SVL fluorescence and duschinskii effect in the S1 state of pyridine

Chemical Physics 54 (1981) 375-382 North-Holland Publishing Company

SVL FLUORESCENCE AND DUSCHINSKlI EFFECT IN THE S I STATE OF PYRIDINE

Yoko MOCHIZUKI, Koji KAYA and Mitsuo IT0 Department of Chemistry, Faculty of Science, Tohoku Univers!ty, Sendai, Japan Received 9 June, 1980

SYL fluorescence spectra from principal bands in the S1 state of pyridine-hs and &s were measured. A large displacement of the normal coordinate origin of ~6~and the Duschir;skti rotation of vloa and zqba were established.

1. Introduction

Pyridine is a prototype of azaaromatic compounds with nonbonding electrons. Because the molecule has been known to be non-luminescent, the information on excited electronic states has been obtained only by the absorption spectrum. Sponer and Stiickien [l] were the first to investigate the absorption spectrum or pyridine vapor, they assigned the lowest excited singlet state (S1) as ?rrr*(Bo). Thls assignment has been revised later by Kasha [2] to be nn’(B1). In our first paper concerning pyridine [3 3, Si states of pyridine-hs (py-hs) and 4s (pyd5) were investigated by the aid of the absorption spectra in various matrices at low temperature and pre-resonance Raman scattering, and the strong vibronic interaction of the vlea mode (CH out-of-plane vibration, a2 symmetry) between S1 and Sz was established. Recently, many of azabenzenes such as pyrazine [4], pyrlmidiie [S] and even triazine [O] were foucd to emit fluorescence from S,(nn*) accompanied by a sharp vibrational structure. Pyridine is not the exceptional molecule. Yamazakl and Baba [7] observed the fluorescence by exciting the S1 state of pyridine. Because they used broad exciting light from Xe arc lamp, the obtained spectrum had only faint structure. In our previous paper [8], we reported the well resolved fluorescence spectra from S1 states of py-hs and ds by using the second harmonics of a YAG laser excited pulsed dye laser as a sharp light source. 0 301-0104/8

i/0000-0000/$02

In this study, we report the results of SVL fluorescence of py-hs and 11* by exciting several principal bands of the So -S1 absorption. Two important conclusions were drawn from this work: the large displiacement of the origin of the Ysa(aal)mode between So and S1; and the Duschinskii rotation of the z’lOa and VI&,modes in Si .

2. Experimental The experimental setup of the SVL fluorescence miaasurement is essentially the same as reported in the previous paper. Pyrldine vapor at 10 Torr was excited with the second harmonics of a dye laser (Rh6G) pumped by a frequency doubled pulsed YAG laser. The exciting light with 3 cm-’ line width (fwhm) was tuned exactly to the peak po&ions of individual vibronic bands of +he So-S1 absorption. As a SHG crystal of the visible dye laser, a temperature controlled ADA or an angle tuned ADP’ crystal was us&d. The fluorescence spectrum was. obrained by a Naiumi 0.75 m monochromator in its second order. The detected signal by an HTV R-542 photomultiplier was averaged by a Brookdeal boxc;lr integrator (9415 and 9425). Commercially available spectral grade py-hs and 4s were purifies by repeated distillations and stored in a dark place. The sample was degassed by freeze-thaw cycles prior to the experiment.

