Steering effects in non-activated adsorption

Classical and quantum calculations of the dissociation dynamics of H2 on W(100) have been performed on an ab initio PES. The results show conclusively that at low translational energies the dissociation is dominated by strong steering in an essentially direct process. Starting from a value near unity, the dissociation falls with increasing energy because the steering has less time to operate and is therefore less effective. No precursor state is involved in the process. By examining the quantum flux we see that molecules oriented with their axes perpendicular to the surface reflect at high translational energy, but not at low translational energy. Some molecular trapping occurs as a result of rotational excitation which gives rise to sharp peaks in the quantum dissociation probability.