A. V. Flegel

Plateau effects in the spectra of electrons scattered from atoms in a strong laser field

An exact quantum solution of the problem of electron scattering from a short-range potential in the presence of a strong elliptically polarized laser field is obtained. The differential scattering cross section as a function of the number of absorbed (or emitted) photons exhibits a plateau caused by the rescattering of electrons from the scattering center. Numerical results for a linearly polarized laser field are presented, and it is shown that the plateau boundaries agree well with classical estimates.

Analytic description of the high-energy plateau in laser-assisted electron–atom scattering

A closed-form analytic formula describing plateau features in laser-assisted electron–atom scattering (LAES) is derived quantum mechanically in the low-frequency limit. The presented formula confirms the classical rescattering scenario for LAES and provides an analytic explanation for oscillatory structures in the high-energy part of LAES spectra.

Rescattering effects in laser-assisted electron–atom bremsstrahlung

Rescattering effects in nonresonant spontaneous laser-assisted electron-atom bremsstrahlung (LABrS) are analyzed within the framework of time-dependent effective-range (TDER) theory. It is shown that high energy LABrS spectra exhibit rescattering plateau structures that are similar to those that are well-known in strong field laser-induced processes as well as those that have been predicted theoretically in laser-assisted collision processes. In the limit of a low-frequency laser field, an analytic description of LABrS is obtained from a rigorous quantum analysis of the exact TDER results for the LABrS amplitude. This amplitude is represented as a sum of factorized terms involving three factors, each having a clear physical meaning. The first two factors are the exact field-free amplitudes for electron-atom bremsstrahlung and for electron-atom scattering, and the third factor describes free electron motion in the laser field along a closed trajectory between the first (scattering) and second (rescattering) collision events. Finally, a generalization of these TDER results to the case of LABrS in a Coulomb field is discussed.

Effects of the atomic structure and interference oscillations in the electron photorecombination spectrum in a strong laser field

Electron photorecombination in the presence of a strong low-frequency laser field has been analyzed using an exactly solvable quantum model. It has been shown that interference patterns in the electron photorecombination spectrum are more sensitive to the details of the atomic potential than those in the case of other non-linear processes. For electron photorecombination on Xe+ ions, the manifestation of the Ramsauer effect in the cross section for the elastic scattering of slow electrons on Xe+ ions has been predicted in the electron photorecombination spectrum modified by the laser field.

Description of harmonic generation in terms of the complex quasienergy. I. General formulation

Using the Hellmann-Feynman theorem for quasistationary quasienergy states of a quantum system in an intense laser field, we present the high-order harmonic generation (HHG) amplitude in terms of the complex quasienergy of a bound electron, thereby avoiding the necessity for an explicit form for the electron wave function in HHG calculations. This formulation for the HHG amplitude confirms use of the dual dipole moment (instead of the dipole moment expectation value) in wave-function-based HHG calculations in order to properly account for both the ionization and the Stark shift of the initial bound state in a strong laser field.

Description of harmonic generation in terms of the complex quasienergy. II. Application to time-dependent effective range theory

A formulation for the high-order harmonic generation (HHG) amplitude [M. V. Frolov et al., Phys. Rev. A 75, 063407 (2007), preceding paper] is employed to provide analytical results for HHG rates within our recently developed time-dependent effective range (TDER) theory (for time-dependent problems involving weakly bound electron systems). Exact and approximate (including quasiclassical) TDER HHG rates are employed to analyze the accuracy of common approximate methods for HHG calculations. For various specific negative ions with s and p outer electrons, numerical results for HHG spectra are presented over a wide interval of laser frequencies (extending from the tunneling to the multiphoton regimes). The role of initial bound state symmetry effects on the HHG spectra is also analyzed. Finally, Coulomb corrections to TDER results for HHG rates are introduced and discussed.

Cutoffs of high-energy plateaux for atomic processes in an intense elliptically polarized laser field

We present analytical estimates for high-energy plateau cutoff positions in the spectra of the most common laser–atom processes (above-threshold detachment/ionization, high-harmonic generation and laser-assisted electron–atom scattering) for the case of an elliptically polarized, low-frequency laser field.

Threshold Phenomena in Electron–Atom Scattering in a Laser Field

Effective-range theory is developed to describe the processes of multiphoton bremsstrahlung absorption and the emission of laser radiation that accompanies the scattering of electrons from atoms in a laser field. It is found that the cross sections for multiphoton absorption in the plateau region increases resonantly for electron energies corresponding to the thresholds of induced bremsstrahlung (i.e., multiples of the photon energy). It is shown that this effect is caused by the laser-field modified threshold phenomena in the cross sections for multichannel reactions (Baz’ threshold anomalies).

Laser-induced resonant structure in electron-atom scattering

Orders of magnitude increases are predicted in the cross sections for electron-atom scattering accompanied by absorption or emission of n laser photons for incident electron energies at which the electron, by emitting μ laser photons, can be captured by the atom to form a negative ion. Resonance enhancements are most significant in the plateau region (n μ) of the scattered electron spectrum, whose shape is predicted to replicate that of the ions (n + μ)-photon detachment spectrum.

Rescattering effects in the multiphoton regime

The plateau features that characterize the low-frequency spectra of fundamental strong-field processes such as harmonic generation, above-threshold ionization and laser-assisted electron–atom scattering are shown to exist also for photon energies Eγ of the order of the energy |E0| of a bound electron. The existence of these rescattering effects in such a high-frequency (and thus nontunnelling) regime is supported by accurate quantum analyses of intense Ti–Sapphire laser interactions with halogen negative ions, for which Eγ ≈ 0.5|E0|.