Sfl6 North-Holland Publishing Company

’

376

Y_ Mochizuki et cl. / SVL fluorescence and Duschinskii effect

3. Results and discussion

6aCal)

12(a,)

9a(a,) A

In this section, we first discuss individual fluorescence spectra from selected vibronic levels. Then discussion will be concentrated on the two important topics, potential displacement along vha and VI2 modes and the Duscbinskii effect of vlOa and Vr&_ In fig_ -.- 1, the absorption spectra of py-& and py+ in the So-S1 (IAr-‘Br, mr*) region are shown. O-O absorption bands are located at 34769 -and 34950 cm- r for py-hs and ds, respectively. Most of the intense bands are attributed to parallel-type bands and can be assigned as fundamentals, overtones and combinations of the totally symmetric vibrations vsa (542 cm-’ in py-h, and ~535 cm-’ in py-ds in S1) and v12 (995 cm -’ in py4rs and 957 cm-’ in py&s in Si)_.Besides, four moderately strong bands appear at 139,33 1,672 and 864 cm-i above O-O for py-?zS _ The 139 cm-i band has a parallel-type band contour and has been assigned as the O-2 transition of Y1eb (br symmetry) by Jesson et al. [9]. The 672 cm-’ band which also has a parallel-type contour was assigned by us as a combination band of the 139 Cm-’ and Vea band. The 33 1 and 864 cm-’ bands

280

265 “m

W1;5 .

5

titi 605cni’ 582

lOa(a2)

2?5 FI,~_ 1_ hficrophotometer of gaseous py?rs and ds-

260

tracirqs of So-S1

285

absorption spectra

1218 886

16a(a2)

-+ -a16t(bl)

+

374 405 329 371 Fig. 2. Vibrational modes active in the absorption and fluorescence spectra of pyfzs and 4,. Frequencies in wavenumber. are those in the ground state.

exhibit perpendicular-type contours with in-plane transition moments. These bands have been assigned as a O-l transition and a combination band with vga of a vibronically active mode. Tentative assignment was made on that vibration as vlOn (a2 symmetry) on the basis of the result of preresonance Raman scattering, which suggested vlOa as the key vibration of the Sl-S2 iibromc interaction in analogy with pyrazine [4] _In pyiIs, the 6a& band splits into doublet by Fermi resonance interaction in the gas phase. The 12 1 and 270 cm- ’ bands correspond to the 139 and 331 cm-r bands of py-hs , respectively. It should be remembered that the absorption spectrum of py-hs or -& has only one false origin at 33 1 or 270 cm-’ above O-O_ In the present work, these principal bands of pyhs and 4s were selectively excited and the observed fluorescence spectra were found to display spectral patterns characteristic of the vibrational modes involved in the emitting levels. In fig. 2, several vibmtions active in the absorption and fluorescence spectra are shown.

3.1. Fhorescence levels i----

ti 1030 1006

-

888 69.0

’

spectra from individual vibronic

3.1.1.

O-O excitation The fluorescence spectra of pyJls and c1s from

the O-O level are shown in fig. 3. As already reported

Y. Mochizuki et al. / SVL fluorescence and Duschinskii effect 6212’: 6212: w-h,

12:

34bOO

34769

12:

I

34 ,950

r

I

1

I

3

4

32600

6a012’ “M n= I

crri’

3

I

n= I

I 34000

I

2.

33600

PY-ds

fjaf

1I

I

2

I 33000

3

y

I 32000

Fig. 3. Zero level fluorescence spectra of py-hs and -ds_ The ordinate indicates the intensity in arbitrary units without correction for spectral sensitivity of photomultiplier and monochromator.

in the previous paper [S] , the vibrational frequencies in the spectra exactly coincide with those of py-hs and 4s in the ground state (So), which clearly evidenced the spectra to be the genuine ones. The fluorescence spectra constitute mostly of fundamentals, combinations and overtones of the totally symmetric modes, ~6~ and viz. The intensities of combination bands 6a’: 12: and 6a!$12? are stronger than those of fundamentals 6a’: and 12’: _This implies that the intensities of 6a!,6a$ and 122 are stronger than that of the O-O band which is obscured by scattering of the exciting light. This spectral feature is analogous to the absorption spectra where 6al, 6ai and 12; bands are more intense than the O-O absorption band. The long progressions of vsa and vr2 modes suggest that the potential displacements of those modes in Sr state relative to Se are quite large. We will discuss this subject in detail later. In addition to them, three bands

