Product selectivity of vibrationally mediated photofragmentation of methanol.

We investigate the photodissociation of CH3OH in the first absorption band (S0→S1) by a two dimensional wave packet study employing the associated internuclear potential energy surfaces obtained from ab initio calculations. The quantum chemical calculations are performed in the complete active space self-consistent field approach including the CH3O and OH bond distances with the CH3–O–H bond angle being fixed. The methyl group is considered as a structureless particle. The nuclear wave functions in the ground and in the excited electronic state are then calculated by using a novel expression for the kinetic energy operator in terms of bond coordinates for fixed bending angle and fixed total angular momentum J=0. The photodissociation of methanol is a very fast process and, in agreement with experiment, leads to a broad and structureless absorption spectrum. Dissociation of the vibrational ground state yields, also in qualitative agreement with experiment, exclusively the products CH3O+H. The main emphasis of the present study, however, is the influence of initial vibrational excitation in the electronic ground state on the fragmentation process. For example, the branching ratio for the chemical channels CH3+OH and CH3O+H is shown to exhibit a drastic dependence on the initial vibrational state of CH3OH(S0) as well as on the energy of the dissociating photon. Thus proper selection of the initial vibrational state and of the dissociating photon frequency can, in principle, be used to exert control of products to obtain branching ratios spanning several orders of magnitude. [ABSTRACT FROM AUTHOR]