The emmission spectrum of Br2 in argon

Volume 84, number 3

CHEMJCAL PHYSJCS LETI’ERS

15 December 1981

THE EMISSION SPECTRUM OF Br2 IN ARGON Joel TELLINGHUISEN, Patrick BERWANGER, J Gad ASHMORJZand K S VISWANATHAN Departmcnt of Chemrstry, Vandcrbdt Umversity NashvrIIe, Tenncssec 37235, USA Recelved 31 August 1981

The emisston spectrum of Brz m Ar 1s studled as a functmn of Bi2 and Ar pressure Evldence IS found for at least 10 electromc tranatmns m addltlon to the dommant D’(2g) - A’(2u) band at 2915 A Included among the new transrhons arc the E -) B system near 3120 A and a broad band near 3550 A assl8ned to D’+ 3h2u

1 Introduction At high pressures of mert buffer gas the enusslon spectra of the halogens are dommated by a smgle electromc trantutlon Tlns transltlon has been made to lase m aUfour homonuclem molecules [l] and m ClF, BrF, and IF [21 In the course of studymg these lasmg systems we have had oLcaslon to examme some of the weaker transltlons m the ernB=on spectra and have found apparent mconslstencles and errors m prevlous analyses For 12 many of our results have already been presented [3-51 The present paper summarnes our fmdmgs for Br, A number of authors have stu&ed the emlsaon spectrum of Br,, both w& 16-91 and wthout [IO121 admmture of buffer gas In the present work we use a tesla dtscharge to examme tbe pressure dependence of the UV enusslon spectrum of Brz m Ar Selected regons have been recorded at hrgh resolubon usmg smgle sotopes of bromme As for I2 [4] we see evldence of many of the predtcted Ion-pan states wtich correlate unth BI-(IS) + Br+(sP, ID, 1s) However, the Br, spectrum 1smore congested than that of I,, so our concluslons remam somewhat more quaht&ve

2 Expernnental ‘Ihe expenments mvolved equlpment and procedures snmlar to these dcscnbed prenously [4,13] 528

Low-resolutlon spectra (-1 i%)were recorded usmg a 0 3 m McPheraon model 218 monochromator v& typlcal sht Hndths of 30 m High-resolutlon spectra (=O 03 A) were photographed usmg a 1 5 m JY spectrometer wlth a 3600 groove/mm gratmg For the lowresolution work the Br2 was obtamed by heatmg dry CuBrz The spectrum was studled at Br2 pressures of 0 21-13 6 Torr, and Ar (Matheson ultra fugh punty) pressures of 0-740 Torr A beam sphtter was used to send a portion of the hght to a second monochromator, wluch contmuously momtored the strongpeak near 29 15 A for mtensity fluctuatmns The hgh-resolufion work employed smgle-sotope (79Br2 and *lBr,) sources [3,14,15] Au sources were operated m open au, at an estunated duzharge temperature of 360 K

P31 In appearance the ducharge was a fmt yelIow& whlte or redduh-wlute w~th Br2 alone, but only a barely wstble vlolet ~nth Ar present We have stutlled only the JN em.ssmn m tfus work Typical low-resolutum spectra are shown for several Ar pressurea m fw 1 and 2 Instnunent~ parameters have been adJuated to gwe a roughly constant 2915 !i peak m fa 1 However the ,rltenaty of tipeak mcreased by rou.$ly an order of magmtude frmn spectrum A to spectxum C Thê mtens@’ is op+zn,d for PB,, = 2 Torr and PAr = 300 Torr, It drops hy a factor of 3 when the Brz pressnre ISmcreaseti to 12 Torr, somewhat leas whén-lfsreduced to 0.3 Torr For Br, pressures below 2 Torr the mam factor determmmg mtenaty is probably the effective-