377

appear at 374,800 and 864 cm-’ from O-O in py-its and they were assigned as 16a?(as), 16b;(br X br) and lOa’:( respectively. As seen in fig. 3, the lOa: band is much stronger than the 16a: band and combinations with vsa and v12 are developed from lOa’:_ The appearance of these two false origins, lOa: and 16a:, in the zero-level fluorescence spectrum is in contrast with the absorption spectrum where there is only one false origin at 33 1 cm-’ above O-O. The same situation arises in the zero-level fluorescence spectrum of pyOs and the corresponding frequencies respectively. ofvrca and v16a are 690 and 329 cm-‘, This asymmetry between the absorption and the zerolevel fluorescence spectra as to the number of false origins suggests the occurrence of the Duschinskii effect for vrca and vIea. This will be discussed in a later section. 3.1.2. 6ah excitation The fluorescence spectrum of py-I2s followed by the excitation of the 6aA level in S 1(542 cm-’ ) is shown in fig. 4. The spectrum mainly contains the fundamentals, OVertOneS and combinations of Vea and v12 _The long progression of the v12 mode has an intensity distribution similar to that in the zero level fluorescence spectrum. Because of scattered exciting light, the intensity distribution of the 6aA(n = 0, 1 ___)progression was estimated from the progression members 127 6aA. The intensity of each band was found to decrease in the order 6ah > 6a: = 6a:. It is expected from a simple calculation of Franck-Condon factors in the harmonic oscillator approximation that if the inten-

72?6a,

649,120,

“=

1

:

:

1 6a5, n= 1

I 35311 Fig.

35

00

4. Fluorescence

34

00

spectrum

I 33000

Cd

from the 6ab level of pysIs

..

378

Y. Mochizukf et ul. / SVL fluorescence and Duschinskii effecf

sity ratio of XA to X$ bands equals an integer m, the intensity distribution of the Xi progressicn shows a minimum at XL, where X stands for a vibration of ar symmetry. As the 6ai/Ga$ intensity ratio in the absorption spectrum is nearly 2, the band 6a: is expected to give a minimu,-- among 6aj!, progression members. The discrepancy between the observation (6a$ I* 6a: > 6a$) and the expectation may be exp!ained by the overlap of 6a&9a! (-12 18 cm-’ ) wirh the weak 6a: band. In the bsorption spectrum of py-& vapor, two prominent parallel-type bands appear at 5 12 and 560 cm” above O-O which have been assigned by us as the Fermi doublet between 6ah and an overtone o.f a nontotally symmetric vibration. The fluorescence spectra excited at these two bands resemble each other with respect to the intensity distribution of the fia; progression. This confirms our assignment of the Fermi doublet. 3.i.3.331 ct?z-‘(pv-hs) and 270 crn-‘(py’ds) band excitatdovs At 331 cm- ’ above the O-O band in the absorption spectrum of py-hd (270 cm-’ in py+), there exists I! perpendicular-type band corresponding to a 0-- 1 transition cf a vibration of a2 symmetry. The fluorescence spectrum obtained by exciting the 331 cm -’ band of py-hd , which is shown in fig. 5, obviously has more complex features compared with the zero-level fiuorercence spectrum. This complexity Is attributed to the appearance of the three origins, which arc located at 374 and 880 cm” shifted to the red fram the exciting light. Because 374 and 880 cm -' are the ground state vibrational frequencies of vrell and urea of as symmetry, hereafter we refer to thcsc origins as Y ’ I Y r IGar and Y’ J . Y is the a2 vibration in the !: J state whose vibrational frequency is 331 cm-‘. Each of these origins is followed by the progressions of the totally symmetric modes u6,, and ~~2, and the intensity distributions of the progressions are slmiinr to those of the zero-level fluorescence spectrum. The appearance of the two transitions, Y’ Ifial and Y’ 1Odr , means that the upper state a2 vibr. on Y does not have one-to-one correspondence to nny slngh vibrational mode of a2 symmetry in the ground state. It should be assigned as a mixed mode 1 Oa

of -k

!ruu gronod sts!e ribrst:ons,

P,~,, and vIea. me

mixlug of vibrational modes of the same symmetry

L 35ioo

34 '00

33bOO

Cd

32 0

Fig.5. Fluorescence spectrum from the O-O + 331 cm-’ band of pysIs.

