Visualizing Electron Motion at Photochemical Interfaces Using Ultrafast XUV Spectroscopy*

Directly observing electron dynamics at surfaces is required to understand and control the material properties that determine efficiency of many applications including efficient energy conversion as well as ultrafast information processing.  Toward this goal, we have developed extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy as a surface-specific analog of XUV transient absorption.  This method combines the benefits of traditional X-ray absorption spectroscopy, such as element, oxidation, and spin state resolution, with surface sensitivity and ultrafast time resolution.  Using this technique, we investigate charge and spin dynamics in materials with applications ranging from photocatalysis to optical control of magnetic switching.  In one example, we describe a systematic comparison of surface and bulk electron polaron formation in hematite showing that surface self-trapping dynamics differ significantly from bulk and that these dynamics can be systematically tuned by surface molecular functionalization offering the possibility for design of photocatalytic interfaces with enhanced carrier transport based on earth abundant materials.  In a second example, we highlight evolving applications of XUV-RA spectroscopy to study spin dynamics at surfaces.  Applications include understanding ultrafast spin crossover in magnetic semiconductors as well as control of spin polarized electron dynamics at chiral photochemical interfaces.  Last, I will describe capabilities that will soon become available at the NSF National eXtreme Ultrafast Science Facility (NeXUS) that is currently under development at Ohio State University.