ATOMIC-HYDROGEN IN INTENSE SHORT LASER-PULSES - A NEW SERIES OF PHOTOELECTRON PEAKS FROM ABOVE-THRESHOLD IONIZATION

We have studied atomic hydrogen in intense, short, visible laser pulses (I less than or similar to 1.2 x 10(14) W cm-2, tau almost-equal-to 0.5 ps, lambda almost-equal-to 608 nm). At high intensities (I > 6 x 10(13) W cm-2) we observe a new series of higher order above-threshold ionization peaks in the photoelectron energy spectrum. We discuss some tentative explanations Two models are described in some detail. One relies on ionization saturation, the second one may be named 'parking of atoms as Rydberg wavepackets'.