SURFACE FEMTOCHEMISTRY: ULTRAFAST REACTION DYNAMICS DRIVEN BY HOT ELECTRON MEDIATED REACTION PATHWAYS. D.N. DENZLER, CH. HESS, S. FUNK, AND G. ERTL Fritz–Haber–Institut der Max–Planck–Gesellschaft, Faradayweg 4–6, 14195 Berlin, Germany. M. BONN Leiden Institute of Chemistry, P.O. Box 9502, 2300 RA Leiden, The Netherlands. CH. FRISCHKORN AND M. WOLF Freie Universit¨ at Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin, Germany. Fundamental insights into the ultrafast dynamics of energy transfer processes at surfaces are of central importance for a microscopic understanding of chemical reactions at solid surfaces, e.g. in heterogeneous catalysis. The detailed investiga- tion of the rates and pathways of energy flow in the adsorbate–substrate system as well as the chemical dynamics has become possible utilizing intense femtosec- ond laser pulses. Surprisingly, the strong non–equilibrium situation that arises upon irradiation with these pulses results in a new reaction channel caused by hot electron excitation for a number of systems investigated. This electron–mediated reaction mechanism is exemplified here for the model reaction H ad +H ad → H 2,g on Ru(001) and discussed in the broader context of other simple reactions such as the formation of H 2 O, CO 2 and the CO desorption on the same surface. 1 Introduction In comparison to the gas phase, where femtosecond (fs) laser pulses have been used to probe the ultrashort timescale of bond making and breaking for nearly two decades 1 , insights into femtochemistry at solid surfaces have just started to emerge 2,3 . The extension of femtochemistry methods to the gas-solid inter- face is complicated considerably by additional experimental challenges: The well-defined preparation and characterization of the initial state of the sam- ple, the low density of reactants of typically one monolayer, as well as the ultrashort lifetimes of excited states due to rapid quenching channels which have to be outpaced by sufficiently large reaction rates, especially on metal surfaces. Furthermore, the chemical mechanisms responsible for surface reac- tions to occur are entirely different to those in the gas-phase: The presence of a solid–state surface opens a variety of possible excitations and relaxation mechanisms for the adsorbed molecule, which lead to much more complex denzler˙wolf: submitted to World Scientific on November 15, 2001 1