HIGH-ORDER ABOVE-THRESHOLD IONIZATION OF ATOMIC-HYDROGEN USING INTENSE, ULTRASHORT LASER-PULSES

High-sensitivity photoelectron energy spectra of atomic hydrogen using strong 50 fs, 630 nm laser fields are investigated. Differences and s imilarities, especially with respect to the plateau observed recently with rare-gas atoms, are reported. An intensity-dependent study reveal s new details for two series of above-threshold ionization peaks which vanish at low energy. In particular, their shift is compatible with t he ponderomotive increase of the ionization limit. The angular distrib utions of the photoelectrons show an angle-dependent red-shift of the spectra with the maximum shift for a polarization of the laser field p ointing in the direction of the detector. We propose an explanation fo r this feature.