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 5pπ → 6p Rydberg state two-photon transition. The produced CH3 I+ (X[combining tilde]2 E3/2 ) 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 CH3 I+ ion. Direct dissociation on a repulsive state from the manifold of states representing the B[combining tilde] excited state leads to CH3 + (X[combining tilde]1 A1 ') + I*(2 P1/2 ), while the CH3 + I+ (3 P2 ) 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.