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Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry logoLink to Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry
. 1967 Nov-Dec;71A(6):587–599. doi: 10.6028/jres.071A.050

Fundamental Energy Levels of Neutral Promethium (Pm I)

Joseph Reader 1, Sumner P Davis 1
PMCID: PMC6658511  PMID: 31824069

Abstract

The spectrum of atomic promethium has been observed with a variety of light sources and spectrographs. The Zeeman effect has also been recorded. Analysis of the spectrum shows that the ground configuration of the neutral promethium atom is 4f56s2. The relative positions (in cm−1) of the low levels of this configuration are:

6H5/2° 0.00 6H13/2° 3919.03 6F5/2° 5872.84
6H7/2° 803.82 6H15/2° 5089.79 6F7/2° 6562.86
6H9/2° 1748.78 6F1/2° 5249.48 6F9/2° 7497.99
6H11/2° 2797.10 6F3/2° 5460.50 6F11/2° 8609.21

This group represents all levels of 4f56s2 expected below 14,000 cm−1. From these results the following values of interaction parameters and their estimated uncertainties have been inferred:

ζ4f=925±20cm1  E3=510±20cm1

Data on 209 upper levels of even parity and 714 classified lines are given.

Keywords: Atomic spectroscopy, electronic energy levels, neutral atom, promethium, rare earth

1. Introduction

In this paper we present the first results of our investigation to determine the electronic structure of neutral promethium.

Promethium was the last lanthanon element to be discovered. Since the early 1900’s it was known that an element lying between neodymium and samarium with atomic number 61 remained to be discovered. A large number of attempts were made to find this element in nature without success. Two false claims of discovery resulted in the early names of illinium and florentium for element 61. The long sought-for element was finally identified in 1947, when Marinsky, Glendenin, and Coryell made a chemical separation of a new element from among the fission products of uranium. They chose the name promethium for element 61 “… after Prometheus, the Titan in Greek mythology who stole fire from heaven for the use of mankind.” This name was chosen not only to symbolize the dramatic way in which the element could be produced in quantity as a result of man’s harnessing of the energy of nuclear fission, but also to “… warn man of the impending danger of punishment by the vulture of war,” [1].1

The first extensive work on the spectrum of Pm was carried out at the National Bureau of Standards by Meggers, Scribner, and Bozman [2], who published a list of 2249 Pm lines excited in arcs and sparks. Their attempts to separate the lines according to ionization state and to find spectral regularities were not successful. Their paper gives many of the details of the history of the discovery of Pm and of earlier spectroscopic studies.

Promethium is an entirely artificial element. All of its isotopes are radioactive, and none has ever been found in nature. In the several investigations of the promethium spectrum to date, including the present one, the samples have consisted of monoisotopic Pm147. This is the only readily available form of Pm. It is produced in nuclear reactors as 2.6 percent of the fission products of uranium. Pm147 decays by β emission (0.22 MeV) into Sm147 with a half-life of 2.6 years.

The original paper of Meggers, Scribner, and Bozman suggested the existence of appreciable hyperfine structure in a number of Pm147 spectrum lines. This hfs was investigated by Klinkenberg and Tomkins [3] with a 9-meter grating spectrograph and later by Reader and Davis [4, 5] with Fabry-Perot interferometers. As a result of these investigations and others by the methods of paramagnetic resonance [6] and atomic beam resonance [7], it is known that the Pm147 nucleus has a spin , a magnetic dipole moment μI = 2.6 nm, and an electric quadrupole moment Q ~ 0.7 barn. These nuclear moments cause many Pm lines to appear with very broad and complex hyperfine structure which impedes attempts to describe the spectrum.

The ionization energy of the neutral promethium atom has been estimated [8] by an interpolation method to be 5.55 ± 0.02 eV.

2. Experimental Procedure

2.1. Light Sources

The construction of light sources was undertaken with the greatest care, because Pm, being radioactive, cannot be handled in the ordinary way. The first light source was a cooled hollow cathode, made for the investigation of hyperfine structure. Its use was essential for that work [4], but it was not used extensively in the present investigation.

The more useful sources were electrodeless discharge tubes, constructed especially for us by Earl Worden at the Lawrence Radiation Laboratory, Livermore. The promethium was obtained from Oak Ridge National Laboratory, and purified either there or at the Lawrence Radiation Laboratory, Berkeley. A spectrographic analysis was made prior to its use. A typical tube was constructed of a section of fused silica tubing 2 cm long with 7 mm o.d. and 5 mm i.d., containing 200 μg of PmI3. A long handle was attached. No filling gas was admitted, since an initial heating of the tube released enough iodine to make subsequent starting of the discharge relatively easy.

Altogether, six of these tubes were used. Each tube was operated inside a microwave cavity, supplied with rf power by a magnetron at a frequency of 2450 MHz. The temperature of the tube (and hence intensity of the spectrum) was regulated by adjustment of power to the cavity and the amount of air-cooling. During operation, the discharge was a brilliant blue color.

2.2. Spectrographs

Our experimental investigation of the spectrum of promethium has extended over several years, and has included the taking of hundreds of spectrograms on several instruments, under many different conditions. As we gained experience and familiarity with the spectrum, each set of spectrograms was taken to provide specific information.

The 6.4-meter concave grating at Berkeley was used for preliminary testing of the light sources and identification of spectrum lines. The instrument has a plate factor of 1.25 Å/mm at 5000 Å in the first order.

The 3-meter Czerny-Turner plane-grating spectrograph at Berkeley was used for accurate wavelength measurements. This instrument has a grating of width 12.5 cm, ruled with 300 grooves/mm. It is used at angles of incidence and diffraction of approximately 64 deg. At 5000 Å the instrument is used in the 12th order, and the plate factor is 0.36 Å/mm.

