Picosecond photofragment spectroscopy. III. Vibrational predissociation of van der Waals’ clusters

Abstract

This paper, last in this series, reports on the picosecond dynamics of vibrational predissociation in beam‐cooled van der Waals’ clusters. Reaction rates have been measured for clusters (1:1) of phenol and cresol (p‐methylphenol) with benzene by the picosecond pump–probe photoionization mass‐spectrometry technique. Dissociation to form phenol (cresol) and benzene takes place from vibrational levels of the S_1 state of phenol (cresol) prepared by the pump laser. The predissociation rates were measured for a number of different excess energies upto ∼2500 cm^(−1), and the reaction threshold was found to be 1400 cm^(−1) above the S_1 origin for phenol–benzene and ∼1795 cm^(−1) for cresol–benzene, respectively. For phenol–benzene, the predissociation rates, following excitation of ring‐type modes, vs excess energy vary more or less smoothly. Cresol–benzene exhibits biexponential decay, with the fast component becoming more dominant at higher energies. A non‐RRKM model involving division of the vibrational phase space is discussed to explain this observation.