X-ray Absorption Spectroscopy (XAS)
When the x-rays hit a sample, the oscillating electric field of the electromagnetic radiation interacts with the electrons bound in an atom. Either the radiation will be scattered by these electrons, or absorbed and excite the electrons.
A narrow parallel monochromatic x-ray beam of intensity I 0 passing through a sample of thickness x will get a reduced intensity I according to the expression:
ln (I 0 /I) = µ x (1) where µ is the linear absorption coefficient , which depends on the types of atoms and the density ρ of the material. At certain energies where the absorption increases drastically, and gives rise to an absorption edge . Each such edge occurs when the energy of the incident photons is just sufficient to cause excitation of a core electron of the absorbing atom to a continuum state, i.e . to produce a photoelectron. Thus, the energies of the absorbed radiation at these
edges correspond to the binding energies of electrons in the K, L, M, etc,
shells of the absorbing elements. The absorption edges are labelled in the
order of increasing energy , K, L I , L II , L III , M I ,…., corresponding to the
excitation of an electron from the 1s (2S ½), 2s (2S ½), 2p (2P ½), 2p (2P 3/2), 3s
), … orbitals (states), respectively.
Bohr Atomic Model
ed K L L L Continuum ge:
2 S ½2 S ½
2 P ½2 P 32III II I
University of Calgary