species during the electronic excitation is known as the Duschinskii effect and the appearance of the cross sequence bands Y’ ?Oal and Y’ 16ar afford:; a definite proof of the Duschinskii effect _In the ex.:itation of the O-O + 270 cm-’ level of py-ds , the same phenomenon as mentioned above was observed in the SpeCtWn. The intensity ratios of Y’ 16a J to Y r 1Oa are 3.0 and -1 .O for py& and &s, respectively. The analysis of the Duschinskii effect will be made in a later section. J

3.1.4. 239 cm-’ (py-hs) and 121 cm-’ @y-ds) bnnd excitations The assignment of the absorption band at 139 cm”(py-hs) or I21 cm-‘(py-d,) above the O-O band has been a puzzling problem [ 101. The fluorescence spectra from these levels ars essentially the same as the corresponding zero-level fluorescence spectra except for the reIative shift of the whole spectra by 139 cm-’ (PySrs) and 121 cm” (pyil$). One cannot draw a conclusion from this result on the controversy whether the bands shotrId be assigned as cold bands (O-2 transition) of Yreb or hot bands of Unknown vibrations. The definite answer will be obtained by the measurement of the absorption spectra in a supersonic cold beam, which is now in progress in our laboratory. 3.1.5. Combination band excirntions There are several unassigned bands in the So-S1 absorption spectra of py-hs and -ds in the wavelength region exceeding 600 cm- 1 above O-O. Among them, the 672 and 864 cm’ r bands of py-hs have been assigned ._ by us as the combination bands. of ~~(542 cm-‘) + 139 cm-‘ and Ye, * 331 cm-l, respectively.

Y. Mocfiizukiet al. / S VL fluorescent‘: and Duscftinskii effect Absorpfion

The fluorescence spectra from these levels exhibit the spectral feature characteristic of the combination banl! excitations and confirms the previous assignmeclts. 3.2. 77re dispibcernent between So and Sz

379

I Go d Y .

of the or&ins of v6a and v1i

C^o

2.0 1.0

d Y

0

In the absorption

and fluorescence spectra of pyridine, two totally symmetric vibrations V6a and v,a were found to form long progressions. From these data, one is able to estimate the displacement of the equilibrium position along that normal coordinate between Se and Sr. Let us denote a totally symmetric vibration as X whose displacement of the equilibrium position between Se and Sr is dx. Then, in a displaced harmonic oxcillator approximation, the intensity ratio of O-l to O-O transrtion is grven by /(O-1)/1(0-0) p = (w;l/w;c)“2

= (21’2p2/l

+ P2)(4r2cw’x/h)

.

O-O

d& (la)

Here wk and C& represent the vibrational frequencies in wavenumber of the vx mode in Sr and Se, respectively. By the aid of the observed absorption intensity ratio of the 1-O to O-O transitions, dx values were evaluated for Vea and v12 nnd they are given in table 1 in comparison with those of pyrazine and pyrimidine. One can easily And that the evaluated displacements of the &a mode in py-h5 (0.52 amu* ‘2A) and pyd5 (0.48 amu’ “A) are particularly large among those of azabenzenes. By using these displaced harmonk oscillator wavefunctions, the intensity distributions of Ye0 and v r2 progressions in the individual fluorescence spectra were calculated in comparison with the observed data. The results were found to be in good agreement with one another. In fig. 6, comparisons of the calculated and experimental

PY-h5

“60 “12

PY+i

“6n “12

pyrazine pyrimidine

“68

“68

of

fluorescence

I

(lb)

Table 1 Dispiocements of the origin

ext.

