ABOVE-THRESHOLD IONIZATION SPECTRA FOR HYDROGEN-ATOMS EXPOSED TO ULTRA-INTENSE LINEARLY POLARIZED HIGH-FREQUENCY LASER-PULSES

The time evolution of H(1s) atoms in very intense (I(o) almost-equal-to 3.5 x 10(18) W W cm-2) high-frequency (omega = 1 au) laser fields is calculated by numerical integration of the time-dependent Schrodinger equation in the Kramers-Henneberger frame. ATI spectra are analysed as a function of pulse duration for a sin2 pulse shape and pulse durations in the sub-picosecond range. A prominent feature in these spectra is a combination of peak broadenings due to interaction time and laser power. For short pulses the spectra are dominated by ionization early during the pulse. For the longer pulses we find an additional series of peaks that correspond to ionization from the Stark-shifted H(nl) levels (n > 1) near the peak of the laser pulse. Such peak positions (for n = 1) were predicted before by Pont et al (1990) using a high-frequency Floquet theory, but have been calculated for the first time in an ab initio approach for finite laser pulses.