Charge transfer, electronic quantum processes, and dissociation dynamics in molecule-surface collisions

In this paper some fundamental processes in molecule-surface dynamics are addressed; charge transfer from a metal surface to an incident or adsorbed particle, non-adiabatic electronic processes, and bond breaking dynamics. These processes are illustrated and discussed by referring to two recent experiments and the associated theories. The first example is Cl-2 molecules impinging on a clean potassium surface [L. Hellberg, J. Stromqvist, B. Kasemo and B.I. Lundqvist, Phys. Rev. Lett. 74 (1995) 4742]. At similar to 7 au from the surface a ''harpooning'' electron is tunnelling from the metal valence band to the diabatic electron affinity level of Cl-2, making a Cl-2(-) ion. This charge transfer can cause dissociation of Cl-2 into Cl and Cl-. As the neutral Cl escapes from the Coulomb repulsion of the Cl- ion, its atomic affinity level is rapidly (<100 fs) shifted downwards in energy, from initially above E(F) to below the bottom of the valence band. This creates, with a certain probability, a deep Cl 3p hole of sufficient energy to cause emission of electrons (exoelectrons) by Auger decay, or photons by radiative decay (surface chemiluminiscence). In the second example a monolayer of potassium on graphite is exposed to monochromatic photons, causing photodesorption of K atoms [D. Chakarov, L. Osterlund, B. Hellsing, V.P. Zhdanov and B. Kasemo, Surf. Sci. 311 (1994) L724]. The picture emerging from calculations is that incident photons create hot electrons by vertical valence-to-conduction band transitions in the graphite bulk. Some of the hot electrons reach the surface, where they scatter into the empty 4s resonance of potassium, converting the ionic adsorbate to a neutral K atom. The latter is a vertical Franck-Condon transition from the ionic ground state to the repulsive wall of the covalent potential energy curve for K-graphite, which causes ejection of K atoms from the surface.