- Email: [email protected]
Laser excited S2 → S1 and S1 → S0 emission spectra and the S2 → Sn absorption spectrum of azulene in solution
CHJZMICALPHYSICS LIrïTERS
VoIume 58. namxber 4
15 October í978
LASER EXCITED S2 ds1 AND S1 + S. EMISSION SPECTRA AND TIIE S2 +S, ABSORPTION SPECTRUM OF A2!UFANE IN SOLUTION Meir ORENSTEIN, Sol KIMEL and Shammaï SPEISER Department of &emïstty, Techm-on-bael íitstiture of Technoiogy, Haifa, laael Renived 21 June 1978 We preses the SI -1 So fluorescentespectrum,between740 and 940 om, of azulenesohitions(íOm3 M ìn methanol) excited wïth a @svitched ruby laser.The nitrogerrlaser excïted Sz + SI fíuorescencespectrum,between 700 and 930 om, is also reporteb Tbe transientSt + Sn s@etrmn between 500 and 6.50 runwas studïed,usingsynchmnousnitrogenlaser and dye Iasere?rätatïon The Ss C’B~(3)) state of arulenewas found to be Iocatedat 45.500 cm-’ and the crosssection 025 of the transïentabsorption & +Ss is estimakd to be 3 x lO_” cm2/mo!ecuk.
1. Introdnction The dominant fluorescente emission of azuïene is associated wïth the S2 -f SO transition El-33 _ The
S1 + So and S2 + ,551 transitiom exhibit low fluorescence quantum yïelds. Varïous other aromatic hydrocarbons also violate Kasha’s rule [2] but the fast nonradiative relaxation of higher excited singlet states s2, s3 .._ causes the S2 -f So emissîon of these compounds to have a much lower quantum yieid than the “normal” Sl + Sg fluorescente. The extremely weak Sl -f SO emïssion of azuiene was studïed in the solid [4] and gas phase [5] and in solution 163_ The S2 + SI fluorescente spectnun was measured in solution [7,8], ín the gas phase [S] and, recently, in a ShpoYskiï matrix [9] _ Conventiond absorption studies of azulene [3,10] are complemented by transient absorption measurements of Sl + S, transitions [I ?] and by emission spectroscopic investigations. A combination of results obtaïned wïth absorption and emïssÏon technïques serves to better specify the energïes of the various excited singlet states S, , which permits a comparïson with cakulated vahres [ 12]_ In this way Jortner and co-
werkers determined the energies of the states SI, S2, S3 and 34 of azulene and also the absorption cross sections of the transitions SI + S2 [SI, SI + S3 and The solutïon studies 163 were performed over a limited spectrr
@on;
the low accuracy was due to
scattered light. We present here additional spectro582
scopic results on azulene in solutions. The SI 4 Sg and $2 + SI fluorescente spectra were measured over a wider spectral range than before and with higher spectral resolution. The transïent S2 + S, absorption spectrum is reported here for the fïrst time.
2. ExperÎmentaI ‘Ihe Q-switched ruby laser and the detection system used for monitoring its output power and energy have heen descrïbed prevïously 1131. The SI + So fluorescence signals were measured, perpendicular to the excíting laser beam, with a Bausch and Lomb 0.5 m monochromator equipped with a Hamamatsu R 777 photomultiplier CJoMeCtedto a Keithley 610 B electrometer_ Filter solutions of CuSO4 were used for attenuation of the excitation intens@. The system parameters were checked by comparing the S2 * Sg emission, resultïng from cons&utïve two-photon absorption SO * Si + S2, with that obtaïned after one-photon excitation to S2 1141. As expected, we have observed a quadratic dependence of the fluorescence signal on the excitation intensity. For examining the S2 + S1 emission a nïuogenlaser based spectrofluorimeter was assembled. The fluorimeter (see fig_ 1) was calïbrated agaïnst published fluorescente data [14]. Emission spectra were recorded wïth a Bausch and Lomb 0.25 m monochromator (resolution 0.5 run). When necessary, eïther to scan beyond 800 nm or to ïncrease the
Volume 58. number
CHEMICAL
4
PHYSICS
LETTERS
15 October 1978
focused by a cylindrical lens into the 1 X 1 X 3 cm sample cell and excited the molecules present along the narrow focal line. The second part of the beam was utilized to pump a dye laser (Molectron DL 200), thus generating 5 11swide pulses synchronous with the exciting nitrogen laser. The dye laser probed the excited species which are produced in the sample cell. At each dye Laserwavelength the laser photometer, operating in its absorbance mode, measured the
