Extracting generalized multiphoton ionization cross-sections from nonperturbative time-dependent calculations: An application in positronium

Ab initio time-dependent (TD) calculations of the behavior of positronium (Ps) under strong: subpicosecond laser pulses are presented. The results are compared with results in H through scaling. Tt is found that a substantial amount of the population call be found in excited states after the pulse. In the perturbative regime, generalized multiphoton ionization cross-sections: are extracted from the results of the time-dependent calculations. The generalized cross-sections are used to predict the response of Ps to nanosecond laser pulses at. wavelengths of current experimental interest. Beyond the application to Ps, the generality of the method for extracting generalized cross-sections from TD nonperturbative calculations is discussed.