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J . Phys. Chem. 1991, 95, 9682-9687
van der Waals Cluster and Excimer Formations of I-Cyanonaphthalene and Methyl-Substituted 1-Cyanonaphthalenes in Supersonic Expansion Michiya Itoh,* Miki Takamatsu, Noriyuki Kizu, and Yoshihisa Fujiwara Faculty of Pharmaceutical Sciences, Kanazawa University, Takaramachi, Kanazawa 920, Japan (Received: March 7, 1991; In Final Form: August IS. 1991)
The fluorescence excitation spectra of I-cyanonaphthalene and methyl-substituted 1-cyanonaphthalenes in supersonic expansion indicate the van der Waals (vdW) dimer, trimer, and cluster ( n > 3) formations of these compounds. The excitation of the cluster bands affords the large Stokes-shifted excimer fluorescence (A,, 410-430 nm) with decay times of 140-200 ns, while only UV resonance fluorescencewas observed in the excitation of the dimer and trimer. From vapor-pressure dependence of the fluorescence intensity of jet-cooled vdW clusters, the critical size of the clusters for detectable excimer fluorescence was suggested to be placed at n 2 4. Similar excimer fluorescence was observed in the excitation of the ground-state dimer of these compounds generated in the rigid glassy solution of 3-methylpentane at 77 K and also of polycrystals of these compounds. The X-ray crystallography of 1-cyano-2-methylnaphthalene indicates partially overlapped *-electron systems with a head-to-tail configuration in a single crystal. Since the full or partially overlapped a-electron system is required for detectable excimer fluorescence even in the jet-cooled elusters, the geometry of clusters n 3 4 may resemble bulk crystalline structures of these molecules, while the dimer and trimer were suggested to have the planar structure with two CN groups facing antiparallel and the planar cyclic structure with the C3, symmetry. respectively.
Introduction
The combination of laser spectroscopy and supersonic free jet provides a new approach to the investigation of the excited states of weakly bound molecular complexes such as van der Waals (vdW) dimers. In general, however, various characteristic nonradiative relaxations including vibrational predissociation take place in the excitation of the vdW dimer especially to the higher vibronic states.'-3 Of the above dimers formed in the supersonic expansion, excimer fluorescence has been observed only in a few cases of fluorene and 9-ethylfluorene dimers4s5 and of p-(dimethy1amino)benzonitrile dimer: though nonfluorescent excimer formation was suggested for well-studied benzene to account for the quenching of resonance fluorescence lifetime and for a biexponential decay of resonance-enhanced two-photon ionization ~ i g n a 1 . l On ~ the other hand, Saigusa and Itoh," and also Saigusa et al.,'* have reported exciplex formation in the excited state of the vdW complex between jet-cooled l-cyanonaphthalene ( 1 - C N N ) and triethylamine (TEA), and remarkable vibrational-energy dependence of the exciplex formation and their transformation dynamics. Further, Castella et aLI3and Haas and his co-workers14 have reported several jet-cooled vdW complexes leading to exciplex formation, and a similar excess vibrational( I ) (a) Levy, D . H.; Haynam, C. A.; Brumbaugh, D. V. Faraday Discuss. Chem. Soc. 1982, 72, 137. (b) Haynam, C. A,; Brumbaugh, D. V.; Levy, D . H . J. Chem. Phys. 1984,81, 2282. (2) Doxtader, M . M.; Topp, M . R. J . Phys. Chem. 1985, 89, 4291. (3) Hopkins, J . B.; Powers, D. E.; Smalley, R. E. J. Phys. Chem. 1981.85, 3739. (4) Saigusa, H . ; Itoh. M . J . Phys. Chem. 1985, 89, 5486. (5) Itoh. M.;Morita, H. J. Phys. Chem. 1988, 92, 5693. ( 6 ) Peng, L. W.: Dantus, M.; Zewail, A. H.; Kemnitz, K. K.; Hicks, J. M.; Eisenthal, K. B. J . Phys. Chem. 1987, 91, 6162. (7) Langridge-Smith, P. R. R.; Brumbaugh, D.V.; Haynam, C .A,; Levy, D. H . J . Phys. Chem. 1981, 85, 3742. (8) Law. K. S . ; Schauer, M.; Bernstein. E . R. J. Chem. Phys. 1984, 81, 4871. ( 9 ) Schauer, M.; Bernstein, E. R . J. Chem. Phys. 1985, 82, 3722. (IO) Shinohara. H . ; Nishi, N . J. Chem. Phys. 1989, 91, 6743 and references therein. ( I I ) Saigusa, H . ; Itoh. M. Chem. Phys. L e f f . 