Journal of Research of the Nationa l Bureau of Sta ndards Vol. 49, No. 2, August 1952 Resea rch Paper 2345 Infrared Spectra of Noble Gases (12000 to 19000 Ar Curtis 1. Humphreys and Henry 1. Kostkowski . The first spectra of heliUl.n, neon, argon, krypton, and xe non, excited by discharges in gelssler t ubes, operated by direct connection to a transformer, have been explored in the ll1frared (1 2090 to 19000 A) . A hi~h-reso lu t.i o n , automatically recording, infrared spec trom­ eter, emploYlllg a. 15009-h~ es -p e r-lllch gratlllg and lead-sulfi de photocond ucting detector, was used as t he dlspersmg mstrument. A new set of wavelength values is reported for all t hese spectra. New data include 18 pre viously unreported lines of neon and 36 of krypton all of which have been. classified . .~h e descriptions of t he spectra of argon, krypton, and xe non represent essent ially a repetit IOn of t he observations of Sitt ner and Peck. Several prev io~ s l y missing classificat ions a re supplied, also a few amended interpretat ions. The analysIs of t hese spectra m ay be regarded as complete. Use of selected lines as wavelength standards is suggested., 1. Introduction mocouple detector were reported by Humphreys and Plyler [2] . These observations covered the same The essentially complete character of both the spec tral region in which the data herein reported were d escription and interpretation of th e photographed obtained, but, because of well-known limitations af­ spec tra of the noble atmospheric gases makes it fecting the precision of spectral data obtained by apparent that any reopening of th e subject can be prism spec trometers with thermal detectors, may be justified only on the basis of th e availability of new' considered as en tirely sup erseded by the presen t sources of information, such as a new technique of work. The earlier paper may be referred to for a observation permitting an ex tension of the observa­ fairly extensive list of references on the first spectra tions into a previously unexplored r egion, leading to of the noble gases, which will not be repeated in full significant addit ions to th e experimental material. here. It. also reported two new levels, designated 4 U Such a technique is the utilization of lead-sulfide ttnd 4 W, in Kr J , computed from pho togra phic da ta by photoconducting detectors in combination with high­ M eggers [3] but requiring confirmation by radiomet­ r esolution gratings for radiometric observation. The ric observation of infrared lines, arising from combi­ lead-sulfide cell extends the r ange of such high­ nations of the same levels. The first, or neutral­ resolution observations beyond th e photographic atom, spectra of all th e noble gases are very rich in limit to the limit of its sensitivity near 30000 A, and infrared lines. Considerable portions of th ese infra­ because it is also sensitive to the visible and ultra­ red spectra lie in the photographically accessible violet as far as 3500 A, at least, use of higher order region, and have been the subj ec t of exhaustive in­ standard lines for comparison is possible. vestigations. R eference is m ade to a few of the more Three of these spectra, argon, krypton, and xenon, recen t publications, which, in addition to those al­ h~ve been observed by this radiometric technique by ready mentioned, will serve as a background and Slttner and P eck, whose reported obser vations and introduction to the current work:, and provide cross analysis [1] 2 cover essentially the same region as references to earlier work as required. A paper en­ those herein presented. These excellent observa­ t itled "The Infrared Spec tra of Neon, Argon, and tions appear to have been essentially complete, and Krypton", published by Meggers and Humphreys [4] the overwhelming majority of the classifications are i!l 1933 brought the analysis of these spectra essen­ correct . In the intervening period, ho\vever, suf­ tially to completion. A sep fJ.rate paper by Hum­ fi cient new information has b een accumulated to phreys and M eggers [5], and of similar scope, appear­ make it appear justifiable to prepare a new publica­ ing a few months earlier, brought X e I up to date. tion, which should complete these analyses as far as Shortly after this, plates incorporating new photo­ any reasonable effort will permit. Discussion of the sensitizing dyes, made availahle by the Eastman specific points of differ ence between this analysis and Kodak Co. [6], permitted photographic observation as that of Sittner and Peck, together with extensions