Femtochemistry at metal surfaces: Nonadiabatic reaction dynamics

Surface femtochemistry is initiated by ultrashort-laser pulse excitation of an adsorbate-covered metal surface. The energy transfer from the initially excited electronic degrees of freedom of the substrate to the nuclear motion of the reactants occurs either directly through electronically nonadiabatic substrate-adsorbate coupling or indirectly via equilibration with the lattice and subsequent coupling to the adsorbate. Surface femtochemistry experiments are performed by applying amplified femtosecond-laser pulses to well-defined metal/adsorbate systems in ultrahigh vacuum (UHV). Femtosecond-laser excitation of an adsorbate-covered metal substrate results in a distinctive relationship between the reaction yield and the absorbed laser fluence. Femtochemistry is usually assumed to be induced by a hot but thermalized electron distribution for a substrate-mediated excitation mechanism. A main advantage of surface femtochemistry is the access to nonadiabatic coupling effects directly in the time domain which allows one to switch on electronic frictional forces by an ultrashort laser pulse. The concept of multidimensional dynamics with electronic frictions provides a simple and appealing approach to extend the established foundations of chemical reaction dynamics on Born-Oppenheimer surfaces to include nonadiabatic coupling to a hot electron transient.