (±0.426)	3.554 (±0.27)	3.238 (±0.394)	3.408 (±0.254)	3.125 (±0.359)
c (10−5)	−7.52165 (±1.33785)	−5.39402 (±2.30266)	−8.0673 (±1.24109)	−5.23207 (±2.21498)	−7.50743 (±1.24003)	−5.12476 (±2.06435)	−7.43963 (±1.20053)	−5.25005 (±1.89709)
R2	0.98939	0.99085	0.9887	0.98963	0.9867	0.98935	0.98552	0.98953

Figure 11 shows the average computed emission spectra for all the neutral and charged even and odd-carbon PAHs considered in this study computed for absorbed photon energies of 4, 6, and 8 eV. The relative intensities of the 6.2, 7.7, and 8.6 µm bands agree with those obtained from observations of class A objects. We should note that when using only highly compact and symmetric PAHs, such are coronene, circumcoronene, and circum-circumcoronene, the 6.2, 7.7, and 8.6 band ratios cannot simultaneously be satisfied (Ricca et al. 2012).

Figure 11.

Average emission spectra for all the species studied in both neutral and charged states for absorbed photon energies of 4, 6, and 8 eV. The inset shows the 3 µm bands. Dashed lines show the dominant peaks of observed spectra, at 3.3, 6.2, 7.7, 8.6, 11.2, 12.7, 13.5, 14.2, 15.8, 16.4, 17.4, and 17.8 µm.

3.4. Comparison with the Observed 7.6/6.2 and 8.6/6.2 Intensity Ratios

Among the possible correlations between the different PAH (sub-)components towards NGC 2023, the 7.6 and 8.6 Gaussian components exhibit the strongest correlation (Peeters et al. 2017). This correlation was subsequently found to hold for irradiated PDRs (Stock et al. 2017)¹. Comparison of the observations with the computed IR spectra of both a sample of oval PAHs and of all the PAHs included in the PAHdb revealed that, while a good agreement could be obtained for the individual 8.6/6.2 and 7.6/6.2 intensity ratios by many PAHs in the PAHdb, very few PAHs could reproduce the observed trend between the 7.6 and 8.6 Gaussian components, when normalized using the 6.2 PAH band (Peeters et al. 2017). Here, we present the intrinsic IR spectra of a larger collection of PAHs with straight edges than currently present in version 3.00 of the computational library in the PAHdb, which allow us to investigate if this particular subsample of PAH structures is able to reproduce the observed trend. For completeness we note that Peeters et al. (2017) also found a weaker correlation between the 7.8 and 8.2 Gaussian components towards NGC 2023, which is confirmed by Stock et al. (2017) to be present in irradiated PDRs. The 8.2 Gaussian component has been attributed to bay regions (Peeters et al. 2017), which are absent in the structures of our PAH sample. Therefore, we focus solely on the 7.6 and 8.6 µm components in this paper.

As in Peeters et al. (2017), we calculated the intrinsic intensities using the same wavelength ranges as those used for the four Gaussian decomposition, i.e. the 7.365–7.8 µm range for the 7.6 Gaussian component, and the 8.408–8.752 µm range for the 8.6 Gaussian component, after applying a 15 cm⁻¹ redshift. These intensities were normalized to the 6.2 µm PAH to avoid a dependence on charge state (when normalized to the more commonly used 11.2 µm PAH). We calculated the 6.2 intrinsic intensity using the 6.2–6.6 µm range in which the most intense band is located for PAHs with a number of carbons greater than 20, and we used an excitation energy of 8 eV, which corresponds to the typical photon energy for the illuminating star.

The calculated intrinsic intensity ratios for all the neutral and charged even and odd-carbon PAHs are reported in Tables 7 and 8. Figure 12 shows a comparison of the computed and observed correlation between the 7.6/6.2 and 8.6/6.2 intensity ratios in three RNe, namely NGC 1333, NGC 7023, and NGC 2023. The green marks in Figure 12 represent the astronomical data: i.e. the observed intensity ratios in spectral maps of three RNe (i.e. we obtained a spectrum for each pixel in the field-of-view positioned at each of the three RNe). These were determined by fitting the PAH emission in the 7–9 µm region with four Gaussians (centered at approximately 7.6, 7.8, 8.2 and 8.6 µm) and computing their intensity and subsequent intensity ratio. The computed ratios that are in agreement with observation are indicated in Tables 7 and 8 with a check mark. Overall, a significantly larger number of PAHs with symmetries ranging from D_2h to C_s have intensity ratios (7.6/6.2 and 8.6/6.2) that match the observed trend compared with our previous study using the PAHdb (Peeters et al. 2017). This PAH (sub-)population spans a wide range of sizes. For NGC 7023 and NGC 2023, the relevant PAHs have a very similar range in size (46 ≤ C ≤ 103–113) whereas for NGC 1333 the range is somewhat larger, i.e. 38 ≤ C ≤ 127. NGC 1333 exhibits smaller 7.6/6.2 values compared to those for NGC 7023 and NGC 2023. This results is a much larger overlap with the intrinsic PAH ratios and a larger PAH size range. Both anions and cations are present with cations being the dominant charge state, while the earlier comparison using the PAHdb revealed that only a handful of PAH anions could reproduce the observed trend for NGC 2023. Furthermore, amongst these PAH cations, odd-carbon PAHs tend to be more prevalent than even-carbon PAHs, particularly for NGC 1333. This comparison therefore suggests that even and odd-carbon PAHs with straight edges and with a broad range of symmetries are viable candidates for the PAH emission seen towards irradiated PDRs.

Table 7.

Intensity ratios (µm) of the even-carbon PAHs for an excitation of 8 eV and after applying a redshift of 15 cm⁻¹. The computed ratios for the cations and the anions that match observations are labeled with a checkmark.

	Even C
PAH	Cations		Observations			Anions		Observations
	8.6/6.2	7.6/6.2	NGC1333	NGC7023	NGC2023	8.6/6.2	7.6/6.2	NGC1333	NGC7023	NGC2023
C22H12	0.07	0.47				0.07	1.04
C24H12	0.07	0.83				0.14	0.98
C28H14	0.12	1.18				0.15	1.19
C30H14	0.05	0.84					0.81
C32H14	0.23	0.96				0.08	2.43
C34H16	0.31	1.54		✓		0.31	0.99
C38H16	0.23	1.33				0.58	0.92	✓
C40H16	0.19	1.79				0.10	2.61
C42H16	0.12	0.42				0.21	1.18
C46H18	0.52	1.74		✓	✓	0.42	0.63
C48H18	0.24	0.88				0.83	0.91
C48H18	0.35	2.97				0.25	2.30
C52H18	0.33	1.11	✓			0.37	1.37		✓
C54H18	0.16	0.53				0.42	1.19		✓
C54H20	0.86	1.86				0.65	0.68
C56H20	0.86	2.99				0.64	1.95		✓	✓
C58H20	0.36	1.07	✓			0.78	0.69
C62H20	0.76	1.89		✓		0.49	0.95	✓
C64H20	0.46	0.98	✓			0.58	0.97	✓
C64H22	1.36	2.72				1.35	2.40
C66H20	0.35	1.14				0.70	2.01		✓	✓
C72H22	1.10	1.68				0.63	0.76
C76H22	0.87	1.03				0.71	0.69
C78H22	0.72	2.52		✓	✓	0.42	1.87	✓	✓	✓
C80H22	0.45	0.86	✓			0.99	0.97
C82H24	0.45	1.98		✓	✓	2.29	4.05
C88H24	1.57	1.13				0.88	0.74
C90H24	0.87	1.20				0.55	0.38
C90H24	1.45	3.20				0.60	1.14	✓
C94H24	0.91	0.97				1.15	0.77
C96H24	0.48	1.01	✓			1.15	1.05
C102H26	2.08	2.35		✓		1.54	1.94
C108H26	1.97	0.98				0.99	0.72
C110H26	0.88	0.62				1.31	0.51
C112H26	0.69	1.35	✓			1.53	1.80
C120H28	0.71	0.89				0.40	0.54
C148H30	1.72	0.67				1.92	0.68

Table 8.

