Electron Impact Excitation of Positive Ions

Abstract

Publisher Summary This chapter discusses the excitation of positive ions by electron impact. Fine structure in the ion ground state can be of importance in calculating dielectronic recombination rates. The direct consequence of dielectronic recombination is to produce atoms in highly excited states. If the particle density is sufficiently low, these states will undergo further radiative decay. At higher densities, it is necessary to consider further collisional processes involving the highly excited states. The density dependence of the recombination rate coefficient is quite different for the case in which dielectronic recombination can be neglected and the case for which dielectronic recombination is the dominant process for radiative capture. In cases, recombination and subsequent stabilization is by the emission of radiation. For the case of no dielectronic recombination, the number of atoms in excited states is smaller than the number for thermodynamic (Saha) equilibrium. The effect of collisions at higher densities is to bring the populations closer to thermodynamic values and hence to increase the recombination rate. When dielectronic recombination is important, the number of atoms in highly excited states is larger than the thermodynamic value. The effect of bringing the populations closer to thermodynamic equilibrium, by collision at higher densities, is now to decrease the recombination rate.