A 3.4-meter Ebert plane-grating spectrograph at the Lawrence Radiation Laboratory, Berkeley, was used for some of the Zeeman spectrograms. The angles of use, orders, and plate factor of this instrument are about the same as for the 3-meter Czerny-Turner spectrograph just described.

The 9-meter concave grating spectrograph at the Argonne National Laboratory [9, 10] was used for the early spectrograms taken for wavelength measurements, temperature classification, and Zeeman effects. Spectrum lines were observed in orders as high as the eighth, although we generally confined our measurements to the third and fourth, in order to avoid overlapping orders. At these lower orders, overlapping could be prevented through the use of filters. The plate factor is about 0.45 Å/mm at 4000 Å in the fourth order.

2.3. Exposures

Three different magnets were used for the Zeeman spectrograms. Most of them were taken at Argonne, by using an electromagnet with iron pole-pieces, producing a field of 24,000 G. The discharge tube was excited in the cavity placed between the poles, perpendicular to the field.

The electromagnet used at the Radiation Laboratory produced a field of 29,000 G. A second magnet used there was a superconducting solenoid [11] operated at a field of 41,000 G. In this case, the source was operated in the cavity, but alined parallel to the magnetic field. A more stable operation of the discharge resulted with this orientation. A small mirror placed inside the solenoid permitted observation of the light emitted in a direction perpendicular to the field.

2.4. Exposures

The exposures made at Argonne were taken on 5 × 45 cm plates, with nine separate tracks on each plate. Thorium standard lines were placed on the top and bottom, with seven promethium exposures in-between. Each Pm exposure was taken at a different lamp temperature to help separate the spectra [12]. The intensities differed by as much as a factor of 80 from the weakest to the strongest exposures. These plates proved to be very valuable for spectrum separation, but the presence of small unexplained shifts made them less useful for absolute wavelength measurements. Since the wavelengths of most of the classified lines given in this paper are based on these exposures, it is possible that the present wavelengths may eventually have to be revised by a few thousandths of an angstrom. The Zeeman exposures included pi, sigma, and no-field lines. The field was calibrated by means of patterns belonging to several lines of Ca, Ag, and Cu which appeared on the plates.

The spectrograms taken at Berkeley for wavelength measurements were made on 10 × 25 cm plates, with the thorium and promethium lines overlapping, to eliminate any errors of measurement. A samarium spectrum was also included on these plates to identify impurity lines due to small amounts of Sm formed by the decay of Pm. Overlapping grating orders were separated by use of an external prism predisperser [13], which prevented light from the unwanted orders from entering the spectrograph. Each grating order was photographed separately.

Eastman Kodak spectroscopic plates were used for all exposures, processed in the recommended manner. Types 103a−O, 103a−F, 103−O, 103−F, and I−N were utilized. All but the first two were prefogged by exposures to weak light, prior to loading in the spectrograph.

2.5. Measurements

The plates were measured on two semiautomatic scanning comparators, one at the University of California and one at the National Bureau of Standards. Wavelengths for the thorium standards were taken from the work of Meggers and Stanley [14]. The estimated uncertainty of the wavelength measurements is about ±0.005 Å. At the present time some of our plates from Argonne and most of the plane-grating plates remain to be measured. When these measurements are complete, we expect to provide a full description of the Pm spectrum.

3. Analysis

In the neutral rare earths the ground configurations are generally of the type 4f N6s2 or 4f N−15d6s2. Since the ground configurations in Nd I and Sm I are 4f 46s2 and 4f 66s2, respectively, it was expected that the ground configuration in neutral Pm would be 4f 56s2. The lowest level of 4f 45d6s2 was not expected to be less than 10,000 cm−1 above the lowest level of 4f 56s2. The levels of 4f 5d6s were expected to start at about 8500 cm−1 above the lowest level of 4f 56s2. Prior to out beginning the Pm I analysis, Conway and Wybourne [15] had published theoretical predictions for the relative energies and g-values of the low levels of 4f 56s2 by using hydrogenic ratios for the Slater parameters and interpolating values of ζ/F2. These predictions proved to be very useful to us in carrying out the analysis. Their results for Pm I are summarized in table 1.

Table 1. Theoretical predictions for the low-lying levels of the 4f 56s2 configuration of Pm i.a.

Level Calculated energy (cm−1) Interval (cm−1) Calculated g-value Percentage composition
6H5/2° 0 0.297 96% 6H
816
6H7/2° 816 0.829 97% 6H
953
6H9/2° 1769 1.071 98% 6H
1046
6H11/2° 2815 1.201 99% 6H
1109
6H13/2° 3924 1.278 99% 6H
1146
6H15/2° 5070 1.327 95% 6H
6F1/2° 4915 −0.649 98% 6F
182
6F3/2° 5097 1.057
381
6F5/2° 5478 1.303
675
6F7/2° 6153 1.389
924
6F1/2° 7077 1.429
1094
6F11/2° 8171 1.451
a

J. G. Conway and B. G. Wybourne; Phys. Rev. 130, 2325 (1963).

The first part of the analysis was carried out through the use of the Zeeman data. A large number of self-reversed lines showed resolved patterns involving levels having J = 5/2 and 7/2 and g-values of about 0.30 and 0.83, respectively. It was clear that these lines were transitions to the and levels. Alter a certain amount of trial and error in searching for repeating differences involving these lines, the interval was found to be 803.82 cm−1. About 15 upper even levels were established in this process. The good agreement between this result and the prediction of Conway and Wybourne showed that the predictions would be useful in extending the analysis.

