) Journal of Resea rch of the Na tional Bureau of Sta ndards Vol. 50, No. I, January 1953 Research Pa per 2380
The Sixth Series in the Spectrum of Atomic Hydrogen1 2 Curtis J. Humphreys
The first member of the sixth series (6h 2IP - 7i 21) in the sp ectrum of atomic hydrogen has been observed at 12.371-', in agreement with the predicted position. Other newly observed lines include the second line of the Pfund series and the third, fOLlr t h, and fifth lines of the Brackett series. The source was a water-cooled discharge tube drawing 640 milliamperes a t 5,000 volts. The t ube was fitted with a ICBI' lens, which served also as a window for end-on observation. An atomic-hydrogen discharge was maintained by admitting water vapol' through a Hopfield leak and pLlmping continuously. A conven tional dispersing and recording system ,ras used, consis ting of a P erkin-Elmer model 12 spectrometer with a d-c breaker amplifi er and Spee::!omax recorder. R ela t iv e -int e n s i t ~ · measurements are included. 1. Introduction only t wo members of his series, and Pfund found only the first member of the eries bearing his name. The last new series to be discovered in the spectrum Table 1 is introduced to point out thc locations of , of atomic hydrogen was reported by Pfwld 3 in 1924. the first two members of known and predicted series. This series originates in transitions to the fif th orbit The latter include all entries below Pfund-a. Pre from those of greater quanLum number . This had d icted po itions of the higher members of infrared been preceded 2 years earlier by the discovery by sories, including and beyond Lbat of Brackett, are B'rackett 4 of the series originating in transitions to gi ven by Swensson.8 the fourth orbit. Both of t.hese contributions fo llowed closely and probably received considerable T ABLE 1. Locations oj the various hydrogen series according impetus from the spectacular e).rpel'im en ts of Wood,5 to the series J01'1nnla v= l?( ~--\ ) in which he succeeded in exciting excepLionally long nl- n2 Balmer series in hydrogen di scharges . This spec trum was of unusual purity, and, becau se of the striking Series 71, '" A v contrast between the brillian t lines and dark back I A cm.-1 ground between, was call ed the "black discharge." Lymnn ___ 2 1215.68 (vac) 82258.31 B rackett inferred correc tly that tbe exci tation con ----- 1 { 3 1025. 73 (vae) 9749 1. 36 Dai ll1er ___ 2 3 6562.79 (air) J 5233.22 ditions favoring the "black discharge" were prereq --- { 4 4861. 33 (air) 20564.79 P ,-lschell _____ 4 J8751.1 5331. 58 uisite to the production of series resulting from 3 5 12818.1 7799.33 5 40511. 4 2467. 77 transitions to levels higher than the second 01' third. I Brackett. 4 ~ 6 26251. 3 3808.29 This condusion depends on the fact that the length Pfund ____ 6 74577.6 13·10. 52 - -- 5 { 7 46524. 7 2148.81 of series and the appearing of series having a hi gh I 6 ______7 123681 808.29 level as the final st,aLe are bo th dependen t on the 6 { 8 7,,001. 5 J 332.89 7 ______8 190569 524.60 , density of population of the more hi ghly excited 7 { 9 113057 884. 27 states. --- The extension of the hydrogen specLrum farther R es ults of a new investigation presented and into the infrared region to include additional series discussed here include obscrvation of the first mem predicted by theory has long consLiLuted a chall engc. ber of an enLircly new series, 6h 2H o-7i 2I , a second , This has become increasin gly insisten t when one member of the Pfund scries, and three addi tional co nsiders tllat in more than a quarter of a cen tury mem bel'S of the Brackett series beyond the two origin of intensive infrared activity, marked by spec tacular ally reported. developments in observing techniques, there has been no further infrared ex Lension of the hydrogen 2 . Experiments spectrum. A review of the presen t state of our The method of exciting the hydrogen spectrum knowledge of this spectrum will be followed by a adopted in these experimen ts was similar to that discussion of the experimen tal conditions req uired employed b y earlier observers mentioned in the for the excitation of higher series and utilized in the references, consisting of a high-current Geissler eurrenL experiments. Of the many compilations di scharge. Modifications were introduced to meet of hydrogen data, one of the most satisfactory is that the special requirements of the observations, t.he given by Grotrian.6 The P aschen series was ob most important of which was the necessity for usi ng served, in the range of photography, as far as the a transparent window on the discharge tube to per eighth member by Poetker.7 Brackett observed mit emergence of energy associ ted with the first
'Curtis.T. Humphreys, J. Opt. Soc. Am.42, 4~2 (1 952). line of the n.ew series at 12.37 1-'. All reported obser 2 Presented at the Symposiu m on M olccular Structure and Spectroscopy, vations of the hydrogen spectrum emphasize the Ohio State Un iversity, 1952 . 3 A. H. Pfund. J. Opt. Soc. Am. 91 , 193 (1924) . difficulty of maintaining an atomic spectrum be • F. S. Brackett, Astrophys. J . 56,154 (1922) . 'R. W. W ood, P roc. Roy. Soc. (London) [A I 97 , 455 (1920); Phil . Mag. 42, cause of the tendency of hydrogen atoms to assoeiate 729 (1921). 6 W . Orotrian. H andbuch der Astrophysik. Kap. 5, Oesetzm~ssigkeiten in der 8 J . W . Swen sso n, P redictions de raies atomiqnes dans I'iofrarouge, extrait d es Scriellsoektren, p. 475 (Springer , B erlin, 1930). M emoires io-8° de la SociHe R oyale des Sciences de L iege, quatrieme serie, 7 A. n . Poetker, l'hys. Rev. 30, 418 (1927). X I, fase. I (L ouvain. 1951).
