SVL fluorescence spectra of sulphur dioxide.

Chcminl Physics 8 (1976) 155-164 0 North-Holland Publishing Company

SVL FLUORESCENCE I. Multiple vibronic

SPECTRA

OF SULPHUR

origins in the 315-308

DIOXIDE.

nm region

R-J. SHAW, J.E. KENT and M.F. O’DWYER Deportment of Clrenristry, hIomd

Urriversity, Clayton. Victor@ 3168. Arrstralia

Received 1 June 1976

SVL fluorescence spectra far bands A, El and C of the x-f system of sulphur dioxide arc prescntcd and analyred in terms of multiple vibronic origins Careach of these bands. The vibrationul structure of the emission from tbc different orifim varies considerably. Horvcver the vibrational content of the origins thcmsclvcs cannot be deduced. On the other hand the data show tint the excited sbtc is not linc;lr. In fact it has much the umc OS0 an@ as in the ground state.

1. Introduction

probably due to the occurrence of other interacting

The first excited singlet system of sulphur dioxide has inspired a number of cxperimcntal and theoretical studies ranging over the last forty years. However, even a fragmentary understanding of the photo-physical processes involved in this system is only beginning to emerge. The absorption spectrum [l] in the 380-260 nm region begins with a scrics of weak bands at the long wavelength end, the first few of which have been clearly identified as belonging to the ;ijB, - %‘At transition [?I _To higher energies the bands become perturbed,

triplet states [3,4]. Further to the violet, another distinct absorption system begins around 340 nm, initially with a series of weak bands, that runs into a succession of sharp, stronger peaks, labelled A, B, C, _..by Clements [5] and later modified by Metropolis [6] _Fig. 1 shows this portion of the SO, absorption spectrum. Although the current interpretation of the available experimental data on the 340-760 nm system is far from satisfactory, there has arisen a number of general conclusions which will no doubt provide the basis for a more complete understanding of the system. These can be summarised as follows:

WAVELENGTH Fig. 1. Low resolution absorption

spectrum of the i-2

(nm)

system of sulpbur dioxide. The main vibronic fearures arc labetted A to 0.

156

R.J. Show et al./SVL jluorescence specrra of sulphurdioxidc. 1

(i) The extinction coefficient (E,, a 200) would suggest the transition is spin allowed and so connects the singlet ground state with an excited singlet clectronit state. In accord with this, the transition is traditionally labellcd X-R presuming. of COUISC,that only one excited state is responsible for the transition. (ii) The observed isotope shifts between the prominent features of the St60, and Sta02 absorption spectra [7] indicate an origin (or origins, provided that if more than one excited electronic state is involved, they have similar vibrational frequencies) at about 28000 cm-‘. (iii) The Franck-Condon (FC) maximum is some 5000 cm-* above this origin(s), suggesting a rather large change in geometry. (iv) A magnetic rotation spectrum (MRS) is observed [S,9] as is also a Zceman effect [lo] _The origin of the inferred angular momentum is by no means clear. (v) Partial rotational analysis of some of the vibronit bands [I l] seems to clearly indicate C-type bands, corresponding to B, vibronic symmetry. These analyses indicate that the vibronic bands are perturbed since much of the rotational fine structure cannot be assigned. The geometry calculated from the rovibronic data, despite fairly large error limits, indicatCs a dccrease in the O-S-O angle and a slight Icngthcning of the S-O bond length. (vi) Emission spectra, lifetime measurements and pressure dependent studies of the latter [1,12-151 show that the lifetime is much longer than expected from integrated absorption measurements. The Iifetime also varies from band (vibronic) to band and is efficiently quenched at higher pressures with producticn of triplet and ground state molecules [ 16,171. (vii) While the vibrational assignments of both Metropolis [6] and Brand and.Nanes [7] incorporate most of the prominent vibronic features of the spectrum, they are by no means unique. Neither assignment can adequately csplain all tbc observed elperimental data and as such they cannot be considered the final word on the matter. Confusion still surrounds two of the most basic properties OFthe excited state, namely the geometry and the electronic symmetry. Hardwick and Ebcrhardt [9] claim-that their MRS data is consistent with a linear excited electronic state while Dixon [Ia] concludes that the rotational analyses require a bent excited state. As discussed above the vibronic symmetry