By continuing to use the Zeeman data and by using an electronic computer to search for constant differences between groups of strong lines, two sets of classified lines were built up. One group represented transitions to the group of levels; the other represented transitions to the levels. However the connection between these two groups was not known. After a considerable amount of searching for this connection, a few weak, resolved Zeeman patterns were found in the ultraviolet which proved to be transitions from upper levels known from combinations with 6F° levels down to the low 6H° levels. This established the energies of the 6F° levels relative to the ground state. The 6H15/2°, 6F9/2°, and 6F11/2° levels were later established, thus bringing our knowledge of the low levels to its present state. According to a recent diagonalization by Conway [16] this group represents all levels of 4f 56s2 expected below 14000 cm−1.

The results are given in tables 2, 3, and 4. Table 2 gives the energies and g-values of the odd levels. Table 3 gives the energies, J-values and g-values for the even levels. The classified lines are given in table 4. The estimated uncertainty in the values of the low levels given in table 2 is ±0.01 cm−1. The estimated uncertainty in the values of the high levels given in table 3 is about ±0.03 cm−1.

Table 2. Low levels of the 4f56s2 configuration of Pm i.

Level Energy (cm−1) Interval (cm−1) g-value
6H5/2° 0.00 0.305
803.82
6H7/2° 803.82 0.831
944.96
6H9/2° 1748.78 1.079
1048.32
6H11/2° 2797.10 1.205
1121.93
6H13/2° 3919.03 1.307
1170.76
6H15/2° 5089.79 1.33
6F1/2° 5249.48 −0.68
211.02
6F3/2° 5460.50 1.051
412.34
6F5/2° 5872.84 1.293
690.02
6F7/2° 6562.86 1.385
935.13
6F9/2° 7497.99 1.440
1111.22
6F11/2° 8609.21 1.458

Table 3. Even levels of Pm I. Levels with asterisk are uncertain.

Energy (cm−1) J g
17104.72 7/2 0.885
20006.04 3/2 0.068
20157.85 7/2 0.503
20265.98 5/2 0.527
20517.96 5/2 0.659
20567.76 5/2 0.910
20660.00 7/2 1.114
20675.81 5/2 1.075
20909.00 7/2 0.929
21100.10 7/2 1.319
21143.06 7/2 0.977
21237.49 9/2 0.841
21348.22 7/2 0.815
21371.05 7/2 0.927
21590.60 3/2 0.135
21625.45 9/2 1.117
21657.89* 5/2 1.01
21666.80 7/2 0.696
21732.93 9/2 1.137
21920.49 9/2 0.986
21946.12 3/2 −0.01
21976.26 7/2 1.218
22013.40 3/2 0.887
22080.08 5/2 0.571
22084.65 7/2 0.858
22205.44 9/2 0.974
22259.21 1/2 −0.32
22294.96 9/2 1.245
22301.24 5/2 0.976
22309.94 7/2 0.850
22355.68 9/2 1.374
22388.06 3/2 0.84
22414.17 11/2 1.12
22425.58 5/2 0.83
22446.20 11/2 1.531
22456.72 9/2 0.936
22522.90 5/2 0.735
22586.77 9/2 1.283
22654.34 7/2 0.84
22656.68 5/2 0.936
22761.33 11/2 1.296
22817.13 11/2 1.134
22905.24 5/2 1.16
22934.70 7/2 1.237
23006.35 11/2
23033.95 11/2 1.0
23178.13 7/2 1.150
23188.54 5/2 1.48
23198.33 11/2 1.12
23276.10 9/2 0.83
23278.90 7/2 1.0
23334.10 5/2 0.571
23337.53 7/2 1.257
23345.07 11/2 1.323
23435.40 9/2 1.00
23443.79 5/2 0.784
23480.63 9/2 1.16
23501.57 11/2 1.283
23538.86 5/2 0.780
23550.60 11/2 1.170
23571.27 9/2 1.26
23584.31 7/2 1.123
23629.06 13/2 1.09
23712.56 7/2 1.181
23732.57 1/2 3.24
23740.42 13/2
23743.96 7/2 1.111
23760.57 11/2
23926.91 3/2 1.754
23938.76 13/2 1.044
24013.29* 13/2 1.29
24038.82 11/2 1.08
24071.03 9/2 1.12
24091.39 3/2 1.395
24122.41 13/2
24180.80 13/2 1.28
24204.37 7/2 0.670
24234.42 9/2
24245.66 13/2 1.14
24338.33 9/2 1.17
24418.44 7/2 1.124
24443.15 13/2 1.17
24443.57 9/2 1.037
24471.10 7/2 0.83
24503.45 11/2 1.22
24520.23 13/2 1.34
24533.27 11/2 1.183
24558.56 9/2 1.39
24627.53 9/2 0.961
24681.68 11/21/2 0.895
24705.25 15/2 1.102
24754.58 9/2 0.888
24770.04 13/2 1.17
24789.86 9/2 1.08
24884.90 11/2
24912.34 11/2 1.29
25104.27 11/21/2 1.15
25306.07 15/2 1.21
25351.46 3/2 0.58
25357.24 11/2
25402.61 11/2 1.211
25405.29 13/2 1.034
25448.28 9/2 1.14
25474.46 15/2
25521.55* 9/2 0.910
25537.36 13/2 1.39
25618.77 15/2 1.13
25755.17 11/2 1.016
25919.50 13/2 1.26
26015.94 11/2
26065.63 1/2 −0.36
26080.99 17/2 1.29
26096.75 17/2 1.25
26101.28 13/2
26103.56 11/2 1.2
26181.98 13/2 1.55
26211.44 7/2 0.715
26237.84 9/2 1.43
26282.20 13/2 0.95
26285.02 7/2 0.877
26300.30 1/2 0.955
26456.26 13/2
26468.80 1/2 −0.45
26479.61 3/2 0.731
26522.35 3/2 1.135
26545.85 17/2 1.14
26555.44 13/2 1.07
26591.40 13/2 1.10
26609.39 5/2 0.56
26630.56 5/2 0.32
26694.38 11/2 1.095
26695.79 5/2
26703.97 13/2 1.30
26725.52 3/2 0.65
26830.74 3/2 0.794
26841.36 5/2 1.38
26955.22 15/2
27036.66 5/2 0.931
27042.18 15/2
27109.75 15/2
27245.99 5/2 0.761
27272.46 {7/29/2 0.8
27304.15* 15/2 1.44
27319.28 7/2 1.274
27334.48 5/2 0.947
27351.42 7/2 0.92
27383.92 15/2
27468.45 3/2 0.21
27476.28 7/2 1.077
27512.95 5/2 0.913
27596.27 13/2 1.29
27621.74 7/2
27685.89* 3/2 2.049
27829.89 9/2 1.024
27919.29 3/2 1.31
27923.37 5/2 0.869
27939.87 9/2 1.025
28008.09 7/2
28030.99 13/2 1.39
28075.94 9/2 1.155
28084.28 5/2 0.904
28086.21 11/2 1.150
28150.73 7/2 1.071
28153.69 5/2 0.632
28169.71 7/2 0.9
28186.31 7/2 1.98
28196.56 1/2 2.44
28273.52 7/2 0.764
28274.21 9/2 1.055
28325.13 7/2 1.38
28338.98 5/2 0.963
28467.52 7/2 0.87
28490.35 9/2 1.123
28565.66 9/2 1.0
28607.33 9/2 1.079
28608.57 3/2 1.740
28657.02 1/2 0.29
28680.26 7/2 0.88
28994.90 11/2 1.30
29002.94 3/2 0.99
29074.03 9/2
29129.60 9/2 1.162
29161.96 9/2 1.0
29242.64 11/2 1.259
29585.21 11/2 1.2
29595.58 9/2 1.04
29648.42* 9/2
29705.77 11/2
29757.69 11/2
29784.08 9/2
29856.72 11/2 1.10
29883.87 7/2 1.199
29908.90 9/2
29960.42* 11/2 1.18
30008.40 11/2 1.28
30063.62 9/2 1.224
30251.50* {11/213/2 1.2
30281.98 13/2
30374.95 11/2 1.226
30457.44 13/2 1.226
30541.28 11/2
30726.26 9/2
30785.03 9/2
31103.24 11/2
31846.70 11/2
32022.32 9/2
32435.06 13/2
33180.50 15/2 1.285
33246.65 9/2

