Two-colour IR+XUV spectroscopies: the “soft-photon approximation”

We address several questions related to the nonlinear ionization processes observed when atoms are in the simultaneous presence of intense and coherent infrared (IR) and extreme ultraviolet (XUV) laser pulses. This topic is of much interest in the context of the current development of new XUV and soft-X-ray coherent sources, either from high-order harmonics or from X-ray free-electron laser (XFEL) devices that can be synchronized with IR lasers. The theoretical description of this class of two- (more generally multi-) colour ionization processes is challenging for theory. Here, we discuss the advantages and limitations of the so-called “soft-photon approximation”, which, we believe, provides most useful insights in the analysis of these processes.