fs
averaged
value of the ratio between
the transmitted
laser intensity and that of ‘ibe incoming beam. Fluctuations in the exciting intensity could be corrected for by simultaneous monitoring of the nitrogen Laseroutput, using a 5% split-off fraction of the beam. The performance of the transient spectrophotometer was tested by recording the SL + S, absorption spectrum of rhodamine 6G (R6G) in ethanol and comparing it with the reported spec:rum dye
E5g.1. Laserspectrofluorimeter.Ns LASER - Pnolectron UV 400 nitrogen laser, C - collimator, B - 5% beam splitter, S - sample ce& AC - achromatic condenser, F - filter, A attenuato:, M - Bausch and Lomb 0.25 m monochromator or Spex 1402 double monochromator, MD - motor drive, PM - Hamamatsu R 777 photomultiplier, PD - PIN photodiode, AMP 1 - Molectron LP 143 amplifiershaper, AMP 2 hfolectron LP 141 amplirir-shaper, XY-R - XY recorder, LASER PHOTOMETER - Molectron model LP 20 (operating in its signal mode and averaging 50 pulses per second).
spectral resolution, a Spex 1402 double monochromator was employed_ The high repetition rate (50 Hz) of the nitrogen Laser(Molectron UV 400) enabled the use of an averaging procedure with a Molectron LP 20 photometer thus obtaining high signal-to-noise ratios. Transient absorption spectra were recorded using the set-up described in fig. 2. The nitrogen laser beam was split into two parts. One part was collimated and
IPI-
Azulene (AR Koch-Light Laboratories) and methanol (spectrograde) were used without further purification.
3. Results ami discus&n Fig. 3 displays the Sl--+ So and the S2 + S1 emission of azulene (10m3 ,U in methanol). The data points represent the intensities of the S1 + SO emission resulting from Q-switched ruby laser excitation. They 2
c =
_ 8-
E7-
I.
lo
A
5o%as
Fig. 2. Excited statespectrophotometer. N2 LASER, B.S. C, PD and LP (laser photometer) as in fe_ l_ DYE LASER Mokctron model DL-200, CL - cylimiricallens,ID - iris diaphragma, RC - reference cell, SC - sample cell, E Keith&y 610 B ekctrometer, Rl and R2 - X-t recorders, AMP - Molectron LP 141 ~%mpnfW-shaper. The laser photometer, operating in its absorbance mode, was averaging 20 pukes per second.
Fig. 3. Fluorescence spectra of axulene lob3 M in methanol. Data points - ruby laser excited St -, Se emission. Full curye - nitrogen laser excited S2 + St emission. The two curves are not drawn to scale.
Volume 58, munber4
CIiEMIC_4L+ PHYSICS LETl-ERS
agree with gas-phase results obtained in a point-bypoint fashion using simiir excitation [S] and also with observations of fluorescence in solutions following excitation with a mode-locked ruby laser or a second-harmonic mode-locked neodymium laser [6] A better resolved spectrum was obtained by Friedman and Hochstrasser in a low-temperature study of azulene-naphthalene mixed crystals [4] _ The full curve in fig_ 3 is rhe S2 + S1 emission spectrum recorded in a continuous fashion with the nitrogen-laser spectrofluorimeter (fig. 1). It shows the same viironic structure as obtained at low resolution in the gas phase IS] _ It is better resolved than the reported solution spectra [7,8] and covers a wider spectral region_ Similar vibronic features were observed, at nigher resolution, in a low-temperature Shpol’skii matrix [9] _ The similarity between the gasphase and soIution qcctra indicate fast non-rzliative transitions in the gas phase from the Sz(u) states to Sz(O), the ground vibrational level of Sz_ The S2 + S, absorption spectrum of azulene is shown in fig. 4. It displays a maximum at 550 nm with a shoulder at 520 run. Because of the fast radiationless decay of vibrational energy the nitrogen laser excitation at 337-l nm prepares azulene molecules in Q(O) at 28300 cm-’ _ The edge of the transient absorption band is at 580 run (: 17200 cm-‘), thus we are exciting to an electronic level which is about 45500 cm-l above the ground state_ This corre-ponds to the S5 (lBl(3)) state, located at 45000 cm-’ according to conventional spectroscopy [IO] _ The present results provide a better assignment of the Sg origin. It is possible to give an estimare for the cross
15 October 1978
Mnm)
Fig_4_ S, -, S, transientabsorptionspectrumof azulene low3 M in methanol.
