A velocity map imaging study of the photodissociation of the methyl iodide cation.

The photodissociation dynamics of the methyl iodide cation has been studied using the velocity map imaging technique. A first laser pulse is used to ionize methyl iodide via a ( 2 + 1) REMPI scheme through the 5p pi 6p Rydberg state two- photon transition. The produced CH3I(+)( (XE3/2)-E-2) ions are subsequently excited at several wavelengths between 242 and 260 nm. The reported translational energy distributions for the methyl and iodine ions present a Boltzmann- type unstructured distribution at low excitation energies as well as a recoiled narrow structure at higher excitation energies highlighting two different dissociation processes. High level ab initio calculations have been performed in order to obtain a deeper understanding of the photodissociation dynamics of the CH3I+ ion. Direct dissociation on a repulsive state from the manifold of states representing the B excited state leads to CH3 +( X(1)A(1)(')) + I*( ?P-1/ 2), while the CH3 + I+( P-3(2)) channel is populated through an avoided crossing outside the Franck- Condon region. In contrast, an indirect process involving the transfer of energy from highly excited electronic states to the ground state of the ion is responsible for the observed Boltzmann- type distributions.