The photoionization rate and emitted electron angular distribution were computed for ground-state atomic hydrogen, in the two-photon regime (6.80–13.6 eV), for laser irradiances between 1013 and 1015 W/cra2, and for linear polarization. A direct numerical solution of the time-dependent Schrödinger equation (TDSE) was performed by a standard method and with slow laser turnon and turnoff pulse-envelope switching functions incorporated. Comparison is made with the predictions of a two-photon ionization rate formula, as described by Klarsfeld [Lett. Nuovo Cimemto 3, 395 (1970)] in second-order perturbation theory, into which a resonance width and threshold shift were inserted. Under the conditions of this problem the width was dominated by the one-photon Rabi rate, and the threshold shift by the ponderomotive energy. Photoionization rates and angular distributions determined from the TDSE agreed well with the values obtained from the extended Klarsfeld formula.
K. J. LaGattuta, "Two-photon ionization rates of atomic hydrogen: comparison of numerical and analytical techniques," J. Opt. Soc. Am. B 10, 958-962 (1993)