Volume 84, number 3

CJEiUICALPHYSJCS WlTJZRS

15 Der_mbm 158J

nesswah whích ,$hdchschargpowe& c&qh?d tiio

tbe’systti, ae~~LJ3r&~es k, qír&c~mg hkclyPbys*~*@$+$$ MXhf!absente of&ff&gâs+iie spectr& show, no apprecWle qúaKtatwe_cbange m tbe pressare range 0 21-13 6 Ton. As Ar is addéd, vost the peakr; J.II fq IA perszst,but they cbang m &aI c end decrease m mfensity relatwe to fhe 2915 A peak The Ar pressure dependence of tbe 3120 and 3550 A peaks 1s shown m more detail m fq 3 Data of tlus type were obtamed also fix tbe peaks at 2200,2270, and 3335A Jn most casu tbe dependance on l/PA, appeared to lx hear, however, for the 3550 A peak defmte curvature was endent forp&, < 2 5 Torr (fig 3) The de pende-nce on PB~* was nood below 2 5 Torr, bdt mtensties cbanged appnnably for Psrz = 12 TOD The mfuute pressure relative mtenstib of these Eve peaks appeartd t0 be independent of &s, WI& the precmon of our measnement7These mtenMes, corrected for mstromental response,are summanzed m table 1 and &scussed furtber below FE 1 Ultravrolet emmon spectra (nat corrected for spectrometer reqonsa) of Br#r mnrfores, from a te& &scJmrge operated at 360 JC,br2 =24Torr,andPAr=OTorr(A), 26Torr(B).and112Tort(C)

100

EO 1

605 40-

/

/

/

/ **

20 _ca-

--. 0

I

1

I

I

I

8

Fg 2. Brz enus.~on spectta fWP&, = 2 4 Ton aud PAl = 26 Torr (AI, 112 Torr (B), and 696 Torr (c)

Fii 3 KeJatwe mtens&sPr (uncorrected) of the 3120 (wdes1 aud the 3550 A peak (souares~ versus l/p~,., for Br2 pressures of 0 21 Ton (JIJJedpomts), 2 4 Twr (unffled), md 120 Ton &alMïlJed) For cJ&y of presentat~on the pomp at 3550 A havebcen lowered 10 urn@

529

.

Volume 84, number 3

CHEhlICALPHYSICSLETTJZRS

Table i Wavelengths of peaks III the em&non spectrum of Br2 m Ar, together with tnfuute pressure mtenslbes (quanta/A), where measured AU mtensttles are tn arbttrary uruts based on 1000 for D’ + A’, with estiated standard errors m parentheses

2200

0 4 (4)

2270 2370 2510

0.4 (4) -

-

2650 2760

2915 3120 3335

3420 3480 3550

1000 6 (1) 1 4 (4)

7 (1)

3 Discussion 3 1. Quahtatwe

features

Our spectra resemble those reported by Venkateswarlu and Verma [6-8,10]_ These authors used d.dTerent excJtatJon sources and dJd not quantJtatJvely study the dependence of the spectrum on pressure. They reported t-hat the emJssJon Js entirely d~ffuse when no buffer gas IS present, and that different, discrete electronJc transJhons develop as Ar 1s added. To the contrary, we consider that most of the systems present JJJ Br2 alone remul as Ar JS added, and that the main effect of the latter is to induce vibratJonal. rotational, and electromc relaxatJon toward a pseudoequihbnum w~tlun the marufold of low-lymg Jon-pau states [4] _ All of the 20 case c Jon-pau states [4] are expected to have relatJvely large internuclear lstances (3 2 i%) and dJssocJntion energJes (35000 cm-J) but small v~brational frequencies (150 cm-l) [ 14,15]_ To the extent that these states reflect the mtervals m the Br+ ion, there will be six near 49000 cm-J, SIX around 53000 cm-l, SE-Lnear 60000 cm-l, and two consJderably higher (assummg that D’ JS m the lowest group see below) [4] _ For transJtJons to the valence states the difference poten&&, V,(R) = V’(R) - V”(R), generally display mmima near R = RL = 3.2 a [ 14,151. These rmJurna translate into long-wavelength maxima in the spectrum. For a gJven transJtJon Jt IS important to note that smce the mmiJnum lies near RH, all u’ levels of the ion-p&r state can access this region of v’(R), hence ah can emit in the long-wavelength extremum of the spectrum. Moreover, the Franck-Condon properties of

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15 December

1981

such transitions [ 16,171 dictate that no u’ levels can emJt sJgr&ca~rtly to the red of thus extremum; they can of course ellzlt to the blue. For iugh LJ’levels the emission at the long-wavelength extremum is entirely bound-free, no matter what lower state Js mvolved. (“HJgh u”’ means possessJJrg vlbratlonal energy greater than or comparable to the dJssociatJon energy of the lower state, which is 16000 cm-t for the ground state, 3000-4000 cm-l for the more deeply bound excited valence states [ 141 .) For those transihons w&h termlnate on bound lower states, vlbratlonal relaxation of the excJted state eventually yJelds dJscrete structure m the extremum. ConcomJtantly the spectral peak narrows, since the emission from lower u’ levels spans a smaller spectral regJon. Thus a smgle electromc transitJon, uutJally characterized by a lugh U’ population dlstnbutlon, can undergo on mcreasmg pressure an apparent conversxon from diffuse to discrete_ These changes are apparent m the 2915 A peak, which dommates the Br? emSslon at high pressures

