The charge transfer excited state of the I−⋅CH3I SN2 reaction intermediate: Photoinduced intracluster dissociative attachment

We describe the reaction dynamics which occur upon excitation of the I−⋅CH3I ion–molecule complex to the first optically allowed excited electronic state. Photoelectron spectroscopy of I−⋅CH3I confirms the identification of the species as essentially charge localized, where the observed vibrational fine structure is found to be consistent with small distortions of the CH3I neutral upon complexation to form a stable intermediate in the SN2 identity reaction. A narrow photofragmentation band lies just below the vertical electron detachment energy and is assigned to the I−⋅CH3I→I...CH3I− charge transfer excited state. The recoil energy imparted to the I− fragment is only about 10% of the available energy, indicating that most of the energy is lost to the methyl group as expected for an impulsive dissociation. The I− photoproduct is preferentially ejected along the electric vector of the laser with an anisotropy parameter β of +0.5±0.2. This requires that photoabsorption occurs to a repulsive state which dissociates on a time scale at least comparable to the rotational period of the complex. While the recoil characteristics suggest a direct transition to a repulsive excited state, which would be expected for a charge-transfer excitation between the I− and CH3I moieties, the I− photofragment action spectrum displays vibronic structure similar to that found in the photoelectron spectrum. These results indicate that the photoexcitation mechanism is not direct charge transfer, but mediated by a weakly bound negative ion state. We develop a picture of the photofragmentation dynamics in the context of the diabatic (charge-localized) states recently invoked to interpret the structure of the (ground state) SN2 potential surface.