X-ray absorption spectroscopy probing hydrogen in metals

Abstract

X-ray absorption spectroscopy (XAS) is a widely used technique for determining the electronic structure of matter. In contrast to X-ray photoelectron spectroscopy (XPS), XAS makes use of photons only, and therefore suffers less from absorption of the probe beam, i.e., electrons or photons, respectively. This is true for hard X-rays probing, e.g., the Kedges of d-metals in metal hydrides (albeit with limited chemical information). Soft X-rays, which are suited to analyze the electronic structure of hydrogen in solids, have a limited absorption length in gases. Photons with energies of less than 50 eV (“hydrogen K-edge” <;20 eV) are absorbed in less than 1 mm at ambient pressure, which is needed for technical hydrides. Recently, we developed a membrane-based approach to study materials exposed to high hydrogen “pressures” while keeping analysis chamber under high vacuum - thus effectively achieving high pressure XPS analysis. In this paper, we demonstrate that the membrane approach originally designed for XPS can be equally well used for XAS. We show first results on the electronic structure of hydrogen in Pd-Ag alloy as measured by in situ XAS using a laboratory extreme ultraviolet (EUV) source.