Thus transitIon was ongmally assigned as E + B [7,9], but has been reassigned as D’(2g) + A’(2u) [3,14]. With no buffer gas (fig. I A) tlus peak 1s broad and centers near 2900 a; it probably extends to ~2000 A but 1s evidently overlapped by several other transitions m the 2000-2800 A region. With mcreasmg Ar pressure the main peak narrows, fifts to 29 15 A, and &splays clear vtbrattonal

structure (even at the low resolution of fig. 1C). Further mcrease m PAr beyond 114 Torr pro-

duces further narrowmg to a linntmg effective halfwidth of 40 A, which JS thought to be characteristic of a Boltzmann vibrational distnbution at =360 K. Ln the h&-pressure hmit the transition is wtually 100% dlsCrete. At low pressure It is probably mamly diffuse but may contain a significant Crete contribution, which, because of extreme congestion, appears to be diffuse, even at lugh resolution [IO] .

Because of the congestJon of the Brz spectrum, Jt is lfficult to estimate the total intensJty m each band, consequently we have not investigated the temperature dependence as we did m I2 [41_ However the dominance of the D’ + A’ band at high pressure is consstent with the notion that D’ IS the lowest ion-pau state. The much steeper slopes for PBra = 12.0 Torr in fig. 3 suggest that the excited states are quenched by Br,, effectively shortening their lifetimes so that a given pressure of Ar is less effective in causmg vibrational

Volume 84, number 3

CHEMICAL

PHYSICS LETTERS

electromc relaxation_ This effect is evldent also m the 2915 A peak, which, for a given Ar pressure, is less completely relaxed (i.e. broader) for PBrz = 12.0 Torr than for PBr2 = 0.2-2-S Torr. SuniJarly, our single-lsotope sources, wluch have an estunated Br, pressure of 10 Torr, show ngmficant emslon from u’ > IO levels m the 3120 A system (see below) [ 151. Moreover, the weak qualitative dependence of the spectrum on Psrz at Pk = 0 mdicates that such quenching occurs at a rate at least comparable to that for BrZ-mduced nbrahonal relaxation. On the other hand, Ar appears to be an insignificant quencher of the D’ state. SumJar results are found for the ion-pair states of the mert-gas habdes [I] . and

3 2 3000-3800

a region

The region 3000-3800

.& shows evidence

of at

least four electronic tranntlons. The strongest peak m the spectra of fig. 2 is that at 3 120 A. Tlus peak shows the sharp long-wavelength extremum typical of discrete Ion pau * valence transitions; and the narrowmg with mcreasing Ar pressure 1s m&catrve of mternal relaxation. Our h3gh-resolution analysis of ttus system verses that the lower state IS B 3 no+“, as concluded earher by Venkateswarlu and Verma [7]. However, our analysts is completely tiferent and places the excited state +350 cm-1 above D’ and 0 03 A to larger R, with almost identical vibrational constants [IS]. From the absence of a Q branch the upper state is O+g. From its posltlon m the spectrum and its mtenslty and shape, this system appears to be the analogue of the 4300 A band of I,, which is designated E + B [4,9] _ Consequently we have suggested adoption of the E label for the excited

state in Br,.

in the 3150-3350 i% re@on to 2 single electronic transition. This system meludes peaks of progressively increasing intensity at 3170,3190,3215,3240,3270,3300,and3335A.’Ihe subskhary peaks to the blue of 3335 A are not well deveIoped m the absence of buffer gas, and only two of these were reported by Venkateswarlu [lo]. At our Jugbest buffer gas pressure the subsidiary peaks again become less pronounced, which we attriiute to effects of vibrational relaxation in the excited state. This type of structure has been observed previously in the B + X and D + X systems of the lnertgas hahdes [l] and the 2880 A system of I2 [4] _ It is characcenstic of translWe attnbute

the structure

tions which terminate on a gently sloping lower potential near the dissociation limit of the latter. Such transitions may be discrete (XeCJ, I*) or diffuse (o-her mert-gas habde molecules, except XeF), depending on whether or not the em&Ion from low u’ levels samples a bound region of the lower state. We have examined the rnaiu peak (3335 A) at high resolution and found no discrete structure. However, it is possible that &Crete structure may be detectable at shorter wavelengths,

where

higher u’ levels emit at larger R.

