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. . scheme. The dial settings corresponding with the emission peaks of the mercur'y lines were de­ III. Observations termined from the averages of six recordings, and The sources used in the earlier observations at can be established to within one-ten th of a scale the National Bureau of Standards were, for the division, The mercury spectrum gives a good most part, Geissler tubes, made by Robert distribution of lines up to nearly 2 fJ.. The wcak­ Gatze, in Leipzig, and of special construction ness of this array in a prismatic spectrum is that permitting end-on illumination of the spectrometer the four stronges t lines at the longer wavelength slit. Several of these tubes were still in excellent end, namely, 1.6919, 1.6935, 1.7073, and 1.7108 /J., condition, and were made available for these appear as two emission peaks, each consisting of observations by the kindness of W . F . Meggers, an unresolvcd pair. The latter pair has a se p­ of the Burea u's Spectroscopy Section. aration of 12 cm- 1 in wave number and is just A Perkin-Elmer model 12A spectrometer, beyond the practical limit of resolu Lion at this equipped with a General Motors amplifier and setting. The averages of the wave numbers of Brown recorder, was used as the dispersing, de­ thc respective pairs were adopted as the values tecting, and recording system. Only slight modi­ for the observed peaks. It is expected that more fi cations of this system were needed . The housing satisfactory standards for the 2-jJ. region can be covering the proj ecting system was lifted, and the found when more intense sources are available. plane mirror rotated by a sufficient amount to Poss ibilities are the helium line at 2.058 /J. and the permit forming on th e entrance slit an image of hydrogen line at 1.875 /J., the latter being the first the emission so urce, which was mounted beside line of Lbe Paschen series. The wave numbers of the global' housing. The first recordings were the mercury lines were plotted against observed made with the rock-salt prism in place. These dial settings to give the calibration curve. served to establish the existence of the emission The noble gas so urces were operated in the usual spectrum in the region between 1.3 and something manner by connecting the tubes directly to the over 2J.1. This, however, is near to the region of secondary of a sign transformer, rated at 12, 000 minimum dispersion for J"o cksalt, so that th e v, 200 va. A variable-voltage transformer was conditions for wavelength measurement arc un­ placed in the primary to control the input voltage favorable. Subsequently, a flint-glass prism cut so as to prevent overloading. The energy output to an angle of 55 degrees was mounted in the of these tubes was extremely small as compared spectrometer . This angle was calculated to per­ with the mercury somce. It was necessary in mit operating the instrument within the range of general to operate the amplifier gain setting as available wavelength dial settings without chang­ high as possible. This required unusually good ing the setting of th e Littrow mirror. By a operating conditions in the room because very proper selection of the prism angle, it is possible slight temperature changes caused a large amount to interchange prisms of different materials with­ of zero drift. The slit settings were kept as small out any readjustment of the other components, as possible, consistent with reasonable deflections, and to maintain the calibration for each prism . but in general it was necessary to operate in the Wavelength calibration was obtained by use of range of slit-width settings between 60 and 120 /J.. an H- 4 mel'CUl'Y lamp. This is a so urce of high Repeated runs were made with different amplifier intensity and is steady in operation. The arc gains and different sli t widths, chosen to give the stream appears as a thin line wh en the source is best record of characteristic features. Thus the

