Ion Collisions with Molecules

Abstract

The most prominent characteristic of an ion is that its mass is much larger than the electron mass. Under the same electric field, the ion is much less accelerated than the electron. An ion loses a large fraction of its energy at a collision with neutral molecules, even in an elastic collision. Thus the ions in a molecular plasma do not much deviate from the thermal equilibrium with the neutral molecules, unless the applied field is too strong. This means that the mean energy (i.e., the temperature) of ions is often close to the gas temperature. According to the estimate by Phelps [134], the ion temperature is less than 10 eV, unless the reduced electric field, E/N, exceeds 103 Td (1 Td = 10?17 Vcm2). In the present chapter, we mostly deal with the ion–molecule collisions at the ion energies below about 10 eV.