I

4

ext. fluorescence

Fig. 6. tlomparison of the observed and calculated rclativc intensit:if,distribution of 6nr(m = 0, 1) progressions in the absorption and fluorescence spectra. * indic?tcs the ~crlup of the 6a3 and 6ai9ap bands. 5~ Observed value. e-w CMculated + .~lue.

results f0.r. the 6a progressions are shown for the fluorescence spectra excited at the O-O and Grak

the normalcoordinate of totally symmetric vibratij)ns between Se and $1 ,.________ .__.-. __ JX(amu”3 A) w’(cm-t) w”(cm-I) f(O- I ,/1(0-O) 542 995 535 957 582 641

601 1030 582 1013 956 681

2.4 1.0 2.0 1.5 1.0 1.0 -

0.52 0.26 0.48 0.32 0.34 0.31 _---

.

fluorescenceand Duschinskiieffect

Y. Miichizuki et al. /SVL

.380

py-hs . So it c-an be concluded that vsa and via modes in SI and So can be treated as independent one-dimensional displaced harmonic oscillators without any influence of the Duschinskii effect.

3.3. Duschinskii effect of vlOa arkd v16a vibrations In section 3.1, the experimental evidences supporting the existence of the Duschinskii effect of vloa and v16a were shown. Here, the Dushinskii effect will be treated quantitatively according to the theories derived by Small [1 I] and Metz et al. [12] _It was pointed out first by Duschinskii [133 that the normal coordinate Qf of a molecule in an excited electronic state is in general rotated and displaced relative to that of a molecule in the ground state, Qf:

spectrum excited at Y1 level and (4) vlea vibrationas the most effective mode of the ground state vibration for the vibronic interaction between S1 and Sa as studied by preresonance Raman scattering. Let us consider a transition moment B&,=1 from the kth vibrational level of the electronic ground state to the Ith vibrational level of the excited electronic state and expand it either in terms of the ground state normal coordinate Q” or the excited state normal coordinate Q’;’ Mgk,d = t

x

(goberls”>ckll~) c

e=s


i

I(a~ja42j-')o I e” ) et- e”,

X (kliQ~ 112)

(4)

=(gOleriso)~klll) QI=

CDijQl i

+di 9

+ c

di is the displacement of the origin of the ith normal coordinate between the two electronic states and Dii is the vth component of the unitary matrix between normal coordinates Q; and Q;-‘_In pyrhiine, because the rotation of the normal coordinates was found to occur only among the vibrations of the same symmetry, it seems reasonable to assume the geometric structure in S1 and S,-, to be the same (Csv)_ Then di is zero for nontotally symmetric vibrations such as vlOa and vi6=_ Pyridine has three vibrations (VIaa. VIea and Vr& of a2 symmetry. Among them, vloJ and v16s form false origins in the fluorescence spectra. So, the mixing of the vibrational modes may be assumed to occur only between vlOa and v16= by neglecting the participation of v1 75 _Under these assumptions, the normal coordinate of the vibration Y whose frequency is 331 cm-’ in the Sr state of py-h5 (270 cm-’ in py-d,) is described by (3) Here Q: denotes the counterpart of QG due to the mixing of Q:‘,, and Q’:,,. Four experimental findings should be explained consistently in terms of the Duschinskii effect. They are: (1) two false origins in the zero-revel fluorescence spectrum, (2) &gle false origin in the absorption spectrum. (3) cross sequence bands in the fluorescence

