Quantum mechanical treatment for the diffusion process of a hydrogen atom on the amorphous water ice surface

Abstract

The diffusion process of a hydrogen atom on the amorphous water ice surface was investigated under very low temperature conditions (10 and 70 K) using both classical and quantum approaches. The model amorphous water ice slab was prepared by the classical molecular dynamics (MD) simulation under the two-dimensional periodic boundary condition with 1000 water molecules in a unit cell. For a H atom thrown onto the surface of the amorphous ice, the sticking and diffusion processes were studied. In the sticking case, the incident H atom initially diffused for 1–3 ps and then became trapped in one of the stable sites on the amorphous ice surface. To estimate the quantum mechanical diffusion constant, a new formalism was developed using the differential diffusion constant. A rate calculation for a H atom diffusing from one trapped site to another on the amorphous water ice was performed. The numerical value was compared with the hopping rate constant for the classical thermal diffusion, and a large quantum effect was found. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 379–385, 1998