Soft X-rays probe buried interfaces

Abstract

Second-harmonic generation (SHG) is a nonlinear optical process in which two photons of a given energy interact with select types of materials and combine to form a single photon with double the original energy. The SHG process, and a closely related one known as sum frequency generation, lie at the heart of a number of spectroscopy methods based on infrared, visible, and ultraviolet laser light. As a result of spectroscopy selection rules, these nonlinear processes are particularly adept at probing interfaces, even ones hidden by many layers of molecules, as is the case for a solid catalyst in contact with high-pressure gas or an electrode in contact with a liquid-electrolyte solution. X-rays with photons in the 100-to-1,000-eV energy range, so-called soft X-rays, can provide valuable information about chemical bonding and structure with elemental specificity. But because of the lack of available light sources with the required intensity and coherence, researchers