233 1. 50- 53- -1 size of .the image projected onto the slit of the ~ spectrometer.. The really unique feature of the ource operatIOn was the manner of admittmg hydro gen. Because the use of wet hydrogen has been den~onstrated to be advantageous, the idea of intro dUClllg pure water vapor suggested itself. This proved to be successful and als? made the replenish- L ment of hydrogen extremely SImple. As indicated TO PUMP a small tube of distilled water was attached to th~ " source .through a Hop~eld leak. This leak, illus trated m fig9-re 1, conSIsts of a crack generated in the small blm~-end t9-be sealed inside the figure-4 "" shape~ tube WIth a rmg seal. The opening of the crack IS controlled by a clamp at the crossover point < of th~ tubing. In practice, the rate of vapor flow was mcreased somewhat by mounting a radiant FIGURE 1. Schematic design of SOUTce tube and Hopjield leak. heater near the water supply. The lens of KBr on the end of the tube was used in the final version to form H2 molecule~, with the attendant develop replacing a flat window. This is one of the lense ' ment. o~ th.e many-hne molecular spectrum. This su~plied for a Perkin-Elmer model 21 spectrometer asso~IatlOn IS favored in the vicinity of the electrodes whICh ha:d. not been installed owing to prevailing 1 and 1 probably caused by a catalytic action by the low" hunlldIty. Actually, the lens made possible a metal. Wood's successful excitation of a pure considerable in the amount of energy that atomic-hydrogen discharge apparently resulted from i~crease ~ c?uld be admItted to ~he optical ystem. In addi two features of his technique: (1) construction of an tIOn to the wate~' coolmg tJ:e tube was wrapped in extremely long tube, of which only the ceD tral portion cotton, over whICh a contllluous stream of water ' wa used for end-on observation through a thinly flowed. The be~t development of higher series wa blown bend, and (2) provision fo~ continuous change found to be realIzed when the tube was operated at of the hydrogen supply, accomphshed by connectinO" the lowest pressure for which a steady discharge a constantly operating exhaust pump to the syste:r: could be maintained. and a:t the same time admitting hydrogen through . All other co~ponents in the setup were conven a cap~lary tube. drawn to a fineness determined by tlOnal commerCIally available pieces of apparatus. experiment to glve the optimum pressure. The source of power was a 2Yz -kva transformer. The More recent investigations have pointed to the output.voltage was rated at 5,000. The transformer adv~ntage of using moist hydrogen. This is made was shghtly overloaded at the maximum source Ob~710U.S by the appearing of the intense Balmer current of 630 rna. A variable-voltage autotrans senes III a vacuum system when the last traces of former of 40-amp capacity was used to control the J moisture are being pumped out. This condition power supply. f~vol'i~g sp~cLrum the atomic is probably caused by A Perkin~Elmer model 12 spectrometer with a hb~ra~lOn of hydrogen III the atomic state upon dis rock~salt prIsm was used as the dispersing system, SOClatlOn of the water molecule. Arnulf and Dun modIfied o~y by rem.oval of the housing over the oyer ~ described th~ design of a tube for producing source optlCS to permIt advantageous placement of ato~rlle-hyd~ogen dIscharge.s .of high intensity, fea the sour~e, and atta~hment of a simple motor-driven tur~ng a sIde tube contalllmg gypsum and from gear tram. for sc~nlllng. ~n the later phases of the " ~hlch small amounts of moisture could be liberated work, an lmprovIsed housmg was mounted so as to lllto the system by heating. For the current series of experiments several tubes enclose the end. of ~he light s~urce a:nd proj eeting system to pm~mlt fillmg the entlre optIcal path with were built and tried out, all of which had the ad OIl-pumped llltrogen for the .p~rpose of eliminating vantageous features established by earlier investi watm: vapor. l!0r the prehmmary experiments a r gato~s to the extent permitted by the