THE DYNAMICS OF HYDROGEN DISSOCIATION ON W(100)-C(2X2)CU

The dynamics of dissociative hydrogen adsorption on the W(100)-c(2 x 2)Cu surface have been studied with a supersonic molecular beam. The surface is observed to present a higher barrier to direct dissociation than W(100) where this channel is non-activated. The removal of the direct channel at low energies reveals a pure indirect channel, allowing for the first time the study of an isolated indirect route to hydrogen dissociation on a metal surface. The initial sticking probability (S-0) via the indirect channel decreases surprisingly slowly with incident beam energy, decaying exponentially up to E(i) similar to 150 meV. Only a weak dependence on surface temperature and angle of incidence exists, while S-0 is constant up to a hydrogen coverage of theta(H) = 0.5. The in-plane scattering distribution is diffuse and peaked at sub-specular angles (towards the surface normal), and exhibits some memory of the incident conditions. No direct scattering component could be observed. The sticking and scattering results cannot be reconciled with the trapping of a molecular precursor through inelastic processes such as phonon excitation or electron-hole pair creation. We suggest that the indirect channel to dissociation takes place via the trapping of the molecule without full energetic accommodation, through a dynamical precursor.