Femtosecond dissociation dynamics of methyl iodide clusters

The photodissociation dynamics of methyl iodide clusters using λ=270 nm as pump and λ=405 nm as probe are studied using a femtosecond two color pump–probe laser arrangement combined with a reflectron time-of-flight (RTOF) mass spectrometer. This enables the A state and 10s Rydberg state of methyl iodide to be accessed with the pump beam. Of particular interest is a comparison of the femtosecond dynamics of the methyl iodide monomer with the clustered species. Clocking of the monomer dissociation shows a transient which is indicative of a fast C–I bond breakage as is to be expected upon excitation of methyl iodide into the fast dissociating A state, or into the predissociative 10s Rydberg state. Clusters, however, show a very different pump–probe transient composed of a fast decay and a subsequent dip in ion signal followed by a rise for pump–probe delay times greater than 2 ps. The cluster ion signal shows an enhancement for pump probe delay times up to 70 ps. The results are interpreted in terms of the e...