Femtochemistry at gas/solid interfaces”. Publisher's VersionAbstract. 1998. “
This thesis studies the chemical reactions on surfaces induced by ultrashort laser pulses. Two sets of experiments are presented. In the first experiment, we study the desorption of O2 and the oxidation of CO from CO/O2/Pt(111) induced by 300-fs laser pulses. We observe the transition between linear and nonlinear fluence dependence of the photoyield. In the nonlinear regime, the yield of O2 is enhanced by a factor of 1000 compared to that in the linear regime, and is a few decades more than the yield of CO2 . The decay of the photoexcitation is found to be around 1 ps. We verify that the nascent photoexcited electrons in platinum to be responsible for the chemical reactions on the surface and the transition in the fluence dependence indicates a change from single to cooperative action of photo-excited electrons. We also study the reaction pathway leading to the desorption and oxidation by isotope labeling. Our data show conclusively that the O2 desorbs molecularly and put constraints on the formation of CO2 . Results from this work elucidate the mechanism of the metal photocatalysis and expand the understanding of electron-mediated pro-cesses at surfaces and interfaces. In the second experiment, we discover that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in the atmosphere of 500-torr SF6 or Cl2 gas. The spikes, which are up to 40-microm tall, have a cross section of about 6 x 10 microm2 near the base and taper down to a diameter of about 0.8-microm near the tip. The tops of the spikes are approximately at the same level as the surrounding surface of the wafer and are capped by a 1.5-microm ball. Sharp spikes are formed only in the presence of a halogen-containing gas; irradiation in vacuum or in the presence of N2 or He produces blunt spikes with irregular sides and rounded tops. We also find that the size of spikes depends strongly on pulse duration and laser fluence. At low fluence or long pulse-duration, the spikes become smaller and denser. Preliminary study suggests that the spike formation is seeded by the growth of silicon islands on the surface, followed by differential removal of substrate materials around the islands via laser ablation and laser-induced halogen gas-assisted etching.