Abstract
The radiation of a linearly polarised single-mode Q switched laser is made to interact with a caesium atomic beam. The laser wavelengths (528 nm and 1056 nm) are chosen so that the authors deal unambiguously with non-resonant processes. Two kinds of laser focusing are used, leading to identical results within the error bars. This procedure takes account of intensity distribution and interaction volume effects. The cross section values are sigma 2=(6.7+or-1.9)*10-50 cm4 s for two-photon ionisation and sigma 4=(7.5+or-2.8)*10-109 cm8 s3 for four-photon ionisation.