Open AccessHydrogen adsorption and diffusion on amorphous solid water ice
Author(s) -
AlHalabi A.,
Van Dishoeck E. F.
Publication year - 2007
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2007.12415.x
Subject(s) - adsorption , amorphous solid , amorphous ice , diffusion , atom (system on chip) , atomic physics , ice ih , hydrogen , physics , surface diffusion , crystallography , chemistry , thermodynamics , molecule , quantum mechanics , computer science , embedded system
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: . 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 Å 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 × 10 −5 cm 2 s −1 at T s = 10 K . The residence time τ 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 τ, 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.