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.

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.