Molecular-beam studies of the precursor-mediated dissociative chemisorption of N2 on W(100)

Abstract

Molecular-beam techniques have been employed to probe various aspects of the dynamics of the dissociative chemisorption of N2 on W(100). The dissociative chemisorption probability has been measured over a wide range of incidence energies Ei, angles θiand surface temperatures Ts. This probability is found to fall rapidly with increasing E, in the range 26–450 meV, where it is also found to fall rapidly with Ts, and to be relatively insensitive to surface coverage at low Ts. In addition, resu l ts are found to be surprisingly insensitive to the incidence angle over the range 0°-70°. This behavior has been interpreted in terms of a precursor model, noting that the trapping process must scale quite closely with the total incidence energy. Support for these conclusions comes from careful measurements of the angular and velocity distributions of scattered molecules which has also revealed that increasing Tsprimarily serves to reduce the fraction of precursor molecules that go on to dissociate, by biasing the kinetics in favor of desorption. In contrast, the trapping probability into the precursor state is found to be relatively insensitive to Ts, accounting for ≤ 20% of the observed effects. Measurements have also been made of the trapping probability of N2 on the W(100) surface covered with N and N2 species. We find that while trapping is insensitive to coverage for Ei <0.1 eV, it varies appreciably with coverage at higher energies; being highest on the surface saturated with both atoms and molecules followed by the clean surface, with the lowest trapping probability on the surface saturated with atoms. © 1989, American Vacuum Society. All rights reserved.