Femtosecond-laser-induced reactions at surfaces”. Publisher's VersionAbstract. 1998. “
We have experimentally studied chemical reactions at the surfaces of platinum and silicon using subpicosecond laser pulses to induce the reactions. The laser pulses stimulate electrons, which in turn stimulate molecules absorbed on the surface. Irradiation of CO/O2/Pt(111) with 300-fs laser pulses yields O2 and CO2. We measured the yields of O2 and CO2 induced by laser pulses of various wavelengths to determine that nonthermalized electrons stimulate the adsorbates. When the fluence in the laser pulses is high, the effective cross section for desorption is high compared to the cross section measured using an arc lamp source. We demonstrated that by controlling the fluence in the subpicosecond laser pulses we can access either the low or the high cross section regimes. We used isotopic labeling to show that the O2 desorption is molecular, and to discover some properties of the pathway to CO2. In many simulations of molecular dynamics following subpicosecond-pulsed laser excitation, the substrate is assumed to be static. We discovered, however, that subpicosecond laser pulses with fluences above 50 J/mm 2 induce reaction between O2 adsorbed on Pt(111) and atoms from beneath the surface – atoms near the surface move. Laser pulses of even higher fluence ablate the substrate. We irradiated silicon surfaces with 10 000-J/mm 2, 100- fs pulses in a chamber filled with halogen gases. We discovered that sharp spikes develop on the surface, and deduced some of the elements of a model for the spike formation. We also report discovery of an electron-beam induced reaction in C6H6/O2/ Pt(111) that yields phenol.