Electronic spectroscopy of toluene–rare-gas clusters: The external heavy atom effect and vibrational predissociation

Abstract

Toluene-X van der Waals clusters (where X = Ne, Ne2, Ar, Ar2, Kr, Xe) have been investigated by fluorescence excitation spectroscopy in the region of the S1-S0 transition. With the exception of Xe, for each rare-gas studied, we have assigned cluster transitions in the region of all the strong monomer vibrational bands up to 1000 cm(-1) above the origin band. We have further investigated the S1 relaxation dynamics for each vibrational level of each complex, via their fluorescence decay profiles. Clustering with neon has little appreciable effect on the vibrationless S1 lifetime. By contrast, the clusters with argon and krypton exhibit markedly shorter fluorescence lifetimes compared with the monomer. The effect is so severe in the case of toluene-Xe clusters that no fluorescence signals were observed. We interpret these results in terms of an external heavy atom effect in which the rate of intersystem crossing in toluene is influenced by the cluster partner. For clusters built upon excited S1 vibrational levels, the situation is potentially complicated by intramolecular vibrational redistribution and vibrational predissociation (VP). The majority of the fluorescence decay profiles were satisfactorily modeled using single exponential decays. The emission following pumping of the 37(1) level in the toluene-Kr cluster, however, is an exception. We have modeled the decay of this level with a simple kinetic scheme including VP and determined a predissociation rate of (1.04 +/- 0.54) x 10(7) s(-1).

Vibrations of the low energy states of toluene ($\tilde X$X̃ 1A1 and $\tilde A$Ã 1B2) and the toluene cation ($\tilde X$X̃ 2B1)

“…The S 1 electronic state of Tol-h 8 has been the focus of a number of studies in the past as described above. The vibrational structure observed here compares well to the early jet-cooled fluorescence excitation spectrum of Hopkins et al, 54 and also to the later spectra reported by both Hickman et al, 3 and Doyle et al 24 Good agreement is also seen with the (1 + 1) REMPI spectrum recently reported by Davies et al 41 and the (1 + 1) REMPI spectrum recorded by Gunzer and Grotemeyer. 43 It is noted that the relative intensities of several bands observed in the (1 + 1) REMPI spectrum reported in the present work differ from those observed in previous studies, particularly the origin band; this is attributed to the varying UV intensity across the wavenumber range of this spectrum.…”

“…The 371.0 cm -1 feature in the Tol-h 8 spectrum was also seen in the fluorescence study by Hickman et al 3 who dispersed the fluorescence, finding a spectrum displaced from that of the origin by 604 cm -1 ; they assigned this band as M 14 M 20 . This feature was also seen by Murakami et al 27 and is evident in the spectrum of Doyle et al 24 Persual of Table II shows that both this and the 289.9 cm -1 feature are likely associated with the two lowest-wavenumber fundamentals, M 14 and M 20 . Clearly neither fundamental can be attributed to this feature, on the grounds of both wavenumber and symmetry; however, combinations and overtones are possible.…”

“…The feature also cannot be attributed to toluene clusters, as these are very broad in appearance; 59 nor can it be due to toluene-argon complexes, as the same band is seen using helium as a backing gas. 24 In Table III the calculated vibrational wavenumbers for the S 1 state are presented and perusal of these shows that the only viable assignment of this band would be to the lowest-wavenumber b 1 mode, M 20 (consistent with the Krogh-Jesperson et al 20 paper). However, it becomes clear from the assignment of the other low-wavenumber features, that the M 20 vibrational wavenumber in combination with other vibrations suggests a value somewhat lower than 165.1 cm -1 (see below).…”

“…Interestingly, the Tol-h 8 band has not been mentioned in recent work, although it can be seen in the fluorescence excitation spectrum of Doyle et al, 24 recorded using helium as the backing gas. It is unfortunate that the signal-to-noise of the fluorescence excitation spectrum in Ref.…”

“…18 There are also two-photon resonant spectra recorded via fluorescence by Vasudev and Brand 19 and via multiphoton ionization by Krogh-Jespersen et al 20 Independent confirmation of some of the vibrational assignments were provided by Balfour and co-workers, who obtained infrared 21 and UV 22 absorption spectra for liquid toluene and seven deuterated isomers thereof. The S 1 state of toluene has also been studied in relation to complexes: Shauer et al 23 report fluorescence spectra of toluene-He and toluene-CH 4 complexes; and Doyle et al 24 report fluorescence spectra of toluenerare gas complexes, while examining the heavy atom effect. Eisenhardt and Baumgärtel 25 reported the band origin energy of toluene from resonance-enhanced multiphoton ionization (REMPI) spectroscopy, but no REMPI spectra were actually shown therein.…”