Effect of a non-resonant irradiation on atomic energy levels - application to light-shifts in two-photon spectroscopy and to perturbation of Rydberg states

Atomic or molecular energy levels are shifted by a nonresonant electromagnetic irradiation. These so-called 'light-shifts' must be carefully taken into account in high resolution spectroscopy experiments since they perturb the atomic Bohr frequencies which one wants to measure or to use as frequency standards. In this paper, we present a brief survey of various types of light-shifts corresponding to different physical situations. We use a dressed atom approach allowing simple physical discussions and simple calculations. Some new suggestions are made. (orig.)