of the state is apparently B,, implying an excited clectronic state of either Bt or A-, symmetry. Comparison of the geometry obtained From the rotational analyses [ll] and the theoretically calculated geometries of Hillier and Saunders_[ 121 has led some authors [ 11,181 to suggest that the A-X system of SO, involves a vibronically induced transition to an A-,&ctronic state. In the hope that the situation might be further clarified, we decided to attempt to measure single vibronic level (SVL) fluorescence spectra from as many bands of this system as possible. The spectra from levels D, E, F and G have previously been reported [20] and will be discussed again in the following paper [I! 11. The present paper concerns the identification and nature of the vibronic origins as determined by the SVL fluorescence spectra, in the vicinity of the A, B and C bands. Though not as yet, this information may in the future lead to a reliable vibrational assignment of this system because of the vastly different vibrational content of the emission spectra from different vibronic orlgms.

2. Experimental The apparatus used to obtain the rpcctra has been described previously [ZO] Extinction coefficients of bands A, B and Care up to an order of magnitude lower than those of bands D through G, necessitating USCof wider excitation bandwidths and higher SO? pressures in order to obtain measurable emission intcnsities. All spectra described below were recorded for an sxcitation bandwidth of 1.6 nm and a pressure of 2 mtorr. Admittedly, these arc only approximate SVL conditions, but nevertheless all features of the spectra can still be confidently assigned. The spectra have been analyscd in terms of the ground state vibrational energy levels calculated from the vibrational constants given by Shelton et al. [22]. In discussing the spectra, the symmetric stretching mode is referred to as u1 (a1 symmetry), the bending mode as u2. (at) and the antisymmetric stretching mode a.~“3 (b,). The measured frequencies are accurate to about 10 cm-l for sharp, clearly defined bands and about 20 cm-t when overlapping and weaker transitions are involved. The band positions given in cm-l have been vacuum corrected.

3. Results The absorption spcctrunl irldi~atcs that bands A, R and C appear to consist of a number of vibronic levels (see fig.. 5). The emission spectra detailed below bear this out ad also indicate that these levels difkr markedly in their vibratiun~ content. Fig- 2(a) shows tic SVL fluorescence spectrum excited at a frequency of 3 19 15 cm-l (-uI~inlurn absorption for band A) together with its assignment. Tables of the band n~JXii1~~ in cm-r and the detailed assignments may be found in ref. [73]_ Only the main progressions Irave been i[ldi~ated in I?g. 2(a) ~ltlrou~~ many other bands, that do not form part of these progrcssions, lnrve also been assigned. Some of these bands, indicated by arrotvs in the figures, involve emission

from origins of lower frequencies thtln the level popu-

either the 31925 cm-r -+ it + w2 transition, which should be 1667 cm-1 below the origin, or the 31325 cm-1 -+ 3~~ transition, which shoufd bc 1534 cm-t below the origin. Since the absorption spectrum suggests that band A may be a superposition of more than one vibronic transitiorl, WCdecided to vary the excitation wavelength over band A, hoping to see some variation in the relative intcn~ties of the various lines. Fig. 2(b) shows the spectrum excited at 3 1840 cm-‘, together with ils assjgnment_ The relative ~ntens~tics do in fact change dramatically. The band at 30306 cm-* is reduced in in~el~sity while that at 30174 cm-l is enhanced. The reverse trend is shown iTthe excitation is-at frequencies

greater than 31925 cm- ‘, indicating that the line at 30306 cm-t comes frown an origin above 3 1925 cm-l, while tllat at 30174 cm-’ comes from one below 319X cm-t_ Assuming both transitions arc to vlfv2, the bands c~t~po~a~e to origins at 31842 em-’ and 3 1973 cm-t. The afternative assignments to 3~2 are ruled out on the basis of the rclativc intensities with varying excitation frequency. Wi~iththese two additional origins all the other bands in both spectra can bc assigned except for a number of weali bands, which

