The role of molecular rotation in activated dissociative adsorption on metal surfaces

The role of molecular rotation in dissociative adsorption of H2 on the activated NiAl(110) metal surface is systematically investigated by means of classical dynamics calculations performed on ab initio six-dimensional potential energy surfaces. The calculations show that molecules rotate abruptly when they are close to the surface and that this rotation allows the molecules to adopt the orientation that is more convenient for dissociation (i.e., nearly parallel to the surface). Also, in reactive sectors of the NiAl(110) unit cell, there is an "angular threshold" below which molecules cannot dissociate. This angular threshold goes down as the incidence energy increases, which explains the rise of the dissociation probability and the fact that it reaches a value close to 1 at incidence energies of the order of 2 eV. The fact that switching on molecular rotation favors dissociation establishes a competition between dissociation and rotational excitation of reflected molecules above the dissociation threshold. Measurements on rotational excitation might thus bring indirect evidence on the dissociation dynamics. Sample calculations for nonactivated Pd(111) and activated Cu(110) metal surfaces suggest that some of these conclusions may be of general validity.