Sticking of Hydrogen Atoms to Crystalline Ice Surfaces:  Dependence on Incidence Energy and Surface Temperature

We present results of classical trajectory calculations on the sticking of hydrogen atoms to the basal plane (0001) face of crystalline ice, I<INF>h</INF>. The sticking probability is found to decrease with both incidence energy (E<INF>i</INF>) and surface temperature (T<INF>s</INF>). At the surface temperatures studied, the sticking probability can be fitted to a simple decay function:  P<INF>s</INF> = 1.5 e^-E<INF></INF><INFSUP>i</INFSUP>^(K)/175 at T<INF>s</INF> = 10 K, and P<INF>s</INF> = 0.85 e^-E<INF></INF><INFSUP>i</INFSUP>^(K)/175 at T<INF>s</INF> = 70 K. In the trapped state, the adsorbed hydrogen atom is located on top of the ice surface, over the center of a surface hexagonal ring, interacting with all water molecules forming the ring. The calculated physisorption energy of the adsorbed atom is approximately 400 ± 50 K. The results of our calculations are compared with the experimental and theoretical data for amorphous ice surfaces. At T<INF>s</INF> = 10 K, our values for the sticking probability are higher than those of Buch and Zhang [Buch, V.; Zhang, Q. Astrophys. J. <BO>1991</BO>, 379, 647], which is attributed to differences in surface topology. Our sticking probability values are lower than those of Masuda et al. [Masuda, K.; Takahashi, J.; Mukai, T. Astron. Astrophys. <BO>1998</BO>, 330, 243], which we attribute to an incorrect implementation of the H−H<INF>2</INF>O potential in their work. The experimental results available on hydrogen formation on amorphous ice are in good agreement with our results, if the assumption is made that all H-atoms that stick will recombine. Our calculations then suggest that the formation of H<INF>2</INF> through recombination of H-atoms adsorbed on the surface is efficient enough to compete with the cosmic destruction of H<INF>2</INF>.