lated by the excitation process. The intensities of these particular b;tnds arc pressure dependent, ~o;~~rining the cxisrcnce of vibrational rctaxation prior to emission. In all casts of rehxed transitions, only those to the zero point lcvcl and to one quan turn of it in the ground stale arc observed. None have been observed to u2, v3 or any combination bands. In addition, no emission bands have been observed that can be att~buted to the excitation of hot ground state levels. The very strong band at the high frequency end of the spectrum in fig. 2(a), is due to scattered exciting light logftfter with resonance emission to tbe zero point level of the ground state. This resonance emission appfars to be about 50% of the tuta~ band intuitsity although it is difficult to allow for some inevitable r~~bs~rption. The two strong bands at 30773 cm-t and 236?9 cm-l, that arc apyroXim;ltCly Il5OCm-1 and 2300 cm--l to the red of the excitation fre~~Iency respectively, arc assigned as transitions to one and two quanta afvf in the ground state. They extrapolate to a common origin at 3 1325 cm-1 . Transitions to three, four and five quanta of tire same vibration can be subsequently assigned, with a nraxirnum Fobs - yGlrcfor this progression of 7 cm-‘. &rwever, the majority of the observed fluorescence bands cannot be assigned on the basis of this particular origin. For example, there are two well resotvcd bands at 30306cm-f and 30174~m-~ that lie 1619cm-1 and 1751 cm-l respectively to the red of the 31925 cm -1 origin. These frequency measurements arc suffi-

and 3 (b) together with their assignments. Again the main origin was located with reference to the very strong transition to ~1 in the ground state, this origin being at 32170cm-‘. A prominent line at 30453 cm-r is too far away from the calculated position of the 32170cmW1 -*v~+-zJ~ transition to be so assigned. The relative intensity of this band varies with e~ci~ti~n ~vavelen~th such that the transition appears to originate from a level below the 32 170 cm-r origin. cunning the transition temlin3tes on v1 + vz in the ground electronic state, a second origin can be located at 32120 cm-l_ For such an origin, many other ob-

ciently accurate to preclude either band belonging to

served emission bands can be satisfactorily assigned..

CXIbe accounted fur by introdu~inE a fourth origin at 3 1775 GIII-~ _ There also seems to be same relaxation prior to emission a5 a progression in Yt from an orrgm at 3 I 7 17 cm-i is observed _This origin (noted by Metropolis 167) is too far to the red of the excitation frequency to be excited optically. ThuSband A seems to consist of four ~broni~ origins at 317X%,31847_, 31325 and 31973Cm.-l. In an analo~oIts way, the SVL fluorescence spectra excited at 32 I 50 cnr- i and 32240 cm-I (both in the ~cin~~~ of band B} have been analysed . They are sho\vn in figs. 3(a)

RJ- Show et aL/SVLfluorescence

158

WAVENUMBER

27.000

28.ooo

I

Fig. 2. (a) SVL tkoxxcncc

&cited at 31840

spectrum

[CM-‘)

30.000

2Tm

I

spectra of mlphur dioxide. I

I

,

I

I

31.000

I

I

I

32.aoa

31973 -4,.nv, 31973- rwl* 31717 -IN

of sulphur dioxide excited at 31915 cm-t _ (b) SVL fluorescence

spectrum

of sulphur dioxide

cm-‘. Relaxed transitions are indicated by an arrow. These sometimes overlap other wigned lines.

R.L Shatv et oL[SVL fluorescence pctrn

of sufphur dioxide. I

159

WAVENUMBER(CM-‘)

WAVENUMBER

(CM-‘1

Fig. 3. (a) SVL fluorescence spectrum of s!lphur dioxide excited at 32150 cn-‘_ (b) SVL fluorescence spectrum o~sulphur dioxide excited at 32240 cm-‘. Relaxed transitions are indicated by an arrow. These sometimes overlap other zsigned lines.

Likewise, comparison of the relative intensities from the two band B spectra (figs. 3(a) and 3(b)) suggests that the band at 31712,31226,31098 and 30581 cm-l come from an origin above 32170 cm-t. When assigned to vz, 2u,, v1 and it + ~2 they give rise to a common origin at 32250 cm-l. Thus band El seems to consist of three vibronic origins at 32120,32 170 and 32250 cm -1. Again there is evidence of vibrational relaxation, involving the levels discussed previously in reference to the band A spectraviz., 31778,31842 and 31973 cm-t. The assignments for the band B spectra show one feature not in common with those of the band A spectra.