Intensity ratios (µm) of the odd-carbon PAHs for an excitation of 8 eV and after applying a redshift of 15 cm⁻¹. The computed ratios for the cations and the anions that match observations are labeled with a checkmark.

	Odd C
PAH	Cations		Observations			Anions		Observations
	8.6/6.2	7.6/6.2	NGC1333	NGC7023	NGC2023	8.6/6.2	7.6/6.2	NGC1333	NGC7023	NGC2023
C31H15	0.20	0.34				0.23	0.37
C35H15	0.24	0.57				0.22	0.57
C37H15	0.41	0.53				0.15	0.68
C43H17	0.27	0.48				0.26	0.63
C45H17	0.30	0.89				0.21	0.83
C47H17	0.39	0.65				0.30	0.64
C51H19	0.30	0.41				0.35	0.69
C55H19	0.43	1.21	✓			0.27	1.08
C57H19	0.34	0.51				0.33	0.65
C59H19	0.46	0.73				0.38	0.62
C59H21	0.34	0.44				--	--
C65H21	0.51	1.19	✓	✓		0.46	1.73		✓	✓
C67H21	0.55	2.14			✓	0.22	1.20
C67H21	0.34	0.38				0.33	0.69
C67H23	0.38	0.69				--	--
C71H21	0.45	0.77				0.39	0.68
C73H21	0.58	0.72				0.54	0.50
C77H23	0.32	0.31				0.34	0.74
C79H23	0.71	2.19		✓	✓	--	--
C83H23	0.40	0.63				0.41	0.83	✓
C85H23	0.57	1.13	✓			0.45	0.90	✓
C87H23	0.58	0.65				0.57	0.46
C87H25	0.32	0.47				--	--
C91H25	0.66	1.93		✓	✓	--	--
C93H25	0.77	2.48		✓		--	--
C95H25	0.39	0.55				0.41	1.03	✓
C95H27	0.53	1.60	✓	✓	✓	--	--
C99H25	0.71	1.33	✓			0.42	1.13	✓
C103H27	0.50	1.48	✓	✓	✓	--	--
C107H27	0.44	0.87	✓			--	--
C107H27	1.10	2.67				--	--
C111H27	0.77	1.69	✓			--	--
C113H27	0.66	1.17	✓			0.50	1.32	✓	✓
C115H27	0.82	2.09				--	--
C115H29	0.55	1.26	✓			--	--
C119H27	0.59	0.67				0.63	0.52
C119H29	0.57	1.30	✓			--	--
C121H29	0.99	1.96				--	--
C127H31	0.63	1.39	✓			--	--

Figure 12.

Comparison of the intensity ratios of even-carbon PAH cations (red triangles) and anions (blue triangles), and odd-carbon PAH cations (orange triangles) and anions (purple triangles), computed for an excitation of 8 eV and after applying a redshift of 15 cm⁻¹, with observations (green circles) of the RNe NGC 1333, NGC 7023, and NGC 2023.

4. DISCUSSION

Our findings that PAHs with an even and an odd number of carbon atoms are potential PAH emitters in irradiated PDRs is supported by combustion studies of fuel-rich flames that show that there are two different paths for the molecular growth, one involving neutrals and the other involving cations (Weilmünster et al. 1999). Neutral PAHs with an even number of carbon atoms are stable molecules with closed-shell electronic configurations in their ground states. Adding or subtracting an electron from the neutral PAH yields an open-shell anion or cation. However, the closed-shell cations with an odd number of carbon atoms are also very common in the flames. For PAHs with less than 50 carbons the cations with an odd number of carbon atoms are, in general, more numerous than similar-sized species with an even number of carbon atoms whereas for PAHs with more than 50 carbons cations with an odd and even number of carbons are present in equal number.

The neutral molecules associated with this second class of closed-shell cations either have an open-shell structure or add an additional H atom to tie up the open shell. Because neutral species with an odd number of carbons are harder to synthesize than those with an even number of carbon atoms, they and their ions have been far less studied spectroscopically. However their presence in fuel-rich flames strongly suggests that they will be formed in the outflows of carbon-rich stars. Furthermore, the concentrations observed in flames, suggests that the odd-number of carbon cations should be about as common as the better-studied even-number of carbon cations.

5. CONCLUSIONS

In the present work we revisit the effect of shape and size on the 7.6 and 8.6 µm bands using a set of PAH molecules with straight edges and with an even and odd number of carbons. The emission spectra are calculated for an excitation of 8 eV and a redshift of 15 cm⁻¹ is applied. The computed 8.6/6.2 and 7.6/6.2 intensity ratios are compared with observations for the RNe NGC 1333, NGC 7023, and NGC 2023 and reveal that several even and odd-carbon PAHs with straight edges and with sizes of ≈ 50–110 carbons can reproduce the observed correlation between the 7.6 and 8.6 Gaussian components normalized to the 6.2 PAH feature. This suggests that even and odd-carbon PAHs are relevant emitting species in illuminated PDRs and this idea is further supported by combustion studies, which conclude that even and odd-carbon PAHs can form in the outflows of carbon-rich stars.

The stabilities of even and odd-carbon PAHs are comparable to those of the most stable PAHs in the PAHdb. Neutral and cation even and odd-carbon PAHs have very similar stabilities for N_C ≤ 70 and then differ, with the stabilities of odd-carbon PAHs converging to lower values. Anionic even and odd-carbon PAHs have very similar stabilities for N_C ≤ 110.

In the 3 µm region, the C-H stretching band positions of even and odd-carbon PAHs are very similar and the largest differences in band intensities are for the cations, followed by the neutrals and the anions. Even and odd-carbon PAHs with 50 carbons or more contribute to the 8.6 µm band intensity and its position is insensitive to size.

The relative intensity of the 7.7 and 6.2 µm bands is greater than 1 for even-carbon PAHs and less than 1 for odd-carbon PAHs. The correlation between the 6.2 and 7.7 µm bands is well established in a variety of environments and within extended sources. However, recent studies suggest that this strong correlation breaks down on small spatial scales toward the massive Galactic star-forming region W49A and toward the giant star-forming region N66 in the Large Magellanic Cloud (Whelan et al. 2013; Stock et al. 2014). Our findings on the different behavior of odd and even-carbon PAHs for the 6.2/7.7 intensity ratio suggest that this may originate in a varying ratio of odd to even-carbon PAHs. The origin of such a varying ratio is however unclear given the similar stability of even and odd-carbon PAHs and warrants further investigation.

A small feature is also obtained at approximately 6.9 µm and it has the largest intensity for the anions of even-carbon PAHs. Its intensity is between 16–33% of the 7.7 µm intensity.

In the 10–15 µm region, the CH_oop of the neutrals peak between 11.17 and 11.26 µm, with the odd-carbon PAHs CH_oop band peaking longward of the band in even-carbon PAHs, the CH_oop band of the cations peaks at 11 µm, and that of the anions peaks at 11.4 µm, which supports the assignment of the 11.0 and 11.2 µm bands to cations and neutrals, respectively. Compact PAHs with 70 carbons or more are plausible candidates for the emission in the 15–20 µm region.

The intrinsic strengths of the C-H modes at 3.3 and 11.2 µm as a function of the number of hydrogens deviate from linearity, with the smallest deviation obtained for the 11.2 µm band of the neutrals. For the cations, the intrinsic strengths of the 6.2 and 8.6 µm bands follow different trends as a function of the number of carbons for round, compact oval, and compact elongated PAHs. The 11.2/3.3 ratios can be used to determine PAH size and they follow a very similar exponential growth as a function of the number of carbons for both even and odd-carbon PAHs. Using our plots we were able to reproduce the PAH sizes as a function of the distance from the star for NGC 7023, namely ≈ 50 carbons at the surface of the PDR, ≈ 70 carbons inside the PDR cavity and ≈ 80 carbons in the molecular cloud.

