Spatial distributions of H 2 desorbed from Fe, Pt, Cu, Nb, and stainless steel surfaces

Abstract

The spatial distributions of H 2 molecules desorbed from polycrystalline Fe, Pt, Cu, Nb and stainless steel (type 304) surfaces were measured with a rotatable ionization gauge. Prior to cleaning the surfaces by ion bombardment or by oxidation, the AES (Auger electron spectroscopy) spectra indicated that all of the sample surfaces were highly contaminated. (S, C, and P were the predominant impurities.) The spatial distributions measured under these conditions were considerably narrower than the commonly-assumed diffuse distribution; i.e., the concentration of desorbed molecules was higher in the vicinity of the surface normal than for the case of diffuse emission represented by cos θ, where θ is the angle of inspection measured from the surface normal. These measured distributions may be described approximately by the form cos d θ, where d ranges from ∼ 2.5 for Nb to ∼ 7 for Fe. However, as the surfaces were cleaned, the spatial distributions for Fe, Pt, and Nb tended to become diffuse (i.e., d → 1), which is consistent with the trend we observed previously for polycrystalline and single-crystal Ni. Copper proved to be an exception to this trend, since its distribution was far from being diffuse (i.e., d≅ 4) when the surface appeared to be clean according to our AES measurements. We suggest that this unusual characteristic of Cu is a consequence of an activation energy barrier associated with the dissociative adsorption of hydrogen on Cu.