Table 4. Classified lines of Pm i.

The wavelengths are in air. Even levels are designated by the energy in cm−1 followed by the J-value in parenthesis. Intensities are visual estimates on a scale of 1 to 1000.
C – complex U – perturbed by close line, but unresolved R2 – widely reversed
L – shaded to longer wavelengths D – double R3 – moderately reversed
S – shaded to shorter wavelengths B – blend R4 – slightly reversed
W – wide H – hazy R5 – barely detectably reversed
P – perturbed by close line, but resolved R1 – very widely reversed * – classification in doubt
λ (A) I σ (cm−1) Classification
6420.171 10 15571.61 6F11/2°24180.80(13/2)*
6355.910 100P 15729.04 6F11/2°24338.33(9/2)
6348.654 8 15747.02 6F7/2°22309.94(7/2)
6335.048 100 15780.84 6F9/2°23278.90(7/2)
6313.796 15 15833.95 6F11/2°24443.15(13/2)*
6313.663 10 15834.29 6F11/2°24443.57(9/2)
6311.586 85 15839.50 6F9/2°23337.53(7/2)
6308.577 20 15847.05 6F9/2°23345.07(11/2)
6302.377 60 15862.64 6F7/2°22425.58(5/2)
6283.239 70 15910.96 6F11/2°24520.23(13/2)
6278.104 2 15923.97 6F11/2°24533.27(11/2)
6268.138 150 15949.29 6F11/2°24558.56(9/2)
6263.942 10 15959.97 6F7/2°22522.90(5/2)
6255.078 10 15982.59 6F9/2°23480.63(9/2)
6246.909 50 16003.49 6F9/2°23501.57(11/2)
6220.118 10 16072.42 6F11/2°24681.68(11/2)
6219.809 50 16073.22 6F9/2°23571.27(9/2)
6214.768 75 16086.25 6F9/2°23584.31(7/2)
6208.146 80 16103.41 6F5/2°21976.26(7/2)
6193.898 200 16140.46 6F5/2°22013.40(3/2)
6186.089 2 16160.83 6F11/2°24770.04(13/2)
6168.403 5 16207.17 6F5/2°22080.08(5/2)
6165.603 70 16214.53 6F9/2°23712.56(7/2)
6153.685 80 16245.93 6F9/2°23743.96(7/2)
6142.459 100 16275.62 6F11/2°24884.90(11/2)
6132.934 40 16300.90 6H7/2°17104.72(7/2)
6132.125 200 16303.05 6F11/2°24912.34(11/2)
6117.382 100 16342.34 6F7/2°22905.24(5/2)
6085.359 150P 16428.34 6F5/2°22301.24(5/2)
6082.102 30S 16437.13 6F5/2°22309.94(7/2)
6053.356 150 16515.19 6F5/2°22388.06(3/2)
6039.646 30 16552.68 6F5/2°22425.58(5/2)
6032.252 90 16572.97 6F9/2°24071.03(9/2)
6015.361 5 16619.50 6F3/2°22080.08(5/2)
6013.146 40 16625.63 6F7/2°23188.54(5/2)
5980.648 100D 16715.97 6F7/2°23278.90(7/2)
5973.373 10 16736.32 6F9/2°24234.42(9/2)
5969.214 20 16747.98 6F11/2°25357.24(11/2)
5960.957 3 16771.18 6F7/2°23334.10(5/2)
5959.738 125 16774.61 6F7/2°23337.53(7/2)
5953.095 100 16793.33 6F11/2°25402.61(11/2)
5951.191 100 16798.71 6F3/2°22259.21(1/2)
5936.504 200 16840.26 6F9/2°24338.33(9/2)
5922.221 3 16880.88 6F7/2°23443.79(5/2)
5909.318 5 16917.74 6F7/2°23480.63(9/2)
5905.899 100 16927.53 6F3/2°22388.06(3/2)
5899.630 20 16945.52 6F9/2°24443.57(9/2)
5892.838 10P 16965.05 6F3/2°22425.58(5/2)
5877.354 30C 17009.74 6F1/2°22259.21(1/2)
5875.337 50 17021.38 6F7/2°23584.31(7/2)
5869.549 30 17032.36 6F5/2°22905.24(5/2)
5859.865 50 17060.51 6F9/2°24558.56(9/2)
5844.718 200L 17104.72 6F5/2°17104.72(7/2)
5833.183 125 17138.55 6F1/2°22388.06(3/2)
5829.407 50 17149.65 6F7/2°23712.56(7/2)
5818.750 3P 17181.06 6F7/2°23743.96(7/2)
5776.992 200 17305.25 6F5/2°23178.13(7/2)
5730.809 200 17444.70 6F3/2°22905.24(5/2)
5724.260 60 17464.66 6F5/2°23337.53(7/2)
5710.075 20 17508.05 6F7/2°24071.03(9/2)*
5671.018 500 17628.62 6F11/2°26237.84(9/2)
5657.260 300 17671.50 6F7/2°24234.42(9/2)
5644.515 10 17711.40 6F5/2°23584.31(7/2)
5639.234 50 17727.98 6F3/2°23188.54(5/2)
5624.182 20 17775.43 6F7/2°24338.33(9/2)
5603.922 125L 17839.69 6F5/2°23712.56(7/2)
5598.944 2 17855.55 6F7/2°24418.44(7/2)
5597.807 10H 17859.18 6F9/2°25357.24(11/2)
5594.077 100 17871.09 6F5/2°23743.96(7/2)
5591.080 35C 17880.67 6F7/2°24443.57(9/2)*
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4543.598 20C 22002.82 6F7/2°28565.66(9/2)
4542.539 75 22007.95 6F3/2°27468.45(3/2)
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4517.306 200 22130.88 6H7/2°22934.70(7/2)