That is, transitions

involving 2v3 occur quite frequently. They are fairly weak, but were not involved at all in the band A spectra. Figs. 4(a) and 4(b) show the spectra, and their assignments, excited at 32389 cm-l and 32450cm--L, the former frequency being approximately the absorp tion maximum of band C and the latter being to somewhat higher energy. Once again each spectrum consists of a number of welt resolved bands that are dominated by one particular band lying approximately 1150 cm-t to the red of the excitation frequency. For 32389 cm-’ excitation this infers an origin at 32388 k-1 and

R-J. Show er aL/SVL fluorescence spec~n of xulphur dioxide. I

WAVENUMBER mu0

I

(CM’) 3o.wo

2%~

1

I1

mm

1

I

32481- PA-WJI WAVENUMBER

(CM-‘)

Fig. 4. (a) SVL fluorcsccnce spectrum of sulphur dicsidc cxcitcd at 32389 cm-‘. (b) SVL fluorescence spectrum of sulphur dioxide excited at 32450 cm-‘. Relaxed tmnsHl(lns arc indicated by an arrow. These sometimes ovcrl~p other assigned lines.

many of the bands in the spectrum can be satisfactorily assigned relative to this origin. Comparing the relative intensities of certain bands in the spectra excited at

to lower levels, all of the bands in these two spectra (figs. 4(a) and 4(b)) can be assigned. There is no firm evidence for the presence of v3 in

32389 cm-t and 32450 cm-t, notably those at 31948, 30805 and 29660 cm-r, it is seen that these bands probably correspond to emissibn from a higher energy origin than 32388 cm-t. With the assignment of the

the band C spectra. The line.at 31018 cm-t could correspond to the 32388 cm-l + p3 transition as well as the 32170 cm-l + q relaxed transition. This, however, is the only assignment where v3 could possibly

bands to ~2, v1 +u2 and 2u, + u2, they extrapolate to a common ongm at 32469 cm-t. Using these two origins and allowing for some vibrational relaxation

be involved and since the 32170 cn-t + 2 q transition is also observed, it is doubtful that u3 is at all involved in either of the band C spectra.

R.J. Showcf al./SVLfluorescemespecrraof mlphurdioxide.I 4. Discussion Table 1 summarizes the main features of the SVL fluorescence spectra from the different vibronic origins contained within the A, !3 and C bands. The relative

Table 1 Vibrational

content of em&s@ from vibronic oridns in the A, B and C bands of SO2 A-X system Vibronic

Progcssions

origin (cm-‘)

pRS.%t

Length

Approximate

(within spectrum) 3

VW

31778

4

W

1 5

w VW

5 4

yw v\”

31842

3 1 4 4 2

YI,’ 1” m m w

31925

5

YS

31973

2 3 3 2

VI” VW vw yw

32120

2 5 3 3 3

VW m m m 1”

32170

intensities of the various progressions are broadly classitied as either very weak (v-w), weak (w), etc. By far the strongest progressions are those involving rrul, being about an order of magnitude more intense than any other progression, and these are simply labelled as very strong (~5). The following major features emerge from a perusal of table 1. 4. I. Origins showing otdy strong rwl progressions

re-

lativc intensity

31717

161

(I) 31925 cnrpr [band A): This vibronic origin shows only a progression in at. It has five members and the iotensity decreases monotonically from II = 1 to n -5. Such an intensity distribution is expected where the emission ongmates from a vibronic level containing at most one quantum of ur and where the displacement of the v1 normal coordinate between the two electronic states is small. The absence of a progression in o2 implies the displacement for the normal coordinate associated with v, is close to zero, i.e., all the intensity is in the 0-O band. (ii) 32170 cnPl (barrd 8): This origin shows only progressions in W, and 2n3 + IIY~,with the former very strong and again monotonical!y decreasing for II = I to II = 3 while the latter is weak and has only two members besides 2~3 _The same FC remarks apply as for band A for both of these progressions. (iii) 3I 717c1n-1: This origin is populated through vibrational relaxation and necessarily shows only very weak emission bands. Only the progression 3 1717 + rrur is detected for II = 1,2 and 3.