Table 13.

Band positions (µm) and intensities (km/mol) of the even-carbon PAHs in the 10–15 µm region. A redshift of 15 cm⁻¹ has been applied.

Neutrals
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
14.881	10.7	13.165	11.7	14.896	22.3	14.225	6.1	14.476	2.1	14.100	23.3
13.887	1.7	11.773	175.7	13.539	21.5	13.530	71.6	13.403	2.5	13.401	16.4
13.532	43.2			12.984	14.1	13.026	4.7	13.123	9.7	12.920	15.6
12.902	3.8			12.467	52.3	11.670	3.8	12.079	74.3	12.637	21.5
12.606	34.9			11.989	7.4	11.340	148.4	11.370	126.4	12.404	1.9
11.461	104.3			11.303	118.3	10.410	4.2	11.154	3.2	12.045	35.7
10.425	3.4			10.673	1.6			10.501	2.5	11.662	6.2
				10.441	3.1					11.240	138.3
				10.002	1.1					10.654	7.7
										10.445	3.9
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
14.245	11.1	14.312	4.9	14.453	10.7	14.554	8.0	14.843	2.2	13.307	4.8
13.367	55.1	13.523	3.1	13.650	3.4	13.931	18.5	14.476	3.8	13.170	9.0
12.970	15.6	13.360	2.1	13.070	7.1	13.428	51.2	13.998	10.6	12.258	8.8
12.353	5.0	13.029	5.5	12.798	23.9	13.041	7.2	13.416	32.3	11.884	105.0
11.704	11.0	11.990	101.1	12.154	28.7	12.267	6.9	12.822	42.4	11.443	5.5
11.257	168.9	11.250	136.4	11.878	9.7	11.715	5.4	12.176	21.8	11.203	158.1
10.850	6.4	10.819	2.4	11.296	176.1	11.573	18.3	11.769	1.9	10.384	1.9
10.417	3.9	10.651	3.6	10.458	3.1	11.222	205.7	11.207	202.8
						10.418	3.6	10.747	13.3
								10.422	3.6
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
13.912	8.5	14.158	22.3	14.734	4.4	13.374	13.1	14.930	3.7	14.916	1.9
13.087	8.7	13.055	43.4	13.795	18.4	13.122	6.3	14.514	10.8	13.928	2.8
12.724	19.0	12.134	10.2	13.417	50.7	12.076	59.3	13.746	20.6	13.672	2.0
12.285	44.7	11.250	220.3	13.031	10.5	11.729	67.4	13.419	38.9	13.497	4.8
11.765	7.3	10.327	2.7	12.401	1.2	11.196	185.0	12.960	19.8	13.043	7.7
11.419	28.1			11.929	3.6	10.636	2.3	12.259	35.6	12.840	20.2
11.208	178.4			11.639	32.5			11.558	18.7	12.240	20.3
10.425	1.1			11.472	27.3			11.197	243.9	12.085	32.1
				11.204	207.1			10.431	3.9	11.391	49.6
				10.432	4.8			10.060	8.4	11.177	191.0
										10.224	1.1
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
14.505	2.4	14.470	1.6	14.463	5.5	14.626	2.7	14.353	4.9	14.669	4.5
14.108	4.2	13.387	17.5	13.615	7.3	14.221	5.7	13.914	3.0	14.418	10.6
13.624	4.6	13.163	5.7	12.943	53.1	13.875	3.4	13.514	4.1	13.663	5.4
12.915	15.1	12.561	6.5	12.497	2.3	13.512	4.4	13.070	13.2	13.441	6.4
12.607	8.7	12.003	80.1	11.888	7.5	13.067	7.7	12.880	12.9	13.033	43.7
12.299	64.9	11.596	42.5	11.186	255.4	12.816	25.1	12.393	27.0	12.583	2.6
11.682	7.9	11.199	217.2	10.107	6.7	12.438	12.6	12.042	47.6	12.148	19.0
11.179	228.0	10.365	1.8			12.160	21.1	11.776	3.0	11.779	4.6
10.316	4.0					11.823	14.3	11.507	5.7	11.308	52.2
						11.400	57.5	11.162	254.4	11.162	230.2
						11.181	218.6	10.432	15.6	10.868	2.1
						10.741	2.9	10.066	5.4	10.401	5.7
						10.461	3.2
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
14.648	2.6	14.948	3.0	14.939	2.2	14.391	2.8	14.503	10.2	14.008	4.5
14.213	2.1	14.480	4.2	14.463	2.2	13.899	2.6	14.118	9.5	13.428	7.7
13.483	6.7	14.071	23.2	13.864	7.0	13.556	16.1	13.858	3.8	12.942	50.4
13.270	2.4	13.423	50.4	13.592	6.5	12.963	19.0	13.358	2.3	12.625	5.2
12.806	61.6	12.979	15.1	12.927	22.2	12.401	18.1	12.997	53.5	12.070	20.6
11.988	4.6	12.786	4.7	12.404	57.8	12.214	8.5	12.478	13.9	11.730	4.5
11.718	4.9	12.466	39.5	11.801	25.3	11.972	55.7	12.149	1.1	11.547	2.2
11.365	16.1	12.288	20.0	11.161	305.0	11.493	19.1	12.008	1.0	11.334	18.7
11.158	266.1	11.912	23.4	10.574	3.4	11.153	294.5	11.787	4.8	11.142	289.5
10.068	2.2	11.172	331.2	10.426	18.6	10.155	1.1	11.157	317.5	10.668	1.7
		10.945	25.3	10.021	1.7			10.414	17.3	10.301	5.6
		10.603	14.8							10.013	2.7
		10.467	11.9
		10.146	3.6
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
14.966	5.7	14.393	13.7	14.799	3.6	14.861	13.2	14.877	4.6	14.943	7.9
13.416	6.0	13.820	6.5	14.314	1.7	14.148	5.0	14.358	2.9	14.081	5.2
12.702	73.8	13.419	5.3	13.984	10.4	13.824	5.3	13.755	1.7	13.808	7.9
12.192	2.7	13.019	59.2	13.371	3.6	13.355	6.1	13.373	5.8	13.463	33.0
11.140	317.2	12.594	3.4	12.892	62.6	13.016	46.1	13.024	11.5	12.970	14.6
		12.412	2.9	12.430	5.2	12.296	4.2	12.783	51.7	12.763	12.7
		11.631	2.3	12.261	5.1	12.035	24.9	12.333	4.3	12.494	35.1
		11.164	351.5	12.008	5.5	11.542	9.2	12.077	13.8	12.170	15.4
		10.417	38.0	11.729	11.5	11.328	25.4	11.816	8.7	11.726	42.5
		10.056	8.5	11.141	352.4	11.136	322.1	11.126	341.2	11.373	30.9
				10.223	3.8	10.285	4.8	10.297	4.8	11.151	329.6
										10.776	14.4
										10.368	28.9
C148H30		average
14.656	5.6	14.455	3.1
14.349	3.3	14.096	4.5
14.023	2.4	13.837	2.1
13.291	9.9	13.437	17.8
12.853	60.4	12.984	31.2
12.580	6.6	12.438	6.5
12.093	15.6	12.267	7.5
11.797	23.6	12.034	21.6
11.393	2.5	11.766	15.3
11.114	402.7	11.168	242.1
10.595	1.4	10.414	6.2
10.332	14.7
10.110	1.8
Cations
