Model study of multiphoton ionisation in strong fields

Recently it was found that when Xe was irradiated by a high-intensity laser the photoelectron spectrum contained peaks which corresponded to absorption of more than the minimum number of photons required to ionise the atom. The authors have used a model approximation involving a single bound state and m continua to calculate excitation probabilities and the effective orders of non-linearity of these higher peaks. Simple analytic expressions are obtained for these quantities which agree qualitatively with experiment.