X-ray Raman scattering

X-ray Raman scattering (XRS) is non-resonant inelastic scattering of X-rays from core electrons. It is analogous to vibrational Raman scattering, which is a widely used tool in optical spectroscopy, with the difference being that the wavelengths of the exciting photons fall in the X-ray regime and the corresponding excitations are from deep core electrons. XRS is an element-specific spectroscopic tool for studying the electronic structure of matter. In particular, it probes the excited-state density of states (DOS) of an atomic species in a sample. XRS is an inelastic X-ray scattering process, in which a high-energy X-ray photon gives energy to a core electron, exciting it to an unoccupied state. The process is in principle analogous to X-ray absorption (XAS), but the energy transfer plays the role of the X-ray photon energy absorbed in X-ray absorption, exactly as in Raman scattering in optics vibrational low-energy excitations can be observed by studying the spectrum of light scattered from a molecule. Because the energy (and therefore wavelength) of the probing X-ray can be chosen freely and is usually in the hard X-ray regime, certain constraints of soft X-rays in the studies of electronic structure of the material are overcome. For example, soft X-ray studies may be surface sensitive and they require a vacuum environment. This makes studies of many substances, such as numerous liquids impossible using soft X-ray absorption. One of the most notable applications in which X-ray Raman scattering is superior to soft X-ray absorption is the study of soft X-ray absorption edges in high pressure. Whereas high-energy X-rays may pass through a high-pressure apparatus like a diamond anvil cell and reach the sample inside the cell, soft X-rays would be absorbed by the cell itself. In his report of finding of a new type of scattering, Sir Chandrasekhara Venkata Raman proposed that a similar effect should be found also in the X-ray regime.