Dependmg on the atomic asymptote of the lower state, the upper state lies near 46600,50300, or 54000 cm-l. We favor the second value, wluch places this state above D’ and E and makes it the analogue of one of the I, states responsible for the three peaks in the 46004800 A region of the I2 spectrum [4]. We have not yet attempted quantitative simulations of this spectrum_ However, it is worth noting that although the structure is loosely associated with the vibrational level structure in the excited state, the peak measurements alone are not a valid assessment of the vIbrational interval 1181. The region 3400-3800 A undergoes some interestmg changes as Ar is added. With no buffer gas there is a single clear peak near 3550 A and a shoulder near 3420 A (both reported also by Venkateswarlu [JO] see fig. 1 A). Addition of 28 Torr Ar sharpens these two peaks and develops a third near 3480 A (fig_ ?A)_ As PAr is increased to 100 Torr, the 3550 A peak dirnirdshes more rapidly than the other peaks in the 3000-3800 A region (fig. 2B). However, with further mcrease of pressure this peak decreases less rapidly (see fig. 3), so that its mfirute pressure intercept is greater than those for the 3 120 and 3335 A peaks. At the same time this peak broadens, and the 3420 and 3480 A peaks clrsappear. We interpret these observations to mdicate that at least two electroruc transitions are emitting in this region. The broad peak which persists in the high-pressure limit appears to be the anaJogue of the 505Q A peak in I,, which has been provisionally assigned as D’ + 3112,, [4,J91, with the lower state correlating with the 2P3j2 + 2P1,2 atomic hmit. This peak has an estimated half-width of 300 A as compared with 40 A for the D’ - A’ band. Together with the limiting intensity ratio (table I), these widths indicate that this band is 5% as strong as D’ 4 A’. For 1, this ratlo was 8.7%. The 35.50 A wavelength implies that the transition ter531

CHEMICAL PHYSICS LETTERS

Volume 84 number 3

mmates on the lower state =lOOO cm-l above the Br + Br* asymptote, in I, th~.~energy was 800 cm-l. The other state or states emitting in the 3400-3800 A regon may be analogues of the states responslb!e for the three peaks III the 4600-4800 A regon in I?_ As far as we can tell the enusslon IS entiely

throughout

diffuse

this repon.

3 3. ZUOO-3000

A and assIgned thus structure

as H + X. At

the resolution of fig. 1A we see only the overall mtensity distribution of the bands and not the fine structure they measured. However, we thmk It hkely that the structure m the 2000-2200 A regon of our spectrum, whether lffuse or &Crete, belongs to a single system. As Ar is added, the structure to the blue of 2200

A disappears,

leavmg a single peak. At Hugh reso-

lution we have seen (but not yet analyzed) dm.xete

in both thus peak and Its neighbor at 2270 A. Lf we assume that the exctted states involved m these two bands are Ion-pair states havmg R, = 3.2 A,

structure

we calculate their energtes as 57000 (58500), 60500 (62000), or 63000 (64500) cm-l, accordmg to whether they termmate on X, A (A’), or B. These energes are around the proper reson for states wluch correlate with Br- + Br+(lD). However, the analysis of Haranath and Rae yielded I”, x 52000 cm-l for the H state, which would require a much smaller RI: value of =2.7 A (comparable to that of the excltad valence states) Such a small R, value seems surprismg, part~culady in view of the low vlbratlcnal frequency of 120 cm-l obtained for the H state. To resolve these questions we mtend to reexamme these systems at high resolution usmg our single-notope sources.

In the limit of infiuute

Besides the 2200 and 2270 A peaks there are peaks near 2370,2510,2650, and 2760 A, aiI of which persist to some extent at the lughest Ar pressures we have investigated. Verma has assigned some bands in the 2590-2660 A regon to a transition terminatmg on the B state [8] _His w: value of 108 cm-l is considerably smaller than the =150 cm-l found for the D’ and E states, and the implied internuclear distance (z3.9 A; from the Franck-Condon parabola and the know-n [20] potenhal curve for the B state) is much

A regzon

We thmk there is evidence of at least SLXelectromc bands m ad&tion to D’ + A’ m the 2000-3000 A reBon. The structure m the 2000-2200 A region 1s remiruscent of some bound-free transltlons m I, and the inert-gas hahdes [ 1 ,161. For Br2 alone HarGath and Rao [ 1 l] reported lscrete structure 111the region below 2100

