J. Phys. Chem. 1993,97, 1851-1862
7857
Rotational Effects on Singlet-Triplet Interaction of pBenzoquinone Vapor Nobuhiro Ohta,'.t Iwao Yamazaki,? Masaomi Sanekata,*ql Isamu Suzuka,* and Osamu Sekiguchir Department of Chemical Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan, Department of Industrial Chemistry, College of Engineering, Nihon University, Kohriyama 963, Japan, and Chemistry Department, Gunma University, Maebashi 371, Japan Received: February 8, 1993; In Final Form: April 20, 1993
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Fluorescence decays are observed following excitation at the individual rotational lines of the ,:O 22;, and 26; vibronic bands belonging to the SO lBt,(n?r*) transition. With excitation at the 22; and 26; bands, fluorescence lifetime varies remarkably along the rotational contour. A comparison between the fluorescence excitation spectra and simulated spectra suggests that the fluorescence quantum yield decreases with increasing J'of the excited level. The rotational state dependence of both the yield and lifetime of fluorescence at 22' and 26l of SIis interpreted in terms of the K scrambling in the triplet manifold reached via the intersystem crossing. With excitation into the vibrationless level, on the other hand, the lifetime is independent of the excited rotational level and the fluorescence excitation spectrum well agrees with the simulated absorption spectrum, implying that singlet-triplet interaction is extremely weak at the 'B1, origin.
Introduction The role that molecular rotation as well as molecular vibration plays in the singlet-triplet interaction has been thoroughly examined through the fluorescence measurements in some azaaromatic molecules whose electronic excited state S1exhibits an intermediate level structure composed of zero-order singlet state and zero-order triplet states.I4 It was found that the fluorescence quantum yields of these molecules decrease monotonically with increasing the rotational quantum number of the total angular momentum of the excited level (J'), and these rotational effects have been interpreted in terms of the K scrambling in the triplet manifold reached via intersystem crossing; Le., the number of the triplet state effectively coupled to the singlet state (Neff)is proportional to 2J'+ 1. Rotational coupling in the singlet-triplet interaction was also reported in p-benzoquinone (PBQ) by ter Horst and Kommandeur,s based on the fluorescence decay measurements across the rotational contour of the so-called vibronic band E. They concluded that Coriolis interaction between singlet and triplet states plays a significant role in the rotational coupling of PBQ and that Ncffin PBQ is given by a function of the square of the rotational quantum number K'of the excited level. Thus, the rotational state dependence of N c given ~ by ter Horst and Kommandeur for the singlet-triplet interaction of PBQ is different from that proposed for azaaromatic molecules. With respect to the mechanism of the rotational coupling of PBQ, however, further study seems to be necessary since the spectral resolution employed by ter Horst and Kommandeur was not enough to resolve the rotational fine structure. In the present study, the rotational state dependence of the fluorescence lifetime and its vibronic level dependence in PBQ vapor have been examined, based on the decay measurements following excitation at the individual rotational lines of three different vibronic bands belonging to the SO 1B1, transition. The rotational state dependence of fluorescence quantum yield also has been examined by comparing the well-resolved fluorescenceexcitation spectra observed across the rotational contour with simulated absorption and excitation spectra. By combining the rotational state dependence of fluorescence quantum yield with that of fluorescence lifetime, rotational effect on intramo-
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Hokkaido University. University. 8 Present address: Institute for Molecular Science, Myodaiji, Okazaki 444, Japan. # Gunma University. t Nihon
0022-3654/93/2091-1851$04.00/0
lecular photophysical processes and its vibrational state dependence at the IB1, electronic state of PBQ have been discussed. Experimental Section PBQ (Tokyo Kasei) was purified by vacuum sublimation. Fluorescenceintensity and decay were measured in a supersonic jet by monitoring undispersed fluorescence. The scattered light was eliminated by a filter. The jet apparatus and experimental procedures are the same as reported elsewhere.6 Briefly, a pulsed beam of a gaseous mixture of PBQ saturated at a temperature of 120 "C and the diluent rare gas (He) was expanded through a nozzle with a diameter of 0.4 mm into a vacuum chamber. A dye laser (Lambda Physik FL2002E) pumped by a XeCl excimer laser (Lambda Physik EMG 103MSC) was used as an excitation light source. The laser light, which has a duration of 10 ns and a spectral width of -0.05 cm-1 with etalon, irradiated the sample 22 mm downstream from the nozzle. Decay measurements were carried out with a digital memory (Iwatsu DM901) by fixing excitation wavelengths at a top of each rotational line.
