Deducing anharmonic coupling matrix elements from picosecond time-resolved photoelectron spectra: application to S1 toluene at low vibrational energy

Abstract

The 6a(1) + 10b(1)16b(1) Fermi resonance in S(1) toluene is studied through picosecond time-resolved photoelectron spectroscopy. Our time and energy resolution, together with the necessary stability to monitor dynamics for many hundreds of picoseconds, enable new and unexpected insight into the dynamics and identity of the prepared wavepacket, and the determination of the coupling matrix elements responsible for those dynamics. In particular we are able to determine the influence of the torsional motion of the methyl group on the dynamics; this motion has long been implicated as an effective accelerator of IVR processes.

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.…”

“…Owing to the improved resolution of the present study, in comparison to that of the previous picosecond study, 41 and improved signal-to-noise when compared to the fluorescence excitation spectrum of Hickman et al, 3 we have been able to resolve several previously unreported features and the assignment of these will be discussed. Assignments of the Told 3 spectrum will be made concurrently with those of Tol-h 8 , since consistency between the two is generally expected.…”

“…Indeed, very recent time-resolved work (to be discussed further below), reports evidence for three contributions to the Fermi resonance. 41 In early work, 54 the two most intense features were assigned as being in Fermi resonance, but only one component was explicitly assigned (as M 11 ). In Ref.…”

“…These experiments are complementary to the time-resolved studies 38,39,41 but owing to the use of a nanosecond laser, which has a considerably narrower bandwidth (0.3 cm −1 ), we are generally able to select single vibrational eigenstates within the intermediate S 1 electronic state cleanly; we shall discuss the time-resolved experiments in the light of our own experiments in the below. We shall also compare our ZEKE results to the dispersed fluorescence spectra of Hickman et al, 3 gaining further insight into the assignment of the S 1 levels.…”

“…Improved spectra of the latter were reported recently in a time-resolved study, with a detailed analysis. 41 Zero kinetic energy (ZEKE) photoelectron spectroscopy employing nanosecond lasers offers far greater resolution than the time-of-flight photoelectron method, and has enabled a detailed study of the torsional levels in the toluene cation by Lu et al 42 Using the closely related mass analysed threshold ionization (MATI) technique, in which ions rather than electrons are detected following the ionization of Rydberg states by a pulsed electric field, Gunzer and Grotemeyer 43 were able to obtain well-resolved spectra from vibrational levels of the S 1 state of toluene.…”