ChargeExchange

Charge exchange is a collisional process in which a positively charged ion, often highly charged, captures one or more electrons from a neutral atom or molecule during a close encounter. The captured electron typically places the ion in an excited electronic state, and as the ion relaxes to lower energy levels, photons are emitted. In many cases, the emitted photons lie in the ultraviolet or X-ray part of the spectrum, especially for highly charged ions.

The effect changes the charge balance of the colliding species: the ion’s charge is reduced by the number of captured electrons, while the neutral donor becomes ionized. Single-electron capture is the most common form at many collision energies, though multiple-electron capture can occur under suitable conditions. The process can proceed via radiative and nonradiative channels; in radiative charge transfer, the electron is captured with simultaneous photon emission, while in nonradiative transfer the energy is dissipated through other pathways.

Charge exchange is important in both astrophysical and laboratory contexts. In space, solar wind ions capture electrons from neutral gas in comets, planetary exospheres, and the heliosphere, producing characteristic X-ray line emission that contributes to the soft X-ray background. In laboratories, charge-exchange reactions are used as diagnostic tools (charge-exchange spectroscopy) to study ion temperatures and flows, and to calibrate instruments in fusion devices and beam experiments.

Spectroscopic signatures from charge exchange help identify the interacting species and the physical conditions of the environment. The cross sections and emitted spectra depend on the ion charge state, the donor species, and the collision energy, making charge exchange a versatile probe in plasma and astrophysical research.