Mechanisms of Electron Losses: Electron-Ion Recombination

The ionization processes were considered as a source of electrons and positive ions, e.g., as a source of plasma generation. Conversely, the principal loss mechanisms of charged particles, the elementary processes of plasma degradation, will now be examined. Obviously, the losses together with the ionization processes determine a balance of charge particles and plasma density. The variety of channels of charged particle losses can be subdivided into three qualitatively different groups.

The first group includes different types of electron-ion recombination processes,in which collisions of the charged particles in a discharge volume lead to their mutual neutralization. These exothermic processes require consuming the large release of recombination energy in some manner. Dissociation of molecules, radiation of excited particles, or three-body collisions can provide the consumption of the recombination energy.

Electron losses, because of their sticking to neutrals and formation of negative ions, form the second group of volumetric losses, electron attachment processes. These processes are often responsible for the balance of charged particles in such electronegative gases as oxygen (and, for this reason, air); CO₂ (because of formation of O−); and different halogens and their compounds. Reverse processes of an electron release from a negative ion are called the electron detachment.

Note that although electron losses in this second group are due to the electron attachment processes, the actual losses of charged particles take place as a consequence following the fast processes of ion-ion recombination. The ion-ion recombination process means neutralization during collision of negative and positive ions.

Finally, the third group of charged particle losses is not a volumetric one like all those mentioned previously, but is due to surface recombination. These processes of electron losses are the most important in low pressure plasma systems such as glow discharges. The surface recombination processes are usually kinetically limited not by the elementary act of the electron-ion recombination on the surface, but by transfer (diffusion) of the charged particles to the walls of the discharge chamber.