Synchrotron Radiation Photoelectron Spectroscopy Study on Oxide Evolution during Oxidation of a Si(111)-7 × 7 Surface at 300 K: Comparison of Thermal Equilibrium Gas and Supersonic Molecular Beams for Oxygen Adsorption

Abstract

The translational energy of gas molecules is a fundamental physical quantity which dominates adsorption processes in gas–solid reactions, and for this reason molecular beam experiments have been widely conducted. Although gas in thermal equilibrium (thermal gas) plays an important role in many beneficial chemical reactions supporting the most advanced technologies, understanding the adsorption processes of thermal gas in terms of translational energy remains an unsolved issue. Here, for thermal-O2 and supersonic O2 molecular beams, we present a comparative study of oxide evolution at early stages of oxidation of a Si(111)-7 × 7 surface at room temperature. Real-time observations of oxides at the surface were conducted by employing high energy resolution photoelectron spectroscopy using high brilliance synchrotron radiation. It was found that ins structure which has one oxygen atom at the backbond of a Si adatom is the common first product for oxidation by thermal-O2 and supersonic O2 molecular beams. Similarities between thermal-O2 and supersonic O2 molecular beams were found for oxide evolution and illustrate the notion that translational energy of thermal O2 can be ascribed to the average molecular kinetic energy defined based on the most probable speed of the Maxwell–Boltzmann distribution. This experimental result suggests that translational energy is the unifying reaction parameter that rationalizes adsorption mechanisms for both thermal gas and supersonic molecular beams.