Infrared Emission Spectra 501 most intense lines could be observed with nalTower of the respective series, all of which are observed slits and lower amplifier gains, whereas it was photographically. The fact that the combinations necessary to increase both to the practical limit in 2Ps-3d4 and 2P9-3d~ show the highest intensities order to obtain the faintest lines. It was en­ among this group is in accordance with expecta­ couraging to find, however, that the sharpest tions. The line observed at 5,930 cm - I is much maxima could be determined with a precision of stronger than any of the others. As noted in the about one-tenth of a scale division from repeated table, it is possible that five different lines may records. The less well-defined maxima arc prob­ contribute to this maximum. Considering the slit ably fixed with a precision about half as great. widths that were required, it is probable that the At 6,000 cm- I , a scale division corresponds to limit of resolution when using the noble gas about 15 wave numbers, when the glass prism is sources was not better than about 25 cm - I , so in place. that these ambiguities cannot be avoided until The portions of the red and infrared spectra of the technic can be improved. krypton and argon falling within the range of photographic observation have been reobserved TABLE I.- List of Kr I lines by the methods r eported in this paper. The 1 ntensity AobB . ).Io b8. 'l' erm comb ination Vral e . wavelengths of the lines so observed were found

to conform with the system of mercury standards j\;fiCT on s cm- 1 cm-1 1. ______and with the noble gas lines of lesser wavelength. ls.-2p lO ______5, 322 5______2p .... 3d; ______. __ 5,503 1. ______Inasmuch as these lines have already been 1. 7820 5,610 2P lO-3d, ______5, 603 1. ______observed with a precision that co uld not be 1. 7347 5,763 2p,....3.;' ______5,756 5,919 matched with the radiometric technic used in 2p ....M 3d, ______5,932 thi s work, they are not listed in this paper except IL ______1. 6859 5, 930 2p,.... 3d•, ______• 5, 917 r2p,-3d"; ______. ___ . 5, 903 where classified on the basis of new observations. 2p6-3d; ______. ____ 5.956 L ______j3drlX ______. 6, 319 1. 5823 6, 318 l3d,....4Z ______. ____ 6. 320 IV. Discussion of Results 4 ______{2P iO- 3d3 ______-- --- 6,519 1. 5326 6,523 2P6-3d, ______. ____ 6, 522 The results obtained for krypton, together with P .... 3di------6, 7S5 3 ______1. 4747 6, 779 t2p.-3d, ______6, 772 their interpretation, are shown in table 1. H ere 2P 6-3s, ______6, 771 are shown observed wave numbers and corre­ tP7-2S4------6, 930 3 ______I. 4424 6,931 2p,-2s, ______. 6, 942 sponding wavelengths, estimated intensities on a 3d;-4X ______6, 942 scale 1 to 10, calculated wave numbers, and inter­ 2 ______1. 3926 7, 179 3d';-4W ______7, 180 !2PS- 3d, ___-- ____ --- 7, 338 6 ______1. 363 1 7,334 pretation as combinations of levels. The first 2p,-2s, ______7,333 2 ______two lines entered in the table were observed, but, 1. 3203 7, 572 2PS-284 ______-- 7, 587 2 ______1. 2883 7, 760 3d,-4U ______7762. 5 inasmuch as the calibration curve obtained from - I. 220454 8191. 43 3d;-4U ______8191. 43 '------mercury lines cannot be extrapolated with reason­ ! - 1.211 781 8250.06 3d;-4W ______8250.06 I able accuracy beyond about l.7 It, only the wave numbers, computed from known level values, are - Wa ve lengths given by W . F. Meggers. listed . There are a number of combinations of the The surprising feature of this spectrum is the type 2p- 2s. However, there is still no evidence failure of the previously unobserved l s-2p com­ for the missing 2s3 term. As it has not been bination to appear wi th appreciable intensity. found from photographic data, only a combina­ The line with adopted wave number 5,322 classi­ tion involving 2P3 or 2P4 could reveal it. fied l S2- 2plO is the only one of this group appearing The remaining lines are classified as com bina­ with sufficien t intensity to be observed by the tiOllS of j -type levels . Two of these designated, Bureau's present equipment. 4U and 4W", are new. It is interesting to note The most intense krypton lines observed are that they explain two previously unclassified com binations of the type, 2p- 3d. These can be lines given by Meggers, at wave numbers 8250.06 classified both on the basis of computed wave and 819 1.43. The level values, in accordance numbers and the intensities of the higher members with the latest revision of the krypton term table

