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 similarities, especially with respect to the plateau observed recently with rare-gas atoms, are reported. An intensity-dependent study reveals new details for two series of above-threshold ionization peaks which vanish at low energy. In particular, their shift is compatible with the ponderomotive increase of the ionization limit. The angular distributions of the photoelectrons show an angle-dependent red-shift of the spectra with the maximum shift for a polarization of the laser field pointing in the direction of the detector. We propose an explanation for this feature.