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
13.695	41.1	13.358	14.3	14.961	34.1	13.519	60.8	13.132	14.0	14.170	26.8
12.381	53.7	13.007	32.2	13.682	1.2	12.938	26.6	11.895	94.8	13.271	19.4
11.189	86.8	12.657	6.9	13.373	22.4	11.660	2.2	11.125	117.1	12.832	5.2
10.862	2.7	11.551	190.4	12.892	2.4	11.090	133.9	10.471	4.6	12.429	54.0
10.603	7.9	10.261	1.0	12.273	75.9	10.150	1.2			11.797	38.0
				11.035	92.4					10.991	116.4
				10.875	6.2					10.601	19.1
				10.595	11.4					10.376	7.5
										10.230	9.8
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
14.337	13.0	14.652	3.3	14.424	10.3	14.545	18.0	14.826	4.7	14.641	1.5
13.407	48.6	14.411	3.9	13.663	1.6	13.926	18.9	14.480	8.5	13.937	1.0
12.858	30.8	13.567	8.8	12.610	41.6	13.408	62.1	14.013	12.0	13.324	15.8
12.352	8.3	13.243	2.5	11.980	46.7	12.935	21.7	13.273	29.8	12.990	8.1
11.566	1.3	12.893	6.6	11.056	165.3	11.980	6.6	12.729	47.4	12.016	20.4
11.405	11.5	11.812	120.2	10.485	11.3	11.322	48.7	12.171	38.4	11.690	118.8
11.030	150.8	10.997	122.1			11.014	166.3	11.221	1.5	11.370	6.8
10.835	3.6					10.717	7.5	11.019	189.7	10.969	130.6
10.171	1.9					10.192	1.8	10.749	24.7
								10.204	1.5
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
13.900	11.4	14.213	21.8	14.775	5.6	14.607	2.7	14.930	13.2	14.881	4.1
12.940	6.1	12.912	54.1	14.231	13.8	13.383	34.1	14.430	12.6	13.941	6.7
12.583	25.4	12.447	1.3	13.457	96.7	13.004	12.3	13.721	74.0	13.396	20.7
12.102	63.2	12.160	13.3	13.226	21.1	12.588	9.9	13.270	46.0	12.702	32.9
11.509	11.8	11.275	1.6	12.938	18.8	12.117	21.2	12.839	28.9	12.102	38.5
11.187	23.0	11.060	218.8	11.891	17.3	11.902	63.9	12.256	53.2	11.881	30.5
10.997	165.2	10.320	46.9	11.632	70.0	11.527	78.4	12.005	12.6	11.189	50.6
10.391	4.8			11.387	44.4	10.969	154.0	11.550	45.9	10.988	171.5
				11.017	186.5	10.627	3.9	11.275	82.8	10.221	1.1
				10.441	1.7			11.022	214.3
				10.207	2.0			10.496	14.8
								10.216	1.2
								10.060	25.9
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
14.997	1.6	14.786	5.6	14.478	7.2	14.648	8.0	14.420	15.0	14.710	6.0
14.445	3.5	14.453	1.8	13.671	7.8	14.142	15.9	13.910	8.5	14.405	13.7
14.124	5.1	13.680	4.5	13.127	1.8	13.881	23.2	13.249	13.1	13.454	6.3
13.585	5.4	13.421	110.2	12.839	57.5	13.493	6.0	12.781	18.2	12.902	52.8
13.068	13.5	13.033	22.9	12.362	10.6	12.786	56.9	12.203	38.2	12.530	7.0
12.775	17.2	12.617	68.4	11.504	17.2	12.210	21.4	11.886	60.0	12.173	5.4
12.121	80.7	11.844	87.4	11.164	37.2	11.983	20.9	10.990	240.8	11.937	22.2
11.348	11.0	11.401	63.6	11.010	205.5	11.602	23.8	10.418	51.2	11.148	64.1
10.983	207.5	10.977	173.9	10.343	9.3	11.199	64.5	10.034	11.9	10.999	210.6
10.290	8.4	10.260	7.4	10.127	5.3	10.995	186.8			10.365	13.7
						10.749	18.6
						10.448	14.6
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
14.669	2.9	14.930	3.4	14.925	2.2	14.403	5.6	14.885	3.6	14.335	5.5
13.508	11.1	14.374	4.4	14.436	3.0	13.891	11.9	14.455	4.3	14.031	3.1
12.892	20.6	13.930	36.5	13.868	24.1	13.602	15.9	14.114	12.1	13.780	1.2
12.694	59.7	13.210	47.8	13.369	7.4	13.346	4.9	13.856	11.4	13.398	5.0
11.970	4.5	12.898	19.9	12.872	33.3	12.862	19.9	13.342	2.9	13.146	8.7
11.617	8.8	12.274	46.2	12.180	78.1	12.198	72.8	13.194	2.5	12.824	56.3
11.241	31.2	11.716	15.9	11.652	25.4	11.806	66.6	12.890	58.1	12.525	3.5
10.988	243.0	11.313	67.6	11.307	35.2	11.563	8.1	12.491	23.8	11.947	22.3
10.059	2.0	11.041	308.1	11.027	363.4	11.156	66.9	12.168	13.9	11.688	11.0
		10.612	69.8	10.547	11.5	10.991	269.7	11.637	9.3	11.397	5.4
		10.455	13.3	10.403	68.0	10.127	1.6	11.434	1.6	11.223	27.0
		10.133	7.1	10.002	13.8			11.146	74.7	10.993	264.9
								11.007	224.6	10.653	3.1
								10.381	40.6	10.302	20.7
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
14.959	5.8	14.368	54.4	14.802	7.2	14.854	16.2	14.899	3.2	14.888	9.6
13.457	8.2	13.761	25.0	14.292	2.6	14.017	7.1	14.170	7.4	14.271	1.5
12.923	16.7	12.958	117.8	13.669	8.5	13.298	9.1	13.650	1.9	14.108	1.2
12.617	76.5	12.467	1.2	13.328	2.9	12.905	55.3	13.373	7.9	13.795	3.0
12.195	5.3	12.171	13.6	13.072	20.5	12.315	4.2	12.708	66.2	13.196	11.4
11.000	299.2	11.872	1.8	12.799	59.0	11.891	27.2	12.323	7.3	12.907	17.8
		11.482	7.0	12.447	18.0	11.600	7.9	11.946	16.0	12.311	45.7
		11.148	85.5	12.250	13.4	11.374	18.3	11.002	323.8	12.028	16.7
		11.016	245.4	12.006	7.1	11.218	27.4	10.435	3.3	11.582	42.0
		10.384	188.2	11.708	28.9	10.987	312.6	10.280	15.6	11.366	3.9
		10.070	68.0	11.017	367.9	10.566	1.2			11.019	331.8
				10.225	29.9	10.291	10.9			10.764	6.7
				10.036	1.3	10.046	3.9
C148H30		average
14.643	9.4	14.384	9.2
14.362	3.5	13.719	9.5
13.986	2.5	13.405	21.5
13.280	11.2	12.887	41.8
12.771	76.4	12.185	25.6
12.129	18.9	11.861	23.1
11.718	37.7	11.561	19.3
11.377	5.2	11.007	234.7
11.008	383.0	10.750	1.2
10.308	27.0	10.606	3.8
		10.386	17.2
		10.046	6.1
Anions
C22H12		C24H12		C28H14		C30H14		C32H14		C34H16