to far as 13000 A in favorable instances. With these th e analyses as r eported in other earlier publications, new plates M eggers [3] reo bserved all the noble gas will .be,included .illthe..se parate .sections_dealing .with .. spectra. to the photographic limit and interpreted the respective spectra. In brief, th ese consist of nearly all the new lines: In the intervening years no i~clu sion of He I and Ne I , presentation of pre­ further extension of the range of photographic sensi­ ViOusly unreported data consisting mainly of 18 new tivity has been accomplished, and no further observa­ lines in N e I and 36 in Kr I , interpretation of nearly tions of noble-gas spectra were made up to the time of all r eproducible previously unclassified lines, and the radiometric investigations at the National Bureau amended classifications in a few instances. of Standards [2] and at Northwes tern University [1]. Observations of Kr I and A I , obtained with a prism spectrometer, equipped with a glass prism and ther- 2. Energy Levels of the Noble Gases Although the analysis of the spec tra of the noble 1 Presented, in part, at the meeting of the Optical Society of America, Buffalo, gases may be r egarded as essentially complete, in the N. Y., Oct. 1949. • Figures in brackets indicate the li terature references at the end of this paper. sense that nearly all the levels predictable from the 73 electron configurations have been found , and that, ically significant. The number wi thin the bracket is in most instances, long series have permitted highly Racah's K or intermediate quantum number obtained precise calculation of absolu te term values, these by vectorial addition of the .j-value of the parent ion spectra are somewhat unusual in their structure in to the l-value of the external electron. In th e that they do not permit arrangemen t into regular instance of the noble-gas configurations, where the multiplets with any reasonable conformity to rules par ent ion has j - valu e = l ~ (unprimed case), ther e regarding intervals or intensities. Paschen [7] can be a maximum of four K-values; and 'where th e adopted a special notation in reporting his analysis parent ion has j-valu e = O~ (primed case), no more of N e 1. This no tation has been retained in all than t wo values of K appear. For each K-value, by subsequen t publications on these spec tra; eviden tly addition or subtraction of the spin moment S of th e because it is no t possible to describe the levels accord­ external electron, always= 016, two possible j-values ing to the currently accep ted notation, which is appear for the resultant vector sum. IVe thus obtain actually meaningful only when vec tor coupling of a pair of levels for each K. A maximum total of LS-type is presen t. This inability to iden tify mul­ t welve levels is possible for any rare-gas configuration tiplets in rare-gas spectra points to the probable where the external electron has l-value 2 or greater. existence of a different type of coupling. This has As might be expected, this is the same total number generally b een supposed to resemble the jj-case but and th e same set of j-values that would be obtained evidence, principally from Zeeman eff ect, indicates with LS-coupling. Table 1 illustr ates th e develop­ that extreme jj-coupling is not realized, and that an ment of th e set of levels and appropriate qu antum intermediate type prevails. Considerable light was numbers associated with t he binding of th e j -electron shed on the problem by a theoretical paper by R acah in the configuration mp5 nj. [8], who discussed an intermediate coupling scheme, designated j l , and discussed the conditions for its T ABLE 1. Development of nolation fo)' j l cou pling, according existence. It was pointed ou t that , whereas ls­ to Racah, illustm ted by i -type levels of noble gases coupling occurs when the spin-orbit interaction is weak compar ed to the electrostatic, and jj-coupling I Ion Icon fi guration h J( J occurs when the electrostatic interaction is weaker, a ---- third possibility, the j l-case, may be realized, 5 according to which the electrosta tic interaction is lYz +1~3 Wi ) +8~ O Yz { 4 weak compared to the spin-orbit interaction of the 4 [3Yz ) .. _......... { paren t ion, but is strong compared to the spin cou­ 3 3 pling of the external electron. The vectorial repre­ [2Yz ) ............ { senta tion of this case is to combine the total angulm' 2 2 [l Yz ) . ...... moment j of the parent ion with the orbital momen t ... - { 1 l of the external electron to form a resultan t K .