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R&3ultS
A. Fluorescence Excitation Spectra and Fluorescence Decay. The fluorescence excitation spectrum of PBQ obtained in a supersonic jet in the region from 19 900 to 21 000 cm-* exhibits essentiallythe same vibrational structure as reported by ter Horst and K~mmandeur.~ It is considered that two transitions from SO to the l(n?r*) states lie in this region, i.e., the So lA,(nr*) and SO lBl,(n?r*) tran~itions.~-~ The optical transition to the vibrationless level of both n r * states from SOis electric-dipole forbidden. However, thevibronic absorption band at 20 045 cm-1 has been assigned as the 0-0 band belonging to the So lBI, transition.' The origin of the SO 'A, transition, which cannot be confirmed directly in the absorption spectrum, is evaluated indirectly from the excitation spectrum to be at 19 99 1 cm-l? The absorption intensity in the region of the SO '(nr*) transitions is induced mainly through coupling of out-of-plane vibrations with allowed l(r?r*) states. Fluorescence excitation spectra were observed across the rotational contour of the 0-0 (O:), 26;, and 22; bands, whose excess energies above the SO lBlg origin are 0, 126, and 926 cm-1, respectively. The latter two bands were labeled as bands A and E, respectively.10 v26 and v22 are assigned as the out-ofplane C-H bending vibration with a, symmetry and the out-of-
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0 1993 American Chemical Society
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Ohta et al.
7858 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 2-
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h 0
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Time ( p ) Figure 2. Fluorescencedecays of PBQ in a jet with excitationat different rotational lines of the 2 4 band. (1)-(4) correspond, respectively, to the excitation positions (1)-(4) shown in Figure 3.
h
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926 cm?
cn
3
T
Wavenumber (cm-' ) Figure 1. Fluorescence excitation spectra of PBQ in a jet across the
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rotational contour of the 0 , 2 6 ; , and 2 g bands belonging to the SO 1B1, transition (from bottom to top). Excitation energy at 0is at 20 045 cm-1, and the 26; and 22; bands are at 126 and 926 cm-l above 0, respectively.
e
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plane C-0deforming vibration with b3u symmetry, respectively.* The rotational spectra obtained at a stagnation pressure of 600 Torr are shown in Figure 1. It is considered that the 0-0 band is induced by magnetic-dipole transition and that the 22; band is vibronically induced electric-dipole transition whose direction is along the 0-0 axi~.~J'J Then, both bands are considered to show the same A-type rotational contour. Note that PBQ is a nearly prolate symmetrictop, and the A, B,and C axes correspond to the long axis along the 0-0direction, short axis in the molecular plane and out-of-plane axis, respectively.lOJ1In contrast to these two bands, the 26; band is expected to show a B-type rotational contour since the 26; band is assigned to a vibronically induced transition whose direction is along the short axis in the molecular plane.10 In fact, the band contour of the excitation spectrum of the 26; band is very different from the others, as is shown in Figure 1. It is also noticed that the band contour of the excitation spectrum of the 0-0 band is different from the 22; band, though both bands are expected to show an A-type contour, as mentioned above. Fluorescence decays were obtained with excitation at the individual rotational lines of these three vibronic bands at a stagnation pressureof 600 Torr. The observed fluorescenceshows a nearly single exponential decay on any excitation, and the quantum beat is superimposed on some decay. Figure 2 shows the decays observed with excitation at different rotational lines belonging to the 22; band. As is shown in Figure 2, fluorescence decay following excitation into the 22l vibronic level depends on the rotational level. A decay modulated with a frequency of 9.2 MHz is also shown in Figure 2. As is shown in Figures 3 and
e
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:
e
e
e .
e
8
e e
e
e
0.8
m e.
.. ..
I -2
-1
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1
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Wavenumber ( cm-'1 Figure 3. Fluorescencelifetime of PBQ across the rotational contour of
the 22; band. Dotted line showsthe excitation spectrum. Numbers (1)(4) correspond, respectively, to (1)-(4) in Figure 2. 4, the fluorescence lifetime markedly varies along the rotational contour both at 22; and at 26; in the range of 0.65-1.10 and 0.65-0.95 ps, respectively. In contrast to the lifetime at these two vibronic levels, the fluorescence lifetime with excitation at O,! which is -0.57 ps at a stagnation pressure of 600 Torr, can be regarded as independent of the rotational level within the experimental error, as is shown in Figure 5 . B. Simulation of Fluorescence Excitation Spectra. The simulation of the absorption and fluorescence excitation spectra across the rotational contour at low temperatures was carried out by employing the rotational constants given by Christoffersen
Singlet-Triplet Interaction of p-Benzoquinone Vapor
The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7859
1.0-
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Wavenumber ( cm-’) Figure 4. Fluorescence lifetime of PBQ across the rotational contour of the 26; band. Dotted line shows the excitation spectrum.
