Hydrogen adsorption and diffusion on amorphous solid water ice

Results of classical trajectory calculations on the adsorption of H atoms to amorphous solid water (ASW) ice, at a surface temperature T-s of 10 K are presented. The calculations were performed for incidence energies E-i ranging from 10 to 1000 K, at random incidence. The adsorption probability P-s can be fitted to a simple decay function: P-s = 1.0e(-Ei(K)/300). Our calculations predict similar adsorption probabilities for H atoms to crystalline and ASW ice, although the average binding energy E-b of the trapped H atoms calculated for ASW of 650 +/- 10 K is higher than that found for crystalline ice of 400 +/- 5 K. The binding energy distributions were fitted to Gaussian functions with full width half-maximum of 111 and 195 K for crystalline and amorphous ice surfaces, respectively. The variation of the H atom binding sites in the case of the ASW surface leads to broadening of the distribution of E-b compared to that of crystalline ice. We have also calculated the 'hot-diffusion' distance travelled by the impinging atom over the surface before being thermalized, which is found to be about 30 angstrom long at E-i = 100 K and increases with E-i. The diffusion coefficient D of thermally trapped H atoms is calculated to be 1.09 +/- 0.04 x 10(-5) cm(2) s(-1) at T-s = 10 K. The residence time tau of H atoms adsorbed on ASW is orders of magnitude longer than that of H atoms adsorbed on crystalline ice for the same ice T-s, suggesting that H-2 formation on crystalline and non-porous ice is quite limited compared to that on porous ice. This is in good agreement with the results of experiments on H-2 formation on porous and non-porous ASW surfaces. At low T-s, the long values of tau, the high values of D and the large hot distance travelled on the ASW surface before trapping the impinging H atom ensure that Langmuir-Hinshelwood and hot-atom mechanisms for H-2 formation will be effective. The data presented here will be important ingredients for models to describe the formation of H-2 on interstellar ices and reactions of H atoms with other species at the ice surface.