Investigations of above-Threshold Ionization Using Subpicosecond
Laser-Pulses
Journal of Physics B-Atomic Molecular and Optical Physics, 24 (2):
325-347 (January 1991)Ev896 Times Cited:176 Cited References Count:76.Abstract
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.