Femtosecond dissociation dynamics of methyl iodide clusters

The photodissociation dynamics of methyl iodide clusters using lambda = 270 nm as pump and lambda = 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) over tilde 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 electronic state diagram of the methyl iodide monomer and effects resulting from clustering of these species, shifts of electronic energy levels and caging of excited species in the cluster. (C) 1998 American Institute of Physics.