1984, 106, 391; J . Chem. Phys. 1984, 81, 5682. (12) Saigusa, H.; Itoh, M.; Baba, M.; Hanazaki, I . J. Chem. Phys. 1987, 86, 2528. (13) (a) Castella, M.; Prochorow, J.; Tramer, A . J. Chem. Phys. 1984,81, 251 I . (b) Castella. M.; Tramer, A.; Piuzzi, F. Chem. Phys. Left. 1986, 129, 105; Chem. Phys. Leu. 1986, 129, 112. (14) (a) Anner, 0.;Haas, Y . Chem. Phys. L e f f .1985, 119, 199. Anner, 0.;Haas, Y . J. Phys. Chem. 1986, 90, 4298. Anner, 0.;Haas, Y . J. Am. Chem. SOC.1988, 110, 1416 and references therein. (b) Anner, 0.;Zaura, E.; Haas. Y . J . Phys. Chem. 1986, 90. 4298.
0022-3654/91/2095-9682$02.50/0
energy dependence in some cases. Recently, Itoh and Sasakii5 have reported similar exciplex fluorescence of vdW complex between methyl-substituted l -cyanonaphthalene ( M C N N ) and TEA and discussed the transformation dynamics of the vdW complex to the exciplex in terms of the role of the methyl substitution on 1-CNN.
The topics of the excited-state interaction of the organic molecular clusters ( n > 2) is a very exciting area of current research. Particularly, it is of great interest that some spectroscopic and photochemical features such as the occurrences of the chargetransfer-state and the excited-state proton transfer appear as the size of a cluster increases. Recently, Syage and Wesseli6aand Wessel and Syage'6b have reported the excitonic interactions in naphthalene trimer and tetramer clusters. Analysis of the resonance two-photon ionization spectra suggests that the tetramer geometry resembles bulk crystalline naphthalene. Very recently, Ebata et a l l 7 have reported the vdW dimer and trimer of 1cyano-4-methylnaphthalene ( 4 - M C N N ) in the supersonic expansion which do not exhibit excimer fluorescence but only UV resonance fluorescence. They have suggested that the dimer and trimer of 4-MCNN have fairly rigid planar structures stabilized by strong dipole-dipole interaction. Further, the present paper is concerned with the cluster formation and excimer fluorescence in 4-MCNN and related compounds in supersonic expansion. This is the first observation of the excimer formation in jet-cooled clusters ( n I4) of aromatic moIecuIes.I* The fluorescence excitation spectra indicate the vdW dimer, trimer, and cluster ( n > 3) formations of I-CNN and, 2-, 4-, and 5-methyl-substituted 1-cyanonaphthalenes (2-, 4-, and 5-MCNN) in supersonic expansion and further multiphoton ionization time of flight (MPI TOF) mass spectra demonstrate the cluster formation of I-CNN and 4 - M C N N up to n = 9 or 10. The visible (390-41 0 nm) fluorescence excitation spectra of these jet-cooled molecules reveal several satellite bands 2-3-nm red-shifted from their respective origin and vibronic band, while the UV excitation spectra indicate the similar dimer and trimer formations to that in 4 - M C N N . These satellite bands of the visible fluorescence excitation spectra were tentatively ascribed to clusters ( n 1 4) from the vapor-pressure dependence of these compounds in the supersonic jet. The excitation of the cluster bands shows a large ( 1 5 ) Itoh, M.; Sasaki, M . Chem. Phys. L e f t . 1988, 149,40: J . Phys. Chem. 1990, 94, 654. (16) (a) Syage, J . A . ; Wessel, J . E. J . Chem. Phys. 1988, 89, 5962. (b) Wessel, J. E.; Syage, J . A . J. Phys. Chem. 1990, 94, 737. (17) Ebata, T.; Ito. M.; Itoh, M . J. Phys. Chem. 1991, 95, 1143. (18) Itoh, M.; Takamatsu. M . Chem Phys. Letr. 1990, 170, 396.
