A spectroscopic study of a He-Cd discharge created in a concentric cylinder hollow cathode structure

Abstract

In this paper, a large spectroscopic study of a hollow cathode discharge produced in a concentric cylinder perforated tube is presented. First, the NCd cadmium ground state and the He(23S) metastable concentrations are determined from laser absorption measurements. Then, the thermal electron energy Te is estimated to be 0.5 eV from LIF observations, which is confirmed by probe measurements. The net thermal electron density (5*1012 to 2*1013 cm-3 for 1012<NCd<1014 cm-3 at 150 mA) is determined from H beta Stark broadening and from the He(33P) total quenching rate measured by LIF on the He 388.9 nm line. Moreover, for several experimental conditions, many Cd, Cd+, He and He+ excited state population densities are deduced from calibrated emission spectroscopy. The population inversions between the 6G and 4F levels and between the 6F and 6D levels are observed. The voltage-current characteristics versus cadmium density and helium pressure are also recorded and related to the atomic excitation in the discharge. From the behaviour of the helium and cadmium excited state population, for different experimental conditions, the part played in excitation professes by the three electron groups (thermal, secondary, primary electrons) can be discriminated. Consequently an estimate of the electron energy distribution function is deduced by combining the absolute population measurement and rate equation for several He or Cd+ excited states chosen to probe a particular range of electron energy. Finally, the populating channels of several Cd+ excited states are investigated. It is determined that the mixed 9S, 9P, 8D, 6F , 6G and 6H levels are mainly populated by a charge transfer reaction with a total rate coefficient of 4.23*10-10 cm3 s-1. The 5F, 5G and 7D levels are essentially populated by radiative cascades and the 4F, 6D and 7P levels are populated by radiative cascades and by electron excitation from the Cd+ ground state.