Infrared Emission Spectra of Krypton and Argon

U. S. Department of Commerce Research Paper RP1790 National Bureau of Standards Volume 38, May 1947 Part of the Journal of Re search of the National Bureau of Standards Infrared Emission Spectra of Krypton and Argon By Curtis J. Humphreys and Earle K. Plyler The analysis of the spectra of the noble atmospheric gases, utilizing descriptive daLa covering the photographicall y accessible region, has long indicated the possibility of a con sid erable exte nsion of most of t hese spectra into the infrared region beyond 1.3 microns. Observations of the spectra of krypton a nd argo n, in the region be tween 1 and 2 mi crons, have been made with a Perkin-Elmer spec trometer, fi tted wi th a flin t-glass prism cut to an anglc of 55 degrees. Tlw sources " 'e re Ceis ler tubes, used in previously reported work. More than ] 5 new lineR of krypton have bee n obse rved. P art of these are blend s of unresolved pairs or groups. The emission maxima have been determined in favorable cases to a precision of two wave numbers, roughly equivalent to one- te nth of the small est scale division on the wavelength drum. All observed lines have been classifi ed, the most inte nse being represented by combinations of the type 2p- 3d, according to Paschen's notaLio n. Two new levels from the confi guration S2 p5J have been found. T he remaining unobserved com binations of the type l s- 2p, occurr ing in this region, arc, with one exception, too weak to be ob erved. The a rgon infrared spectrum was observed by Paschen. More of its predic ted combinations a re ill t he photographic region than in the case of krypton. A few lines near 1.4 microns ha\'e bee n observed, I. Introduction ning in 1926 [2]. In 1929 Gremmer [3] publi shed The spectra originati.ng in th(' neu tral atoms of the classification of the more prominen t lines. the noble atmospheric gases, neon, argon , krypton, lIe also extended the analysis of n eon and a rgon and xenon, are among the most thoroughly studied in the photographically accessible infrared [4]. of all lin e spectra observed and described to date. In 1929 also, Meggers, de Bruin, and Humphreys Examination of published data reveals vcry few [5] published a detailed analysis of the first lines of appreciabl e intensity that have not been spec trum of krypton. Improved observations fitted into a scheme of energy levels in accordance making available double the number of lines per with the theory now universally aGcepted. mitted a further extension of this classification in The first detailed and relatively complete ana.ly 1931 (6], with some minor revisions. A new paper sis of any of the first spectra of the noble gases by Gremmer [7] showed how all discrepancies between his analysis and that of Meggers, de was that of neo n, published by Paschen [1] 1 in 1919. Pasch(' n used a special notation to r epre Bruin, and Humphreys could be reconciled. sent the spectral terms of neon. His system has Additional lines in the red and infrared were been ge nerally fo llowed by other investigators reported by R asmussen [8] . reporting on the first speetra of the r emaining The first spectrum of xenon was class ified by noble gases, and is used in this paper. }VI eggers, de Bruin, and Humplu'eys (9], with a The first sp('ctrum of argon was classified by subsequent revision and extension by Meggers Meissner and reported in a series of papers begin- and Humphreys (10]. Gremmer (11] also pub li shed an analysis in essen tial agreement with that 1 Figures in brackets indicate the literature references at the end of this paper. of the authors just named. Rasmussen (8] Infrared Emission Spectra 499 extended the classification and contributed addi lowest j-electron configuration, and possibly as tional infrared data. many as eight combinations of the type, 2p- 2s. The above resume of work on the first spectra In argon I, fewer l s- 2p combinations are in the of the noble gases is by no means completc. It visible region than in neon I, but all of them can includes references only to papers in which the be photographed . About half of the 2p- 3d com analyses were brought to essentially their present binations are in the photographic range. The form. The publications cited, however, give the others are to be expected between 1.3 and 3 /1. complete history of the subject back to the dis The d-j combinations involving lowest possible covery of these clements. total quantum numbers are almost all just beyond As soon as the energy-level systems for the the photographic region. Combinations of the noble gas atoms were fairly well established, it type 2p- 2s are in approximately the sam