Strong-Field Threshold Ionization: An Asymptotic Wavefunction Ansatz

Abstract

We discuss an analytic model of threshold ionization in an intense laser field. It is based on a strong-field wavefunction Ansatz that satisfies the long range asymptotic behavior of Coulomb wave plus the laser field at large distances. Results of illustrative calculations of the energy and momentum distributions of the electrons, from ionization of a hydrogenic atom by intense mid-infrared laser field, are presented and discussed. The results indicate that the energy distribution of ionization reaches a finite non-zero value at the threshold (at zero kinetic energy) and, approaches it in an oscillatory way. The parallel momentum distribution close to the polarization axis is characterised by a two-peak structure and show rapid high-amplitude oscillations very close to the threshold. Integrated over small values of the perpendicular momentum, it shows a sharp maximum at zero parallel momentum. The perpendicular momentum distribution close to a direction transverse to the polarization axis shows a cusp-like fall-off at the threshold momentum. Integrated over small values of the parallel momentum, the transverse distribution is dominated by the sharp cusp-structure at the threshold. These prominent qualitative behavior of the ionization energy and momentum distributions at and near the threshold are associated with the long-range Coulomb interaction included in the present wavefunction Ansatz. It contrast, they are absent in the corresponding distributions given by the usual “strong-field approximation” (or SFA) that is based on the final plane-wave Volkov state.