practical HOl'lllg thermopIle, as used in the early Perkin-Elmer reqUIrements of the work. The desirable charac models, was used. This was l~ter rCJ:ilaced by one of te!istics .i~cluded ~rovision for cooling, associatcd the fast-response a-c couples m current production. WIt? abIlIty to WIthstand stea~y high currents, A d-c sy~teI? was used throughout the experiment sn:tlsfactory attachment of a sUItable transparent for. amph~Ylllg and re?ording the voltage output. WIndo,,,:", and. geom~tric design to provide a region ThIS conSIsted of a Llston-Folb breaker amplifier of maXImum mtensIty and to permIt illumination of and a Speedomax recorder. All of the vi ible com large-aperture optics: :rhe tube used in obtaining pone~ts are readily identifiable in figure 2. the reported da~a.I ill9-strated schematically in It I~ apparent that the e ~erjmental setup doe figure 1. The prmcIpal dIfference between this and not dIffer mal:kedly f~om that used by previou o~her preliminary designs is in the size of the central ob~eryers, partIcularly m regard to operating charac dl charge tu,?e. The improved intensity resulting tens.tICs of the source and specifications of the dis from. a ~elatIvely mall capillary, approximately 4 persm~ system. The use of an amplifying and mm III dIameter, more than makes up for the small reco~'~l~g system does, however, provide a margin of A. ArnuIC and L. Dunoyer, Rev. Opttque 9, 510 (1930). senSItIvIty that conld not be .matched in the older 2 a
12.75 12.50 12.25 12.00 - ). MICRONS z '"o .... FIGURE 2. Layout of apparat1lS. u b W ...J '" W point-by-point observations with a galvanometer. o The observation of the first line of the sixth series
howD on each figure is to show the rclation of de- 10 J. Opt. s oc. Am. 42,432 (1952). 3 Although the observation of the sixth series of hydrogen was the principal experimental objective, additional records were obtained for the purpose of extending known series and obtaining intensity comparisons within and between series. The entire spectral range available with the rock-salt prism installation was covered. This range extended from ! below Balmer-,B to somewhat beyond the new line of the SL'l::th series. Survey records described in the 'i appropriate captions are reproduced in figures - 1 1 ) Ie:: l --t---- Bo-,8 Bo-a 0, 0, 0, Po-/" 01 Po-,8 PO-ct 5 and 6. +-)'A 4861.33 6562.79 10938.1 12 818 .1 18751.1 Figure 5 shows the entire region from Balmer-,B to 1 Pfund-a, with some featureless sections removed. At the top is shown the first two members of the I SLIT 200f.l Balmer series, illustrating the intensity decrement. 1 \ This is followed, after adj ustment for drift, by a record of tbe region from Balmer-a to Paschen-a. SLIT 5001-1. Three members of the latter series ma,v be picked out, \ with a fourth probably distinguishable from noise. 'rhe other profiles are unresolved multiplets of 0 I. rJ~I 1 The lower section rovers the region including Br-,8 Br-'" PI-Q. Brackett-a and Pfund-a. The slit width was in !l 74577.6 creased from 70 to 200 after traversing Paschen-a, • ~).A 26251.3 4051 1.4 and the r ecording was again interrupted to widen the I FIGURE 5. R ecord covering interval from Balmer-fJ to Pfund-a. slit t o 500-J..L and adjust the zero after passing Brackett-a. The recording was made without any attempt to remove H 20 vapor from the path, and the curve shows the characteristic profile of the 6.2-J..L water-vapor band with a sli t too wide to reveal rotational structure. Figure 6, representing a record run in the opposite direction, covers the same features. There is slightly less noise because of th e fact th at th e a-c thermopile of greater sensitivity was used, permitting operation ---+------~------4------\ on lower gain settings fo r the sam e sli t openings. PI-a:: H2 0 Br-a: This run WliS made with dry nitrogen in the spec ->-A 74577.6 6 . 