Dust can be important for the interstellar chemistry in the gas phase. Take for instance the process where an atom or molecule lands on a dust grain, diffuses on the grain and meets another atom or molecule to form a new species, which can then desorb form the grain. This is essentially a description of how heterogeneous catalysis occurs. The importance of this process would depend on the diffusion rate of at least one of the adsorbed species and the products desorbing from the grain. This is particularly relevant for H2 formation. Atoms and molecules adsorbed on grains may be modeled as sitting in a local potential energy minimum. The diffusion can then be thought of as occurring through consecutive jumps from one minimum to another. The rate of the diffusion will depend on the heights of the potential energy barriers between the minima. Quantum chemical electronic structure calculations are important in this context as these can be used to compute adsorption energies and barrier heights. Once these quantities have been obtained, it is possible to estimate transition rate constants between adjacent minima using for instance transition state theory. Such rate constants can in turn be used in Kinetic Monte Carlo simulations to obtain diffusion rates. Light atoms, particularly hydrogen atoms, can tunnel through potential energy barriers. Tunneling may therefore substantially increase the rate of transition from one minimum to the next and thus the diffusion rate. Deuterium tunnels less efficiently than the lighter isotope and kinetic isotope effects (KIEs) are thus expected. Laboratory experiments have been carried out where either H atoms or D atoms diffuse on amorphous or polycrystalline ice at 10 K. Interesting KIEs were then obtained. In my talk I will discuss the above raised topics from a theoretical point of view.

Chemical kinetics and tunneling on dust grains

254th National Meeting (2017)

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