X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) is an established experimental technique for studying the electronic and local geometric structures of materials. As a short-range order structural probe, it can be applied to all states of matter: crystalline or amorphous solids, liquids and gases. The method is element selective and highly sensitive, with little compromise required to integrate complex sample environment set-ups. These characteristics make the technique suitable for applications in a range of scientific disciplines, from chemistry and catalysis to environmental science, materials science, physics, biology, medicine and cultural heritage. An XAS spectrum is obtained by measuring the modulation of the sample absorption coefficient as a function of the incident X-ray beam energy. Data are usually collected in transmission detection mode, although fluorescence and electron yield detection modes are often used. The XAS spectrum is divided into two regimes: X-ray absorption near-edge structure and extended X-ray absorption fine structure. In this Primer, an overview of XAS fundamentals is given, together with a description of the experimental set-ups, sample requirements, data analysis and possible applications. X-ray absorption spectroscopy is an element-specific and orbital-specific technique that can probe local atomic and electronic structures, without the need for long-range order. This Primer discusses the background principles, experimental methods and data analysis processes used in X-ray absorption spectroscopy to derive oxidation states, coordination and bond lengths of solids, liquids and gases.