4.2. Or&insfor ~hiclz than 12~~progressiom

nv,

progmsiorn are srrollger

32250

32388

31778 cm_1 and 32388 cm-1 (barld C): Both these origins show progressions in&v, which are much stronger than the other progressions present, namely nv2, v1+ w2, 2vr + rrv2 and 3r~r + ~2 (the latter only in the case of the 31778 cm-l origin). For the 32388 cm-t origin the ratio of intensities (ny, to the other progressions) is very much greater than for the 3 1778 cm-l origin. The nvt progression for band C goes to n = 5, with n =3 missing. FC considerations suggest a quantum or two more of ut in C than for bands A and B

RJ. Show et oL/SVL @orescence

162

4.3. Origins for which nvl progressions are weak or missing, yet muI + nv2 progressions are present (i) 31973 cm-’ and 32469 cm-l: These two origins show no nvl progressions, but only ones in u2 and u2 in combination with up to three quanta of v1 _This pattern seems to indicate a strong Duschinsky effect [24] with regard to these origins (see below). (ii) 31842 cm-l, 32120 cm-I and 32250 cm-l : These three origins shaw nz~, nu2 and muI + nv2 progressions with the latter two being stronger than the first one, especially for the 32120 cm-l origin. This origin possibly also shows activity in u3, although nearly ail the lines so assigned have alternative assignments. Again the intensities in these spkctra suggest a considcrable Duschinsky effect. Thus, so far we have shown that each of the A, B and C bands of the x-x system of sulphur dioxide consists of a number of vibronic origins, and that these origins show different types of SVL emission spectra. Fig. 5 shows the locations of these origins in a medium resolution spectrum of the A, B and C band region. They are tabulated in table 2 and compared with those obtained from previous vibrational analyses of absorption spectra. Ii should be noted that Metropolis [61

spectm of sulphw dioxide I Table 2 Comparison of published absorption frequencies (cm-‘) the SVL fluorescence origins Metropolis

Brand

[61

[41

31717 31778

31717

31717

31763 31786

31778

100 BO 7s

31862

31842

90

31936

31925 31973

31840 31855 31940 31991 32033 32119 32140 32198 32235 32276 32371 32438 32479

SVL

Designation

250 20 25 30 40 400 25

32181

32170 32250

B

32394

32388

C

32469

Relative intensity

A

32120

with

15 600

120 15

has assigned a number of subsidiary absorption maxima as vibronic origins, that have not shown up in the SVL spectra. It is possible that these levels absorb and emit very much more weakly than those observed, but it is more likely that they are not true vibronic ongms. Some comments on the most recent analysis by Brand and Nafies [7] arc perhaps in order. This analysis contained no quanta of u;, that is certainly needed for some of the vibronic bands ir_order to be compatible with the emission spectra. Also, even though bands A and B were not specifically assigned, the most obvious continuation of the Brand and Nanes assignment would be for band A to be 2~; + 7~; and band B to be 3~; t 5~;. These assignments are obviously not compatible with emission spectra involving only very strong progressions in vi, with the very much weaker one involving 2~‘; for band B as well. Finally, some of the vibronic origins observed in SVL emission are not included in the Brand and Nanes assignment. Our attempts at alternative vibrational assignments, have

been unsuccessfulso far. 316

314

312

WAVELENGTH

no

(nml

Fig. 5. Medium resolution absorption spectrum of sulphur dioxide in the region of the A, B and C bands. The ~IIIOWS in@ate location of the vibronic origins deduced from the SVL tluorescencespectra.