Y,
0.8
T
0-0
v
E t
al
-4
.-e
J
0.6-
?!
Y
LL
4
0.05 cm-l at 10 K.
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0
2
Figure6. Rotational contours of the absorption spectra of PBQ with type A (a), type B (b), and type C (c) simulated with a spectral resolution of
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0
Wavenumber (cm”)
al
c
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0.4 -
level, emission excitation spectra are expected to be the same as the absorption spectra. Otherwise, excitation spectra are different from the absorption spectra. The rotational state dependenceof the fluorescence lifetime implies that the fluorescencequantum yield depends on the rotational level excited. As in the case of pyrazine or pyrimidine, where singlet-triplet interaction belongs to the strong coupling limit, therefore, fluorescence excitation spectra of PBQ were simulated by assuming that the fluorescence quantum yield is inversely proportional to (W’ 1). i.e., by calculating the absorption spectrum after each of the intensities of rovibronic transitions concerned was multiplied by (W’+ 1)-1. Here, prime indicates the excited state. Similarly, excitation spectra were simulated by calculating the corresponding absorp tion spectraafter eachof the intensities of the rovibronic transitions concerned was multiplied by (Ki2 1)-1, The results for A- and B-type contours are shown in Figures 7 and 8, respectively. The average value of J’of the excited level, Le., 7,was also obtained as a function of excitation energy across the rotational contour of type-A or -B band. Here, ?is defined as ~J[f(J‘,Ki,Kc’,v,T)/wTJ:K,’,Ki,Ki,v,T), wherejlJ’,Ki,Ki,v, is the probability that a molecule is excited into a rotational level with rotational quantum numbers J’,K i , and K i at an excitation frequency of Y and at a temperature of T,and the sum is taken over all the appropriate values of J’, K.’, and Ki. Detailed procedures are described elsewheree6The results are shown in Figure 9.
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-1
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Wavenumber (cm-’) Figure 5. Fluorescence lifetime of PBQ across the rotational contour of the 0-0 band. Dotted line shows the excitation spectrum.
and Hollas,ll Le., A”= 0.1754, B”= 0.056 33, and C”= 0.042 63 cm-1 in the ground state, and A ’ = 0.1837, B’= 0.055 03, and C’ = 0.042 43 cm-* in the excited state. The absorption spectra simulated by assuming a temperature of 10 K with different transitiondipoles, Le., type A, B, and C bandcontours,respectively, areshowninFigure6. Itisimmediatelynoticed that thesimulated absorption spectrum with an A-type contour well agrees with the excitation spectrum of the 0-0 band (cf. Figures ICand 6a). On the other hand, the excitation spectrum at 22; is quite different from that shown in Figure 6a (cf. Figures l a and 6a). though an A-type contour is expected according to the vibronic assignment of 2%. Obviously, the rotational contour of the excitation spectrum of the 22; band is different from the simulated absorption spectrum with a B- or C-type contour. When emission quantum yield is independent of the excited
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Mscwsion The observation of phosphorescence both in the vapor phase and in solid with excitation into the excited singlet state indicates that singlet-triplet interaction is very strong in the excited state
Ohta et al.
