Rescattering effects in the multiphoton regime

The plateau features that characterize the low-frequency spectra of fundamental strong-fi eld processes such as harmonic generation, above-threshold ionization and laserassisted 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|. The rescattering scenario (RS) [1–3] is central to current understanding of fundamental strong-fi eld processes such as above-threshold ionization/detachment (ATI/ATD), harmonic generation (HG) and multiple ionization owing to the insight it provides on the physics of strong laser-atom interactions, especially on the origin of plateau structures in high-energy ATI and HG spectra [4]. In the RS for these processes, the initial step is the laser ionization of a bound electron, which is assumed to escape by means of tunnelling, so that it has initially zero kinetic energy, E = 0. The next (main) step of the RS assumes that the electron may be returned by the driving laser fi eld along a closed classical trajectory to the same (spatial) point where it was “born.” Quasiclassical analyses [4] show that the energies acquired by the electron from the laser fi eld along its various trajectories have relative maxima, Ei, at return times τi (τ0 < τ1 < · · ·, where τ0 is of the order of a laser period). The maximum energy gained is Ei max ≡ Ecl ≈ 3.1732Up, where Up = e 2F2/(4mω2) is the ponderomotive (or mean quiver) energy of a free electron in a laser fi eld of amplitude F and frequency ω. The energy Ecl, a key quantity of the RS, is intimately related to the tunnelling step since it is the maximum energy that can be gained by a classical electron having zero initial energy; it corresponds to the trajectory having the shortest return time, τ0. In the fi nal (re-collision) step of the RS, the laser-accelerated electron may (1) expend some of its accumulated energy E to excite a core electron (as in multiple ionization) or (2) recombine with the atomic core, converting its energy to harmonic photons (of maximum energy Ω ≈ |E0| + Ecl , where |E0| is the ionization energy) or (3) scatter from the atomic core, thus contributing to one of the high-energy peaks on the ATI plateau, whose cut-off energy, Ecut ≈ 10Up [4], is also closely related to Ecl (cf equations