Molecular beam study of the dissociative chemisorption of oxygen on W(110)

Abstract

Molecular beam techniques have been used to study the dissociative chemisorption of oxygen on W(110). Chemisorption probabilities have been measured as a function of incidence angle, θi kinetic energy, Ei , and of surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions, and LEED has been used to examine surface structure. Sticking probabilities are determined as a function of coverage from the time dependence of reflected beam intensities, using Auger electron spectroscopy and LEED to calibrate the surface coverage. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling with En=Ei cos2 θi . This probability is ∼10% at En=0.1 eV, rising to essentially unity above En=0.4 eV. In most cases, the sticking probability is found to fall roughly linearly with increasing surface coverage. However, a less rapid falloff is observed for En≥1 eV and for En≤0.03 eV; the sticking probability actually rises with increasing coverage, reaching a maximum at ∼0.2 ML. For En≤0.3 eV, the chemisorption probability falls to <3% for a coverage of ∼0.5 ML; however, this apparent saturation coverage rises in a smooth step to 0.75 ML at ∼0.25 eV and to ∼1.0 ML in another step centered at ∼0.85 eV. These ‘‘favored’’ coverages of 0.5, 0.75, and 1.0 ML are found to be associated with p(2×1), p(2×2), and p(1×1) LEED patterns, respectively. Angular scattering distributions recorded with a differentially pumped rotatable mass spectrometer revealed predominantly quasispecular peaks. The energy dependence of the sticking probability and the observed falloff of the sticking probability with increasing coverage are not compatible with a classical precursor model, although such a process may indeed dominate for near-zero incidence energies at low surface temperatures.