0 1991 American Chemical Society
van der Waals Cluster and Excimer Formations
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9683 Dimor
b)
Cluster
T
31200
v/cm+
31600
32000
Figure 1. Fluorescence excitation spectra of jet-cooled I-CNN at nozzle temperature of approximately 340 K, (a) monitored at 340 nm and (b) at 370-400 nm.
Stokes shifted excimer fluorescence (& 400-410 nm) with decay times of 140-150 ns, while that of the dimer band does only resonance fluorescence. In low-temperature (77 K) rigid solution of 3-methylpentane (MP), these compounds exhibit also a large Stokes shifted excimer fluorescence (A, 410-420 nm). The concentration dependence of the fluorescence and excitation spectra suggests that the excimer in rigid solution at 77 K may be generated by the relaxation of the excited state of the ground-state dimer of these compounds. On the other hand, X-ray crystallography indicates the partially overlapped head-to-tail conformation in crystal structure of 1-cyano-2-methylnaphthalene (2-MCNN), of which polycrystalline sample reveals the excimer fluorescence similar to that observed in the supersonic expansion. Therefore, the excimer formation and their excited-state relaxation in the jet-cooled clusters are discussed in terms of the tentatively proposed size and structure of clusters.
Experimental Section The pulsed supersonic jet apparatus and procedures (a nozzle diameter 0.4 mm, H e 1-2 atm) are similar to those described in the previous papers.'2Js An excimer laser pumped dye laser (Lambda Physik EMG 53MSC/FL3002) and a frequency doubling control unit (FL30T/FL532B) were used for laser-induced fluorescence spectroscopy. The laser beam was roughly focused by using a lens (f= 50 cm) at 10-12 mm downstream from a nozzle. In the fluorescence excitation and dispersed fluorescence spectra, a monochromator (Nikon G250 or Ritsu MC-ION) was used, and the output of a photomultiplier (Hamamatsu R1246) was processed by a boxcar integrator (PAR M162/164). Fluorescence decay times were determined by using a digital oscilloscope (Tektronix 2430) connected to a personal computer. For a preliminary detection of the cluster size by multiphoton ionization time-of-flight mass spectra, a KrF excimer laser (248 nm, Lambda Physik EMG 50E)as an exciting light source and an approximately 35-cm flight tube were used. Mass resolution was approximately 5 m / e in the mass number region of (IC")*. The cluster size of (1 -CNN), was detected up to n = 9 or 10 by this equipment. However, details of MPI T O F equipment and procedures will be described elsewhere. 1 -Cyanonaphthalene (Tokyo Kasei) was purified by vacuum distillation, repeated recrystallization, and sublimation. 1Cyano-2-methyl-, I-cyano-4-methyl-, and I-cyano-5-methylnaphthalenes were prepared and purified as described in the previous paper.15 The X-ray diffraction analysis of 1-cyano-2-methylnaphthalene was carried out in the Crystallography Lab of our Faculty. Crystal data and parameter are as follows: space group, P21In;a = 7.669 A, b = 7.1 10 A, c = 16.570 A, a = 90.00,,3 = 92.09, y = 90.00 R = 0.066, R, = 0.071; goodness of fit, 2.089. Results and Discussion The Excimer Formation in the Jet-Cooled Clusters. Figure 1 a shows the UV fluorescence excitation spectrum of jet-cooled
t, Figure 2. Schematic illustrations of the assumed structures of the dimer, trimer, and clusters (tetramer) of 1 -CNN.