4516.418 20CP 22135.23 6F5/2°28008.09(7/2)*
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4364.583 100 22905.26 6H5/2°22905.24(5/2)
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4357 943 100S 22940.16 6H7/2°23743.96(7/2)
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4280.448 75 23355.47 6H9/2°25104.27(11/2)
4279.787 50U 23359.07 6F1/2°28608.57(3/2)
4275.061 3 23384.90 6H11/2°26181.98(13/2)
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4270.924 50C 23407.55 6F1/2°28657.02(1/2)
4269.908 10 23413.12 6F11/2°32022.32(9/2)
4264.321 300R3 23443.79 6H5/2°23443.79(5/2)
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4225.504 60 23659.15 6H11/2°26456.26(13/2)
4224.049 75R4 23667.30 6H7/2°24471.10(7/2)
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4218.308 50R4 23699.51 6H9/2°25448.28(9/2)
4215.984 60 23712.58 6H5/2°23712.56(7/2)
4210.411 15 23743.96 6H5/2°23743.96(7/2)
4208.727 35 23753.46 6F1/2°29002.94(3/2)
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4119.631 10 24267.17 6H9/2°26015.94(11/2)
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4074.455 100 24536.23 6F9/2°26285.02(7/2)
4057.716 5 24637.45 6F11/2°33246.65(9/2)
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4031.260 100 24799.13 6H11/2°27596.27(13/2)
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3986.768 200 25075.88 6H13/2°28994.90(11/2)
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3953.158 50 25289.08 6H11/2°28086.21(11/2)
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3943.420 25 25351.52 6H5/2°25351.46(3/2)*
3940.912 200 25367.66 6H15/2°30457.44(13/2)
3934.714 20 25407.61 6H7/2°26211.44(7/2)
3930.629 15 25434.02 6H7/2°26237.84(9/2)
3926.704 75 25459.44 6F7/2°32022.32(9/2)
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3916.820 200 25523.69 6H9/2°27272.46
3904.745 75 25602.62 6H9/2°27351.42(7/2)
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3833.101 500R5 26081.14 6H9/2°27829.89(9/2)
3831.891 150 26089.37 6H13/2°30008.40(11/2)
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3810.932 400 26232.85 6H7/2°27036.66(5/2)
3807.093 25 26259.30 6H9/2°28008.09(7/2)
3804.538 75 26276.94 6H11/2°29074.03(9/2)
3803.370 400 26285.01 6H5/2°26285.02(7/2)
3797.292 75 26327.08 6H9/2°28075.94(9/2)*
3796.513 300 26332.48 6H13/2°30251.50*
3795.800 500 26337.43 6H9/2°28086.21(11/2)
3792.125 100 26362.95 6H13/2°30281.98(13/2)
3791.842 50 26364.92 6H11/2°29161.9619/2)
3783.806 100 26420.92 6H9/2°28169.71(7/2)
3781.429 200 26437.52 6H9/2°28186.31(7/2)
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3780.284 150 26445.53 6H11/2°29242.64(11/2)
3776.986 250R4 26468.62 6H7/2°27272.46
3775.419 300P 26479.61 6H5/2°26479.61(3/2)
3770.312 100P 26515.47 6H7/2°27319.28(7/2)
3769.334 75S 26522.35 6H5/2°26522.35(3/2)
3768.994 200P 26524.74 6H9/2°28273.52(7/2)
3768.895 150 26525.44 6H9/2°28274.21(9/2)
3767.052 40 26538.42 6H13/2°30457.44(13/2)
3765.747 300 26547.61 6H7/2°27351.42(7/2)
3761.678 200 26576.33 6H9/2°28325.13(7/2)
3757.007 75 26609.37 6H5/2°26609.39(5/2)
3755.190 50 26622.24 6H13/2°30541.28(11/2)
3754.020 5 26630.54 6H5/2°26630.56(5/2)
3748.122 150 26672.45 6H7/2°27476.28(7/2)
3746.530 25 26683.78 6F7/2°33246.65(9/2)
3744.846 35 26695.78 6H5/2°26695.79(5/2)
3742.973 300R4 26709.14 6H7/2°27512.95(5/2)
3741.629 150 26718.73 6H9/2°28467.52(7/2)
3740.679 300 26725.51 6H5/2°26725.52(3/2)
3738.433 200 26741.58 6H9/2°28490.35(9/2)
3731.939 200 26788.10 6H11/2°29585.21(11/2)
3727.938 300 26816.86 6H9/2°28565.66(9/2)
3726.009 500 26830.74 6H5/2°26830.74(3/2)
3724.534 100 26841.36 6H5/2°26841.36(5/2)
3722.153 400 26858.53 6H9/2°28607.33(9/2)
3715.219 200 26908.66 6H11/2°29705.77(11/2)
