New aspects of the ‘‘channel three’’ problem in benzene, as revealed by multiphoton ionization photoelectron spectroscopy

Abstract

Combining a photoelectron spectroscopic technique with a (1+1) resonant ionization method, we have investigated intramolecular decay processes of benzene in its S1 state under collision-free conditions. Photoelectron spectra were obtained by selective excitation of benzene with a pulsed UV laser at several single vibronic levels of the S1 state up to an internal energy (ΔE) of 5000 cm−1. These spectra strongly suggest that the excitation of benzene at the vibronic bands above the onset of the ‘‘channel three’’ is followed by intramolecular vibrational redistribution within the S1 state. It is concluded that there are no decay channels faster than this redistribution process up to ΔE=5000 cm−1 at the first decay stage. The results of the integrated multiphoton ionization intensity distribution over the vibronic bands, as well as the internal-energy dependent spectral changes observed in the photoelectron spectra, also strongly suggest that the channel three is initiated by the redistributed vibrational modes, which lead to a fast internal conversion leads to the ground electronic state.