PRECURSOR DYNAMICS IN DISSOCIATIVE HYDROGEN ADSORPTION ON W(100)

A study of the dynamics of dissociative adsorption on H-2 of W (100) has been carried out using a supersonic molecular beam source. For beam energies in the range 200-13 meV, a 30% increase in the initial sticking probability so is observed with decreasing translational energy. At low beam energies, a complex incident angular dependence of s(0) is also observed. The sticking probability s exhibits a linear decrease with hydrogen coverage at higher energies, however in the lower energy regime a more complex coverage dependence is exhibited, manifested in an initial independence, or increase, in s with increasing coverage. This behaviour is also associated with the contribution of an indirect route to dissociation at lower energies, in addition to a direct channel which predominates above approximate to 150 meV. The value of s(0) at a beam energy of 13 meV, where the contribution of the indirect channel should be greatest, appears to be independent of surface temperature within the accessible range 150 less than or equal to T-s(K)less than or equal to 370. We suggest that the probability that the precursor will go on to dissociate rather than desorb depends on the surface structure.