7860 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 I
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Wave n umber ( c m? ) Figure 7. Rotational contours of the emission excitation spectra of PBQ with type A simulated with a spectral resolution of 0.05 cm-I at 10 K by assuming that the quantum yield is inversely proportional to W'+ 1 (a) and KLZ + 1 (b). of PBQ and that intersystem crossing (ISC) to the triplet states is a significant photophysical process in the excited singlet states of PBQ?JZ-14 The presence of the quantum beat in fluorescence decays also suggests that singlet-triplet interaction of PBQ is very strong, and beat frequencies are regarded as corresponding to the energy separation between singlet-triplet mixed states. Then, the fluorescencequantum yield (@f) and lifetime (v)may be expressed as follow^:^-^
h/Tf('YS-T)
9 YT
+ YdNeff
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-3
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-1
0
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(2)
where rs is the radiative width of the excited singlet state, Neff is the effective number of the triplet state coupled to the singlet state, 7 % is ~ the average width of the singlet-triplet mixed states, and YT and ys are the energy width of the singlet and triplet states, respectively. Here, Neff is given by15 Neff= 2"PT2vS-$ (3) where p~ is the level density of the triplet state coupled to the excited singlet state and V%T is the coupling matrix element of the spin-orbit interaction. The increase of the vibrational level density of the triplet state with increasing the excess vibrational energy above the 1B1, origin (AE)is regarded as responsible for the lifetime lengthening as a function of AE. In fact, the average value of 71 across the rotational contour seems to increase with increasing AE (see Figures 3 - 9 , as reported by ter Horst and Kommande~r.~ The rotational state dependence of q at 22l and 26' indicates that N,~dependsalso on the rotational level at these vibronic levels of PBQ. As mentioned in a previous paper: the following three cases are noteworthy.
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Wave number ( c m-' Figure 8. Rotational contours of the emission excitation spectra of PBQ with type B simulated with a spectral resolution of 0.05 cm-l at 10 K by assuming that the quantum yield is inversely proportional to 2J'+ 1 (a) and KL2 1 (b).
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(I) Y T > ys/Nefi In this case, q is invariant while Of is inversely proportional to Nee. Case 3 is excluded at 22' and 26l of PBQ since Tf varies across the contour. As will be mentioned below, case 2 is applicable to the singlet-triplet interaction of PBQ at the vibronic levels of 221 and 26l of the lB1, state. A comparison between the plot of q a s a function of excitation energy and the distribution of ?indicates that q both at 221 and at 26l has a tendency to increase with J', though rfdoes not vary with J'monotonically (cf. Figures 3 and 9a and cf. Figures 4 and 9b). These results are completely different from those at 00 of the 'BI, state, where rf is nearly independent of the excited rotational level (see Figure 5 ) . The variation of Tf across the contour of the 22; and 26; bands is regarded as resulting from the rotational state dependence of Nen; Le., Nem has a tendency to increase with increasing J'both at 221 and at 261. The simulated absorption spectrum shown in Figure 6a is very similar to the observed spectrum of the 0-0 band shown in Figure IC. The similarity between these two spectra indicates that the 0-0 band shows an A-type rotational contour, in agreement with the assignment of this band as the origin of the long axis polarized magnetic-dipole-allowed 'A, 1B1, transition,7.10 and that @fat the lB1, origin is independent of the rotational level. It is noted, however, that both the intensity and frequency of each rotational line of the observed spectrum are not completely identical with those of the simulated one probably because the rotational constants derived for the 221 level, which may be a little different from those at 00,were used in the simulation. The rotational
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Singlet-Triplet Interaction of p-Benzoquinone Vapor
The Journal of Physical Chemistry, Vol. 97, No. 30, I993 7861
Wavenum ber ( cm? 1 Wavenum ber (cm-’ 1 Figure 9. Average value of J’(J), across the rotational contour of the simulated absorption spectrum of PBQ with type A (a) and type B (b). Dotted
line shows simulated absorption spectra.