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
14.282	54.5	13.425	16.4	14.995	17.3	14.136	72.9	14.497	3.4	14.296	44.1
13.906	6.1	13.087	17.7	14.088	24.0	13.023	5.6	13.787	1.4	13.978	8.7
13.510	12.0	12.842	5.3	13.780	12.9	11.889	175.5	13.240	1.2	13.751	12.1
13.021	1.3	12.550	164.7	13.339	22.1	10.858	1.7	12.770	9.2	13.344	21.5
12.618	4.9	10.933	1.8	12.935	7.4			12.188	195.5	13.082	12.4
12.288	135.7			12.599	22.5			10.555	6.4	12.483	22.8
11.451	18.9			11.967	165.5					12.323	38.9
10.995	1.0			10.909	2.1					11.747	155.9
10.718	2.2			10.710	2.3					10.695	8.8
				10.079	50.5					10.438	35.0
										10.310	45.8
C38H16		C40H16		C42H16		C46H18		C48H18		C48H18
14.430	27.3	14.715	8.0	14.539	8.3	14.573	22.5	14.810	2.7	14.102	2.2
13.753	37.6	14.358	5.5	13.349	9.8	14.120	40.7	14.541	6.2	13.554	3.6
13.256	11.8	13.609	4.0	12.992	7.7	13.680	29.4	14.192	28.0	13.179	7.2
12.291	21.5	12.433	44.8	12.575	10.7	12.989	15.5	13.678	16.5	12.855	2.8
11.667	194.9	11.912	198.7	11.875	207.4	12.318	3.1	13.160	26.1	12.261	62.1
10.910	48.0	10.898	1.3			11.975	37.1	12.984	18.2	11.734	219.0
10.092	5.6					11.537	193.7	12.207	73.0	10.389	4.4
						11.366	137.5	11.509	202.6
						10.745	7.1	11.268	4.6
								10.784	31.4
C52H18		C54H18		C54H20		C56H20		C58H20		C62H20
14.975	4.0	14.215	17.7	14.754	6.7	13.899	1.1	14.943	8.0	14.937	1.2
14.079	2.4	13.444	14.1	13.937	36.2	13.490	6.6	14.601	13.8	13.646	8.6
13.782	3.9	13.231	9.6	13.611	30.7	12.647	7.9	13.753	79.3	13.194	13.1
13.434	1.6	12.865	1.9	13.134	13.7	12.436	37.1	13.407	15.5	12.999	4.0
13.149	4.4	12.628	5.6	12.424	14.5	12.105	46.9	13.113	13.4	12.710	3.8
12.731	27.1	12.095	50.3	11.699	300.1	11.625	240.9	12.762	12.3	12.458	26.8
12.226	22.4	11.608	186.0	11.459	214.5	10.654	13.9	12.293	100.9	12.214	3.0
11.681	220.8	10.355	1.5	10.715	3.0			11.886	23.9	11.918	33.2
10.446	4.4			10.490	14.8			11.574	57.2	11.555	239.9
								11.418	295.3	11.069	2.6
								10.728	2.6	10.260	2.8
								10.529	1.7
								10.111	43.3
C64H20		C64H22		C66H20		C72H22		C76H22		C78H22
14.571	1.9	14.582	1.5	14.537	4.6	14.665	6.3	14.476	8.1	14.710	7.0
13.644	4.0	13.755	9.6	13.643	6.5	14.318	3.2	13.928	2.5	14.480	7.6
12.999	6.6	13.443	6.6	13.134	35.0	13.537	3.4	13.563	4.9	13.546	9.2
12.634	47.9	13.063	9.8	12.694	1.1	13.125	10.0	13.294	2.2	13.228	26.5
12.035	28.0	12.575	21.2	12.375	3.4	12.855	27.2	13.002	15.6	12.845	4.1
11.526	239.3	12.274	73.5	11.780	41.1	12.427	17.3	12.749	14.1	12.478	3.4
10.686	16.5	11.939	9.2	11.543	227.1	12.154	4.6	12.364	37.7	12.189	6.9
10.336	9.0	11.551	270.8	11.178	6.0	11.736	75.4	11.825	61.7	11.996	1.8
		11.077	10.0	10.409	4.6	11.478	250.5	11.432	243.6	11.492	301.5
		10.362	17.3	10.194	5.2	11.197	1.7	10.927	16.6	10.908	8.0
						10.774	9.2	10.444	20.2	10.425	6.9
						10.492	28.6	10.070	3.8
C80H22		C82H24		C88H24		C90H24		C90H24		C94H24
14.676	2.1	14.977	3.7	14.832	3.5	14.349	6.3	14.830	1.5	14.345	4.0
13.499	5.4	14.537	3.9	14.512	1.6	13.926	1.9	14.499	3.8	14.055	1.7
12.933	45.8	14.162	50.6	13.904	2.9	13.545	14.6	14.142	7.0	13.455	7.0
12.042	1.8	13.759	35.5	13.450	9.3	13.113	24.6	13.873	1.1	13.111	32.7
11.467	317.3	13.454	7.2	12.927	24.0	12.682	5.6	13.473	5.6	12.786	9.1
11.095	13.5	13.115	16.2	12.729	41.9	12.213	65.2	13.129	40.5	12.478	8.7
		12.794	5.6	12.527	17.2	11.851	41.6	12.492	8.8	11.985	32.8
		12.497	35.9	12.066	28.2	11.740	48.0	11.447	349.8	11.408	324.1
		12.240	16.2	11.705	60.9	11.361	294.9	10.432	20.9	11.191	11.5
		12.050	31.2	11.371	266.6	10.183	19.6	10.236	2.3	10.681	6.5
		11.674	33.4	11.082	20.9					10.334	5.4
		11.320	270.3	10.437	19.5					10.019	1.8
		10.967	20.7	10.039	4.4
		10.629	6.4
		10.473	17.7
		10.158	11.6
C96H24		C102H26		C108H26		C110H26		C112H26		C120H28
13.576	10.2	14.372	18.1	14.826	6.2	14.883	11.2	14.881	3.6	14.890	13.8
12.845	65.9	13.902	4.1	14.358	1.4	14.172	2.7	14.680	4.3	14.073	18.5
12.213	1.5	13.435	13.2	13.961	6.6	13.831	4.2	14.424	1.6	13.207	51.2
11.525	155.3	13.179	26.5	13.423	6.3	13.189	70.8	14.102	1.4	12.752	41.0
11.320	207.1	12.960	29.1	12.975	50.9	12.798	6.7	13.416	8.6	12.353	22.5
		12.621	1.7	12.460	2.8	12.276	50.9	13.108	17.7	11.916	55.0
		12.164	11.8	12.262	5.0	11.952	3.6	12.935	34.1	11.575	62.7
		11.942	23.2	12.005	5.9	11.752	19.0	12.469	18.9	11.287	294.7
		11.408	364.1	11.692	11.8	11.361	352.4	11.851	22.4	10.981	49.3
		11.183	8.9	11.364	386.5	11.057	3.7	11.490	72.2	10.727	8.5
		10.785	3.8	11.095	13.1	10.878	2.0	11.330	284.3	10.385	10.9
		10.448	98.8	10.397	1.6	10.306	3.1	10.945	6.3
		10.068	8.6	10.247	5.2	10.065	2.2	10.465	16.0
C148H30		average
14.667	19.7	14.164	17.5
14.033	1.6	13.723	8.3
13.326	9.3	13.430	5.8
13.001	73.9	13.134	30.5
12.588	15.3	12.527	14.4
12.270	16.0	12.249	31.3
11.825	35.9	11.895	33.6
11.285	433.4	11.408	247.8
10.975	2.7	10.948	4.9
10.342	27.8	10.735	3.3
10.118	9.6	10.446	11.8
10.006	14.4	10.092	5.0

