Terahertz Spectroscopic Analysis of the Minium Pigment: Evidence of Low-Temperature Phonon Splitting

Abstract

Terahertz spectroscopy has been increasingly utilized as an effective nondestructive tool for the diagnosis, analysis, and restoration of artworks. In particular, in the case of artist’s pigments, the terahertz probe reveals the vibrational modes that are unique to a given pigment species under study, motivating the ongoing efforts to establish a comprehensive terahertz spectral database of representative pigments. Standard archived spectra are typically acquired at room temperature and susceptible to spectral broadening, which often renders pigment identification difficult, if not impossible. In this paper, we report the frequencies of the vibrational modes of minium (Pb₃O₄, red lead) by performing terahertz time-domain spectroscopy at room temperature and also at low temperatures. Clear absorption peaks appear at 54.9, 62.1, 71.3, and 83.9 cm^–1 at room temperature and blue-shift as the temperature decreases. In addition, new absorption peaks of 59.8 and 66.4 cm^–1 are observed below 150 K, which signify a structural phase transition occurring at 170 K in minium. Our results are expected to enhance our understanding of the vibrational activity of minium and suggest a future direction for how to improve and refine the existing terahertz spectral databases for pigment analysis.

Introduction

Terahertz spectroscopy is a relatively new technique for analyzing paintings and other artworks,¹ but there is a growing demand for this method due to several unique advantages over other, more traditional methods. Terahertz spectroscopy is nondestructive, which is highly desirable for the diagnosis of invaluable artworks. At the same time, terahertz light can penetrate deeply into the bulk of a specimen, allowing access to the layers beneath its visible surface.² Furthermore, the terahertz wavelength couples primarily to intermolecular vibrational modes (lattice modes and librational modes) of the medium under investigation that can be effectively used to identify historical and modern pigments used in paintings.³ As such, terahertz spectroscopy is expected to form a critical component in future analysis tools dedicated to the conservation and restoration of cultural heritage.⁴

In this connection, a number of well-known pigments have been studied with terahertz spectroscopy in the interest of identifying absorption peaks that can be used as fingerprints that are unique to individual pigment species. Azurite,⁵⁻⁷ calcium carbonate,⁸ copper phthalocyanine,⁹ magenta,¹⁰ malachite,⁵⁻⁷ minium,^6,7,11−13 verdigris,⁵ vermilion,^7,11,12,14 and quinacridone¹⁰ have been extensively analyzed, and several terahertz spectral databases have been set up to archive the key spectral features of such representative pigments. Typically, these databases and the source terahertz studies only report room-temperature absorption spectra. The absorption spectrum is dominated by the absorption coefficient or by the extinction coefficient k, while the refractive index n is rather concealed. However, the latter quantity is quite essential in the analysis of artworks in connection with the measurement of the physical (rather than optical) thicknesses of underlying layers in paintings via, e.g., terahertz imaging.¹⁵ There are also complications due to the spectral broadening inevitable in data acquired at room temperature, leading to an overlap of absorption peaks of multiple pigment species nearby.

In order to overcome the aforementioned shortcomings, we have recently conducted a terahertz spectroscopic study of the vermilion pigment, which can hint at future directions in the field of terahertz analysis of artists’ pigments.¹⁴ First, pigment samples in both free-standing and pellet forms have been studied to obtain “pure” spectral responses of vermilion, including the refractive index spectrum in addition to the absorption coefficient. Second, transmission measurements were extended to low temperatures. This allowed for the clear identification of a pair of peaks that had often been regarded as a single peak in past reports. Third, a detailed comparison with density functional theory (DFT) calculations was made, which revealed the subtle nature of the observed vibrational modes such as their asymmetric line shapes that were ultimately traced to their unusual vibrational patterns.

In this paper, we report on our new terahertz study of minium (Pb₃O₄, red lead) in the frame adopted in our previous study of vermilion.¹⁴ Standard sets of transmission and absorption spectra have been acquired by using terahertz time-domain spectroscopy. We confirm the presence of four absorption peaks in the terahertz region at room temperature, as reported by Kleist and Korter.¹¹ These authors have assigned these modes to the infrared-active transverse optical phonon modes with E_u and A_2u symmetry. At low temperatures, we find new absorption modes at 59.8 and 66.4 cm^–1 and connect this observation with a structural phase transition at 170 K in minium.¹⁶ We present the detailed evolution of the phonon modes with temperature and compare our data with the existing DFT calculations. Overall, we believe that the present study will be a valuable addition to the current database of color pigments studied by terahertz spectroscopy.

