On the validity of the quasi-particle approximation in photoelectron spectroscopy

It is shown that a decaying vacancy is able to give an isolated line in an atomic or molecular photoelectron spectrum although the usual condition of validity of the quasi-particle approximation is violated. A more general condition is proposed which is based on the ability of the one-hole Green's function expansion in the vicinity of a complex pole. On the basis of this condition experimental photoelectron spectra are analysed. A modified expression for the spectroscopic factor of a decaying vacancy is presented.