1-CNN. The spectrum exhibits satellite bands 116 cm-l redshifted from the origin (31418 cm-') and each vibronic band at nozzle temperature of 330-340 K. The intensity ratio of satellite band to the origin was dependent not only on the nozzle temperature but also on the delay time between the nozzle opening and the laser excitation. The facts suggest that the satellite bands may be attributable to the dimer of 1-CNN. As mentioned in the introductory section, Ebata et a1.I' have reported the vdW dimer and trimer formations of 4-MCNN in supersonic jet and proposed their coplanar structures. On the other hand, Kobayashi et al.I9 reported the dimer of jet-cooled benzonitrile (BCN) exhibiting a sharp dimer band red-shifted 98 cm-l from the origin band of this compound. They suggested that the structure of the vdW dimer of BCN is planar with two C N groups facing each other in an antiparallel geometry by analysis of the rotational contour. Therefore, the dimer of I-CNN may be similarly coplanar structure with C N groups facing each other in an antiparallel way, as shown in Figure 2, and the observed satellite band red-shifted 116 cm-l from the origin band may be ascribed to the origin band of the dimer, (1-CNN),. If it is the case, the dimer band may be due to the transition to the antisymmetric Davydov component of the exciton model. Further, very weak bands were detected a t 31 183 and 31202 cm-] in the lower energy region of the origin band. They may be attributable to the trimer exhibiting some intermolecular vibration with a spacing of -20 cm-'. The structure of the trimer of 1-CNN may be the planar cyclic structure in which the C N groups form a regular triangle, as suggested by Ebata et a1.l' for (4-MCNN)3. Figure 4 shows dispersed fluorescence spectra in the excitations of the dimer (a) and trimer (b) bands of I-CNN. These fluorescence spectra are attributable to UV resonance fluorescence of these vdW complexes, though considerably weak 400-nm fluorescence was observed in the trimer-band excitation at 3 1 184 cm-l. The weak 400-nm fluorescence in Figure 4b may be attributable to the consequence of the excitation of the weak cluster band superimposed on the trimer band in the almost same wavelength region, as will be mentioned later. The fluorescence excitation spectrum monitored a t 380-400 nm reveals several dispersed bands 2-3 nm red-shifted from an origin and each vibronic band, as shown in Figure lb. Figure 3 shows the visible fluorescence excitation spectra at several nozzle temperatures. The spectra remarkably increase in intensity with increasing temperature of a sample reservoir compared with those of origin and vibronic bands of bare molecule. Since it is almost impossible to determine the very low vapor pressure of this compound in the sample reservoir, the intensity of the origin band in the excitation spectra was considered to proportional to the vapor pressure. If it is the case, the logarithmic plot of a peak intensity at 31 193 cm-' versus that of the origin band show a steep slope (19) Kobayashi, T.; Homma, K.; Kajimoto, 0.;Tsuchiya, S. J . Chem. Phys. 1987, 86, 1 1 I 1.
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The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
1-CNN
b)
b
cluster
dimer 300
hlnm
400
341 K
t 400
log l o
3'000
v,cm-l
500
31400
Figure 3. (a) Fluorescence excitation spectra of I-CNN clusters at several nozzle temperatures monitored at 400 nm, and (b) logarithmic plots of peak intensities ( I , ) of the main cluster (31 193 cm-I) and the dimer (31 302 cm-I) bands versus that (I,) of the origin band (31 418 cm" ) ,
of 3.8, as shown in an inset of Figure 3. The fact suggests that the dispersed excitation spectra at 31 193 cm-' region may be attributable to clusters ( n > 3) of this compound, while satellite bands a t 3 1302 and 3 1 184 cm-' are ascribed to the dimer and trimer of this compound, as mentioned above. These dimer, trimer, and cluster formations were demonstrated by the MPI TOF mass spectra (248-nm excitation), as will be mentioned later. Figure 4 shows dispersed fluorescence spectra (c and d) in the excitations of the cluster bands together with those of the dimer and trimer. The excitations of the cluster bands exhibit a large Stokes-shifted fluorescence, while only UV resonance fluorescence was observed in the excitation of the dimer (31302 cm-I) band as shown in Figure 3a. Figure 4b shows a fluorescence spectrum of the trimer-band excitation (320.67 nm, 31 184 cm-I), which looks like exhibiting a weak large Stokes-shifted fluorescence in addition to the UV resonance one. However, since almost similar fluorescence peaked at 380-400 nm was observed in the excitation of very close proximity region of the trimer band, the weak dispersed fluorescence (380-420 nm) in Figure 4b may be attributable to the consequence of excitation of the weak cluster band superimposed on the trimer band. Therefore, both dimer and trimer are confirmed to show only UV resonance fluorescence. Since the Stokes shift of the visible fluorescence is approximately 5000-6000 cm-' from the excitation bands of the clusters, the visible fluorescence may be attributable to an kind of excimer. I n general, the excimer is restricted to the large Stokes-shifted fluorescence from the excited-state dimer generated from an encounter collision between ground- and excited-state molecules and also from the excited state of the ground-state dimer in the solution and/or solid. However, the large Stokes-shifted fluorescence observed here was also ascribed to a kind of excimer. It is noteworthy that the clusters n > 3 exhibit the excimer fluorescence, while the dimer and trimer show only UV resonance fluorescence. Decay times of the excimer fluorescence were measured in the several excitation wavelengths of the cluster bands. The peak band excitation at 3 1 I93 cm-I of the clusters indicates an approximately single exponential decay with decay time of 145 ns. However, the excitation of other cluster bands at 31 152-32056
Figure 4. Dispersed fluorescence spectra of the dimer, trimer, and clusters of jet-cooled I-CNN; (a) the dimer excited at 31 302 cm-l, (b) the trimer at 31 184 cm-', where a 380-420-nm fluorescence may attributable to the excimer by the consequence of excitation of the weak cluster bands superimposed on the trimer-band region (see text), (c) the excimer fluorescence in the excitations of the cluster bands at 31 193 cm-'. and (d) a t 32053 cm-'.
