Ray Absorption by Bound Atoms

The x-ray absorption spectra of an element recorded for free atoms, atoms in molecules, or atoms in various chemical compounds exhibit small but significant differences. Apparently the photoabsorption by an atom depends on its chemical environment. Much progress has been made in the interpretation of the detailed features of absorption spectra and nowadays absorption measurements provide a powerful tool to extract information on a variety of materials including three-atom molecules and small clusters. Principles and results of experimental investigations have been compiled by several authors.  

Basically, the near edge structure of the absorption spectrum makes it possible to determine the chemical binding of an atom, whereas the structure extending above the edges allows determination of the local atomic environment, i.e., kinds and distances of surrounding atoms. 

The structure of edges in an absorption spectrum allows a precise determination of inner-shell binding energies (see Section 3.2). Inner-shell energies are mainly determined by the strong nuclear potential, but to some extent they also depend on the electron density in outer shells, which, in turn, depends on the specific chemical state. Interesting cases are the transition and rare-earth elements with partly unfilled 3d and 4/ states, respectively. We discuss some studies of the occupancy of the 4/ valence band. 

The valence change on solidification can be investigated in detail by embedding the atoms in a noble gas matrix at different concentrations. For example, the valence of samarium is 2 for the free atom and 3 for the solid. 

F ie 20. L3 absorption spectra of atomic (dotted lines) and m Iic (solid lines) cerium, samarium, gadolinium, and erbium. The positions of the Fermi levels Bp, the atomic 54 excitation energies and the ionization energies Bf are indicated. The dashed lines are Lorentzian white lines. (From Ref. 76.) 

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