Kollisionsionisation

Kollisionsionisation is a term used in plasma physics to denote the ionization of atoms or ions caused by energetic particle collisions. The most common driver is electron impact ionization, in which a free electron with sufficient kinetic energy transfers energy to a bound electron, ejecting it and creating a higher charge state. Colliders may also be protons or heavier ions, particularly in hot, partially ionized plasmas.

Mechanisms include direct ionization by impact and indirect processes such as excitation to a state that autoionizes and contributes an ionization pathway. In dense plasmas, three-body collisions can also contribute to the overall ionization rate.

Quantitatively, ionization rates are described by cross sections σ(E) and rate coefficients ⟨σv⟩, which are integrated over the velocity distribution of the colliders (often a Maxwellian in laboratory or astrophysical plasmas). These rates determine the ionization balance alongside recombination processes.

In many contexts, the concept of collisional ionization equilibrium (CIE) is used, where ionization by collisions and recombination reach a steady state. The resulting ionization fractions depend mainly on temperature and density and are key to modeling hot astrophysical plasmas, such as the solar corona and the interstellar medium, as well as laboratory plasmas in fusion devices.

Kollisionsionisation is distinct from photoionisation, where photons provide the energy to ionize atoms. In practice, many plasmas experience both processes, and modeling often uses collisional-radiative frameworks to account for both mechanisms and their interplay.

See also: collisional ionization, excitation-autoionization, ionization cross section, collisional-radiative modeling.