Stereodynamics of Chemical Reactions on Solid Surfaces Induced by Molecules and Radicals

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One of the ultimate goals of surface science is to be able to design surfaces with particular catalytic reactivity. This entails a need for an atomic-level understanding of the fundamental principles (elementary processes) underlying the bond-making and bond-breaking at surfaces. The orientation of a molecule incoming on the surface is expected to play an important role in such elementary processes. We have been developing a new machine of ultra-high-vacuum (UHV) compatible oriented-molecular beam line, in order to investigate for initial orientation-effect of an incoming molecule in the surface chemical reactions. The purpose of the present research project is to probe the surface-chemical reaction, especially on Si surfaces, in detail by controlling the molecular orientation of impinging molecules. We report results of a study on the incident-energy and the surface-temperature dependence of the steric effects in the dissociative adsorption of CH_3Cl on a Si(100) surface. The initial sticking probability of CH_3Cl on Si(100) was measured by the King-Wells method. Data presented show that the initial sticking probability for the Cl-end collision is larger at an incident energy of 120 meV than that in the CH_3-end collision. Furthermore, this steric preference is quite sensitive to the kinetic energy and the rotational state of CH_3Cl and the surface temperature. This study shows that the non-equilibrium surface trapping plays a key role in the initial step of the decomposition of CH_3Cl on Si(100). We also introduce the on-going experiments using another new apparatus equipped with the oriented-molecular beam line and the reaction chamber for X-ray photo-emission spectroscopy. In the reaction of oriented NO molecules with Si(111), we found that a N-end collision is more effective in the dissociative adsorption than O-end collision. In addition, we also elucidate the oxide-formation processes on Cu surfaces with oxygen molecular beam.