Time-resolved study of the symmetric SN2-reaction I−+CH3I

Time-resolved photoelectron spectroscopy of negative ions has been applied to study the title reaction as a model system for gas phase SN2 reactions. Starting from the precursor cluster I2−⋅CH3I, the interaction of the reactants I− and CH3I is initiated by a pump pulse and the subsequent dynamics are observed with a delayed probe pulse used to detach the excess electron and measure their photoelectron spectra. Using two different pump photon energies, which lead to different amounts of internal energy available to the reaction complex, a number of dynamical features have been observed. For small internal excitation, the reactants only form stable, albeit vibrationally excited, I−⋅CH3I complexes. However, with increased internal excitation, complexes are formed that exhibit biexponential decay back to I− and CH3I reactants with time scales of 0.8 and 10 ps. Similar dynamics are expected for entrance channel complex formed in the first step of a gas phase SN2 reaction.