The intramolecular vibrational redistribution (IVR) dynamics following the excitation of a mode in the first electronically excited states of toluene, toluene-d3 and p-fluorotoluene that has predominantly C-CH3 stretching character and an internal energy of ~1200 cm(-1) have been compared using picosecond time-resolved photoelectron imaging spectroscopy as a probe. Temporal changes in the intensities of spectral features in each molecule have enabled IVR lifetimes of 12, 15 and 50 ps, respectively, to be determined. Our measurements show that doorway states are critical in mediating the IVR dynamics in toluene and toluene-d3, and we deduce that these doorway states, which are assigned in the course of this work, are also instrumental in reducing the IVR lifetimes of these molecules relative to p-fluorotoluene.

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.

We commence by presenting an overview of the assignment of the vibrational frequencies of the toluene molecule in its ground (S 0 ) state. The assignment given is in terms of a recently proposed nomenclature, which allows the ring-localized vibrations to be compared straightforwardly across different monosubstituted benzenes. The frequencies and assignments are based not only on a range of previous work, but also on calculated wavenumbers for both the fully hydrogenated (tolueneh 8 ) and the deuterated-methyl group isotopologue (α 3 -toluene-d 3 ), obtained from density functional theory (DFT), including artificial-isotope shifts. For the S 1 state, one-colour resonance-enhanced multiphoton ionization (REMPI) spectroscopy was employed, with the vibrational assignments also being based on previous work and time-dependent density functional theory (TDDFT) calculated values; but also making use of the activity observed in two-colour zero kinetic energy (ZEKE) spectroscopy. The ZEKE experiments were carried out employing a (1 + 1 ) ionization scheme, using various vibrational levels of the S 1 state with an energy <630 cm -1 as intermediates; as such we only discuss in detail the assignment of the REMPI spectra at wavenumbers <700 cm -1 , referring to the assignment of the ZEKE spectra concurrently. Comparison of the ZEKE spectra for the two toluene isotopologues, as well as with previously reported dispersed-fluorescence spectra, and with the results of DFT calculations, provide insight both into the assignment of the vibrations in the S 1 and D 0 + states, as well as the couplings between these vibrations. In particular, insight into the nature of a complicated Fermi resonance feature at ∼460 cm -1 in the S 1 state is obtained, and Fermi resonances in the cation are identified. Finally, we compare activity observed in both REMPI and ZEKE spectroscopy for both toluene isotopologues with that for fluorobenzene and chlorobenzene.

We report (nanosecond) resonance-enhanced multiphoton ionization (REMPI), (nanosecond) zero-kinetic-energy (ZEKE) and (picosecond) time-resolved slow-electron velocity map imaging (tr-SEVI) spectra of fully hydrogenated toluene (Tol-h8) and the deuterated-methyl group isotopologue (α3-Tol-d3). Vibrational assignments are made making use of the activity observed in the ZEKE and tr-SEVI spectra, together with the results from quantum chemical and previous experimental results. Here, we examine the 700–1500 cm−1 region of the REMPI spectrum, extending our previous work on the region ≤700 cm−1. We provide assignments for the majority of the S1 and cation bands observed, and in particular we gain insight regarding a number of regions where vibrations are coupled via Fermi resonance. We also gain insight into intramolecular vibrational redistribution in this molecule.