Non-adiabatic photodissociation dynamics of vinyl iodide from nσ* and nπ* transitions

The photodissociation dynamics of vinyl iodide upon photoexcitation at 199.2 and 200 nm are investigated in a joint theoretical and experimental study. The gas-phase absorption spectrum measured by Fourier transform spectroscopy along with the use of synchrotron radiation is reported and a reassignment of the excited electronic states responsible for the absorption at the energy range of interest is proposed. Femtosecond time-resolved velocity map imaging in conjunction with resonance enhanced multiphoton ionization detection of the I(²P_3/2) and I*(²P_1/2) photofragments have been carried out. The experimental results are discussed in view of high-level <i>ab initio</i> calculations including potential energy curves and semiclassical dynamics. Three conical intersections (CIs) governing the dynamics are identified in a search for stationary points using spin-orbit gradients. Based on these results, a complete picture of the photodissociation dynamics of vinyl iodide is obtained. Photoexcitation at 200 nm, associated with a n_I(⊥)σ* transition, leads to a fast dissociation occurring in a repulsive potential energy surface, which is mediated by a CI with a low-lying excited electronic state. This mechanism resembles the typical dissociation of alkyl iodides in the first absorption A-band. In contrast, one-photon excitation at 199.2 nm into a well-defined vibronic structure of the absorption spectrum is assigned to a n_I(‖)π* transition. The subsequent dissociation dynamics from that state features an ultrafast electronic predissociation with sub-200 femtosecond reaction time. State-switching at a first CI with a low-lying electronic state governing the mechanism involves states of completely different character, occurring in less than 20 fs. This remarkably fast process takes place through an initial stretch of the CC bond, followed by a C-I elongation with subsequent vibrational activity in the CC stretch mode.