Materials and Methods

Sample Preparation

Our minium samples were made with pure minium powder (Natural pigments Inc, PR 105) and polyethylene (PE) powder. Due to the strong absorption of minium in the terahertz range, free-standing samples were not amenable to standard terahertz transmission measurements. Hence, we prepared pellet samples of minium mixed with PE. PE exhibits a weak absorption peak at 75.6 cm^–1 in the terahertz range,¹⁷ but it does not significantly overlap with the absorption peaks of minium. Samples prepared in pellet form were pressed under a weight of 10 tons for 1 min. The weight ratio of minium and PE powder in our pellets was 1:29. The thickness of the pellets was 2.75 mm as measured by a micrometer. A pure PE pellet was also prepared under the same conditions.

Terahertz Time-Domain Spectroscopy

Terahertz transmission measurements were conducted on a TERA K15 (Menlo Systems GmbH, Germany) spectrometer in the frequency range of 10–100 cm^–1. A femtosecond laser source with a 1.5 μm center wavelength delivered 90 fs pulses at a 100 MHz repetition rate. When a laser pulse arrives at an InGaAs photoconductive emitter, electron and hole pairs are copiously created. The voltage applied across the emitter electrodes accelerates the electrons and holes to opposite ends, creating a pulsed current and hence a pulsed terahertz wave. The terahertz pulse, polarized in the direction in which the charges are accelerated, transmits through the sample and arrives at an InGaAs detector. The incoming terahertz pulse can be mapped in the time domain by monitoring the current across the detector electrodes where the detector is optically gated by a split-off laser pulse traversing through a delay line. The acquired terahertz electric field is then converted into the amplitude and phase spectra through Fourier transformation. All terahertz measurements were conducted inside a plexiglass housing purged with dry nitrogen to remove water absorption peaks in the terahertz range. The low-temperature experiment was conducted with a SpectromagPT magnetooptical cryostat (Oxford Instruments, UK), which can control the temperature from 1.5 to 300 K.

Results and Discussion

Terahertz Time-Domain Spectroscopy at Room Temperature

Figure 1a shows the terahertz electric field waveforms transmitted through the reference aperture (blue) and a PE-mixed minium pigment pellet sample (red) acquired at room temperature. The corresponding absorbance spectrum is presented in Figure 1b. The absorbance spectrum of the PE-mixed minium pellet sample clearly shows characteristic absorption peaks of minium in the terahertz range at 54.9, 62.1, 71.3, and 83.9 cm^–1. Our experimental peak positions agree fairly well with DFT calculations,¹¹ which predict infrared-active E_u phonon modes at 58.22, 60.23, and 85.77 cm^–1 and an infrared-active A_2u phonon mode at 71.3 cm^–1, all of which fall into the spectral range of our measurements. The small differences are attributable to thermal expansion.

Figure 1.

(a) Terahertz electric field waveforms transmitted through the reference aperture (blue) and a PE-mixed minium pigment pellet sample (red). (b) Absorbance spectrum of the PE-mixed minium pigment pellet sample. All measurements were conducted at room temperature.

To perform terahertz transmission measurements, the minium pigment was mixed with PE to form a pellet sample (Figure 2a). The absorbance spectrum of this sample is compared with that of a pure PE pellet sample in Figure 2c. We found that PE exhibits a weak absorption peak at 75.6 cm^–1 at room temperature, which is negligible compared to the strong absorption peak of minium located at 71.3 cm^–1. The PE peak seems to form a weak high-frequency shoulder of the minium peak at 71.3 cm^–1, slightly distorting that mode. Also, since the PE-mixed minium pellet sample needs to be fixed to a copper sample holder in a cryostat for low-temperature measurements to be described later, it was wrapped with Kapton tape (Figure 2b), and the absorbance was remeasured for comparison. Although there was a difference in the overall absorbance level mainly due to the thickness difference and internal reflections caused by Kapton tape, there was little change in the observed absorption peak positions (Figure 2d).

Figure 2.

(a) Photograph of a PE-mixed minium pellet sample. (b) PE-mixed minium pellet sample wrapped with Kapton tape. (c) Absorbance spectra of the PE-mixed minium pellet (red) and a pure PE pellet sample (blue). (d) Absorbance spectra of the PE-mixed minium pellet sample [red, the same as in (c)] and of the same pellet wrapped with Kapton tape (green). All measurements were conducted at room temperature.

