Velocity Distribution ofH2, HD, andD2Molecules Desorbing from Polycrystalline Nickel Surfaces

Velocity distributions of desorbing molecules have been measured as a function of both desorption angle, $\theta$, and surface temperature, $T_s$. The mean energy, $\overline{E}$, of ${\mathrm{D}}_2$ desorbing from a half-sulfur-covered nickel surface at $T_s=1143\mathrm{}$ K decreases from $3.35k{T}_s$ at $\theta =0\mathrm{}°$ to $2k{T}_s$ at glancing desorption, whereas the speed ratio increases from values < 1 to values ≥ 1. Over the range $940 \mathrm{K}<T_s<1040\mathrm{} \mathrm{K}$, $\overline{E}$ was proportional to $T_s$. No differences in mean energy and speed ratio for ${\mathrm{H}}_2$, HD, or ${\mathrm{D}}_2$ were observed.