Dynamics of direct and indirect channels to dissociative adsorption

Two limiting dynamic processes leading to dissociative adsorption of hydrogen on W(100) are described. An indirect channel to dissociation is evident at low incident translational energies, and the probability that the "precursor" will go on to dissociate depends in a complex fashion on the coverage of atomic hydrogen. In addition to the indirect route, a direct channel to dissociation also contributes on the clean surface. Pre-adsorbed nitrogen in the c(2 x 2) structure is shown to block the direct channel to dissociation at low incident translational energies. However, the indirect channel to dissociative adsorption remains intact, and leads to a significantly high initial dissociation probability at low energies (60% at 3.4 meV) to conclude that the precursor must be formed at the W(100)-c(2 x 2)N unit cell.