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.
The time evolution of a resonance peak is studied for three- and four-photon ionisation of the caesium atom with square and Gaussian laser pulses. In the domain 10 -13 -10 -7 s two different regimes are defined according to whether the time is smaller or larger than a characteristic time t c . For t<t c the ionisation probability varies as t 3 whereas it is proportional to t when t>t c . The corresponding resonance width varies as t -1 in the first case becomes intensity independent in the second. The effects of the spatial distribution of the intensity in the focal volume are discussed and an excellent agreement is found between the theoretical and experimental values of the order of non-linearity, the resonance width and the absolute magnitude of the ion signal.