Numerical calculation of above-threshold-ionization of hydrogen atoms in short intense laser pulses

The time-dependent Schrodinger equation in the Kramers-Henneberger frame has been integrated numerically in Cartesian coordinates in order to calculate the time evolution of ground-state hydrogen atoms in short, intense laser pulses with linear polarization. Energy distributions of the ionized-electrons were calculated by the spectral method. For moderate intensities they show many above-threshold-ionization peaks. For ultra-intense laser fields the electron energy distributions acquire a uniform shape. Results are shown for laser pulses of 20 and 40 cycles with a frequency of omega =0.4 au (wavelength lambda =114 nm), peak intensities in the range of 10 14 -10 20 W cm 2 and a sin 2 -shaped envelope. The total ionization yield for these pulses shows little dependence on the pulse duration. Non-perturbative behaviour is observed in the contributions towards ionization from above-threshold-ionization peaks of order 1 to 5 as a function of peak intensity.