structure of the absorption band of 22; in a bulk gas at room temperature has been also assigned as an A-type contour. However, the observed excitation spectrum of the 22; band shown in Figure l a is quitedifferent from thesimulated absorption spectrum of type A shown in Figure 6a; e.g., the relative intensity at around the band center is much larger in the observed spectrum than in the simulated one. These results show that @f at 221 depends on the rotational level and/or that the 22; absorption band gives a different rotational contour from type A. A comparison of this excitation spectrum with the simulated spectra shown in Figure 6, b and c, indicates that the rotational contour of the 22; band is different from type B or C, and so @f at 221 is considered to depend on the rotational level. Thus, @f as well as rf following excitation into 22l are considered to depend on the rotational level. As mentioned above, the rotational state dependence of 71 suggests that Ncn increases with increasing J’. If YT is comparable to ys/Nen, Le., in case 2, @f decreases with increasing J’ (see eq 1). Otherwise, @f is independent of the rotational level, Le., case 1 is applicable. When the excitation spectrum of the 22; band is compared with the simulated spectra from this point of view, it is noticed that the observed spectrum is very close to the one simulated by assuming that @f is inversely proportional to (2J’+ 1) (cf. Figures l a and 7a). Especially, the strong band near the center, indicated by an arrow in Figure la, is well reproduced with this assumption. It is also noticed that the relative intensity at the excitation position far from the band center in the observed spectrum is not so weak as expected from the simulated one, implying that the J’dependence of @f is smaller than that expected from (25’ + l)-l dependence. When Neffis proportional to 2J’+ 1, @f is expected to be inversely proportional to 25’ + 1 only in case 3, where 71 is determined by YT and regarded as independent of the rotational level. In case 2, where both @f and rf vary with J‘, on the other hand, the J’dependence of Of will be smaller than that expected from the linear relation between 91and (2J’+ l)-l, even when Ncn is proportional to 25’ + 1 (see eqs 1 and 2). Thus, the rotational state dependence of @f and rf at 22l of PBQ is well understood by assuming that Ncff has a tendency to increase with increasing J’and that the relation between YT and rs/Ncff belongs to case 2. As mentioned previously, r f at 261 also shows a tendency to increase with J‘. We cannot confirm that the J’dependence of @f at 26l is so crucial as 22’ even when the observed excitation
spectrum is compared with the simulated one. However, the intense band near the center indicated by an arrow in Figure l b seems to be reproduced by assuming that @f is proportional to (W’+ l)-l (cf. Figures 6b and 8a), implying that @f at 261 has a tendency to decrease with J’, as well as at 221. Eventually, it is concluded that Ncn at 261 shows a tendency to increase with J‘, as well as at 221. The rotational state dependence of 01and rr observed at 221 and 26l of the 1B1, state of PBQ is very similar to that of s-triazine vapor, where the yield and lifetime of the slow component of fluorescence emitted from the initially prepared vibronic level decrease and increase, respectively, with increasing J’ As in the case of s-triazine and other azaaromatic molecules,l4 therefore, the rotational state dependenceof fluorescenceproperties of PBQ seems to be interpreted in terms of the K scrambling in the triplet manifold; various nonrigid body couplings as well as asymmetry lead to extensive scrambling of the rotational states of different K’ in the triplet manifold reached via ISC from 221 and 261 of the lB1, state. As a result of the Kscrambling, Ncnis proportional to (2J’+ l), though 27~~TV%T*, which corresponds to the energy width in which the singlet-triplet mixed states are distributed, may be independent of the rotational level. Actually, Ncn does not increase monotonically with increasing J’but fluctuates with J’both at 22l and at 26l, though Nenshowsa tendency to increase with J’. The lack of the complete linear relation between N d and 2J’+ 1 may come from the low vibrational level density of the triplet state, since the linear relation resulting from the K scrambling seems to be satisfied only when a large amount of vibrational levels of the background triplet states are located near the excited singlet states. The 3Bl,(na*) state is confirmed to be located at 18 683 2 cm-l above the SOorigin,9J0J6i.e., at 1362 cm-l below the 1B1, origin. Another 3 ( n ~ *state, ) i.e., 3Au(n?7*),is also considered to be located below the 1B1, origin. There is still controversy about the location of the latter state, but the energy separation between these two triplet states seems to be very small, e.g., the separation was reported by Goodman and Brus to be as small as 11 cm-1.9 The rotational invariability of rf and @f at the 1B1, origin implies that the interaction between the 1B1, origin and isoenergetic levels belonging to these triplet states is negligibly small. The energy separation between 00 and 26l of the 1B1, state is as small as 126 cm-1, and the vibrational level density of
Ohta et al.
