THEORY OF SINGLE-PHOTON BOUND-FREE TRANSITIONS - EXTENSION OF THE POLE APPROXIMATION

A theory of single-photon ionization, detachment, and dissociation of quantum systems (atom, molecule, ion, etc) is presented. A unified tre atment of excitation from the ground and excited states is performed, via a consideration of the rotating-and counter-rotating-wave terms on equal footing. The familiar pole approximation (PA) is extended to ob tain higher-order corrections, which allows one to establish the PA va lidity conditions and provide a unified description of the cases of ab ove-and below-threshold excitation. As a specific example, photodetach ment of negative ions is considered. An exact long-time solution is ob tained for the general case. A familiar model of photodetachment is us ed to validate the present PA formalism. The latter has allowed us to derive the PA validity condition, which holds for all values of the pa rameters, involving quite different situations. The applicability of t he existing versions of the rotating-wave approximation has been check ed.