section a25 associated with the transient absorption S2 + Ss in azulene. The relative value of aln of the Sl+ S, transition in R6G, measured with the present set-up, was first caliirated against the known value of al”, as deduced from the reported absorbance A of R6G [15].Thus
10~51azulene
= IA/eClazulene
[~co~n/Al R6G,
where E is the extinction coefficient at 337.1 run and c is the concentration. The resulting value 025 = 3 X 10ml* cm2/molecule seems somewhat high compared to cross sections of other transient absorptions in azulene [5,11]_ Combining the present results with those obtained from the transient absorption spectrum S1 + S, ]l l] and the conventional absorption spectrum [3,10] the steady state properties of the frost five singlet levels of azulene can be summarised as shown in table 1.
TabIe 1 Ener,q levels 2nd transientabsorptioncross sectionsfor azulene
State
S1 ‘B,(l) S2 ‘Al(2) S3
‘B1(2)
-41(3) Ss ‘B1<3)
s4
z
Calculated=) (cm-‘)
13900
14400
24800 33200 37800 45200
2830d) 32700 40300 45000
‘)Ref_ [121b)So-S~spectrum e&s2 tran&ion [Sl.
584
One-photon spectrumb) (=-‘I
[lo].
%~+Sn~~m[il];
Transientabsorptionspectrum energy (cm-‘)
crosssection (cm2~molecuIe)
33 600’) 36 200’) 45 sood)
5 x Id22 5 x lo-2’ 3 x 16’8
Lo-2o e) =) =) d,
dk2-rSnspectrum,presentwork.
From the similarity of the soM.k~
and gas phase
spectra we may conclude that vibrational relaxation processes within the S1 and S2 manifolds of the zzulene molecule in the condensed phase are comparable to those occurring in the gas phase are 15--100 torr pressure together with 250 torr inert buffer gas). I&is is in agreemenr w&h conclusions based on direct measurements of Lifetimes of vibronic levels of azulene molecules in the gas phase and in solution [16-181. Furthermore, we note that consecutive absorption of nitrogen and dye laser radiation enables, in a si.mpIe ftion,
to explore the vacuum UV spectrmn of
dissolved species.
[4] J.&I_ Friedman and RM. Hochstrasser, Citem. Phys. 6 (1974) 14s. [S] D. Huppert, J. Jortner and P.hf_ Rentzepis, J. Chem. Phys. 56 (1972) 4826. P.M. Rentzepis, C&em. fhys. Letters 3 (1969) 717. P.M. Rentzzpis, J. Jortn=r and R.R. Jones, Chem. Phys. Letters 4 (1970) 599. D. Huppert, J. Jortner and P&i_ Rentzepis, Chem. Phys. Letters 13 (1972) 255. C.D. Giiispie and E.C. L&n, J. Chem. P%ys_65 (1976) 4314. D-E. Mann, J.R. Platt and H.B. Kievens, J_ Chem. Phys. 17 (1949) 481. A. Bergmanarid J. Jortner, Chem. Phys. Letters 20 (1973) 8. R. Pariser, I. Chem. Phys. 25 (1956) 1112.
S_Speiserand S. Kimel, J. Chem.Phys.51
Finally, we would like to point out that this study presents, for the first time, a record of a molecular absorption spectrum originating from the S2 state. Refkrences [I ] M. Beer and
NC. Longwzt-Higgins, J. Chem. Phys. 23 (195x) 1390. [Z] GViswana th and Ii%_Kasha. J. Chem. Phys. 24 (1956) 574. [3 J J.W. Sk&an and DS. McClure, J. Chem. Phys. 24 (1956) 757.
w1
(1969)
5614; 53 (1970) 2392. I.B- Berlman, Handbook of fluorescence spectra of aromatic molecules, 2nd Ed_
320.
El61 P. Wirth, S. Schneider and F. D&r, Chem. Phys. Letters 42 (1976) 482. 1171 EP. Kppen, C-V. Sh.w& and R.L. Woerner, Chem Phys. Letters 46 (1977) 20. El81 D. Huppert, 3. Jortner and P.M. Rentzepis, Israel J. C&m. 16 (1978) 277.
