Photoinduced electron transfer and solvation in iodide-doped acetonitrile clusters

We have used ultrafast time-resolved photoelectron imaging to measure charge transfer dynamics in iodidedoped acetonitrile clusters I ^-(CH₃CN)_n with n ) 5-10. Strong modulations of vertical detachment energies were observed following charge transfer from the halide, allowing interpretation of the ongoing dynamics. We observe a sharp drop in the vertical detachment energy (VDE) within 300-400 fs, followed by a biexponential increase that is complete by ̃10 ps. Comparison to theory suggests that the iodide is internally solvated and that photodetachment results in formation of a diffuse electron cloud in a confined cavity. We interpret the initial drop in VDE as a combination of expansion of the cavity and localization of the excess electron on one or two solvent molecules. The subsequent increase in VDE is attributed to a combination of the I atom leaving the cavity and rearrangement of the acetonitrile molecules to solvate the electron. The n ) 5-8 clusters then show a drop in VDE of around 50 meV on a much longer time scale. The long-time VDEs are consistent with those of (CH ₃CN)_n - clusters with internally solvated electrons. Although the excitedstate created by the pump pulse decays by emission of a slow electron, no such decay is seen by 200 ps.