(so KaH/aQ:)o Ie” )

c

e=s i

g - E”,

X
Here Ilk> and [[I) represent thevibrational wavefunctions of the kth quantum level of Q,r’ and the Zth quantum level of Qi, respectively. In the optical transitions such as O-O, k-Z transition of a totally symmetric mode and u-u transition of a nontotally symmetric mode, the first term of eq. (4) determines the intensity. On the other hand, the intensity of the O-l transition of a nontotally symmetric mode is determined by the second term which comes from the vibronic interaction. As far as the Sr state of pyridine is concerned, the vibronic interaction of the a2 symmetry vibration can be safely assumed to occur dorninantly with S2 (l Ba, m-r*) [3] _Then, the state e” in the second term of eq. (4) is restricted to Sz. Under this approximation, the relative intensity ratio of the two faIse origins lOa: and 16a: in the zerolevel fluorescence spectrum is given by Ilo,‘: -I 16$ =-

_

Vloa(O IQion I 1) =

I

V,,,
VlO,

V 16Z3 II

I 1)

I

2 46a -77, WlOa

IGo, = (Sl I(aH,aQ:o,)o

(5) IS,)

,

Y. Mochizuki et

al. / SVL fluorescence and Duschinskii effect

~$0~ and w:sa are the frequencies of vlOa and vIea vibrations in So which are known to be 880 cm” (690 cm-‘) and 374 cm-’ (329 cm-‘) for py-ils (py-~‘~), respectively. From the observed intensity ratios of 5.0 and 33 for py-h5 and dS, the relative magnitudes of the vibronic interaction I Vlo$V16, I were estimated to be 3.43 and 2.79 for py-h5 and ds_ This implies that the vibronic interaction between S1 and S2 is dominated by the vlOa vibration in terms of the ground state vibrational mode. This concIusion derived from the zero-level fluorescence spectra is in good agreement with the previous conclusion obtained from the preresonance Rarnan study that among three a2 vibrations of the ground state moIecuIe, the Raman intensity of vlOa vibration exhibits enormous resonance enhancement. in contrast to the appearance of the two false origins in the fluorescence spectra, there appears only one false origin at 33 1 cm-’ above O-O of py& (270 cm-’ in py-ds) in the absorption spectrum,

381

which we have denoted as Y1 . The search for the counterpart of Y’ as denoted Z’ up to 1000 cm-’ above O-O resulted in the conclusion that the transition OZ’ is by far weaker than OY’ -0 denotes a vibrationless state in the ground state. This situation can be simply described by &y’/[oz1

= I vy/v,

I= w’z/w;

2> 1

(6)

in the excited state normal coordinate representation. Eq. (6) indicates that in terms of the excited state a2 vibrations, the vibronk interaction between S1 and S2 is contributed mostly by the vibration Y. A question arises; what is the normal mode Y or Z in the excited state? To answer this question, it is necessary to determine the Duschinskii coefficient Dij. Dii values are determined by the intensity of the cross sequence bands, Y1 16a, and Y’ lOal , in the fluorescence spectrum from Y’ level of py-?zs or -ds. The intensity of the cross sequence band comes from the first term of eq. (4) and is determined by the

Fig. 7. Possible vibrational modes of uy and YZ in S1 in comparison with the ground state a2 vibrations vloa and ~16~.

382

Y. Mochizukl et al. / SVL fluorescence

and Duschinskii

effect

transition moment (So lerl S, > and the Franck-Condonfactor(16a1 IIY’)or~lQalllY1).Then,the intensity ratio of Y * 16a1 to Y1 1Oal is given by

titative treatment of vibronic interaction between S1 (nn”) and S2(7r7r*) seems to be necessary.

IIY 16a#Y110q

4. Conclusion

= I(16at

IIY’ P/(lOaI IIY’ )I*

(7) Because thu OZ’ absorption band is not observed, the frequency of the Z vibration wL is not known. It was assumed to be equal to G&,. This seems to be an appropriate approximation because w; is almost equal to G.&,~. Under this assumption, the absolute values of Df/ were evaluated from the observed intensity ratios of 3.O(py-hs) and 1 .O(py-d,). ID, I I - 0.89 * l&t

1’0.73

I

lDll

I = 0.47 for py-hs ,

lOi

; - 0.68 for py&

.