585
VoIume 58. namxber 4
15 October í978
LASER EXCITED S2 ds1 AND S1 + S. EMISSION SPECTRA AND TIIE S2 +S, ABSORPTION SPECTRUM OF A2!UFANE IN SOLUTION Meir ORENSTEIN, Sol KIMEL and Shammaï SPEISER Department of &emïstty, Techm-on-bael íitstiture of Technoiogy, Haifa, laael Renived 21 June 1978 We preses the SI -1 So fluorescentespectrum,between740 and 940 om, of azulenesohitions(íOm3 M ìn methanol) excited wïth a @svitched ruby laser.The nitrogerrlaser excïted Sz + SI fíuorescencespectrum,between 700 and 930 om, is also reporteb Tbe transientSt + Sn s@etrmn between 500 and 6.50 runwas studïed,usingsynchmnousnitrogenlaser and dye Iasere?rätatïon The Ss C’B~(3)) state of arulenewas found to be Iocatedat 45.500 cm-’ and the crosssection 025 of the transïentabsorption & +Ss is estimakd to be 3 x lO_” cm2/mo!ecuk.
1. Introdnction The dominant fluorescente emission of azuïene is associated wïth the S2 -f SO transition El-33 _ The
S1 + So and S2 + ,551 transitiom exhibit low fluorescence quantum yïelds. Varïous other aromatic hydrocarbons also violate Kasha’s rule [2] but the fast nonradiative relaxation of higher excited singlet states s2, s3 .._ causes the S2 -f So emissîon of these compounds to have a much lower quantum yieid than the “normal” Sl + Sg fluorescente. The extremely weak Sl -f SO emïssion of azuiene was studïed in the solid [4] and gas phase [5] and in solution 163_ The S2 + SI fluorescente spectnun was measured in solution [7,8], ín the gas phase [S] and, recently, in a ShpoYskiï matrix [9] _ Conventiond absorption studies of azulene [3,10] are complemented by transient absorption measurements of Sl + S, transitions [I ?] and by emission spectroscopic investigations. A combination of results obtaïned wïth absorption and emïssÏon technïques serves to better specify the energïes of the various excited singlet states S, , which permits a comparïson with cakulated vahres [ 12]_ In this way Jortner and co-
werkers determined the energies of the states SI, S2, S3 and 34 of azulene and also the absorption cross sections of the transitions SI + S2 [SI, SI + S3 and The solutïon studies 163 were performed over a limited spectrr
@on;
the low accuracy was due to
scattered light. We present here additional spectro582
scopic results on azulene in solutions. The SI 4 Sg and $2 + SI fluorescente spectra were measured over a wider spectral range than before and with higher spectral resolution. The transïent S2 + S, absorption spectrum is reported here for the fïrst time.
2. ExperÎmentaI ‘Ihe Q-switched ruby laser and the detection system used for monitoring its output power and energy have heen descrïbed prevïously 1131. The SI + So fluorescence signals were measured, perpendicular to the excíting laser beam, with a Bausch and Lomb 0.5 m monochromator equipped with a Hamamatsu R 777 photomultiplier CJoMeCtedto a Keithley 610 B electrometer_ Filter solutions of CuSO4 were used for attenuation of the excitation intens@. The system parameters were checked by comparing the S2 * Sg emission, resultïng from cons&utïve two-photon absorption SO * Si + S2, with that obtaïned after one-photon excitation to S2 1141. As expected, we have observed a quadratic dependence of the fluorescence signal on the excitation intensity. For examining the S2 + S1 emission a nïuogenlaser based spectrofluorimeter was assembled. The fluorimeter (see fig_ 1) was calïbrated agaïnst published fluorescente data [14]. Emission spectra were recorded wïth a Bausch and Lomb 0.25 m monochromator (resolution 0.5 run). When necessary, eïther to scan beyond 800 nm or to ïncrease the
Volume 58. number
CHEMICAL
4
PHYSICS
LETTERS
15 October 1978
focused by a cylindrical lens into the 1 X 1 X 3 cm sample cell and excited the molecules present along the narrow focal line. The second part of the beam was utilized to pump a dye laser (Molectron DL 200), thus generating 5 11swide pulses synchronous with the exciting nitrogen laser. The dye laser probed the excited species which are produced in the sample cell. At each dye Laserwavelength the laser photometer, operating in its absorbance mode, measured the
fs
averaged
value of the ratio between
the transmitted
laser intensity and that of ‘ibe incoming beam. Fluctuations in the exciting intensity could be corrected for by simultaneous monitoring of the nitrogen Laseroutput, using a 5% split-off fraction of the beam. The performance of the transient spectrophotometer was tested by recording the SL + S, absorption spectrum of rhodamine 6G (R6G) in ethanol and comparing it with the reported spec:rum dye
E5g.1. Laserspectrofluorimeter.Ns LASER - Pnolectron UV 400 nitrogen laser, C - collimator, B - 5% beam splitter, S - sample ce& AC - achromatic condenser, F - filter, A attenuato:, M - Bausch and Lomb 0.25 m monochromator or Spex 1402 double monochromator, MD - motor drive, PM - Hamamatsu R 777 photomultiplier, PD - PIN photodiode, AMP 1 - Molectron LP 143 amplifiershaper, AMP 2 hfolectron LP 141 amplirir-shaper, XY-R - XY recorder, LASER PHOTOMETER - Molectron model LP 20 (operating in its signal mode and averaging 50 pulses per second).