e region became evident that many relatively intense lines as 2p- 3d, and all the 2s levels have been estab should occur in the infrared. The discovery of lished by photographic observations. improved sensitizers for photographic plates has The structure of krypton closely resembles permitted a number of extensions of tlwse spectra that of argon with a somewhat greater displace in the direction of greater wavelengths. Meggers ment of analogous combinations toward greater and HumphTeys published a paper on the infrared wavelengths. Five of the ls- 2p combiliations arc spectra of neon, argon, and krypton in 1933 [12]. beyond the range of photography, and extend as Photographic obscrvations as far as 12,000 A were far as 2}'-. The 2p- 3d combinations are predicted reported. The paper also listed radiomctric in the range between 1 and 10 },-, a few being observations previously published by Paschen of still greater wavelength. A number of these [13] on argon, and by Hardy [14] on neon. Com transitions have been observed photographically, plete term tables were given, incorporating the namely, in instances where the p-level series con extensions permitted by the new data. These verges to the lower, and the d-level to the higher tables should be consulted in order to understand ion limit. The d-j combinations of first members the discussion of thc location of infrared lines as of these series are, with a few exceptions, of a given in succceding paragraphs. A later publi wavelength too great for photography. The cation by Meggers [15] gives wavelengths and second members of the s-series are of about the classifications of helium, neon, argon, krypton, same magnitude as the first members of the d and xenon lines as far as 13,000 A. These obser series. This brings the 2p- 2s combinations within vations were made with Z-type 'Eastman plates the same range as those of the type 2p- 3d. and represent the present limit of photographic The predicted distribution of xenon lines in the observations, utili zing silver halide emulsions infrared is considerably different from that of the incorporating photosensitizing dyes. other noble gases. Five l s- 2p combinations have not been observed. They are all too far out in II. Location of Infrared Emission Lines the infrared for the radiation to be transmitted In conformity 'with generally observed char by the glass enclosures of the sources used in this acteristics of the spectra of homologous elements, worle They are also combinations between there is a shift of analogous term combinations levels converging to different limits and not toward longer wavelengths as one goes from ele expected to be very intense. The 2p and 3d ments of lower to those of higher atomic number levels are of about the same magnitude, placing in the array of noble gases. In the first spectrum the combinations for the most part beyond the of neon most of the ls- 2p combinations are in range of glass transmission. The interlimit tran the yellow, orange, and red regions and give the sitions of the 2p- 3d type fall mostly in the photo discharge its characteristic color. The 2p- 3d graphic rang'e. This results from the very large combinations are in the photographically acces difference between the ion limits amounting to sible infrared region, and appear with great 10,540 em-l. The d-j combinations fall in the intensity on plates treated with special sensitizers. photographic range. Only one of the 2s levels The only combinations, in neon I , out of the is known. range of photography in the infrared, are those It may be concluded that, of these spectra, of the 3d levels with the levels originating in the krypton I , and argon 1, may be expected to have 500 Journal of Research a considerable number of intense combinations warmed up, permitting a la rge concentration of between IJ.1 and 2J.1 . Almost all intense neon lines energy in the slit OpeniJlg. With this Olll'ce it may be photographed, whereas xenon has a very was possible to work with slit openings of 15 J.I, sparsely populated region betwern the photo and in some instances down to 12 /J.. The mer graphic limit and about 5J.1. These considerations cury spectrum in the infrared is weH known from indicate that krypton and argon arc the most the observations of :rvLcAli tel' [J 6], who use d a promising of the noble gases for study in the glass multiprism spectrometer of high resolution, region just beyond the limits of pbotography, obtaining wave numbers that agreed rema[,kably but in which the glass enclosures of fI,vailable well with those calculated from the known level sources are transparent.

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