Higher-Order Effects in Resonant Multiphoton Processes

Abstract

The multiphoton transitions in the hydrogen atom are investigated by taking into account the higher-order terms arising from the $S$-operator expansion. These higher-order effects originate induced shifts and induced breadths on the virtual-atomic-energy levels. It is shown that these perturbations play an important role for resonant multiphoton processes. In this case, the theory predicts a particular flux dependence of the transition rates. Furthermore, the well-known dispersion curves are strongly modified in the neighborhood of any resonance. Thus the transition rates can exhibit several peaks. Numerical results are given for a three-photon bound-free transition from the $2S$ state of hydrogen.