Kenneth L. Andrew

Coupling Considerations in Two-Electron Spectra*

The energy level structure, relative line strengths, and Lande g factors of two-electron configurations are discussed for four important types of pure coupling: LS, LK(Ls), jK(jl), and jj. Transitions from one type of coupling to another are discussed in detail, the configuration pf being used as an example. The appropriateness of LS- and jj-coupling notation in two-electron spectra is quite limited for atoms of medium atomic weight, where nearly all excited configurations show a strong tendency toward pair (LK to jK) coupling. For other atoms, pair coupling occurs mainly for high values of orbital angular momentum of the excited electron: the coupling may then be near LK for small values of the principal quantum number of this electron, approaching pure jK as this quantum number increases. Either LK or jK notation can serve unambiguously to identify levels throughout the range of intermediate pair couplings, but neither will correctly designate the nature of the quantum states in all cases because of exchanges of the L (and of the j) compositions of certain states which occur as the coupling conditions change from pure LK to pure jK. Examples are discussed from the spectra of N, P, Ge, and the rare gases.

Arc Spectrum of Silicon

The arc spectrum of silicon has been reinvestigated from 1976 to 12 270 A by using plane and concave grating spectrographs and Fabry–Perot interferometers. Electrodeless discharge lamps containing silicon–halogen compounds, and a silicon arc operated in argon at reduced pressure, were used to obtain measurements on 395 wavelengths, 192 of them interferometric. The total number of newly observed lines is 179, and the number of new classifications 174. Forty-two new even levels were established, principally from the 3s2 3p np (n = 6, 7) and 3s2 3p nf (n = 5, 6, 7) configurations. With the aid of vacuum-ultraviolet data provided by A. G. Shenstone, 38 new odd levels were found. The interferometric and grating investigation has led to the calculation of 92 provisional Ritz standards between 1580 and 2000 A with uncertainties of less than 0.002 A. The new wavelength data have also made possible the identification of 76 previously unidentified solar lines as due to Si i. Nineteen other solar lines have been identified as possibly attributable to Si i. A limit of 65 747.5±0.6 cm−1 has been derived for the Si ii level P212∘ from the nd and nf level series. This corresponds to an ionization potential of 8.15116±0.00008 V.

Arc Spectrum of Lead

New grating and interferometric measurements of the first emission spectrum of lead have been made using electrodeless discharge tubes containing halides of 208Pb. In the spectral region from 1977 to 12561 A, 370 classified lines were measured, of which 90 are new. Nineteen new even levels and four new odd levels have been established; the classifications of nine levels have been changed to different electron configurations. The lower series limit of lead was determined from three Rydberg series to be 59819.4 ± 0.3 cm−1, and the separation of the two limits, 14081.074 ± 0.004 cm−1, was measured directly from the forbidden Pb ii transition, 6pP122−6pP322.A series of hydrogenic levels observed by Gieseler and Grotrian in 1925 but omitted from AEL was confirmed for n = 5 to 9 and extended to n = 10. We have identified this as the 6png1/2[7/2]3 series of levels, the observed lines being forbidden Δl = 3 transitions to the J = 2 members of the 6p2 and 6p7p configurations. This is believed to be the first ng series identified in a two-electron neutral spectrum. In addition to the formerly observed even-even forbidden transitions between individual members of the 6p2 ground configuration, we also observed such transitions to 6p2 from 6p7p and 6p5f levels.

Germanium Vacuum Ultraviolet Ritz Standards

The number of Ge i Ritz standards below 2000 A has now been increased to approximately 100 lines with estimated uncertainties not exceeding 0.0009 A, 68 of which have estimated uncertainties of 0.0003 A. This extension is the end product of a systematic Fabry–Perot interferometric investigation which has extended the former interferometric region of 2019–4685 A to 12 069 A. Ninety-five interferometrically determined levels of Ge i are given, 72 of which have been improved by this effort.Twenty-three lines of Ge ii were also measured interferometrically leading to 20 improved energy levels of this spectrum and to estimated splittings of the ng2G (n=5, 6, 7) terms. From the improved levels, twelve Ge ii Ritz standards from 999 to 1966 A have been calculated with estimated uncertainties not exceeding 0.0009 A. An improved series limit of Ge ii calculated from the nf2F and ng2G term series is 128 521.3 cm−1. This value made use of improved levels of this work and as yet unpublished data furnished by Shenstone.

Optical study of the hyperfine structure of the rubidium resonance lines

We have developed a rubidium atomic-beam light source that emits Rb lines with a Doppler width of only 0.0015 cm−1. When the light from this source was measured with a scanning Fabry–Perot interferometer and the data were reduced by digital computer utilizing numerical deconvolution techniques, we were able to determine a hyperfine level splitting in natural rubidium as small as 0.0014 cm−1 with an uncertainty of 0.0001 cm−1. We have measured the relative intensities and positions of 19 of the 20 hfs components of the two resonance lines, 7800 and 7974 A, of natural Rb. From these data, we calculate isotope shifts σ87−σ85 = 0.00268 cm−1 for 52P32 and σ87′−σ85′ = 0.00259 cm−1 for 52P12.

