Ultrafast Internal Conversion Dynamics Of Benzyl Chloride By Femtosecond Time-Resolved Photoelectron Imaging

The ultrafast internal conversion of benzyl chloride (BzCl) was studied with femtosecond time-resolved photoelectron imaging (TRPEI) coupled with time-resolved mass spectroscopy. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. Upon absorption of two 400 nm photons, benzyl chloride was excited to the S-4 and S-2 states at the same time. The time evolution of the parent ion with different pump-probe delays can be well described by biexponential decay. The fit yielded T-1=50 fs and T-2=910 fs. By analysis of time-resolved photoelectron kinetic energy distributions, it is concluded that the excited S-4 state has coupled with and decayed to the S-2 state in a short time scale and then converted to the S-1 state through ultrafast internal conversion (IC). Within 50 fs, the molecule electronically relaxes into S-1 through IC and from there, decays to the S-0 ground state with the relatively slow time constant of 910 fs. The anisotropy parameters of photoelectron angular distributions changed from 0.87 at the delay time of 0 fs to 0.94 at 25 fs and then to 0.59 at 190 fs, which also reflects the coupling from the S-4 state to the S-2 state and the following IC to the S-1 state.