ABSORPTION-SPECTROSCOPY ON THE ARGON 1ST EXCITED-STATE IN AN EXPANDING THERMAL ARC PLASMA

Absorption spectroscopy was used to determine the density of argon atoms in the first excited state in an expanding cascaded arc plasma jet, in order to estimate their relative contribution to the energy exchange processes in plasma jets used for deposition purposes. In the spectroscopic setup a stagnant cascaded arc was used as a high-intensity continuum light source and optical multichannel detection was employed. Measurements were done on the strong Ar(3p(5)4s-3p(5)4p) radiative transitions in the range of 794.8-852.1 nm. An efficient numerical method for obtaining radial density and temperature profiles, using line of sight integration instead of Abel inversion, has been developed. For a purely argon plasma the Ar(3p(5)4s) metastable and resonant state densities lie in the range of 10(18)-10(16) m-3 at a chamber pressure of 40 Pa. The observed decay of the densities in axial direction was also modeled using the quasistationary continuity equation. It appears that the population density of the argon 4s states is determined mainly by the production due to the three-particle recombination and by the partial trapping of resonance radiation. The total densities of the argon first excited state are approximately a factor of 10 lower than the argon ion densities, so the contribution of these states to energy exchange processes in expanding cascaded arc deposition plasmas is limited compared to that of the argon ions.