recolliding

Recolliding, in the context of physics and atomic/molecular interactions, refers to the process by which a charge carrier that has been driven away by an external field returns to collide with its original bound system. In strong-field laser physics, recollision is a central mechanism that underpins several nonlinear phenomena and is often described using the three-step model.

The three-step model describes the sequence: first, an electron is liberated from the atom or molecule by the intense laser field (often via tunnel ionization). Second, the free electron is accelerated by the oscillating electric field of the laser. Third, the electron is driven back toward the parent ion and collides with it, transferring energy to the ion or emitting radiation upon recombination. The likelihood and outcome of the recollision depend on laser parameters such as intensity, wavelength, and polarization; linear polarization is most conducive to return, while circular polarization largely suppresses it.

Recollision drives several important strong-field processes. High-harmonic generation arises when the returning electron recombines and emits a high-energy photon, producing a characteristic plateau in the spectrum with a cutoff approximately at Ip + 3.17Up, where Ip is the ionization potential and Up is the ponderomotive energy. Nonsequential double ionization occurs when recollision provides sufficient energy to eject a second electron. Recollision also enables laser-induced electron diffraction, offering a route to ultrafast imaging of molecular structure.

Historically, the concept was developed in the 1990s as part of the three-step framework and has since become foundational in attosecond science and strong-field physics.