Table 14.

Band positions (µm) and intensities (km/mol) of the odd-carbon PAHs in the 10–15 µm region. A redshift of 15 cm⁻¹ has been applied.

Neutrals
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
13.701	34.1	13.891	21.1	14.749	1.1	14.376	3.0	14.192	5.6	14.461	4.4
13.238	2.0	13.503	12.0	14.438	2.3	14.108	5.2	13.552	20.5	14.140	7.4
12.763	14.2	13.179	9.9	13.719	2.1	13.693	13.0	13.038	7.7	13.092	9.0
12.353	41.1	12.755	2.5	13.122	2.4	13.457	17.7	12.827	25.7	12.812	13.8
11.663	77.0	12.601	2.8	12.477	49.0	12.853	8.2	12.210	10.8	12.273	40.9
11.388	26.9	12.290	2.1	11.993	3.3	12.531	1.3	11.966	11.8	11.473	69.6
11.187	37.2	12.050	36.8	11.489	170.9	12.117	34.3	11.306	184.5	11.313	126.5
10.484	3.5	11.477	75.4	11.198	1.3	11.957	10.4			10.395	3.2
10.030	1.4	11.349	76.5			11.788	17.1
		10.549	2.6			11.421	92.8
						11.215	73.1
						10.734	1.2
						10.457	1.7
C51H19		C55H19		C57H19		C59H19		C59H21		C65H21
14.329	6.2	14.380	16.9	14.877	1.8	14.393	7.0	14.554	2.0	14.552	1.1
13.613	17.0	13.759	1.8	13.672	6.5	14.008	2.7	14.219	5.9	13.992	11.9
13.416	19.4	13.499	17.1	13.043	5.6	13.684	5.1	13.784	7.5	13.455	17.1
13.139	5.0	13.057	7.5	12.753	27.6	13.021	28.3	13.517	14.4	12.922	26.1
12.887	3.7	12.837	12.8	12.162	31.8	12.419	27.1	13.419	16.9	12.355	5.5
11.995	47.5	12.486	13.0	11.770	4.1	11.869	14.4	12.905	5.9	12.117	27.9
11.816	5.7	12.270	20.5	11.413	134.8	11.280	222.9	12.424	3.1	11.772	3.2
11.602	31.1	11.750	16.0	11.194	96.5	10.331	4.3	12.070	18.2	11.579	23.4
11.373	103.4	11.250	207.3	10.084	1.4			11.942	22.8	11.263	224.9
11.191	72.9	10.448	1.9					11.714	6.3	10.943	1.1
10.454	2.7							11.553	41.4
								11.364	199.9
								10.953	1.9
								10.466	1.2
C67H21		C67H21		C67H23		C71H21		C73H21		C77H23
14.459	10.3	14.451	2.0	14.723	4.0	14.451	6.1	14.780	1.1	14.188	6.3
14.047	6.2	13.682	4.5	13.970	7.1	13.900	1.4	13.761	2.8	13.799	3.5
13.873	10.1	13.510	4.3	13.454	32.3	13.521	5.8	13.521	5.3	13.399	3.6
13.475	16.0	13.062	6.8	13.168	4.0	12.938	33.5	12.893	55.6	12.832	27.3
12.917	13.3	12.832	15.9	12.920	2.8	12.516	9.2	11.913	5.0	12.255	38.4
12.273	47.8	12.330	30.6	12.604	1.7	12.219	21.9	11.486	8.6	11.792	10.3
11.681	14.1	11.970	16.5	12.195	10.2	11.882	3.3	11.249	264.9	11.371	184.0
11.235	230.5	11.383	171.6	12.012	34.9	11.705	13.8			11.189	105.9
10.323	2.7	11.159	92.8	11.369	279.8	11.204	247.5			10.499	1.8
10.102	1.0			10.448	1.8	10.918	1.2
						10.643	1.7
						10.395	2.4
						10.137	2.2
C79H23		C83H23		C85H23		C87H23		C87H25		C91H25
14.756	4.5	14.848	2.2	14.405	1.2	14.903	8.1	14.890	1.1	14.990	3.1
14.415	7.1	14.428	6.1	14.118	2.5	13.437	6.7	14.529	1.4	14.386	13.9
13.935	8.8	14.077	2.8	13.401	8.9	13.012	20.2	13.918	4.6	13.803	9.1
13.708	5.0	13.904	2.8	12.855	39.9	12.713	38.8	13.364	7.7	13.382	16.9
13.425	15.2	13.417	6.6	12.522	2.3	12.250	3.8	13.075	5.6	12.947	12.8
12.935	15.9	13.016	28.9	12.105	28.3	11.945	3.2	12.865	12.9	12.461	16.3
12.467	13.5	12.661	10.7	11.798	9.5	11.696	2.3	12.588	13.1	12.338	16.9
12.104	36.0	12.323	15.3	11.602	12.2	11.183	298.8	12.203	31.3	11.896	21.9
11.606	26.0	11.992	17.9	11.188	273.4	10.806	1.8	11.916	4.6	11.543	47.1
11.257	245.8	11.589	22.7	10.065	5.0			11.697	1.4	11.264	255.2
10.417	3.3	11.269	152.6					11.378	190.5	10.406	4.2
10.126	1.3	11.176	119.5					11.276	131.2
		10.651	1.2					10.971	2.3
		10.394	5.2					10.256	1.4
C93H25		C95H25		C95H27		C99H25		C103H27		C107H27
14.799	1.4	14.846	1.7	14.815	3.0	14.919	7.3	14.925	1.8	14.409	8.2
14.393	6.7	14.451	7.1	14.079	10.0	13.744	6.5	14.298	10.8	13.937	2.3
13.877	11.9	13.897	5.6	13.545	21.2	13.376	6.1	13.963	9.0	13.732	2.6
13.580	5.9	13.362	4.7	12.827	27.4	12.937	31.8	13.526	12.4	13.321	7.1
13.365	10.5	13.004	34.9	12.225	25.6	12.607	11.7	13.339	8.1	13.026	31.2
12.972	21.1	12.384	26.1	11.901	4.7	12.370	2.4	12.968	16.9	12.470	7.5
12.495	5.0	12.149	5.7	11.469	104.1	12.054	36.3	12.347	36.8	12.282	27.6
12.000	43.9	11.837	7.7	11.284	243.1	11.557	21.6	11.769	18.0	11.843	3.9
11.585	41.2	11.509	33.5			11.187	295.9	11.507	72.3	11.467	41.2
11.263	259.2	11.231	293.4			10.852	2.3	11.261	258.9	11.259	312.9
10.102	3.6	10.407	7.1			10.035	3.1	10.651	1.6	10.397	8.2
		10.085	3.2					10.398	4.1
C107H27		C111H27		C113H27		C115H27		C115H29		C119H27
14.932	9.0	14.706	3.6	14.691	1.4	14.905	10.9	14.399	12.4	14.308	3.6
14.510	2.2	14.063	2.8	14.304	3.8	14.459	3.6	13.833	11.5	13.966	2.5
13.966	9.9	13.671	15.3	14.041	3.4	14.116	2.3	13.514	13.1	13.667	1.4
13.695	12.4	13.266	10.0	13.721	2.2	13.776	1.3	13.284	8.4	13.351	6.4
13.308	9.2	12.968	17.5	13.340	6.4	13.316	5.5	12.963	15.6	12.997	28.0
12.970	20.0	12.615	6.5	12.920	36.1	13.006	29.3	12.472	10.4	12.775	28.7
12.636	7.5	12.069	54.0	12.447	12.1	12.639	11.4	12.223	30.3	12.473	3.9
12.382	1.5	11.715	23.8	12.120	16.7	12.287	15.1	11.703	13.6	12.044	16.0
12.176	14.6	11.456	107.6	11.750	13.6	11.872	23.5	11.482	99.2	11.150	362.7
11.882	29.7	11.284	135.0	11.510	33.7	11.509	27.1	11.264	261.7	10.325	3.4
11.545	58.9	11.161	125.5	11.225	320.0	11.208	321.3	11.006	1.4
11.268	269.5	10.201	15.6	10.375	1.2	10.824	3.6	10.397	4.2
10.806	1.8
C119H29		C121H29		C127H31		average
14.438	8.8	14.541	7.5	14.447	12.0	14.413	4.8