2~ 40511.4 trometer and in the improvised housing enclosing the source optics. The 6.2-J..L water-vapor band can hardly be seen. The upper and middle sections arc part of a continuous run. The lower section illus trates the condition of par tial removal of water vapor. Table 1 shows that Pfund-,B is located at wave length 4.65 J..L . On the basis of decrements found in other series, it is expected to be about one-sixth to one-seventh as intense as Pfund-a and abou t 2 per cent of Brack ett-a. Observation of Pfund-{:l under conditions of the earlier experiments (Brack ett, SLIT 70j..l Pfund) was quite hopeless because of inadequate sensitivity, coupled with th e fact that it wo uld have been completely masked in the wide-slit image of Po-a. 0,- Po-p 0, Po r OJ OJ 01- Ba -a, Brackett-a at 4.0 J..L. Figure 7 shows the results of -->- A 18751.1 12818.1 10938.1 6562.79 a very limited amount of work with a PbTe cell. The Pfund-,B line can clearly be seen to the left of Brackett-a with in tensity close to that estimated. In the lower CUl've the same line is shown with increased amplifier gain. In this case overloading prevented a satisfactory representation of the pro SLIT 500j..l file of Brackett-a. A difficulty of observing with a ! telluride cell in this r egion is that the maximum of PI-a, Br-a, temperature radiation from the tube comes at abou t 5 J..L. The telluride detector is very sensitive to this ->-A 74577.6 40511.4 continuous background. The trouble was overcome FIG URE 6. Records covering interval f r· om Balmer-{3 10 Pfund-a, by greatly diminishing the power input to th e source showing effect of Te nwval of waler vapor f rom optical palh. . at some sacrifice of signal strength. 4 a: UJ o a: o u UJ a:
Br-fj Br- r B-" Po-a: - >-A 26251.3 216552 19 4417 IB751.1
FICURE 8. R ecords showing the h':gher members of lhe B mclcelt series as observed with f' PbS cell (s lit 50 ,.,. ). Pf-fj Br-lL ->-A 4 6524.7 40511~ FIGURE 7. R ecords showing PJund-fj and B raclcett-Of. as observed with 0. PbTc detector (s lit ;'00 ,.,. ). ' The fact thn.t all of the Brackett srries beyond the first memb~l: l~ es 1Il .thc wavelcngth region of maxi mum scnsltivlty of lead-sulfid e photoconductive cells made the exploration of this scries beyond the second. m em~e[' , as observed by Brackett, appear prolntSmg. Ii Ig Ur e ~ shows the development of th is sen es as observed W I th a PbS detector. Brack eLL-a is not included , being out of the range of sensitivity. Pa-a 01 Br-€ 01 Pa -p 01 01 Po-it" en Pa-b Po-( 0, Po-! (ot ten.) The {3- and ')'-lines are well developed. Paschen-'a, -->. A 18751.1 18174.0 12 818.1 10938 .1 10049:4 9546.0 9229.1 fallJl1 g J li st b etwee n Brackett-a and Brackett-€ partly ov.erlaps those lines with the scale of c\ i s p e r s io~ F IGU IU~ 9. Heconl of Paschen series observed with (L PbS Inherent In the system. It is cer tain that thev could celi (s lit 50 ,.,. ). be clearly ~e pal'a te d wi th a hi gh-resolu tion spectrom eter. As In the other records shown the lines not for new seri es, exLen sion of known serie a nd deter ~dentifiable with tbe hydrogen specLrum originate mina Lion of excitation ('ond i Lions requir ~cl for maxi mum devel.opnwnt of infrared se ri es. The study of l~ 0 I, as w~)Uld be expected when water vapor is the excltatIOn of the hydrogen emission pectl'llm is di ssocIated 1Il the source. In the lower record Paschen-a and its companion on the short wave of great astrophysical interest in its relation to the th e ore~ical e~planation of the long Balmer series with s ~de are attenuated b.v usin g. a dirl'erent gain setting. small m tenslty decremen t that occ ur in the olar ~ he upper short sect l ~n dLsplaced vertically only chromosphere, stars with extensive gaseous envelopes ~ s a subsequent rlln. WI thout attenuation, starting ] li St be.