Nonadiabatic photodissociation dynamics of the hydroxymethyl radical via the 22A(3s) Rydberg state: A four-dimensional quantum study.

The quantum mechanical nonadiabatic photodissociation dynamics of the hydroxymethyl (CH2OH) radical in its lowest absorption band is investigated for the first time on a set of coupled diabatic potential energy surfaces determined by accurately fitting a large set of ab initio data. In this two-state approximation, only the ground and first excited states of CH2OH, which are coupled by conical intersections, are included. The reduced-dimensional dynamical model includes the CO stretch, the COH bend, the HCOH torsion, and the O-H dissociation coordinate. The experimentally measured hydrogen atom kinetic energy distribution is satisfactorily reproduced. The calculated product state distribution of the H2CO(X) fragment indicates strong vibrational excitation in the CO stretching mode, resulting from the relatively large difference in the C-O bond length between the ground and excited electronic states of CH2OH due to the photo-induced promotion of an electron from the half-occupied π*CO antibonding orbital to a Rydberg orbital. In addition, the bimodal kinetic energy distribution is confirmed to originate from nonadiabatic transitions near the conical intersection along the O-H dissociation coordinate.