Recollision

Recollision is a phenomenon in strong-field physics in which an electron that has been freed from an atom or molecule by a intense laser field is driven back toward its parent center by the oscillating electric field. Upon returning, the electron can scatter elastically or inelastically with the ion, or it can recombine with the ion, producing observable ultrafast phenomena. The concept is central to the three-step model of strong-field interactions: tunnel ionization, acceleration of the freed electron by the laser field, and return to the ion when the field reverses.

The returning electron can lead to several outcomes. In high-harmonic generation (HHG), recombination emits a photon with energy equal to the sum of the ionization potential and the kinetic energy of the recolliding electron, producing attosecond-scale light bursts. The maximum photon energy is commonly described by the cutoff rule IP + 3.17 Up, where IP is the ionization potential and Up is the ponderomotive energy of the oscillating field. In non-sequential double ionization (NSDI), the recollision transfers energy to a second bound electron, enabling simultaneous or sequential multiple ionizations.

Beyond atoms, the recollision picture informs processes in molecules and solids, where returning electrons or holes can scatter from molecular orbitals or electronic bands, contributing to spectroscopy and ultrafast dynamics. The concept, proposed and developed in the 1990s, remains a foundational framework for interpreting attosecond science, laser-induced electron diffraction, and strong-field chemistry.