spectral resolution, a Spex 1402 double monochromator was employed_ The high repetition rate (50 Hz) of the nitrogen Laser(Molectron UV 400) enabled the use of an averaging procedure with a Molectron LP 20 photometer thus obtaining high signal-to-noise ratios. Transient absorption spectra were recorded using the set-up described in fig. 2. The nitrogen laser beam was split into two parts. One part was collimated and
IPI-
Azulene (AR Koch-Light Laboratories) and methanol (spectrograde) were used without further purification.
3. Results ami discus&n Fig. 3 displays the Sl--+ So and the S2 + S1 emission of azulene (10m3 ,U in methanol). The data points represent the intensities of the S1 + SO emission resulting from Q-switched ruby laser excitation. They 2
c =
_ 8-
E7-
I.
lo
A
5o%as
Fig. 2. Excited statespectrophotometer. N2 LASER, B.S. C, PD and LP (laser photometer) as in fe_ l_ DYE LASER Mokctron model DL-200, CL - cylimiricallens,ID - iris diaphragma, RC - reference cell, SC - sample cell, E Keith&y 610 B ekctrometer, Rl and R2 - X-t recorders, AMP - Molectron LP 141 ~%mpnfW-shaper. The laser photometer, operating in its absorbance mode, was averaging 20 pukes per second.
Fig. 3. Fluorescence spectra of axulene lob3 M in methanol. Data points - ruby laser excited St -, Se emission. Full curye - nitrogen laser excited S2 + St emission. The two curves are not drawn to scale.
Volume 58, munber4
CIiEMIC_4L+ PHYSICS LETl-ERS
agree with gas-phase results obtained in a point-bypoint fashion using simiir excitation [S] and also with observations of fluorescence in solutions following excitation with a mode-locked ruby laser or a second-harmonic mode-locked neodymium laser [6] A better resolved spectrum was obtained by Friedman and Hochstrasser in a low-temperature study of azulene-naphthalene mixed crystals [4] _ The full curve in fig_ 3 is rhe S2 + S1 emission spectrum recorded in a continuous fashion with the nitrogen-laser spectrofluorimeter (fig. 1). It shows the same viironic structure as obtained at low resolution in the gas phase IS] _ It is better resolved than the reported solution spectra [7,8] and covers a wider spectral region_ Similar vibronic features were observed, at nigher resolution, in a low-temperature Shpol’skii matrix [9] _ The similarity between the gasphase and soIution qcctra indicate fast non-rzliative transitions in the gas phase from the Sz(u) states to Sz(O), the ground vibrational level of Sz_ The S2 + S, absorption spectrum of azulene is shown in fig. 4. It displays a maximum at 550 nm with a shoulder at 520 run. Because of the fast radiationless decay of vibrational energy the nitrogen laser excitation at 337-l nm prepares azulene molecules in Q(O) at 28300 cm-’ _ The edge of the transient absorption band is at 580 run (: 17200 cm-‘), thus we are exciting to an electronic level which is about 45500 cm-l above the ground state_ This corre-ponds to the S5 (lBl(3)) state, located at 45000 cm-’ according to conventional spectroscopy [IO] _ The present results provide a better assignment of the Sg origin. It is possible to give an estimare for the cross
15 October 1978
Mnm)
Fig_4_ S, -, S, transientabsorptionspectrumof azulene low3 M in methanol.
