Real time chemical dynamics at surfaces

It is a major goal in surface science to make movies of molecules on surfaces, in which the reaction of the molecules on the surface can be followed on a femtosecond time scale, with sub-nanometer resolution. By moving the actors (the molecules) to precisely determined positions on the stage (the surface) at some well-defined moment in time, and subsequently making a space- and time-resolved documentary of what happens next, we would be able to understand the reactive interactions between molecules on surfaces in the greatest possible detail. This would enable us to set the stage and bring together the actors in such a way as to produce the chemical outcomes our society needs, by improving existing catalysts and designing novel catalysts, and by engineering novel reactions on surfaces. Any future director of such movies needs to know which techniques (i.e., which theoretical and experimental methods) hold promise for movie making, what has been done with these techniques, and what can be done with appropriate extensions. The methods we discuss are: (i) the time-dependent wave packet method, which is a theoretical method for simulating molecule-surface reactions with sub-nanometer resolution on a femtosecond time scale, (ii) molecular beam experiments, which allow detailed investigation of the molecule-surface interaction at a molecular level, and (iii) time-resolved laser pump-probe experiments, which allow reactions to be studied with femtosecond resolution. In particular, we discuss (i) theoretical studies of the dissociation reaction of hydrogen on metal surfaces, the reactive system presently understood at the greatest level of detail, (ii) the reactive and non-reactive scattering of heavy diatomics (NO,CO) from metal surfaces, and (iii) the competition between reaction of coadsorbed CO with 0 and desorption of CO, again on a metal surface. We examine possibilities to extend these methods to make movies at the desired level of detail. We also discuss which reactions are likely to provide good material for plots of movies that will be exciting for future generations of surface scientists, (C) 2001 Elsevier Science B.V. All rights reserved.