It should be noticed that the cross sequence band can be observed only Jn the allowed electronic tran&ion like the So-S, transition of pyridins. In the case of a forbidden trnnsition like the So-S1 transition of benzene, the appearance of ihese bands is prohibited and the evaluation of D+ or equivalently the proof of the occurrence of the Dus&inskii rotation, Is difficult. From tic evnluated D,/ values, it is con: eluded that the vibration Y of pyhs in the S1 state has dominantly :he character of ulGDwith a considerable contribution from +OG, whereas in py-ds tt160 and ulOa mix almost equally to form Y. The normal mode analysis of the out-of-plane vibrations of pyridins was investipted by KakiuchJ et al. [ 141. By the oio or L matrix obtained by them for the ground state pyridine, the L matrix in SI can be evaluated from the known D/l by L = Ls, 0-1 , 93) 81

l

However, because D,, were obtained in their absolute vaiuts, two possible forms of vibration exist for Y or I, acr iding to the relative blgn of D1 I and D12. in fQ. 7, &he possible two modes of Y and Z vibration in !!h BIG schematica~y shown in comparison with those of UIO~and Q,Q, in rhe ground state. The vibronic __. jn$er,.ztlon of Y mode bstwcen S1 and S2 is known lo be larger than that of 2 mode as discussed previously. So, In c;rder lo select lhc two choices, a quan-

Through the SVL fluorescence study, valuable information on the Sr state of pyridine was obtained. Especially, a large dispIacement of the origin of v6a in S1 r&lative to So and tile Duschinskii mixing of vlOa and vr6n vibrations in S, were established. Acknowledgement

Stimulating discussions with Drs. Mikami and Udagawa are greatly acknowledged. This work was partly supported by a grant-in-aid for scientific research from the Ministry of Education. References [I1 H. Sponer and H. Stiicklen. 9. Chem. Phys. 14 (1946) 101. [2] M. Knsha. Discussions Faraday Scrc. 9 (1950) 14. [3] Y. Mochlzuki. K. Kaya and M. Ito, J. Chcm. Phys. 65 (1976: 4163. 14 J L.M. Iiogan and I.G. Ross, J. Chem. Phys. 43 (1965) 2903; A. Frad. F. Lohrnani. A. Tramer and C. Tric, J. Chem. Phys. 60 (1974) 4419; I. Suzuka, N. Mikami and M. lto. J. Mol. Spectry. 52 (1974) 21; Y. Udogawo. M. Ito and I. Suzukn, Chem. Phys. 46 (1980) 237. IS] A.E.W. Knight, C.M. Lawburgh and C.S. Parmenter, J. Chem. Phys. 63 (1975) 4336. [6] A.E.W. Knight and CS. Parmenter. Chem. Phys. 43 (1979) 257. [7] Y. Yamazoki and H. Balm. J.Chem. Phys. 66 (1977) 5826. [S] Y. Mochizuki. K. Koyn and M. Pto, J. Chem. Phjs. 69 (1978) 935. [9] J.P. Jcsson, H.W. Kroto and D.A. Romsay. J. Chcm. Phys. 56 (1972) 6257. [ 101 G. Herzberg, in: Electronic spectra and electronic structure of pofyotomic molecules (Van Nostrand. Princeton, 1966) p. 554. [ 111 G.J. Small. J. Chetn. Phys. 54 (1971) 3300. [ 121 F. Metz, M.J. Robey, E.W. Schlog and F. Dorr. Chem. Phys. Letters 51 (1377) 8. [13J F. Duschinskii, Actn Physicochim. URSS 1 (1937) 551. 1141 Y. Kokiuchi, M. Akiyamn. N. Saito nnd H. Saito. J. Mol. Spectry. 61 (lY76) 167.