Multi-electron excitation

The assignment of a spectral feature as multi-electron excitation depends on the level of theory, i.e. how the electronic states are described. A general definition can be stated as follows: If there exists no one-electron operator that gives a matrix element unequal to zero between the reference state and an excited state, the latter must be multielectron in nature. The reference state is in most cases the ground state of the system. Some spectral features in XAFS are commonly denoted multi-electron spectral features and referred to as 'shake-up', ‘shake-down’ and 'shake-off' excitations. The amplitude reduction factor [math]S_0^2[/math] in the EXAFS equation is included to consider multielectron excitations.