Direct inelastic and trapping-desorption scattering of N 2 from polycrystalline W; Elementary steps in the chemisorption of nitrogen

Abstract

Abstract Scattering of N 2 from a clean polycrystalline W surface is studied with a time-of-flight molecular beam apparatus. The time-of-flight spectra are used to characterize the N 2 -W energytransfer and condensation, allowing inferences to be made about the initial steps of N 2 chemisorption, thought to proceed via a molecular precursor state. The sticking coefficient on our sample for N 2 to chemisorb to an atomic nitrogen bound state was 0.5 ± 0.1 5 for a 600 K beam and a 450 K surface temperature. Unreacted N 2 scattered into direct and trapping-desorption channels. The direct channel is shown to be entirely inelastic with temperature independent differential energy accommodation coefficients that average 0.46 for normal and specular scattering at 45° incidence angle. The fraction of trapping-desorption scattering diminishes significantly with increasing surface and beam temperature. The observed decrease in sticking coefficient with increase in surface temperature is shown to be due to a diminution of the N 2 condensation coefficient as well as an increase in desorption of the N 2 , recursor relative to its migration-chemisorption.