3712.074 35 26931.46 6H9/2°28680.26(7/2)
3708.064 75 26960.58 6H11/2°29757.69(11/2)
3704.436 200 26986.99 6H11/2°29784.08(9/2)
3699.079 75 27026.06 6H7/2°27829.89(9/2)
3697.626 300R5 27036.68 6H5/2°27036.66(5/2)
3687.381 200 27111.80 6H11/2°29908.90(9/2)
3686.328 250 27119.54 6H7/2°27923.37(5/2)
3684.088 25 27136.03 6H7/2°27939.87(9/2)
3677.558 10 27184.21 6H13/2°31103.24(11/2)
3674.848 200 27204.26 6H7/2°28008.09(7/2)
3673.896 250 27211.31 6H11/2°30008.40(11/2)
3669.218 300R5 27246.00 6H5/2°27245.99(5/2)
3666.455 15 27266.54 6H11/2°30063.62(9/2)
3665.702 20 27272.14 6H7/2°28075.94(9/2)
3664.588 25 27280.43 6H7/2°28084.28(5/2)
3658.578 75 27325.24 6H9/2°29074.03(9/2)
3655.898 100 27345.27 6H15/2°32435.06(13/2)
3655.678 50 27346.91 6H7/2°28150.73(7/2)
3655.284 50 27349.86 6H7/2°28153.69(5/2)
3655.077 20 27351.41 6H5/2°27351.42(7/2)
3653.144 300 27365.89 6H7/2°28169.71(7/2)
3646.844 300 27413.16 6H9/2°29161.96(9/2)
3641.368 100 27454.39 6H11/2°30251.50*
3639.503 300 27468.45 6H5/2°27468.45(3/2)
3639.338 350 27469.70 6H7/2°28273.52(7/2)
3639.249 150 27470.37 6H7/2°28274.21(9/2)
3637.328 35 27484.88 6H11/2°30281.98(13/2)
3636.136 20 27493.89 6H9/2°29242.64(11/2)
3633.617 25 27512.95 6H5/2°27512.95(5/2)
3632.512 3 27521.31 6H7/2°28325.13(7/2)
3630.678 50 27535.21 6H7/2°28338.98(5/2)*
3625.065 200 27577.85 6H11/2°30374.95(11/2)
3619.304 5 27621.74 6H5/2°27621.74(7/2)
3613.816 400R5 27663.69 6H7/2°28467.52(7/2)
3610.835 75 27686.53 6H7/2°28490.35(9/2)
3603.332 15 27744.18 6H11/2°30541.28(11/2)
3601.038 100 27761.85 6H7/2°28565.66(9/2)
3595.645 100 27803.49 6H7/2°28607.33(9/2)
3591.392 75 27836.41 6H9/2°29585.21(11/2)
3590.049 250 27846.82 6H9/2°29595.58(9/2)
3586.236 50 27876.43 6H7/2°28680.26(7/2)
3583.252 50 27899.64 6H9/2°29648.42(9/2)*
3580.731 20 27919.29 6H5/2°27919.29(3/2)
3580.209 100 27923.36 6H5/2°27923.37(5/2)
3579.465 100 27929.16 6H11/2°30726.26(9/2)
3577.348 25 27945.69 6H13/2°31846.70(11/2)
3575.902 100 27956.99 6H9/2°29705.77(11/2)
3571.949 100 27987.93 6H11/2°30785.03(9/2)
3569.377 50 28008.10 6H5/2°28008.09(7/2)
3569.271 100 28008.92 6H9/2°29757.69(11/2)
3565.911 40 28035.32 6H9/2°29784.08(9/2)
3559.695 500R5 28084.27 6H5/2°28084.28(5/2)
3558.878 100 28090.72 6H15/2°33180.50(15/2)
3553.264 300 28135.10 6H9/2°29883.87(7/2)
3550.916 300 28153.70 6H5/2°28153.69(5/2)
3550.109 35 28160.10 6H9/2°29908.90(9/2)
3546.808 200 28186.31 6H5/2°28186.31(7/2)
3543.627 50 28211.61 6H9/2°29960.42(11/2)*
3536.282 50 28270.21 6H7/2°29074.03(9/2)
3535.869 300 28273.51 6H5/2°28273.52(7/2)
3531.795 75 28306.12 6H11/2°31103.24(11/2)
3530.711 200 28314.82 6H9/2°30063.62(9/2)
3529.426 2 28325.12 6H5/2°28325.13(7/2)
3527.701 250 28338.97 6H5/2°28338.98(5/2)*
3525.319 75 28358.12 6H7/2°29161.96(9/2)
3511.772 20 28467.51 6H5/2°28467.52(7/2)
3505.794 100 28516.05 6H13/2°32435.06(13/2)
3494.457 5 28608.56 6H5/2°28608.57(3/2)
3492.308 15 28626.16 6H9/2°30374.95(11/2)
3485.722 50 28680.25 6H5/2°28680.26(7/2)
3472.223 10 28791.75 6H7/2°29595.58(9/2)
3472.132 25 28792.50 6H9/2°30541.28(11/2)
3465.864 35 28844.58 6H7/2°29648.42(9/2)*
3449.970 50 28977.46 6H9/2°30726.26(9/2)
3449.639 25 28980.24 6H7/2°29784.08(9/2)
3442.984 50 29036.25 6H9/2°30785.03(9/2)
3437.803 15 29080.01 6H7/2°29883.87(7/2)
3420.721 75 29225.22 6H11/2°32022.32(9/2)
3416.680 40 29259.79 6H7/2°30063.62(9/2)
3416.484 50 29261.47 6H13/2°33180.50(15/2)
3405.661 3 29354.45 6H9/2°31103.24(11/2)
3373.083 75 29637.95 6H11/2°32435.06(13/2)
3345.32/ 20 29883.85 6H5/2°29883.87(7/2)
3341.011 50 29922.46 6H7/2°30726.26(9/2)
3334.464 75 29981.21 6H7/2°30785.03(9/2)
3319.548 25 30115.92 6H9/2°31846.70(11/2)
3302.264 25 30273.53 6H9/2°32022.32(9/2)
3283.174 25 30449.56 6H11/2°33246.65(9/2)
3202.305 25 31218.48 6H7/2°32022.32(9/2)

