Exact solution of a realistic model for two-photon ionization

A realistic model for resonant two-photon ionization of an atom is developed which can be solved exactly to give the ionization probability as a function of time, photon intensity, and a few atomic parameters. The exact solutions are compared to various commonly used perturbation results in order to ascertain the regions of validity of the perturbation calculations. It is shown that there are combinations of the parameters for which ionization rates cannot accurately describe the correct results, and that one commonly used perturbation development is not generally applicable to resonant two-photon ionization. Extension of the model to $\mathrm{}(n+m)\mathrm{}$-photon ionization having an $m$-photon resonance is discussed. The model is applied to two- and three-photon ionization in Cs.