Electronically Induced Diffusion of Oxygen on Pt(111)

Electronically induced diffusion of atomic oxygen on a stepped Pt(111) surface has been studied using ultrashort pulses of near-infrared light for the generation of a hot electron distribution at the surface. This study represents the first time-resolved investigation of electronically induced diffusion at surfaces and demonstrates the possibility of initiating diffusion of a strongly chemisorbed adsorbate on a metal surface at low substrate temperatures by an electronic excitation. The dissociative adsorption of molecular oxygen at steps on the Pt(111) surface has been used to generate a non-equilibrium distribution of an adsorbate by decorating the step edges selectively with chemisorbed atomic oxygen. The coverage at the step edges during adsorption and induced diffusion has been monitored by exploiting the sensitivity of optical second-harmonic generation (SHG) on surface symmetry which is macroscopically broken by regular steps. The high sensitivity of this method made it possible to measure rates for hopping from a step site onto an initially empty terrace site down to 10^-7 per laser shot. A UHV chamber has been builded for the realization of the optical experiments as well as for the preparation and the characterization of the platinum surface. The diffusion process has been studied as a function of laser fluence and time delay between two pump-pulses, which made it possible to investigate the energy transfer dynamics from the optical excitation to the frustrated translation. These experiments revealed a strong nonlinear dependence of the diffusion-rate on laser fluence (~F^15) and a fast electronic coupling of the initial excitation and the adsorbate motion with a time-constant of 1.5 ps. Both findings suggests that the excitation mechanism of diffusion on metal surfaces is the same as for the well-studied process of desorption induced by femtosecond laser excitation, which is driven by the hot laser-excited electrons of the metallic substrate. Even for complicated reaction pathways, such experiments can be successfully described by models in which the electronic coupling of the adsorbate to the metal surface is represented by a constant electronic friction coefficient. The extreme nonlinear dependence of the hopping-rate on laser fluence, however, indicates that the energy transfer mechanism is more complicated than for the case of desorption. This is reflected by the fact that a consistent description of the experimental data within a generalized electronic friction model cannot be achieved with a constant electronic friction and a reasonable value for the diffusion barrier, but requires the introduction of a temperature dependent electronic friction coefficient. It is suggested that this temperature dependence appears due to the neglect of the coupling between different vibrational modes. A mechanism of an indirect excitation by anharmonic coupling to the O-Pt stretch is proposed, that introduces a coupling which depends on excitation density and would therefore explain the observed effective dependence of the electronic friction on electron temperature.