During the X-ray absorption (XA) process, a core-hole is created by X-ray irradiation. The excited electron from the core orbital can be trapped by one of the previously unoccupied bound states or reach the vacuum continuum. The transition prababilties depend on the local density of states (DOS) obeying the dipole selection rules. XAS is able to probe the DOS of the unoccupied orbitals as well as their symmetry properties. It has to be kept in mind that the presence of the core-hole modifies the DOS and their energies, i.e. excited state information is revealed.
The following sections describe XAS related methods: