Chemistry Department Colloquium

"Chemical Kinetics and Tunneling on Interstellar Dust Grains"

Presented by Professor Gunnar Nyman, Dept. of Chemistry and Molecular Biology, University of Gothenburg, Goteborg, Sweden, Sweden

Monday, August 28, 2017, 11:00 am — Hamilton Seminar Room, Bldg. 555

Dust can be important for the interstellar chemistry in the gas phase. 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 from the grain 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 in interstellar space.

Atoms and molecules adsorbed on grains may be modeled as sitting in a local potential energy minimum. Diffusion can then be thought of as occurring through consecutive jumps from one minimum to another. The transition rate constants between adjacent minima can be estimated by for instance transition state theory. Such rate constants can in turn be used in Kinetic Monte Carlo simulation 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 obtained.

Hosted by: Greg Hall

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