Photodissociation dynamics of phenol: Multistate trajectory simulations including tunneling

We report multistate trajectory simulations, including coherence, decoherence, and multidimensional tunneling, of phenol photodissociation dynamics. The calculations are based on full-dimensional anchor-points reactive potential surfaces and state couplings fit to electronic structure calculations including dynamical correlation with an augmented correlation-consistent polarized valence double-ζ basis set. The calculations successfully reproduce the experimentally observed bimodal character of the total kinetic energy release spectra and confirm the interpretation of the most recent experiments that the photodissociation process is dominated by tunneling. Analysis of the trajectories uncovers an unexpected dissociation pathway for one quantum excitation of the O-H stretching mode of the S₁ state, namely, tunneling in a coherent mixture of states starting in a smaller R_OH (∼0.9-1.0 Å) region than has previously been invoked. The simulations also show that most trajectories do not pass close to the S₁-S₂ conical intersection (they have a minimum gap greater than 0.6 eV), they provide statistics on the out-of-plane angles at the locations of the minimum energy adiabatic gap, and they reveal information about which vibrational modes are most highly activated in the products.