Franck-Condon factor calculations can be used in conjunction with assigned SVL fluorescence spectra to infer the vibrational energy content of the emitting origins. However, such calculations are usually based on a number of assumptions that must be appreciated before a reliable comparison of theory and experiment

R.J. Shaw et ol./SVL fluorescence

is possible. Some of the assumptions that are particularly relevant are that: (i) the electronic transition moment is independent of the vibrational energy content of the excited vibronic level (see [4] for a discussion with regard to the lowest triplet state of SO,), (ii) the Duschinsky effect [24] is not operative; (iii) the transition is dipole allowed, that is, not vibronically induced and there are no interference effects due to the fact that the transition is allowed by more than one mechanism [25]. Such effects can result in very different patterns for absorption and emission spectra, and (iv) the electronic tranistion is not noticeably perturbed by the presence of other nearby electronic state(s). The transition moment for the A-2 transition in SO, will be largely dependent on the O-S-O bond angle (compare 141). Hence, if the vibronic origins detected within the A, B and C bands contain many quanta of the bending mode, vI, the assumption of a constant transition moment will need to be examined when calculating vibronic intensities. However, all the v2 progressions observed in emission are quite short,

spectra of mlplrur dioxide. I

163

If the excited electronic state is B,, then only zero or at most even quanta of v3 should appear in either absorption or emission. Any transition involving Zu, must be very weak. However, if the excited electronic states

suggesting that the emtttmg orlgms contain at most only a few quanta of u2 _

is Al, then first order Herzberg-Teller selection rules [27] require that all bands observed in absorption should contain one quantum of u3. Emission from the same vibronic bands should then produce two sets of progressions of totally symmetric vibrations, one built onto zero quanta of IJ~ and the other built onto two quanta of v’;_ Comparison of the appropriate Herzberg-Teller matrix eIements gives the well-known result (27) that the intensities of the latter set of progressions should be twice the first assuming no change in the U3 frequency between the two electronic states. As mentioned above, transitions involving 2~‘; in emission are not observed at all from bands A and C. In band B they are weak compared with the corresponding transitions built onto zero quanta of U’;. Thus, at first sight, the SVL fluorescence spectra suggest that the system corresponds to the dipole allowed t B, state rather than a vibronically induced transition to a ‘A, electronic state. However, if the Frequancy of u3 changes in the excited state, the above ratio of intensities changes, decreasing to about 0.5

As referred to above, the fact that the progression intensities for the mUt +nUz progressions are nothing like simple products of themvt andrrUZ progression intensities implies a strong Duschinsky effect. The normal COordinate associated with u; may well contain much more of the bending symmetry coordinate than does that associated with u;. We have carried out Franck-Condon calculations [26] in the normal way and while these point to some general features as mentioned above, we feel that a fit with excited state displacements and frequencies will not be obtained unless the Duschinsky effect is included. The rotational analyses [I I] allegedly reveal type C contours, consistent with B, vibronic symmetry. The permissible electronic symmetries are either Bt or A2 provided that in the latter case, the transition is

when u’/u” = 0.5. In particular, Meter [I l] has suggested that ~3 decreases from 1362 cm-’ to 600 cm-t in the excited state. Hence, it is difficult to decide if the rela tive intensity of the transition to 29 in the band B spectrum indicates a B, or an A2 electronic state. Hamada and Merer [I I] have also suggested that the SO, A-Z system contains two electronic states. The first of these being the AZ, vibronically induced state with an origin at approximately 358 nm and the second being the allowed Bt state, with an ongm at approximately 320 nm. The region of the spectrum at wavelengths shorter than 320 mn is then a mixture of these two states. The difficulty of finding a definite vibrational assignment and the complex lifetime data suggest that the system is highly perturbcd. Furthermore, the superficial view of our FC analysis of the

vibronically included by the b2 vibration, ~3. The intensities of transitions involving v3 will be markedly different for thesetwo possible electronic states. In discussing these v3 transitions, we can safely neglect any hot bands in U3 since the frequency of this mode in the

SVL spectra indicates that the vibronic bands in the A, B and C band region (-32000 cm-l) cannot have vibrational energy contents as high (4000 cm-‘) as required for an electronic origin or origins at 28000 cm-l that is suggested by the isotope shift data [7] _

ground state is 1362 cm-’

While there seems little doubt that the lA2 state has

and is only 0.1% populated.

