FREQUENCY-EFFECTS AND CLASSICAL PATHS IN STRONG-FIELD IONIZATION

The ability of the SFA (strong-field approximation) to predict the ionisation of atoms at all frequencies is explored at low frequency by comparison with experiment. Excellent agreement is found over a very wide range of high intensities. At high frequency, where no precision strong-field experiments are available, a comparison is made between predictions of the SFA and a high-frequency theory due to Gavrila. Agreement in transition rates is very good. The disagreement in the assignment of energy conditions at high frequencies is explained as a difference in interpretation brought about by the gauge transformation employed by Gavrila. An examination of semiclassical path behavior of a photoelectron after ionization gives insight on the lower limits of intensity for which the SFA is applicable, and makes transparent the meaning of a recently applied Coulomb correction to the SFA for circularly polarized light. A related examination for linearly polarized light gives an effective high energy limit for intense-field photoelectron spectra.