INVESTIGATIONS OF ABOVE-THRESHOLD IONIZATION USING SUBPICOSECOND LASER-PULSES

At very high light intensities, the electron energy spectrum in multiphoton ionization (MPI) spectroscopy of even the simplest atoms changes from a single, well defined threshold peak into multiple peaks, separated from one another by the photon energy. This phenomenon is generally referred to as 'above-threshold ionization' (ATI). The original experiments investigating ATI used relatively long laser pulses, with the result that amplitudes, energy widths and angular distributions of the individual photoelectron peaks depended on the laser intensity. In addition, the widths of peaks, as well as their absolute energy positions, changed according to the temporal width of the laser pulse. These dependencies were not intrinsic to the ionization process, but rather were all eventually ascribed to ponderomotive forces exerted on free photoelectrons by the laser focus. The ponderomotive effects frustrated comparisons between theoretical calculations and experimental data. More recent studies have shown that a dramatic simplification occurs when MPI is studied with extremely short laser pulses: both the energies and the momenta of the ATI electrons become independent of either the laser energy or the pulse duration. Under these conditions, comparisons between theory and experiment can be made in sufficient detail to discriminate between competing models of the high-intensity ATI process.