Time-resolved photoelectron imaging of S₂ → S₁ internal conversion in benzene and toluene

Ultrafast internal conversion of benzene and toluene from the S₂ states was studied by time-resolved photoelectron imaging with a time resolution of 22 fs. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. The photoelectron kinetic energy distribution exhibits a rapid energy shift and intensity revival, which indicates nuclear motion on the S₂ adiabatic surface, while the ultrafast evolution of the angular anisotropy revealed a change in the electronic character of the S₂ adiabatic surface. From their decay profiles of the total photoelectron intensity, the time constants of 48 ± 4 and 62 ± 4 fs were determined for the population decay from the S₂ states in benzene and toluene, respectively.