Introduction: NEXAFS - Near Edge X-ray Absorption Fine Structure and XANES - X-ray Absorption Near Edge Structure
If an X-ray has just sufficient energy to excite a core level, then the resultant photoelectron will leap into unoccupied states. This is the region that is explored by NEXAFS and XANES and is generally regarded as being the energy region between the absorption edge and where the EXAFS oscillations begin.
NEXAFS has particular application to chemisorbed molecules on surfaces. Information concerning the orientation of the molecule can be inferred from the polarisation dependence. NEXAFS is sensitive to bond angles, whereas EXAFS is sensitive to the interatomic distances. NEXAFS spectra are frequently dominated by intra-molecular resonances of pi or sigma symmetry. The energy, intensity and polarisation dependence of these resonances can be used to determine the orientation and intramolecular bond lengths of the molecule on the surface.
For other experiments using the near edge region (up to 40 eV above the edge), the name XANES usually applies. XANES can provide information about vacant orbitals, electronic configuration and site symmetry of the absorbing atom. The absolute position of the edge contains information about the oxidation state of the absorbing atom. In the near edge region, multiple scattering events dominate. Theoretical multiple scattering calculations are compared with experimental XANES spectra in order to determine the geometrical arrangement of the atoms surrounding the absorbing atom. Hence the technique provides complementary information to EXAFS.