Probing the dynamics of highly excited toluene on the fs timescale

Abstract

Investigation of the dynamics of toluene-h₈ (C₆H₅CH₃), toluene-d₈ (C₆D₅CD₃) and toluene-α,α,α-d₃ (C₆H₅CD₃) has been performed utilizing the VUV pump-IR probe technique on the fs timescale. Using the 5th harmonic (∼160 nm) of a Ti:sapphire laser as the pump beam, two superimposed electronic states, the valence S₃ and the Rydberg 4p, were excited by one-photon absorption, followed by ionization and dissociation induced by the probe beam (800 nm). Analysis of the transient signal of the parent (P⁺) and fragment ions ([P–H]⁺ or [P–D]⁺) implies the existence of two different relaxation processes: (i) from the Rydberg and (ii) from the S₃ valence state. Using a rate equation model, the decay times have been determined and comparison between the different isotopologues has been made. Conclusions on the relaxation path, the relative displacements of the potential energy surfaces and the activation energies needed have been drawn from the decay times. The signals corresponding to the fragment ions present a small in amplitude, but nonetheless, unambiguous periodical modulation, which is attributed to out-of-plane bending oscillation, involving also the methyl group. The dynamics of the H- and D-loss channels has been investigated. Especially for the case of toluene-α,α,α-d₃, where both channels are in operation, it was found that the ratio of the abundance of H/D-loss dissociation reactions decreases as the pump–probe delay time increases.