The motion of slow positive ions in gases - Critical review

Abstract

A full understanding of electrical discharges and plasma requires knowledge of the motion of electrons and ions in gases in electric fields. For electrons, a large amount of published information (see, for example, Townsend 1915, 1925; Thomson & Thomson 1928; 1933; Healey & Reed 1941; Allis 1956; Bates 1962; Hasted 1964; McDaniel 1964) exists concerning collisional properties such as the mean free path, mean energy, diffusion coefficient and mobility. For ions, on the other hand, basic data on the various collisional properties other than mobilities is much more scanty. Positive ions, however, play an important role in the mechanism of electrical discharges not only in the initial breakdown of the gas but also in the development of the current and in the maintenance of the discharge (Llewellyn Jones 1953, 1957). The various collisional processes involving ions such as the liberation of secondary electrons from the cathode, space charge formation, ambipolar diffusion, recombination and charge exchange have been discussed by Mott & Massey (1949) and by Massey & Burhop (1952). Although positive ion mobilities have been studied by many investigators (see Bates 1962) there are considerably fewer results available about the other collisional properties, especially in weak electric fields. This is due to the formidable difficulties encountered in both the theoretical and experimental studies especially in the case of slow ions. Theoretical calculations relating to the collisional properties of positive ions require a knowledge of the forces of interaction exerted between ion and molecule. When the period of interaction is comparatively long, as in the case of slow positive ions colliding with gas molecules, these forces of interaction are often unknown. This is particularly so for positive ions travelling through their parent gas where the phenomenon of charge exchange occurs. Hence, in these circumstances, accurate experimental data concerning such slow collisions are of particular value.