Recombination of Ions and Electrons

the formation of neutral atoms or molecules from free electrons and positive atomic or molecular ions; it is a process inverse to ionization. Recombination occurs primarily in ionized gases and in plasmas and, in the absence of counteracting factors, results in the almost complete disappearance of charged particles. The recombination of ions and electrons produces atoms and molecules not only in the ground state but also in excited states. The energy W released in a recombination event (minus the kinetic energy of the recombining particles) may therefore be different. Recombination is characterized by the recombination coefficient α; when multiplied by the product of the charge concentrations, α gives the recombination rate, that is, the rate of disappearance of charged particles.

In radiative recombination, W and the kinetic energy E of the captured electron are released in the form of the energy of a quantum of electromagnetic radiation:

A⁺ + e → A* + hv

where A⁺ is a singly charged ion, e is an electron, h is Planck’s constant, ν is the frequency of the radiation, and the symbol * indicates that the atom A may be in an excited state.

Nonradiative recombination of atomic positive ions and electrons, or dielectronic recombination, is possible if the atom A has discrete energy levels that exceed its normal ionization energy. When an electron is captured at such a level, recombination is stable only if the electron moves to a lower level (radi-atively, for example) sufficiently quickly after capture.

In dissociative recombination, the capture of electrons by molecular positive ions causes the formation of unstable neutral molecules, which are subject to dissociation. For diatomic ions this process is

AB⁺ + e → AB* → A^→ + B^→

where the symbol → indicates that W is partially converted into the kinetic energy of the particles produced, which may be in excited states.

In three-body recombination, a third particle takes part in the recombination of a positive ion with an electron. W is converted into kinetic energy:

A⁺ + e + B → A^→ + B^→

In a plasma with low ion and electron concentrations, an atom or molecule may be the third particle. In a plasma with a high degree of ionization, a positive ion or second electron may also be the third particle.

Recombination in a decaying plasma is accompanied by the emission of spectral lines by the atoms formed in excited states. Optical techniques are used to determine α from observations of this afterglow of the plasma; probe methods are also used. The study of recombination plays an important role in the determination of various plasma parameters and the detailed investigation of different types of electrical discharge in a gas. Recombination is evidenced in a number of astrophysical phenomena occurring, for example, in galactic nebulae and the plasma of the solar corona. Recombination also has a considerable influence on processes in the ionized layers of the earth’s upper atmosphere (seeIONOSPHERE).