Three-dimensional density profiles of argon metastable atoms in a direct current glow discharge: Experimental study and comparison with calculations

Three-dimensional density profiles of argon metastable atoms (Ar-m*) have been measured by laser-induced fluorescence in a direct current glow discharge with flat cathode, for a range of voltages, pressures and currents (i.e. 700-1200 V, 0.7-1.6 torr, 0.6-3.3 mA). The primary excitation line was taken to be the 794.818 nm line, whereas the 852.144 nm line was used as the fluorescence line. The profile is characterized by two distinct peaks, at 2-4 mm and about 12 mm from the cathode respectively. These peaks are explained as being caused by local Ar-m* production and loss processes, giving rise to local maxima which are not completely spread out by diffusion. The experimental data have been compared with the results of a mathematical model consisting of a balance equation with many production and loss terms. The theoretical profile also shows two peaks, but at somewhat different positions, and the first peak is much more intense. This suggests that the model is not yet able to describe the behavior of metastable atoms exactly, and that the glow discharge is hence more complex than often assumed. Nevertheless, comparison of the overall Ar-m* number density in the rest of the discharge volume indicates to us that a generally reasonable agreement has been reached between experiment and theory. (C) 1997 Elsevier Science B.V.