4. Discussion

The total number of self-reversed lines in Pm I is 122. Of these 120 have been classified as transitions to 4f 56s2. This makes it certain that the ground configuration of Pm I is 4f 56s2.

The eigenvectors given by Conway and Wybourne as a result of their diagonalization of 4f5 show the 6H° and 6F° terms to be nearly pure in LS coupling. We therefore would have expected a somewhat regular variation in intensity of the lines making transitions from a single upper level to several lower levels. However, according to our observations this is not the case. Figure 1 gives some of the more striking examples of the irregular intensities. Of special note are the lines from 26725.52(3/2). In this case the transition to is just barely visible on the plates. The complete absence of the line from 28186.31(7/2) to is also very striking. A similar set of puzzling intensities has been observed by Shenstone [17] in the 3d64s − 4p transitions of Co III. Here the anomalous intensities were found in groups of lines connecting terms of different multiplicity. In this connection Shenstone noted “Especially difficult to understand is the not uncommon habit of intersystem combinations of missing the central of three levels of successive J.” Although this phenomenon has not yet been investigated theoretically, it is clear that it stems from the lack of pure LS coupling in the upper configurations. It is likely that a theoretical study of the transition probabilities for the 3d64s − 3d64p array in Co III would shed more light on this problem.

Figure 1. Anomalous intensities in Pm I.

Figure 1.

No observed transition = X.

No real effort has been made yet to understand the origin of the known upper levels in Pm I. However we note the following points. There are only two configurations which can make transitions to 4f 56s2 with appreciable intensity, namely 4f 56s6p and 4f 45d6s2. For the 4f 56s6p configuration, the strongest transitions to the 4f 56s2 6H° and 6F° levels will originate from levels of the type 4f 5(6H)6s6p and 4f 5(6F)6s6p. If we consider the levels of the type 4f 5(6H)6s6p(J = 5/2), we would expect them to fall into two groups: six of the type 4f 5(6H)+6s6p(3P) and two of the type 4f 5(6H)+6s6p(1P). This type of coupling (first described by Shenstone [18] in the case of the 3d 94s4p configuration of Cu I) will hold approximately here because the parameter which determines the 1P − 3P splitting, G1(sp), is expected by interpolation from other rare earths to be about 2640 cm−1, whereas ζp and ζf are only about 1000 cm−1. The f-s and f-p interactions are much smaller (see Smith and Wybourne’s treatment of the 4f7(8S)6s6p configuration in Eu I [19]) and for our purpose may be neglected. If one interpolates a value for the 4f 5(6H)6s2− 4f 5(6H)6s6p energy difference and uses the above parameters to estimate the level positions, one finds that the six 3P type levels with J = 5/2 will lie in the region 14000−16000 cm−1 and the two 1P type levels with J = 5/2 will lie at about 20000−21000 cm−1 [20]. Of the 27 observed upper levels with J = 5/2, 22 make strong transitions to the 6H group of lower levels. These 22 levels are distributed as follows: 14 between 20250 and 23550 cm−1, 8 between 26600 and 28350 cm−1. Thus of the observed levels with J = 5/2, only 2 would be expected to belong to 4f 5(6H)6s6p. It should also be noted that none of the lines classified so far in Pm I shows the appreciable hyperfine structure which would be expected if one of the configurations contained a single 6s electron. Judd [21] has shown that it is possible for lines from certain levels of 4f N6s6p configurations to 4f N6s2 to show no hyperfine structure. However, there are too many levels here whose transitions show no hfs to believe that the theory is applicable here. The absence of hfs in these lines more likely is evidence of a closed 65 shell in both upper and lower configurations. In this case the hyperfine structures due to the 4f electron in the upper and lower levels have the same sign and about the same magnitude, so that the observed line shows no resolved structure. For these reasons we believe that most of the known upper levels belong to the 4f 45d6s2 configuration.

