Abstract
Model calculations are performed to investigate the kinetic isotope effect of hydrogen and deuterium atom diffusion on hexagonal ice and amorphous solid water. Comparisons with experimental results by Kuwahata et al. (Phys. Rev. Lett., Sep. 2015, 115 (13), 133201) at 10 K are made. The experimentally derived kinetic isotope effect on amorphous solid water is reproduced by transition state theory. The experimentally found kinetic isotope effect on hexagonal ice is much larger than on amorphous solid water and is not reproduced by transition state theory. Additional calculations using model potentials are made for the hexagonal ice, but the experimental kinetic isotope effect is not fully reproduced. A strong influence of temperature is observed in the calculations. The influence of tunnelling is discussed in detail and related to the experiments. The calculations fully support the claims by the Kuwahata et al. (Phys. Rev. Lett., Sep. 2015, 115 (13), 133201) that on amorphous solid water the diffusion is predominantly by thermal hopping while on the polycrystalline ice tunnelling diffusion contributes significantly.
Highlights
H2 is the most common molecule in the interstellar medium
The hydrogen atoms on the grain surface do not have to come from the gas phase
In the present work we theoretically study the kinetic isotope effect between deuterium and protium atom diffusion by estimating the transition rates of H and D atoms hopping between adjacent local minima on the grain surface
Summary
H2 is the most common molecule in the interstellar medium. Its formation is assumed to occur on grains as the gas phase formation is too slow to explain its abundance. In early model calculations, using two types of adsorption sites (i.e. weak and strong adsorption sites), by Hollenbach and Salpeter (1970,1971) the tunneling was efficient enough to allow the adsorbed hydrogen atoms to diffuse over most of the grain surface before desorbing. In the present work we theoretically study the kinetic isotope effect between deuterium and protium (i.e. the common hydrogen isotope) atom diffusion by estimating the transition rates of H and D atoms hopping between adjacent local minima on the grain surface. This is done by employing simple models that are intended to represent the hexagonal ice and the amorphous solid water.
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