7862 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 the triplet state at around the lBl, origin is considered to be not so different from the one at 221. If the rotational invariability of Nenat 00comes only from a low density of the vibrational level located at around the lB1, origin, similar invariability seems to be expected at 221. The presence of the rotational state dependence of Ncfiat 221, therefore, implies that some rigorous prohibition holds for the interaction between the lB1, origin and isoenergeticvibrationallevels belonging to these two triplet states. Then, T f at Oo, Le., -0.58 ps, may be regarded as the lifetime of the zero-order singlet state given by h/ys. ter Horst and Kommandeurs also reported the rotational state dependence of r f with excitation into the 22l level of the IB1, state, though their resolution was not enough to resolve the rotational structure. By comparing the fluorescence excitation spectra across the rotational contour of the 22; band detected with different time delays after excitation, they concluded that T f increases with J'. As far as the 22; band is concerned, the present results agree with their conclusion in the sense that 7f shows a tendency to increase with J'. However, 7fdoesnot increase steadily with J'but fluctuates with J'both at 221 and at 26l, as mentioned above. ter Horst and Kommandeur had interpreted the rotational state dependence of 71 in terms of Coriolis interaction, and the matrix element for the coupling between 22l T of the 1B1, state and background triplet manifold; Le., V ~ was proposed to be a function of the rotational quantum number K'; Le., Ncfi is proportional to Ki2. If parallel Coriolis interaction plays an important role in the singlet-triplet interaction of PBQ, as suggested by ter Horst and Kommandeur, @f is expected to decrease as a function of Ki2 in case 2, since Nefiis regarded as proportional to K8)2 (see eqs 1 and 3). However, the excitation spectrum simulated by assuming that @f is inversely proportional to Kp12 + 1 is very different from the observed spectrum (cf. Figures l a and 7b), suggesting that Coriolis interaction does not play a significant role in the singlet-triplet interaction at 22l of PBQ. Note that @f was assumed to be inversely proportional to K i 2 1in the simulation,instead of Ki2, to prevent a divergency. The excitation spectrum of the 26; band is also very different from the spectrum simulated by assuming that @f is inversely proportional to Ki2 1 (cf. Figures Ib and 8b). Thus, parallel Coriolis interaction does not seem to play an important role in the singlet-triplet interaction both at 221 and at 261 of PBQ, in contrast with ter Horst and Kommandeur. Actually, the K scramblingin the triplet manifold via ISC plays a significant role
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in the singlet-triplet interaction at 22l and 26l of the lB1, state of PBQ, as mentioned previously.
Conclusion Fluorescenceof PBQ vapor shows a remarkable rotational state dependence at the 221 and 261 vibronic levels belonging to the lBl,(nr*) state. A comparison of the variation of sf with the distribution of the averaged value of J'along the rotational contour indicates that T f at these vibronic levels shows a tendency to increase with J'. Fluorescence excitation spectrum across the rotational contour of the 22; band is different from the simulated absorption spectrum, and the observed spectrum can be reproduced by assuming that @f decreases with increasing J'. The J' dependence of @f and q a t 22l and 26l is interpreted in terms of the K scrambling in the triplet manifold following ISC, with which Nefiis proportional to 2J'+ 1. The lack of the monotonic J'dependence in @f and 71 expected from the K scrambling may come from a low vibrational level density of the triplet state at around the excited singlet state. In contrast with excitation into 22l or 261, both @rand71 following excitation into the lB1, origin do not show the rotational state dependence, and the fluorescence excitation spectrum agrees well with the simulated absorption spectrum. The lB1,origin is considered to interact with the triplet states very weakly, even if exists.
References and Notes (1) Baba, H.; Fujita, M.; Uchida, K. Chem. Phys. Lett. 1980,73,425. Baba, H.; Ohta, N.; Sekiguchi, 0.; Fujita, M.; Uchida, K. J. Phys. Chem. 1983,87, 943. (2) See, for a review of pyrazine: Kommandeur, J.; Majewski, W. A,; Meerts, W. L.; F'ratt, D. W. Annu. Rev. Phys. Chem. 1987,38,433. (3) Saigusa, H.; Lim, E. C. J. Chem. Phys. 1983, 78,91. (4) Ohta, N.; Baba, H. Chem. Phys. Letr. 1981,84,308; Chem. Phys. 1983, 82, 41. ter Horst. J.: Kommandeur. J. J. Chem. Phvs. 1982. 76. 137. Sekiguchi, 0.;Ohta, N.; Baba, H. Luser Chem. 1987, 7,213. ter Horst, J.; Kommandeur, J. Chem. Phys. 1979, 44, 287. Dunn, T. M.; Francis, A. H. J. Mol. Spectrosc. 1974, 50, 14. Goodman, J.; Brus, L. E. J . Chem. Phys. 1978,69, 1604. Hollas, J. M. Spectrochim. Acra 1964, 20, 1563. Christoffersen, J.; Hollas, J. M. Mol. Phys. 1969, 17, 655. Jayswal, M. G.; Singh, R. S.Spectrochim. Acta 1965, 21, 1597. Brus, L. E.; McDonald, J. R. J . Chem. Phys. 1973,58,4223. Itoh, T. Mol. Phys. 1985, 55, 799. Freed, K. F.; Nituln, A. J. Chem. Phys. 1980, 73, 4765. Koyanagi, M.; Kogo, Y.; Kanda, Y. Mol. Phys. 1971,20, 747.