502 Journal of Research [12], and respective inner quantum numbers arc intensity for observation, and of operating with as follows: available apparatus and methods. Work is in 4U, 6926.04, j = 4 progress on the design and constru ction of new apparatus for study of line emission spectra in the 4W, 6867.41, j = 3. infrared. The principal uni t is to be a gr it ti ng The experimental evidence for these new levels spectrometer equipped wi th a grating ruled Lo is the appearance of two constant differences, concentrate the radiant energy in a elecLed th e first, between the lines 8191.43 fi nd 7,760 region. Several such gratings will be available. A second improvement that must be made is the cm- 1, equal to the difference between 3 d~ and utilization of sources permitting very much 3d4, and the second, between the lines 8,250.06 and 7,179 em-I, equal to the difIerence between greater concen tration of incident radiation at the 3d: and 3d~. Each of these differences estab­ entrance slit of the spectrometer. lished a new level. Additional evidence that re­ VI. References moves any doubt il,S to the reality of these levels, is that extrapolation of the hitherto incomplete r1] 1'. Paschen, Ann. Physik [4] 60,405 (E1l9) . [21 K \x,r ::\Ieissner, Z. Physik. 37, 238 (1926); 39, 172 series, designated U and W, back to their respec­ (1926); <1 0, 839 (HJ27). tive first members, gives values practically identi­ [3] 'N. Grpmme r, Z. Physik. 5<1, 190 (1 929). cal with those experimentally determined for the [4] \,V. Gremmer, Z. Physik. 50, 721 (1928). two new levels. It becomes apparent therefore, (5]W. F . Megge rs, T. L. deBruin, and C. J . Humphrpys, that th ese levels arc the first members of the BS J . Resea rch 3, 129 (1929) RP89. (6] Y'Y. F . Meggers, T. L. deBruin , and C. J. H umphreys, U and W series, and the reason for the choice of BS J . Resea rch 7, 643 (193 1) RP364. designation is obvious. These .i-type serie are (7] W. Gre mmer, Z. Physik. 73, 620 (1932). nearly hydrogenic and can be readily extrapolated ( ] E . Rasmusse n, Z. Physik. 73, 779 (1932). by means of a Rydberg table [17]. [9] W. F . i\[eggers, T. 1.. deBruin, a nd C. J. Humphreys, V cry few new data were obtained for the argon BS J. Research 3, 731 (1929) RP1l5. (10] C. J. Humphreys a nd W. F . ~I egge r s , BS J . Resea rch spectrum, since most of it is in the photographic­ 10, 139 (1933) llP52l. ally accessible region. Two of the lines observed [11] W. Gemmer, Z. Physik. 59, 154 (1930). by Paschen, 7402.9 and 7287.3 cm- t, arc those 112] W. F . Meggers a nd C. J. Humphreys, BS J . ll e~e a r c h of greatest wavelength to appear with appreciable 10, 427 (1933) RP540. (13] F. Paschen, Ann . Physik [4] 27, 537 (1908). intensity. An emission peak at 7,477 cm- l is r1 4] J. D. Hardy, Phys. Rev. 38, 2162 (1931). probably 2P6-3d; , or 2p2- 2s2 or a blend of these. r15] W. F. Megge rs, Zeeman Ve rhancleli nge n, p. 190 to 200 (Mar t"in Lls N ijhor-r , The Hague, 1935); J . Research V. Conclusion NBS H, 487 (1935) RP781. (16] E. D. McAlister, Phys. R ev. 3<1, 1142 (1929). The infrared measurements reported in tIllS [1 7] Rydberg Interpolation Table (Depart ments of Phy­ paper are to be regarded as preliminary. The sics a nd Astronomy, Princeton University, Prince­ investigation ,vas intended to demonstrate the ton, N . J., 1934). feasibility of exciting these spectra with suffi cien t W ASHING'l'ON, JANUARY 23, 1946.

Infrared Em.ission Spectra 503