General Physics Seminar Wednesday 26 June 2002


Quantum scattering dynamics of Eley-Rideal reactions : H + H/graphite and H + Cl/Au(111)

Didier Lemoine

Physique des Lasers, Atomes et Molecules, Universite de Lille 1, France



Langmuir-Hinshelwood (LH) recombination on a subtrate occurs between diffusing reactant species, both in thermal equilibrium with the surface. The competing Eley-Rideal (ER) mechanism involves a gas phase particle impinging on an adsorbed particle. ER reactions are often strongly exothermic. Perhaps the best studied class of ER reactions is that of H2,HD recomnination on single-crystal metal surfaces. Another important problem is to understand the formation of H2 in the interstellar medium, which is believed to take place on carbonaceous dust particles. Several studies have relied on a model graphite substrate in order to investigate the relative importance of LH and ER processes. Electronic structure calculations based on the density functional theory (DFT) indicate the presence of both weak physisorption and chemisorption sites for the H adsorption on graphite. Ensuing quantum scattering simulations reveal that the ER reaction pathway is highly efficient provided that this channel is open. Accordingly, the LH events would be rare. The ER scattering dynamics of H atoms with Cl adsorbates on Au(111) were also investigated, using both quantum and quasiclassical methods, and with a DFT interaction model as well. The single-adsorbate reaction cross section is large, i.e. 2-3 Angstroms2, which is unexpected in view of the fact that for H2,HD recombinations on metals, cross sections are typically no more than 0.5 Angstroms2. In addition, one might have speculated that the heavy Cl, relative to the fast H motion, would introduce some inertia to reaction. The counterintuitive results and the comparison with experiment will be addressed. Owing to the advanced DFT interaction and quantum scattering models, our results represent the first realistic attempts to asses the importance of ER reactions for both targeted systems.