its origin at 28 000 cm-l, the A, B, and C bands seem to derive a significant portion of their intensity from a state much higher in energy. Certainly, a IBl state originating at ~320 nm, could fulfil this role. Having presented the data For the vibronic levels near bands A, B and C and having concluded that a vibrational analysis of the absorption spectrum is still not viable, what positive conclusions can be made from the SVL fluorescence data? Firstly, the progression lengths in all the SVL fluorescence spectra are fairly short, some going to five members, but ivith an average length of three or four. Wh reference to those involving v2, th% means that the equilibrium geometry pf the excited state cannot be linear at lcast for these vibronic bands. If it were the uz progressions would be much longer and stronger. A rotation of normal coordinates in the excited state could hardly bring the progression lengths down to three or four, and certainly bands A and B must have very nearly the same bond an& in both the ground and excited states. Secondly, FC considerations suggest the presence bf a Bt state at -320 nm, with the weaker absorptions between 340-320nm, being attributed to the A, state. The interaction between two such states will no doubt be significant, producing complex absorption and emission properties. Conventional treatment of the system is then out of the question and calculations of the vibrbnic interaction, as discussed by Robi;.xn and Langhoff [28] will be necessary to unravel the radiative processes.

Such calculations

are at present

under

way

and will be the subject of a future report.

References [I 1 SJ. Sxickler and D.B. Howcll, J. Chcm. Phys. 49 (1968) 1947. [2[ J.C.D. Brand, V.T. Jones and C. DiLuro, trosc. 40 (1971) 616.

J. Molcc. Spcc-

]31 J-CD. Brand, V.T. Jones and C. DiLauro, J. hiolec. Spcctrosc. 45 (1973) 404. ]41 J.P. Vikcsland and S.J. Stticklcr. J. Chcm. Phyr 60 (1974) 660. ]51 J.H. Clcmcnts, Phys. Rev.47 (1935) 224. [61 N. hic~ropolis, Phys. Rev. 60 (1941) 295. I71 J-CD. Bnnd and R. Nancs, J. Molec. Spcctrosc. 46 (1973) 194. 181 P. Busch and F.W. Loomis, Phys. Rev. 55 (1939) 850. [91 J.L_ Hndwick and W.H. Ebcrbardt. private communication. ]lOl D.R. Humphrey, 29 th Symposium on Molecular Structure ;~nsSpectroscopy, Columbus, Ohio (1974). IllI Y. Hamada and A-T. blerer, Can. J. Phys. 52 (1974) 1443; 53 (1975) 2555. [‘II K.F. Grcenough and A&I;. Duncan, J. Am. Chem Sot. 83 (1961) 555. 1131 t1.D. Mettcc, J.Chcm. Phys. 49 (1968) 1784. 1141 R.B. Calon and A.R. Gangadharan. Can. J. Chcm. 52 (1974) 2389. 1151 L.E~ Brus ;Ind J-R. hIcDonald, J. Chcm. Phys. 61 (1974) 97. 1161 T.N. Rno and J.G. Calvcrt, J. Phys. Chcm. 74 (1970) 681. I171 A. ttorowitz and J.G. Calvcrt. Int. J. Chcm~ Rinct~ 4 (1972) 191. 1181 R.N. Dixon and M. HalIE. Chcm. Phys. Lett. 22 (1973) 450. 1191 I.H. llillicr and V.R. Saunders, Molec. Phys_ 22 (1971) 193. 1201 J.E. Kent, X1.F. O’Dlvyer md R.J. Sllaw, Chcm. Phys. Lett. 24 (1974) 221. 1211 RJ. Shaw, J.E. Kent and h1.F. O’Dwyer, Cbcm Phys. I8 (1976) 165. [22] R.D. Shelton, AIL Nielsen and WJI. Flctchcr. J. Chcm. Phys. 21 (1953) 2178;22 (1954) 1791. v31 RJ. Sluw. PI1.D. Thesis, Monasb University. Australia (1975). [24] I’. Duschinsk$. Acta. Physicocbim. USSR 1 (1937) 551. [X] D.P. Craig and C~J. Small. J. Cbem. Pbys. 50 (1969) 3927. 126) IJ. Kim, Physics Department, Univcrsily oTRochcster, USA., private communication. (271 G. Henberg and E. Teller, 2. Physik Cbcm. I321 (1933) 410. [28[ CA. Langhoff and G-W Robinson, Cbcm. Pbys. 4 (1974) 34.