Time-resolved second-harmonic generation as a probe of surface dynamics

Understanding surface dynamics requires knowing the microscopic details of the dynamical pathways open for surface chemical reactions. The time scale of atomic motion, bond breaking, electronic vibrational coupling, and substrate-adsorbate coupling all require using surface probes with 100 fs time resolution. The 100 fs laser pulse pump-probe technique provides such a tool if the second-harmonic generated by the probe pulse (SHG) is used as the probe of the surface structure. SHG is sensitive to surface symmetry, adsorbate coverage, and adsorbate orientation. Because the electronic- vibrational coupling time is >100 fs, the 100 fs pump pulse can heat the electronic system without heating the vibrational system. Such excitation leads to a new class of reaction mechanisms that are non-thermal. We have used time-resolved SHG to observe a 100 fs order-disorder transition in bulk Si following the 100 fs excitation of ~4% of the valence band. We have also observed CO molecules desorb from Cu(111) <350 fs after a 100 fs pulse excites the substrate electronic temperature to 3600 K while leaving the lattice temperature rise <50 K. These reactions occur too fast to be thermally or lattice- vibrationally driven and are probably driven by the cooperative pumping of vibration excitation upon repeated electronic excitation-deexcitation cycles. These observations suggest that lasers can process materials and stimulate chemical reactions in new and unexpected ways.