An experimental technique for the determination of the electron collision cross sections for momentum transfer of alkali metals at low electron energies. (Plasma conductivity measurement)

Abstract

The authors describe a means of determining with high accuracy the electron collision cross sections for momentum transfer of alkali metals at electron energies approximately 0.1 to 0.3 eV. The technique takes advantage of the fact that, at such low energies, the collision cross section or argon is approximately three orders of magnitude less than that of the alkali metals. Argon at atmospheric pressure is 'seeded' with 1 or 2% of the appropriate alkali metal vapour and the mixture heated in a furnace to 1000 to 2000 K. A plasma is formed in which virtually all the electrons are produced by ionisation of the alkali metal, and in which the argon atoms play an almost insignificant role in the electron scattering processes. The electrical conductivity of the plasma is therefore almost totally a function of the alkali metal seed, the argon playing a purely passive role. The conductivity of the plasma is accurately measured using an electrode system incorporating a guard ring and having facility for varying the electrode separation. From a knowledge of the plasma temperature, the electron density can be deduced using Saha's equation; knowing the electron density and the plasma conductivity, the electron collision frequency can be calculated. The collision cross section for momentum transfer of the alkali metal can then be deduced if the partial pressures of the seed and argon are known.