cm-I affords double or multiexponential decays of excimer fluorescence (90-350 ns). Therefore, in addition to the main cluster exhibiting a strong band at 31 193 cm-l, several size of clusters of this compound may be generated in the supersonic expansion, which exhibit similar excimer fluorescence with considerably different decay times. However, taking account of the strong cluster band at 31 193 cm-I, it is likely that a certain size of cluster (probably n = 4 or 5) of this compound is mainly generated and exhibits strong excimer fluorescence in the supersonic expansion. The spectral data including fluorescence decay times of the monomer, dimer, trimer, and clusters in the origin band region are summarized in Table I. In the clusters of a-and /3-naphthol-(NH3),, Cheshnovsky and Leutwyler20 and Droz et aL2' reported that fluorescence excitation spectra and multiphoton ionization time-of-flight (MPI TOF) mass spectra are remarkably dependent on the delay times between the valve opening and the laser excitation, and they obtained mass selective fluorescence and MPI TOF spectra. Following opening of the valve, the beam density increases to a maximum value and the beam temperature drops, then the cluster nucleation and growth shift the maximum cluster size to increasingly higher values.22 In the dimer and trimer of I-CNN, the intensity ratio of the fluorescence excitation spectra was considerably dependent on the delay time and H e pressure, as mentioned above. However, the excitation spectrum of the excimer fluorescence of I - C N N clusters (n > 3) was not significantly dependent on the delay time between the valve opening and the laser excitation and also on the stagnation pressure of He, though these are somewhat dependent. Since the cluster formation does not seem to occur inside (20) Cheshnovsky, 0.; Leutwyler, S. Chem. Phys. Leu. 1985, IZI, I . Cheshnovsky, 0.;Leutwyler, S. J . Chem. Phys. 1988, 88, 4127. (21) Droz, T.;Knochenmuss, R.; Leutwyler, S. J . Chem. Phys. 1990, 93. 4520.
( 2 2 ) Amirav. A,: Even, U.; Jortner, J. J . Chem. Phys. 1981, 75, 2489.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9685
van der Waals Cluster and Excimer Formations
CN
I
4-MCNN
2.0 "
0
-
-
CI
i
b)
clurtrr
7'
Figure 6. (a) Fluorescence excitation spectrum of jet-cooled 5-MCNN
monitored at 340 nm (nozzle temperature approximately 330 K),and (b) monitored at 380-410 nm (approximately 340 K).
I
330K
30SOO
v/c m-'
31100
'a0
log l o
2*o
31300
Figure 5. (a) Nozzle-temperature dependence of the fluorescence excitation spectra of jet-cooled 4-MCNN clusters monitored at 400 nm, and (b) logarithmic plots of peak intensities of the cluster (31 029 cm-I) and dimer (31 184 cm-l) bands versus the origin band (31 271 cm-I).