Low-Temperature Terahertz Spectra

Figure 3a shows the temperature-dependent terahertz absorbance spectra of the PE-mixed minium pellet sample measured over the temperature range of 5–300 K. At each temperature, the peak position was obtained through Lorentz fitting, and Figure 3b shows the representative fit results for 5 K (top) and 300 K (bottom). To fit the low energy of the absorbance spectrum, an additional Lorentzian at 45 cm^–1 was added (not shown). Since all absorption peaks fitted fairly well with Lorentzians, we do not consider asymmetric line shape issues addressed in our previous study of vermilion.¹⁴ The phonon modes of minium predicted by a previous DFT calculation,¹¹ i.e., E_u modes at 58.22, 60.23, and 85.77 cm^–1 and A_2u mode at 71.3 cm^–1, correspond to our experimentally confirmed phonon modes A (E_u, 54.95 cm^–1), C (E_u, 62.17 cm^–1), E (A_2u, 71.32 cm^–1), and F (E_u, 83.87 cm^–1) in Figure 3b. Below 150 K, two new absorption peaks appear at about 60 cm^–1 (black arrows) and 67 cm^–1 (blue arrows). We note the presence of a fairly strong but somewhat broad absorption peak of PE at 78.4 cm^–1 in the 5 K absorbance spectra of our sample. In fact, at room temperature (Figure 2c), the PE peak actually slightly distorts the peak E (A_2u, 71.32 cm^–1) of minium, but the corresponding PE peak at low temperatures, e.g., at 5 K (Figure 3b, top), is by far as strong as the minium absorption peaks. This underscores the necessity of identification and pre-characterization of mixing material components (such as PE¹⁷) when studying the pellet samples of pigments.

Figure 3.

(a) Absorbance spectra of the PE-mixed minium pellet sample at various temperatures between 300 and 5 K. Below 150 K, new absorption peaks appear at about 60 cm^–1 (black arrows) and 67 cm^–1 (blue arrows). (b) Absorbance spectra at 5 K (top) and 300 K (bottom), respectively, fitted with Lorentzians: experimental (dots), fitted (black), and Lorentzian components (colors). (c) Peak positions of the absorption peaks as functions of temperature as determined by the Lorentzian fitting. As the temperature decreases, all peaks blue shift. The dashed vertical line indicates the temperature of 170 K at which the phase transition from the tetragonal phase (above 170 K) to the orthorhombic phase (below 170 K) occurs in minium.

The temperature-dependent positions of the absorption peaks of minium are shown in Figure 3c. The positions of the two new peaks B (orange circles) and D (blue triangles), appearing below 150 K, are also presented. As the temperature decreases, all peaks blue shift. The appearance of the two new peaks is inconsistent with the aforementioned DFT calculations¹¹ based on the tetragonal phase of minium at room temperature. We attribute the emergence of new modes to the phase transition at 170 K in minium from the tetragonal (above 170 K) to the orthorhombic (below 170 K) phase.¹⁶ The structural transition from the tetragonal to the orthorhombic phase presumably lifts the degeneracy of the E_u modes in minium. According to a previous study of minium,¹⁸ the E_u mode in the room-temperature tetragonal phase is split into B_2u + B_3u in the low-temperature orthorhombic phase. Since the absorption peak of the E_u mode in the tetragonal phase of minium at 62.17 cm^–1 at room temperature is dominated by the translation of the Pb and O atoms along the (equivalent) directions of the a and b axes,¹¹ in the orthorhombic phase at low temperatures, it seems natural to split into B_3u and B_2u infrared-active modes along the (now inequivalent) a and b axes, respectively.¹⁸ We expect future DFT calculations to address this issue and elucidate the detailed mechanism underlying the unusual phonon splitting at low temperatures, as found in the present study. As minium exhibits many more phonon modes in the far infrared range, experimental measurements need to be extended beyond 100 cm^–1 to enable a detailed comparison between theory and experiment and a fuller understanding of the lattice dynamics of minium.

Conclusions

In this paper, we conducted a terahertz spectroscopic study of minium and presented the detailed evolution of the infrared-active phonon modes with temperature. The room-temperature data show good agreement with previous experimental results and DFT calculations. The low-temperature spectra of minium reveal an interesting phonon mode splitting, which is attributable to a structural phase transition at 170 K from the high-temperature tetragonal phase to the low-temperature orthorhombic phase. Our results should guide future DFT calculations and offer a new way to study terahertz signatures of representative pigments, emphasizing the necessity of low-temperature measurements and identification of mixing material components. Overall, we believe that our results will be a welcome addition to the existing effort to construct a more sophisticated and enriched terahertz spectral database of artists’ pigments dedicated to the conservation of invaluable artworks.

Glossary

Abbreviations

PE

polyethylene

DFT

density functional theory

Author Contributions

N.Y.B. and J.K. contributed equally. N.Y.B., J.K., and J.E.L. designed the project. N.Y.B. performed the terahertz transmission measurements. J.K. analyzed the experimental data and constructed a theoretical interpretation. J.E.L. assisted terahertz experiment and data analysis. J.H.K. guided the research. All authors discussed the results and wrote the manuscript together.

The authors declare no competing financial interest.