Surface chemical reactions induced by molecules electronically-excited in the gas phase

摘要

We present a model suggesting high chemical activity of electronically-excited molecules colliding with an isolator surface. Initial photochemical event is accounted for as the result of molecular evolution on the electronically-excited potential energy surface (PES), where acceleration and alignment are taking place, guiding all the molecules towards the intersections with the ground state PES, where transitions to the ground state PES will occur with minimum energy dissipation. The accumulated kinetic energy may be used to overcome the chemical reaction barrier. While recombination chemical reactions in a gas phase require participation of a third body, this strong limitation on the reaction rates is removed upon interaction with a surface.rnTo observe the predicted phenomenon, we suggested a new experimental approach, Evanescent Wave Photocatalysis~', based on application of total internal reflection on an insulator surface. Laser photoexcitation is localized in a narrow boundary gas layer just above the interface; majority of the excited molecules can reach the surface before the relaxation. The experiments are performed at high gas pressures, so that dense fluxes of the excited reagents can be readily produced. Products of chemical adsorption and/or chemical reactions induced within adsorbates are aggregated on the surface and observed by light scattering. We will demonstrate how pressure and spectral dependencies of the chemical outcomes, polarization of the light and interference of two laser beams inducing the reaction can be used to distinguish the new process we try to investigate from chemical reactions induced by photoexcitation within adsorbed molecules and/or gas phase photolysis.