14.096	2.7	13.933	12.2	14.223	4.8	13.470	20.9
13.684	4.8	13.600	13.2	13.761	8.5	12.958	25.6
13.247	6.1	13.242	5.9	13.493	20.7	12.241	20.4
13.004	30.4	12.979	19.8	12.970	14.1	12.129	18.4
12.563	6.8	12.472	11.7	12.385	27.2	11.257	287.3
12.327	19.0	12.134	23.5	12.086	18.2	10.400	2.4
12.134	12.9	11.745	21.7	11.684	9.7
11.840	3.4	11.509	82.7	11.468	131.6
11.448	59.6	11.269	275.6	11.264	262.5
11.260	324.8			10.391	5.2
10.945	3.0			10.010	2.8
10.390	5.4
10.013	5.6
Cations
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
14.979	25.3	13.759	10.0	14.343	2.8	14.347	3.2	14.378	1.1	14.445	5.7
13.611	35.0	13.374	17.9	13.708	5.5	14.069	2.2	14.096	1.6	14.124	8.5
13.210	3.0	13.165	6.1	12.082	109.4	13.585	6.2	13.464	7.9	13.408	9.6
13.091	1.8	12.900	24.7	10.957	130.3	13.358	21.2	13.160	12.5	13.125	2.8
12.677	6.2	12.364	1.8			13.106	5.9	12.700	47.7	12.883	8.6
12.346	64.9	11.809	47.1			12.902	18.3	12.031	19.4	12.674	19.3
11.641	19.5	11.464	15.4			12.533	8.4	11.147	68.2	11.976	68.3
10.909	93.9	11.130	62.5			12.226	3.4	10.970	120.1	11.047	103.5
10.625	23.4	10.896	63.5			11.891	24.6	10.712	6.9	10.842	65.1
10.257	1.0	10.572	3.3			11.566	48.1			10.358	13.8
10.141	2.6					11.405	17.0
10.015	3.5					10.995	72.3
						10.877	53.3
						10.683	8.4
						10.414	1.3
C51H19		C55H19		C57H19		C59H19		C59H21		C65H21
14.932	1.3	14.374	8.4	14.817	1.4	14.968	4.0	14.558	3.7	14.802	5.6
14.314	5.9	14.110	4.3	14.162	14.8	14.353	10.0	14.142	27.6	13.953	10.6
13.466	21.3	13.550	11.0	13.637	4.9	13.976	5.8	13.684	3.5	13.550	15.4
13.294	14.1	13.113	12.9	12.897	1.1	13.667	5.8	13.358	58.4	13.305	6.6
13.046	34.9	12.908	9.4	12.610	36.5	12.875	42.1	12.197	43.3	12.781	42.9
11.995	7.5	12.718	26.1	12.058	35.6	12.044	54.0	11.853	40.1	12.470	1.6
11.761	44.6	12.446	1.9	11.647	24.9	11.474	3.0	11.557	33.0	12.160	19.4
11.425	85.0	12.098	32.0	11.215	53.6	11.287	12.9	11.340	69.0	11.966	35.5
11.148	1.7	11.624	5.8	11.014	102.1	11.073	75.8	10.942	126.1	11.567	1.8
10.935	117.4	11.199	55.5	10.790	57.4	10.894	122.6	10.771	31.3	11.212	90.7
10.730	19.9	10.967	156.8	10.070	2.0	10.322	17.3	10.195	1.0	10.945	166.5
10.153	2.0	10.715	7.6							10.712	5.5
		10.355	2.4							10.219	9.3
										10.003	3.7
C67H21		C67H21		C67H23		C71H21		C73H21		C77H23
14.430	17.6	14.984	7.4	14.990	37.8	14.986	2.0	14.789	1.2	14.434	3.8
14.198	12.7	14.516	14.8	14.106	6.0	14.674	2.2	13.883	7.7	14.154	5.9
13.701	8.5	13.672	1.6	13.885	5.5	14.418	7.2	13.521	5.9	13.797	8.6
13.342	8.2	13.335	9.4	13.356	66.2	13.899	2.9	13.014	11.2	13.342	1.7
13.055	13.7	12.728	28.4	12.623	47.2	13.488	7.7	12.781	63.3	13.160	1.7
12.900	22.4	12.450	1.6	12.340	23.1	12.870	45.3	11.481	32.2	12.719	32.3
12.067	46.7	12.082	44.7	11.878	28.7	12.086	38.0	11.271	58.9	12.402	1.8
11.726	1.0	11.563	45.6	11.751	34.2	11.758	24.3	10.935	199.0	12.161	36.5
11.421	15.1	11.235	79.3	11.279	98.1	11.005	169.2	10.097	3.2	11.787	89.7
11.011	230.1	10.981	101.0	10.957	169.5	10.855	70.4			11.244	115.1
10.341	9.5	10.785	58.8	10.411	1.7	10.378	9.5			10.966	122.8
10.045	22.6	10.483	2.3	10.259	2.5	10.133	9.0			10.799	54.2
										10.390	21.8
										10.146	1.3
C79H23		C83H23		C85H23		C87H23		C87H25		C91H25
14.791	2.7	14.892	1.7	14.588	1.1	14.888	10.9	14.676	8.7	14.689	21.8
14.249	30.3	14.323	13.5	14.186	13.5	13.699	3.5	14.059	2.8	14.403	7.9
13.922	7.2	13.918	3.8	13.398	7.3	13.430	7.8	13.829	3.2	13.988	61.6
13.661	7.8	13.380	7.7	13.103	1.4	12.845	31.9	13.543	8.4	13.721	19.9
13.291	13.1	12.898	46.4	12.758	42.5	12.716	49.0	13.273	5.9	13.314	14.1
13.011	36.2	12.375	1.1	12.117	18.1	12.222	5.4	12.726	72.0	13.053	43.8
12.488	3.8	12.126	39.0	11.822	56.1	11.953	5.5	12.192	85.8	12.207	51.1
12.200	28.6	11.918	10.6	11.238	6.0	11.695	11.9	11.937	86.3	11.488	71.6
11.865	27.7	11.500	20.0	10.925	255.6	11.241	47.4	11.236	132.8	11.203	67.8
11.547	5.0	11.120	77.8	10.274	3.3	10.967	247.3	10.972	176.0	10.996	260.5
11.004	282.7	10.995	129.2	10.042	20.4			10.835	47.6	10.827	19.0
10.445	25.8	10.824	66.7					10.554	2.6	10.430	37.9
10.137	13.6	10.326	28.7							10.284	34.9
										10.010	18.3
C93H25		C95H25		C95H27		C99H25		C103H27		C107H27
14.995	42.8	14.434	9.2	14.697	9.1	14.852	13.9	14.912	17.5	14.451	20.7
14.403	7.7	14.021	6.4	14.300	56.1	14.150	1.0	14.658	8.2	13.695	36.2
14.000	32.3	13.784	5.2	13.984	29.4	13.868	3.4	14.207	118.0	13.446	40.8
13.587	1.7	13.618	2.9	13.598	62.5	13.755	2.8	13.567	45.6	12.943	44.8
13.250	8.1	13.092	25.7	13.165	18.1	13.356	9.3	13.134	50.5	12.555	7.8
12.902	39.0	12.925	48.1	12.791	39.2	12.850	40.9	12.514	12.9	12.198	40.6
12.517	17.5	12.429	4.2	12.305	116.9	12.523	3.3	12.162	45.5	11.985	36.0
12.210	21.2	12.157	41.0	11.930	20.1	12.143	33.6	11.966	63.3	11.791	24.5
11.846	36.6	11.662	14.6	11.659	10.3	11.860	30.5	11.748	8.9	11.401	27.5
11.590	25.1	11.377	23.9	11.207	184.0	11.541	20.9	11.416	135.8	11.150	181.3
11.104	156.6	11.141	135.9	10.989	247.1	11.222	17.1	11.008	340.6	10.981	173.7
11.000	164.2	10.982	114.0	10.234	2.5	10.996	292.3	10.762	54.0	10.848	51.1
10.406	8.5	10.815	77.2			10.220	2.2	10.663	33.6	10.537	17.6
10.114	8.8	10.408	41.3			10.008	6.6	10.433	58.7	10.405	49.4
		10.029	11.2					10.227	19.3	10.181	16.9
								10.124	20.5
