Abstract:
X-ray spectroscopy is a tool to probe the electronic and magnetic structure of matter. The annihilation of the x-ray photon emits a core electron and the screening reaction of the valence electrons in molecules and solids gives information on the chemical bonding and the electron correlations. X-ray absorption spectroscopy (XAS) measures the excitation of a core electron to an empty state. Because the core hole has a femtosecond life expectancy, XAS spectra are always 200 meV broad. This resolution was reached experimentally in 1987 implying that in the last 30 years there has been no change in the measured XAS spectral shape.
Recent experimental progress in x-ray spectroscopy is related to the combination of XAS with spatial resolution, time resolution and resonance studies.
- SPACE: Nanoscale chemical imaging of catalysts under working conditions is possible with transmission x-ray microscopy. The 20 nm resolution combined with powerful chemical speciation by XAS and the ability to image materials under reaction conditions opens up new opportunities to study many chemical processes. A comparison is made with the atomic resolution obtained in electron microscopes.
- TIME: x-ray free electron lasers and high-harmonic laser sources allow XAS experiment with femtosecond time resolution. Femtosecond x-ray pulses also allow the measurement of sensitive materials because the XAS process itself takes place faster than 1 fs.
- RESONANCE: One can use the XAS excitation energy to measure a lower energy excitation resonantly. This resonant inelastic x-ray scattering (RIXS) technique allows the measurement of vibrations, magnetic and electronic excitations with ~20 meV resolution making use of the coherent absorption and emission of x-rays. A recent example is the possible determination of excitonic insulator ground states in cobalt perovskite systems.