Optical studies of high temperature surface reactions in ultrahigh vacuum

Abstract

Optical diagnostics, including spontaneous Raman scattering and second harmonic generation (SHG), allow real time in situ measurements of surface reactions at elevated temperatures. We report measurements of oxidation and other reactions on single-crystal stainless steel surfaces in ultrahigh vacuum. Raman spectroscopy provides identification of surface chemical and isotopic composition during high temperature oxidation. By combining Raman spectroscopy with ion sputtering removal of previously grown oxides, depth profiles of chemical and isotopic composition are obtained, allowing identification of diffusion mechanisms operative during oxide film growth. Through use of gated detection and pulsed laser excitation, data can be obtained, even from weak scatterers, at temperatures exceeding 1000°C. Optical second harmonic generation primarily probes the contribution due to surface layers at the interface between two centrosymmetric media. It has demonstrated sensitivity to submonolayer coverages of adsorbates on these single-crystal surfaces. We report the use of SHG to study surface segregation and other phenomena on these surfaces. These optical techniques allow information concerning surface composition and structure to be obtained at elevated temperature and at atmospheric pressure. Potential applications include the study of catalytic reactions and corrosion in realistic exposure conditions.