Theory of Femtochemistry at Metal Surfaces: Associative Molecular Photodesorption as a Case Study

Abstract

Reactions on surfaces are of great scientific interest because of their diverse applications. Well-known examples are the production of ammonia on metal surfaces for fertilizers and reduction of poisonous gases from automobiles using catalytic converters [1]. For these applications, metal surfaces -most notably, those consisting of transition metals -are particularly interesting as they offer a great variety of electronic structure and, therefore, tunability toward specific targets.Photoinduced reactions at surfaces were also studied in detail [2][3][4][5]. These are useful, for example, for photocatalysis and nanostructuring of surfaces. Some of these reactions are occurring on femtosecond (1 fs ¼ 10 À15 s) timescale [6]. The advent of femtosecond lasers -that is, ultrashort and intense laser pulses -offers the possibility to monitor these motions in real time, for example, by two-pulse correlation techniques. Moreover, femtochemistry at metal surfaces can be very distinct -on quantitative and qualitative scales -from more traditional surface photochemistry driven by conventional light sources [5,6].Photochemistry at metal surfaces may or may not proceed indirectly, that is, by initial excitation of the substrate electrons rather than by direct excitation of the adsorbate. One way to distinguish between the direct or the substrate-mediated route is through a dependence (direct) [7], or lack thereof (substrate mediated) [8,9], of the reaction cross section on the polarization of the incoming light. In particular for initiating photons in the UV/vis regime, which can deeply penetrate into metal surfaces [10], indirect mechanisms have been suggested to be operative in many cases. Further, when femtosecond lasers (FLs) are used, hot metal electrons that are attached to the adsorbate are believed to often trigger the reaction rather than phonons [6, 11] (see below). For the reaction of interest below, photodesorption of H 2 from Ru(0001), experiments suggest a substrate-mediated, hot-electron-driven mechanism [12].The indirect, substrate-mediated excitation can also be categorized according to its fluence dependence. For the prototypical case of photodesorption, for example,