One- and two-electron numerical models of multiphoton ionization of helium

We discuss the calculation of ionization rates of helium using time-dependent solutions of the full-dimensional Schroedinger equation in conjunction with time-dependent solutions of a single active electron (SAE) model of helium. The SAE model is a one-electron atom with a non-Coulombic effective potential that can be tuned in certain limits to give near quantitative agreement with the results of the full-dimensional integration. We show how the tuned SAE model can be used to improve the accuracy and reliability of the calculations of ionization cross sections. In addition we consider a case in which failure of the SAE model can be attributed to strong correlation effects in laser-driven helium, specifically interaction with an intermediate doubly excited (autoionizing) state of helium. (author)