Direct evidence of ponderomotive effects via laser pulse duration in above-threshold ionization

Abstract

The energy spectra of electrons produced in strong-field above-threshold ionization are shown to be shifted and broadened when very short laser pulses are used. The shifts crucially depend on the ratio of the pulse duration to the lapse of time spent by the electron to escape from the focal spot. Red shifts up to 0.9 eV were measured with 30-ps pulses at 1064 nm and found to vary linearly with the light intensity. A theoretical model, based on a classical description of the electron behavior in strong fields, accounts for most of the experimental data.