Spectroscopic study of the positive column in a helium-cadmium capillary glow discharge

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Abstract
The purpose of the paper is the determination of the main mechanisms populating the upper level BL of the 441.6 nm laser line in optimum experimental conditions in the positive column of a He-Cd discharge. The problem needing to be clarified is whether the stepwise electron collision or the Penning process is the dominant excitation process for the BL state. For this spectroscopic study, the experimental cell the authors use is similar to the Nagoya group one in order to have an available comparison. The n=2 He excited state concentrations are measured as functions of cadmium density by self-absorption, laser absorption and laser induced fluorescence methods in a He-Cd discharge. The decrease of the He(n=2) concentrations with cadmium density is dominated by the simultaneous decrease of electron energy. Two kinds of model are used to estimate the electron temperature, the Cd II ion concentration and the Cd II excited states population densities. By combining experimental results with modelling, the population rates by electron collisions and Penning ionisation of the upper level (BL) of the 441.6 nm laser line are determined. Finally, in the conditions of maximum emission of the 441.6 line, BL appears to be excited by 75% electron collisions and 25% Penning collisions. But if the electron collision quenching rate of the BL level is taken into account, these percentages becomes 60% and 40%. Thus the two processes, electron collision and Penning ionisation, are similarly important in the balance equation. It is necessary to consider the electron collision quenching of the BL level, because this elementary process redistributes the population between the neighbouring sub-levels and limits the population inversion. All the experimental and modelling results are compared with the works of the Nagoya group and a fairly good agreement is obtained.

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