Collisional ionization ($A+e^{-}\to A^{+}+2e^{-}$)

In collisional ionization, an energetic collision frees a bound electron. The process consumes kinetic energy and changes the electron and ion populations controlling plasma cooling. In a thermal gas its rate competes with recombination and depends strongly on the ionization threshold relative to temperature.

Electron impact can excite an ion into a state above an electron-emission threshold, which then ejects a bound electron by autoionization. This two-stage excitation-autoionization contributes to collisional ionization in addition to direct impact removal.

In collisional ionization equilibrium, collision-driven ionization and recombination balance for the populated ion stages, determining their relative abundances at a given temperature. The resulting cooling curve includes the loss of line-emitting bound states as temperature increases. Rapidly cooling gas or external photoionization can invalidate this equilibrium assumption.

The coronal approximation treats an optically thin, low-density plasma with electron-impact ionization balanced by radiative and dielectronic recombination. Excited-level populations are determined mainly by collisional excitation and radiative decay, not by a Boltzmann distribution over levels. Ion fractions can depend primarily on electron temperature while metastable populations retain density dependence; this distinction enables density diagnostics.