The indirect channel to hydrogen dissociation on W(100)-c(2×2)Cu. Evidence for a dynamical precursor

The dynamics of dissociative hydrogen adsortion on the W(100)-c(2×2)Cu surface have been studied with a supersonic molecular beam. The alloy surface presents a higher energy barrier to direct dissociation than W(100), and the removal of the direct channel at low energies reveals for the first time a pure indirect channel to dissociation. The initial indirect dissociation probability exhibits a slow exponential decay with beam energy, and is substantially insensitive to surface temperature and hydrogen coverage. These results, together with measurements of the scattered flux, suggest that molecular trapping may take place without full accomodation through a dynamical precursor, and that dissociation of this species at defects is responsible for the indirect channel.