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 CH₃I⁺(X[combining tilde]²E_3/2) 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[combining tilde] excited state leads to CH₃⁺(X[combining tilde]¹A₁') + I*(²P_1/2), while the CH₃ + I⁺(³P₂) 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.