CRITICAL DISCUSSION OF AN ABSOLUTE MEASUREMENT OF MULTI-PHOTON IONIZATION CROSS-SECTIONS IN CESIUM

Four-photon ionisation of caesium requires strong electromagnetic fields and the usual interpretation in terms of generalised cross sections has to be questioned. A theoretical calculation in the experimental conditions of a recent measurement provides a new interpretation of the results of Morellec and Normand (1979): the effect of light shifts masks the effect of saturation and leads one to overestimate the multiphoton ionisation cross section. The expected overestimation is only 25 per cent for the quoted experiment, but could be larger in some other physical cases.