Comparison of the observed positions of the 4f 56s2 6H° and 6F° levels with the calculations of Conway and Wybourne shows that their predictions for the intervals within each term are very good, generally within ±30 cm−1 of the observed levels. However, the predicted positions of the 6F° levels are too low by nearly 7 percent in every case. This discrepancy results from the fact that the calculated intervals within the individual terms are very sensitive to the value of the spin-orbit parameter, but rather insensitive to the electrostatic parameters. On the other hand the separation between the barycenters of terms is governed primarily by the electrostatic parameters. At the time of Conway and Wybourne’s work there was no neutral rare earth with more than one known term of 4fN6s2. Therefore, information about the electrostatic parameters could be inferred from the known levels only through second order effects. Thus, a difference of only 7 percent between the predicted and observed 6H° − 6F° separation in Pm I can be considered to be fairly good agreement.

It is not possible to obtain reliable values for the electrostatic parameters E1, E2, and E3 from the known levels, because only one term separation is available. However, we note that since the 6H° and 6F° terms are nearly pure in LS coupling, to a first approximation the energy difference between the 6H° and 6F° barycenters ΔE(6F, HH) will be equal to 9E3 [22]. If we include the Trees αL (L + 1) correction [23], to a very good first approximation we then have:

ΔE(F6,H6)=9E318α.

If we use the known positions of the 6F° and 6H° levels to determine ΔE (6F, 6H), we find ΔE (6F, 6H) = 4167.92 cm−1. If we set α = 30 cm−1 as indicated by the theoretical interpretation of the spectra of Ce III [24] and Pr III [25] we find E3 ~ 520 cm−1.

A preliminary value of ζ4f can be obtained by considering the total widths of the 6H° and 6F° terms. To a first approximation the sum of these two widths is 91 ζ4f, which gives ζ4f ~ 930 cm−1.

Crosswhite [26] has made a least squares fit of the 6F° and 6H° levels to the theoretical energy formulas by using hydrogenic ratios for the electrostatic parameters and a fixed value of 20 cm−1 for α. The parameters E3 and ζ4f were allowed to vary. A diagonalization with E3 = 510 cm−1 and ζ4f = 914 cm−1 gave a mean error of 25 cm−1. This could be reduced to about 8 cm−1 if slightly different values of ζ4f were used for the two terms: 912 cm−1 for 6H and 938 cm−1 for 6F. Cross-white notes that this is probably caused by a spin-other-orbit interaction. The J-dependence of this interaction is the same as that of the spin-orbit interaction. However, the spin-other-orbit interaction constant varies from term to term. Thus, this interaction will cause the spin-orbit constants derived from different terms of a configuration to appear to be slightly different. This effect was first treated for lN configurations by Horie [27]; the principal aspects of the theory have been summarized by Wybourne [28]. Since spin-other-orbit effects cannot be observed by studying only one term of a configuration, the present results for Pm I provide a first opportunity to view their magnitude for the neutral rare earths.

The residual errors in the above calculation have a form very close to that expected from neglect of a spin-spin interaction. If an estimate of the spin-spin interaction energy is made by using Judd’s [29] matrix elements and interpolated values of the radial integrals from the calculations of Blume, Freeman, and Watson [30], the mean error can be further reduced to about 2 cm−1. When this is done the values of ζ4f are changed to 910 cm−1 for 6H° and 940 cm−1 for 6F° [26].

In view of the uncertainties in the ratios of the electrostatic parameters, the value to be used for α, and the Hamiltonian needed to describe the levels, we give the values of E3 and ζ4f for the 4f 56s2 configuration of Pm I as:

ζ4f=925±20cm1E3=510±20cm1

That this value of ζ4f fits in well with other values of ζ4f in the rare earths is shown by the plot in figure 2.

Figure 2. Values of £4f in the rare earths.

Figure 2.

The solid circles represent values derived from 4fN6s2 configurations of neutral atoms; the open circles are derived from 4fN configurations of doubly ionized atoms. References: N= 1, La I, H. N. Russell and W. F. Meggers, J. Res. NBS 9, 625 (1932); N= 1, La III, J. Sugar and V. Kaufman, J. Opt. Soc. Am. 55, 1283 (1965); N = 2, Ce III, N. Spector, J. Opt. Soc. Am. 55, 492 (1965); N = 3, Pr III, R. Trees, J. Opt. Soc. Am. 54, 651 (1964); N = 4, Nd I, J. G. Conway and B. G. Wybourne, Phys. Rev. 130, 2325 (1963); N=5, Pm I, this paper; N=6, Sm I, J. G. Conway and B. G. Wybourne, Phys. Rev. 130, 2325 (1963); N= 12, Erl, J. Reader, unpublished calculations based on data of L. C. Marquet and S. P. Davis, J. Opt. Soc. Am. 55, 471 (1965). This value of ζ4f of 2237 cm−1 is nearly identical to the 2236 cm−1 value of ζ4f derived from the 4f126s configuration of Er II by Z. Goldschmidt, J. Opt. Soc. Am. 53, 594 (1963); N = 13, Tm I, W. F. Meggers, Rev. Mod. Phys. 14, 96 (1942). For simplicity, several values of ζ which have been published for 4fN6s configurations of singly ionized rare earths have not been included.

Acknowledgments

We acknowledge the very generous assistance of John Conway and Earl Worden of the Lawrence Radiation Laboratory in preparing the electrodeless lamps and in other phases of our experiment. We also thank Mark Fred and Frank Tomkins of the Argonne National Laboratory for their help in photographing the first set of wavelength and Zeeman effect plates, and Henry Crosswhite of the Johns Hopkins University for his suggestions concerning the calculation of the parameters. Our success in making the recent plane-grating spectrograms is due in large measure to the efforts of Richard J. Wolff. Some of the Zeeman plates were measured and reduced at the Argonne National Laboratory by Thomas Dickinson. The Zeeman plates taken at M.I.T. by L. Johnson were made available to us by Lee C. Bradley, III.

Part of this work was supported by a grant from the National Science Foundation and an optical research equipment grant from the Advanced Research Projects Agency to one of the authors (SPD).

Footnotes

**

University of California, Berkeley 94720.

1

Figures in brackets indicate literature references at the end of this paper.

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