15 December 1981

P& the 2200 and 2270 A

larger. In our high-resolution, single-isotope spectra we have found a few features which may not belong to the D’ -+ A’ system. However we have not obtamed an altematlve asslgrunent. At the 200 Torr pressure of our sources these peaks are already quite weak, so it IS possable that these other systems are better studied at much lower Ar pressures_

4. Conclusion The emission spectrum of Br, m Ar shows evidence of at least 11 electromc transitions, which probably ongmate from 10 of the 20 pre&cted Ion-pair states In some cases these states have been characterized quantitatively; m others only qualitative conclusons can be drawn at present. It seems hkely that f%rther h&-resolution work will yield precise information about several more of these states and their transitions.

In addmon it should be pomble to study the low-lying ion-pour states through sequential two-photon expenments (e.g. E f B + X) l&e ‘those used by Kmg and coworkers for I, [21,22] . However, such expemnents will require conslderably shorter probe laser wavelengths around 3400 probably

A, since the lowest ion-pair

states

he near or sltghtly above D’ at %49000

cm-*.

Acknowledgement This work was supported by the Vanderbilt Umversity Research Council.

peaks almost van& (table l), which means that in effect these systems are quenched by Ar. However, the

excited states are so far above D’ that thermal equ&bratlon v&h the latter would yield vanishingly small Boltzmann factors, so tlus quenching could still be in the form of electromc conversion to lower ion-pair states. 532

References [ 11 J. Telhnghuaen, m. Apphed atomrc colhsiin phyacs,

VoL 3. Gas lasers, eds. H S.W.Massey, B. Bederson and E.W. McDaruel (Academic Press, New York), to be published

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CHEMICAL PHYSICS LETTERS

[21 M. Dregelmann, HP. Crrenetsen, K. Hohla, X.-J. Hu, I31 r41 [51 161 171 181 191 [lOI r111 r121

J. Krasmski and KL Kompa, Appl. Phys 23 (1980) 283. J. Telhnghuisen, Chem. Phys. Letters 49 (1977) 485. A L. Guy, KS Viswanathan, A. Sur and J. TelhngJmrsen, Chem. Phys. Letters 73 (1980) 582 KS. Vrswanathan, A. Sur and J. Teilmghutsen, J. Mol. Speciw. 86 (1981) 393 P. Venkateswariu and R D. Verma, Proc. Indian Acad. SCI. 46 (1957) 251. P Venkateswarlu and R D. Verma, Proc. indtan Acad. Sa 46 (1957) 416. R-D Verma, Proc. Iadran Acad. Ser. 47 (1958) 196. K Wreland. JB Telhnghutsen and A_ Nobs. J. MoI. Spectry. 41 (1972) 69. P. Venkateswarlu, Proc lndran Acad. Sa 25 (1947) 138. P.B.V. Haranath and P-T. Rao, 3. Moi Spectry. 2 (1958) 428 Y.V Rao and P. Venkateswarlu, J. Mol Spectry. 13 (1964) 288

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1981

A. Sur, A.K. Hui and J. TeiIirqhukn, Rev. Sci tnstr. 50 (1979) 1136. [14] A. Sur and J. TeUmghlwn, J. MoL Spectry. 88 (1981) 323. [ 151 P. Berwanger, KS. Vlswanathan and J. Telllnghuisen, I. MoL Spectry., submitted for pubiication. [16j J. Teilinghutsen, Chem. Phys. Letters 29 (1974) 359. II 71 J. TelhnUnken, G. Plchkr, W-L. Snow. BfE. HilIard and [ 131 M R McKeever,

RJ.E-&n.Chirn.Phys.Sb (1980) 31i 1181 J. Tellinahutsen. A.K. Hays J.kf. Hofiman and G C. Tine, J. khem. l?hbi 65 (1076) 4473. [ I/] H. Hemmati and G_J Collins. Chem. Phys. Letters 67 (1979) 5. 1201 R.F. Barrow.TC.Clark,

Mol. Spectry.

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K.K.Yee,

1.

5 1 (1974) 428.

[21] hf.D. Danyluk and G.W. King, Chem. Phys. 22 (1977) 59. [22] G.W. King, LM. LIttlewood and J.R. Robins.Chem. Phys. 56 (1981) 145.

533