Abstract

The experimental realization of powerful sources of coherent X-ray radiation [1] opens for the first time the possibility to perform two-photon bound-bound absorption experiments involving inner shells in heavy atoms or ions. This would considerably broaden the scope of traditional, (single-photon) X-ray Absorption Spectroscopy (XAS) and related techniques. From the theoretical point of view it is clear that the usual non-relativistic dipole treatment of the atom-field interaction is no longer valid: By considering the two-photon |1²S_1/2〉 → |2²S_1/2〉 transition in hydrogenic systems, we test the inherent limitations of such a simplified approach as compared to a treatment taking full care of relativistic and retardation effects.

© 1997 Optical Society of America