Measurements of the Reflection, Adsorption, and Desorption of Gases from Smooth Metal Surfaces

The scattering of thermal energy beams of ⁴He, H₂, and D₂ from the (111) surfaces of epitaxially grown Ni and Ag single crystals has been measured. Also measured were the angular dependences of the desorption of HD resulting from the dissociation–recombination exchange reaction of H₂ and D₂ on active (111) Ni surfaces. The angular desorption dependence of HD is contrary to the Knudsen law and is describable analytically by ${\cos }^d{\theta }_r$ where $\text{2.5\hspace{0.17em}<\hspace{0.17em}d\hspace{0.17em}<\hspace{0.17em}4.0}$ depending on the condition of the particular surface under study. Comparison of these results with the complementary nonreacted scattering distributions, i.e., direct reflection on (111) Ni, indicates that the two measured components, reflection and desorption, combine to yield the total scattered flux from the surface. An artificial synthesis of the total flux distribution expected from the surface assuming an isotropic hydrogen source and the measured reflection and desorption distributions yields a cosine dependence, as expected, at equilibrium. The reflection distributions for the other systems studied are also consistent with this result if a desorption distribution per unit surface area of the form ${\cos }^d{\theta }_r$ is assumed. The results further indicate that adsorption proceeds as ${\cos }^a{\theta }_{ı}$ where $\text{a\hspace{0.17em}≈\hspace{0.17em}d\hspace{0.17em}−\hspace{0.17em}1}$ .