Improved Cu ii Standard Wavelengths in the Vacuum Ultraviolet

Interferometric wavelength measurements of sixty-one copper spark lines in the region 2884 A to 1979 A have been carried out by means of a Fabry-Perot interferometer equipped with 4, 12, and 20 mm spacers. The light source used was a water-cooled hollow cathode Schuler tube operated principally with circulating helium but occasionally with circulating neon.The measurements were employed to establish the 4s and 5s even levels and 4p odd levels of Cu ii. After fixing these low levels, their values were combined with the grating measurements of Carter and Shenstone to establish the values of certain higher odd and even levels. Use was then made of the Ritz combination principle to calculate precise wavelength values of approximately sixty copper lines in the region between 1663 A and 984 A with estimated accuracies which varied from ±0.0003 A to ±0.001 A.

Vacuum Ultraviolet Spectrum of Neutral Silicon

Measurements have been made on about 200 Si i lines in the region 1548 to 2000 A excited by a low-pressure source, resulting in wavelength values with uncertainties of less than 0.0015 A on the average. Of these vacuum ultraviolet wavelengths, 88 have been calculated by Radziemski and Andrew [ J. Opt. Soc. Am.55, 474 ( 1965)], and a comparison of measured and calculated values gives an average deviation of 0.0008 A. A plot of the differences shows the scatter to be much greater than any systematic difference. This indicates that the calculated wavelengths are compatible with the set of Ge i lines used as standards in measuring the Si i lines. The new data have also led to an extension and a slight revision of the odd-level analysis. New measurements on some lines of C i, Ge ii, N i, and Si ii are also reported. Spectrograms were taken in the first order on a 10.7-m Eagle vacuum spectrograph with a reciprocal dispersion of 0.78 A/mm. The source was an electrodeless discharge lamp containing SiCl4 and GeCl4 vapor with argon or helium carrier gas at a total pressure no greater than 0.5 torr.

Proposed Standard Wavelengths in the Vacuum Ultraviolet. Spectra of Ge, Ne, C, Hg, and N†

The spectrum emitted from a germanium hollow-cathode discharge was photographed at high dispersion in the third order of a 21-ft concave grating spectrograph. Emission lines of Ge II, Ne II, Hg I, Hg II, C I, and N I were measured against Ge I Ritz standards in the regions 1550–2041 A and 2325–3060 A. The Ge I Ritz standards were investigated as to their suitability, and it was found that 56 lines in the region 1650–2041 A may be used with confidence to ±0.0004 A. Fifty-eight interpolated lines of Ne, Hg, C, N, and Ge may be used as auxiliary standards to ±0.001 A in the region 1550–2041 A. The interpolated measurements in the region 2325–3030 A provide 74 lines with a standard error of ±0.002 A which also may be useful. The present investigation of the Ge ii spectrum has provided revised energies for 15 levels. From these levels and 12 additional levels from the recent work of Kaufman and Andrew, the wavelengths of 22 lines in the region 837–1602 A have been calculated with an estimated uncertainty of ±0.0004 to ±0.0018 A.

Arc Spectrum of Germanium

The arc spectrum of germanium has been investigated in the region 1546 A to 11 252 A with concave grating spectrographs and interferometers by the use of various arc and hollow cathode sources. Wavelength measurements supplied by other laboratories are included. The wavelength material has been greatly improved and extended from 184 lines to 475 lines. Of these about 200 visible and infrared lines were found by the development of a special low pressure Ge arc source.An extended analysis of this spectrum has led to 94 new levels and nearly 300 newly classified lines. At present there are 68 even levels belonging to the 4s2 4pnp and 4s2 4pnf configurations and 87 odd levels belonging to the 4s2 4pns, 4s2 4pnd, and 4s4p3 configurations. Improved series limit calculations give the value 63 715±10 cm−1 for the 4s2 4pP212° level of Ge II with respect to the 4s2 4p2 3P0 level of Ge I, corresponding to an ionization potential of 7.899±0.001 volt.

Vacuum ultraviolet wavelength standards and improved energy levels in the first spectrum of silicon.

Vacuum ultraviolet Si i wavelengths have been recalculated using all available low-pressure-source data. One hundred calculated and forty-one measured wavelengths, all with uncertainties less than 0.002 A, should be useful as wavelength standards in the region 1560 to 2000 A. Seventy-seven of the proposed standards were originally calculated by Radziemski and Andrew [ J. Opt. Soc. Am.55, 474 ( 1965)]; although the two sets of wavelength values agree to within their uncertainties, the new set has a greater internal consistency. Included also is a complete list of low-pressure-source levels for Si i, which contains new values for 30 odd levels previously determined exclusively from relatively high-pressure (450–740 torr) arc data.