\·ond bu t not Includll1g Paschen-a. a nd planetary nebulas . .J\. brief di scussion of thi ~ A final record, figure 9, is in seited to show the m ~tte l' with pertinent re!erence material is given by Kless and Humphreys II III a report on the intensities development ?f the Pa~ch e n seh es under the experi mental conclitLOns descnbed and·with a PbS detector. l~ the fl ash spectl'.um obse rved at the solar eclipse The series may be traced to six members. Poetker of ] 940. CorrelatIOn of astrophysical with labora tory daLa is needed . . Brackett i2 regarded the hy photograp~ed it up to and including the eigh th member. It appears certam that better results could droge.n. spectrum prod uccd in his laboratory source be obtained with a grating instrument because of the as ansmg to a large extent from inelastic collision without ionization. H e offered an explanation that l?o ss ibilit~ · of ~eparat ing .weak~.rdro~en and oxygen was s L~ppor ~e d by the unexpec tedly high ratio of the IUlcs ~Y utlhzmg both hlgher dlspersLOn and greater mtenslt.v of Paschen-a to Balmer-a, this reaching resol Vll1g power. The strong'er lines ineludino' P aschen-a "'ere attenuated. The O'ain 'settinO' wa~ 4 to 3 m Brackett's experiments. This rat,io was changed in some instances after the profil e~ had r;ot attained in these experiments, the maximYI.Jl started to develop, or before the background level 1. aschen-a to Balmer-a bemg 4 to 5, but even th iS lS sUlYl'ising, considering the behaviour of the other \~a s rea:ched. This is the. explanation of the sharp plpS adjacent to some profiles. sen es. The. work on hydrogen to date is to be regarded The most significant information for the inter- a~ pI:ehmll1ary t~ more detailed investigation, which 11 c. C. Kiess and C. J . Humpbreys, Tat. Geographic Soc., Con tributed Tech. Papers, Solar Eclipse Sp r. 2, p. 53 (1942) . Will mclude prec ise evaluation of intensities, search li F . S. Brackett, Astropbys. J . 56, 154 (1922). 5 pretation of excitation conditions responsible for any T .~ B LE . 2. Observed l'el J,tive intensities of hyd'/'Ogen lines observed hydrogen spectrum is to be found in inten sity relationships, including decrement ratios wi thin a eries and intercomparisons between different ___~~ _~ ____A__ I i~i~~~\~~ series. R adiometric measurements with a thermo .-1 pile readily lend themselves to intensit.\~ determina Balmer-p ____ 4861. 33 150 tions owing to the essen tially nonselective character Balmer- a. . ____ 6562.79 1000 of this detector. Table 2 gives a summary of the Paschen-o ___ -_. 10019.4 6 Paschen')' 10938.1 28 relative intensities of several hydrogen lines as esti Paschen-/l __ 12818. 1 140 mated from the observations. Except for instances P aschell-a. ___ 18751.1 700 where photoconductive cell observations were made Brackctt-')' __ 21655.2 13 Brackett-/l 26251 .3 40 'within relatively narrow wavelength intervals, as, BrackeLt-a __ 40511. 4 120 for instance, the region from Brackett-{3 to Brack ett -~ , Plund-/l ___ . 46524 . 7 3 these intensity comparisons are based on thermo Plund-a ____ 74577. 6 20 pile observations and should be reasonably precise, Sixth series-a. 123684 3 s ubj ect to some corrections, including those for reflection and scattering by the optical parts. For instance, the firs t line of the sixth series, Pfund-~ , Brackett--y, and Paschen-o, all originat,e in a common These corrections, which vary with wavelengths upper leveL The table shows that these intensities ' h ave not been made in the preparation of this report. fall within the same order of magnitude. Mechanical The exi t slit was imaged within the boundaries of the limitations of the spectrometer prevented extending receiver for all slit wid ths used. this array to Balm e r~~. This intercomparison is Over a year ago E. U. Condon suggested to the pointed out as a very sketchy beginning of something that should be can'ied out more prcc i seh~ and in a uthor that it would be of great interest to inter greater detail. compare intensities of h ydrogen lines originating in a common upper state. Such intercomparison of In conclusion, the au thor acknowledges with course provides data for evaluating relative probabi thanks the valuable help .of L. G. ~1undie and A . .J _ Cussen in all phases of the experimental work. lities of transition to various lower states. The data o f table 2 may be used to provide such information . CORONA, CALIF. , September 3, 1952 .
6