AVS 46th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS2-WeA

Paper SS2-WeA1
Dissociative Sticking of Small Oxidant Molecules on a Simple Metal; NO, O@sub 2@, CO and N@sub 2@ on Al(111)

Wednesday, October 27, 1999, 2:00 pm, Room 607

Session: Gas-Surface Dynamics
Presenter: I. Zori@aa c@, Chalmers University of Technology, Sweden
Authors: H. Ternow, Chalmers University of Technology, Sweden
I. Zori@aa c@, Chalmers University of Technology, Sweden
B. Kasemo, Chalmers University of Technology, Sweden
Correspondent: Click to Email
Motivated by our combined interest in molecular dissociation mechanisms at surfaces and early stages of surface oxidation, we are studying the dissociative sticking of small oxidizing molecules on Al(111). Hydrogen dissociation on metal surfaces has been clarified in detail by a close interplay between state resolved molecular beam experiments and calculated dynamics of dissociation on multidimensional potential energy surfaces. In contrast, dissociation dynamics of O@sub 2@ on simple metal surfaces, e.g. Al(111) and Ag(110, is not well understood. Molecular beam experiments indicate in both cases activated dissociation, while the calculated PES-s so far do not show such features. We have in this work investigated dissociation of several small molecules on Al(111) surface to elucidate the nature of the activation barrier and to explore these molecules' properties as oxidants for oxide film formation. The observed dissociative sticking for the two isoelectronic molecules, CO and N@sub 2@, is consistent with the presence of a very high activation barrier. In contrast, dissociation of NO and O@sub 2@ shows a rather low apparent activation barrier. Vibrational excitation of the molecules enhances dissociation. In the case of NO dissociation an energy dependent N/O ratio on the metal surface is observed. These features are discussed referring to the known symmetry and spatial extention of the frontier orbitals on the two species. The alternative - or complementary - view of the dissociation event, based on a harpooning mechanism, is also discussed.