Measurements of the spatial distribution of H 2 desorbed from Ni surfaces: Effects of surface composition and crystal orientation

Abstract

The effects of surface composition and crystal orientation on the spatial distribution of H 2 molecules desorbed from Ni surfaces have been examined. In this continuation of the work of Dabiri et al., we have obtained data for both polycrystalline and singlecrystal [(111) and (110)] Ni surfaces that are contaminated to varying degrees with impurities. The surface composition was detected by Auger electron spectroscopy and varied by ion bombardment and by controlled deposition of S, C, Si and Au. The spatial distribution of the desorbed molecules was measured with a rotatable ionization gauge. The measured spatial distributions may be described approximately by the form cos d θ, where θ is the angle of inspection measured from the surface normal. Prior to cleaning the Ni surfaces by ion bombardment, the distributions are essentially the same (i.e., d ≈ 4) for the polycrystalline and single-crystal samples and for sample temperatures ranging from 925 to 1225°K. In this case, however, the Auger spectra indicate that the surfaces are unintentionally contaminated with S and, to a lesser degree, with C. After the surfaces are cleaned by ion bombardment, the distributions are essentially diffuse (i.e., d ≈ 1). Diffuse distributions are also observed when pure layers of S, C, or Si are deposited on the Ni surfaces. These results indicate that the spatial distribution of H 2 desorbed from Ni is independent of crystal orientation and temperature, but strongly dependent upon surface composition. Non-diffuse distributions (i.e., d ≠ 1) were observed only for impure surfaces (i.e., surfaces composed of more than one element). Based on these results, we suggest that the non-diffuse distributions reported by Van Willigen, Palmer et al., and Dabiri et al. correspond to impure surfaces.