C107H27		C111H27		C113H27		C115H27		C115H29		C119H27
14.916	17.3	14.839	9.7	14.669	1.6	14.892	14.2	14.635	49.1	14.979	2.3
14.548	49.5	14.069	8.3	14.271	6.8	14.378	11.0	13.910	79.1	14.932	3.5
13.765	37.0	13.672	22.6	13.951	5.1	13.736	4.3	13.712	33.8	14.355	8.3
13.317	16.0	13.455	7.5	13.454	22.1	13.205	15.1	13.316	16.2	13.970	3.7
12.908	50.9	12.873	24.8	12.840	41.6	12.908	42.0	12.992	48.4	13.637	1.9
12.265	36.0	12.395	18.9	12.560	20.4	12.294	25.3	12.617	14.6	13.323	7.0
12.053	25.0	12.258	20.3	12.189	16.9	12.019	44.1	12.273	104.1	12.880	42.4
11.895	23.7	11.827	77.2	11.960	32.2	11.600	20.3	11.841	48.5	12.690	29.4
11.596	44.1	11.616	13.2	11.743	32.2	11.233	50.8	11.566	108.5	12.450	8.3
11.016	391.3	11.430	75.8	11.377	36.0	10.971	314.3	11.324	60.8	11.860	17.9
10.293	9.8	10.979	306.2	11.012	323.9	10.505	2.2	11.061	454.9	11.312	42.0
10.011	91.6	10.194	21.9	10.704	7.3			10.661	1.7	10.987	326.4
				10.433	10.3			10.436	115.5	10.312	7.3
								10.210	5.9
C119H29		C121H29		C127H31		average
14.263	68.3	14.883	24.0	14.613	71.4	14.573	8.4
13.988	16.1	14.594	52.4	13.970	62.7	14.229	13.1
13.732	19.4	13.808	62.7	13.459	17.1	13.986	9.0
13.278	43.4	13.161	36.5	13.235	62.6	13.628	14.6
12.943	39.3	12.913	24.2	12.791	37.0	13.310	11.1
12.665	27.2	12.653	8.5	12.602	54.3	12.880	48.9
12.274	59.7	12.222	47.7	12.067	106.8	12.092	52.7
12.090	51.5	11.795	176.8	11.833	42.2	11.822	25.5
11.752	32.3	11.493	64.0	11.451	189.5	11.434	36.3
11.555	111.0	11.042	390.3	11.044	388.0	10.987	287.5
11.152	187.2	10.637	95.2	10.741	13.8	10.424	22.6
10.987	223.9	10.450	20.9	10.617	2.1
10.416	115.7			10.441	114.3
10.045	38.0			10.231	21.3
				10.012	15.7
Anions
C31H15		C35H15		C37H15		C43H17		C45H17		C47H17
λ	I	λ	I	λ	I	λ	I	λ	I	λ	I
14.116	37.0	14.560	5.1	14.743	6.8	14.259	18.8	14.782	1.5	14.495	2.6
13.617	18.8	14.114	25.2	13.291	6.2	13.916	21.1	14.265	5.5	14.188	7.2
13.001	11.8	13.731	3.9	12.775	18.5	13.364	3.7	13.831	20.6	13.333	10.2
12.593	25.6	13.457	10.0	11.910	198.6	13.079	7.9	13.186	22.0	12.958	9.3
12.270	42.6	12.747	4.7	11.230	4.5	12.553	3.0	12.930	11.0	12.583	19.8
11.895	108.4	12.432	32.5			12.337	42.8	12.258	31.0	12.118	5.7
11.472	39.7	11.858	102.2			12.044	10.9	11.957	2.3	11.788	141.1
10.825	26.0	11.632	76.6			11.719	129.5	11.557	195.2	11.555	77.7
10.097	32.9	11.404	4.0			11.459	71.6	10.797	16.8	10.425	5.1
		11.260	2.0			11.162	27.6
		10.830	1.7			10.775	4.3
		10.601	9.6			10.460	4.1
		10.009	12.5
C51H19		C55H19		C57H19		C59H19		C65H21		C67H21
14.952	2.3	14.401	16.5	14.172	2.2	14.457	6.0	14.461	6.4	14.558	11.4
14.409	9.2	13.852	19.3	13.767	5.5	13.732	6.0	14.100	7.7	14.168	11.9
13.854	25.3	13.572	1.8	13.177	16.4	13.173	20.8	13.860	17.1	13.833	21.5
13.563	8.9	13.175	8.8	12.903	13.1	12.682	21.7	13.604	19.8	13.001	13.0
13.428	7.8	12.917	21.2	12.425	22.4	12.146	14.4	13.348	2.2	12.780	11.2
13.120	3.4	12.498	27.6	12.162	2.7	11.502	239.7	13.077	22.6	12.418	52.2
12.817	3.3	11.986	34.1	11.674	171.1	10.900	6.8	12.615	5.3	11.846	34.1
12.165	50.2	11.682	10.0	11.384	84.6	10.349	5.2	12.240	47.8	11.368	237.1
11.891	23.8	11.446	209.6	10.124	10.6			11.875	54.6	10.765	1.7
11.623	181.0	10.756	11.6					11.388	234.7	10.351	14.2
11.396	82.1	10.411	14.7					11.120	3.3	10.100	7.1
11.187	2.7							10.959	24.3
10.740	2.2							10.745	1.7
10.473	7.0							10.282	7.9
10.181	1.4
C67H21		C71H21		C73H21		C77H23		C83H23		C85H23
14.518	5.3	14.541	5.1	13.521	6.4	14.463	2.2	14.616	8.7	14.440	3.4
13.628	5.6	13.611	8.7	13.007	43.6	14.239	5.4	14.180	3.3	14.184	2.6
13.163	13.1	13.103	29.1	11.922	19.5	13.889	3.3	13.918	3.0	13.528	6.2
12.968	8.1	12.750	8.2	11.654	21.8	13.591	2.2	13.530	6.8	13.360	6.4
12.598	24.4	12.463	13.5	11.459	255.2	13.153	12.7	13.135	26.2	12.963	34.1
12.220	10.4	12.134	6.9	11.203	9.4	12.888	13.7	12.839	16.4	12.338	25.0
11.953	4.2	11.923	17.0			12.467	35.0	12.588	10.6	11.815	50.1
11.596	205.7	11.493	179.0			11.809	40.8	12.213	16.5	11.346	275.2
11.339	91.5	11.380	86.1			11.538	208.7	12.018	5.2	10.979	2.5
10.908	4.2	10.929	3.4			11.324	109.1	11.773	33.9	10.430	2.4
10.523	7.9	10.680	1.0			10.960	12.6	11.435	278.8	10.320	3.8
		10.410	2.7			10.437	20.1	10.786	2.9	10.092	8.6
		10.349	3.4					10.408	16.8
		10.165	1.1					10.009	3.7
C87H23		C95H25		C99H25		C103H27		C119H27		average
14.925	5.1	14.846	1.0	14.654	1.6	14.923	1.3	14.948	4.3	14.453	5.0
14.671	2.6	14.428	6.1	13.740	7.6	14.650	3.6	14.405	5.2	14.122	6.6
13.461	8.9	13.755	12.1	13.468	8.5	14.071	13.9	13.963	2.2	13.831	7.4
13.172	15.0	13.602	6.0	13.014	47.1	13.468	30.6	13.672	2.4	13.534	6.6
12.827	33.0	13.077	46.6	12.516	3.2	13.006	32.0	13.407	9.1	13.011	29.7
12.463	2.6	12.599	27.0	12.092	45.4	12.615	29.6	13.110	25.2	12.408	16.9
12.213	7.1	12.341	3.6	11.722	48.1	12.229	15.5	12.858	28.0	12.268	14.9
11.965	8.7	11.986	6.8	11.310	291.6	11.659	90.1	12.413	8.3	11.872	38.3
11.435	316.4	11.678	66.1	10.868	3.5	11.291	311.1	12.137	21.8	11.355	246.1
11.077	3.9	11.340	302.0	10.247	9.6	10.988	2.1	11.794	4.4	10.947	2.5
10.708	3.1	10.817	6.3	10.044	4.6	10.731	7.5	11.655	5.8	10.803	4.2
		10.452	14.0			10.399	2.9	11.284	380.8	10.426	6.8
		10.072	8.0			10.182	14.7	10.711	1.3	10.096	4.8
								10.337	10.3
								10.005	3.7