section a25 associated with the transient absorption S2 + Ss in azulene. The relative value of aln of the Sl+ S, transition in R6G, measured with the present set-up, was first caliirated against the known value of al”, as deduced from the reported absorbance A of R6G [15].Thus
10~51azulene
= IA/eClazulene
[~co~n/Al R6G,
where E is the extinction coefficient at 337.1 run and c is the concentration. The resulting value 025 = 3 X 10ml* cm2/molecule seems somewhat high compared to cross sections of other transient absorptions in azulene [5,11]_ Combining the present results with those obtained from the transient absorption spectrum S1 + S, ]l l] and the conventional absorption spectrum [3,10] the steady state properties of the frost five singlet levels of azulene can be summarised as shown in table 1.
TabIe 1 Ener,q levels 2nd transientabsorptioncross sectionsfor azulene
State
S1 ‘B,(l) S2 ‘Al(2) S3
‘B1(2)
-41(3) Ss ‘B1<3)
s4
z
Calculated=) (cm-‘)
13900
14400
24800 33200 37800 45200
2830d) 32700 40300 45000
‘)Ref_ [121b)So-S~spectrum e&s2 tran&ion [Sl.
584
One-photon spectrumb) (=-‘I
[lo].
%~+Sn~~m[il];
Transientabsorptionspectrum energy (cm-‘)
crosssection (cm2~molecuIe)
33 600’) 36 200’) 45 sood)
5 x Id22 5 x lo-2’ 3 x 16’8
Lo-2o e) =) =) d,
dk2-rSnspectrum,presentwork.
From the similarity of the soM.k~
and gas phase
spectra we may conclude that vibrational relaxation processes within the S1 and S2 manifolds of the zzulene molecule in the condensed phase are comparable to those occurring in the gas phase are 15--100 torr pressure together with 250 torr inert buffer gas). I&is is in agreemenr w&h conclusions based on direct measurements of Lifetimes of vibronic levels of azulene molecules in the gas phase and in solution [16-181. Furthermore, we note that consecutive absorption of nitrogen and dye laser radiation enables, in a si.mpIe ftion,
to explore the vacuum UV spectrmn of
dissolved species.
[4] J.&I_ Friedman and RM. Hochstrasser, Citem. Phys. 6 (1974) 14s. [S] D. Huppert, J. Jortner and P.hf_ Rentzepis, J. Chem. Phys. 56 (1972) 4826. P.M. Rentzepis, C&em. fhys. Letters 3 (1969) 717. P.M. Rentzzpis, J. Jortn=r and R.R. Jones, Chem. Phys. Letters 4 (1970) 599. D. Huppert, J. Jortner and P&i_ Rentzepis, Chem. Phys. Letters 13 (1972) 255. C.D. Giiispie and E.C. L&n, J. Chem. P%ys_65 (1976) 4314. D-E. Mann, J.R. Platt and H.B. Kievens, J_ Chem. Phys. 17 (1949) 481. A. Bergmanarid J. Jortner, Chem. Phys. Letters 20 (1973) 8. R. Pariser, I. Chem. Phys. 25 (1956) 1112.
S_Speiserand S. Kimel, J. Chem.Phys.51
Finally, we would like to point out that this study presents, for the first time, a record of a molecular absorption spectrum originating from the S2 state. Refkrences [I ] M. Beer and
NC. Longwzt-Higgins, J. Chem. Phys. 23 (195x) 1390. [Z] GViswana th and Ii%_Kasha. J. Chem. Phys. 24 (1956) 574. [3 J J.W. Sk&an and DS. McClure, J. Chem. Phys. 24 (1956) 757.
w1
(1969)
5614; 53 (1970) 2392. I.B- Berlman, Handbook of fluorescence spectra of aromatic molecules, 2nd Ed_
320.
El61 P. Wirth, S. Schneider and F. D&r, Chem. Phys. Letters 42 (1976) 482. 1171 EP. Kppen, C-V. Sh.w& and R.L. Woerner, Chem Phys. Letters 46 (1977) 20. El81 D. Huppert, 3. Jortner and P.M. Rentzepis, Israel J. C&m. 16 (1978) 277.
585