I 300
a heated (>350 K) nozzle or sample reservoir, the cluster nucleation of considerably large molecule such as l - C N N seems to take place in the close proximity of the pulsed nozzle in the supersonic free jet. In this case, the size and distribution of clusters seem dependent on the temperature within the nozzle or sample reservoir rather than the delay time between nozzle opening and laser excitation, though a little dependence on the delay time was observed. However, the actual size and distribution of clusters should be characterized by MPI T O F mass spectroscopy. A preliminary observation of MPI T O F mass spectra (one-color excitation at 248 nm) indicates really the cluster formation up to n = 9 or 10 for (1 X N N ) , , in the supersonic expansion. Unfortunately, however, no two-color mass-selected MPI excitation spectrum corresponding to the excitation spectra of the excimer fluorescence was detected a t the present stage, because of very weak ion signal due to dissociation and/or fragmentation of clusters.23 As mentioned in the previous paper,18 the UV fluorescence excitation spectrum of jet-cooled 4-MCNN exhibits two weak satellite bands at 31 184 and 31088 cm-', which are 87 and 183 cm-' red-shifted from the origin band at 31271 cm-' of this compound. Further, the vis fluorescence excitation spectrum monitored at 390-410 nm reveals the dispersed cluster band at 31029 cm-'. These energies (wavenumbers) of the dimer, trimer, and main cluster bands in the origin band region together with those of other compounds are summarized in Table I. Figure 5 shows the remarkable temperature dependence of the fluorescence (390-410 nm) excitation spectra, together with the logarithmic plots of intensities of the main cluster band versus that of the monomer band at variable temperatures. The slope of the loga(23)
Since the vertical ionization potential of 1-CNN was reported to be
8.61 eV (Utsunomiya, C.; Kobayashi, T.; Nagakura, S.Bull. Chcm. Soc. Jpn. 1975.48, 1852), the e x w s ionization energy imparted to I-CNN clusters is believed to be more than 1.86 eV (1 1 188 cm-I) in the two-photon excitation at 248 nm, and then the efficient fragmentation of 1-CNN as well as dissc-
ciation of the clusters was observed. The observation of mass-selected twocolor ( I + 1) resonance two photon ionization spectrum was expected. However, no significant spectrum was detected at the present stage because of too weak of an ion signal. Details will be reported elsewhere.
I
A/nm
400
Figure 7. Dispersed fluorescence spectra of jet-cooled 5-MCNN; (a) the dimer excited at 30984 cm-l at the nozzle temperature, approximately 330 K. Dispersed fluorescence spectra of the cluster band excitations (nozzle temperature, 340 K), (b) at 30893 cm-I, (c) at 30830.8 cm-I, and (d) at 31 497 cm-I).
rithmic plot was obtained to be -3.8, which suggests approximate size of cluster (n > 3) of this compound, though the obtained slope does not mean any accurate size of clusters. The excitations of these cluster bands exhibit the large Stokes-shifted fluorescence, while those of the dimer and trimer bands afford no significant visible fluorescence but only UV resonance fluorescence, as reported previously.17J8 The other methyl-substituted 1-CNN, 5-MCNN, in supersonic expansion shows similar fluorescence behavior to 1-CNN, whose fluorescence excitation spectra are shown in Figure 6. Temperature dependence of a sample reservoir and also delay-time dependence between the nozzle opening and laser excitation indicate that weak band at 30984 cm-', 164 cm-' red-shifted from the origin band, is attributable to the dimer of this compound. Further, very weak bands with several low frequency vibrational structures were observed at 30 745 cm-I,
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The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
TABLE I: Fluorescence Excitation Spectra of Monomer, Dimer, Trimer, and Cluster (Main) of Methyl-Substituted 1-CNN in the Origin-Band Region and Fluorescence Decay Times 1-CNN
u,
cm-l
Au. cm-I nsr
T,
cm-' Au, cm-I
2-MCNN
u,
4-MCNN
u,
5-MCKN
Y,
ns'
T,
cm-I Au, cm-' nse
T,
cm-I Au, cm-I
dimer"
31 418 22 31 118
31 302 - I I6 46 31 055 -63
-
-
31 271 53 31 148
31 184 -87 62 30 984 -164 27
-
58
nse
T,
monomer"
clusterb
trimer' 31 184' -234 .-3098gd --I30
31 193 -225 145 =30 722 (very broad) 31 029 -242 150 30 830 -318 205
-
31 088( -183 51 30745' -403 -
*
'Fluorescence was detected at 340 f 5 nm (approximately). *Excimer fluorescence was detected at 420 5 n m (approximately). 'With a few vibrational structures (several tens cm-I). With many low-frequency vibrational structures (several tens and several cm-I). eErrors of decay times arc approximately
