Characterization of excited species in low-density plasmas

Excited species in low density rare gas discharges have been characterized using both optical emission and laser techniques. A thorough investigation of the A2Σ+-X2πi origin band of OH radical at 306.4 nm was carried out in both argon and helium reduced pressure inductively coupled plasmas. The A2Σ+ state rotational population distributions have been studied under a variety of conditions of power and gas flow rate. The rotational state population distributions are generally nonlinear but have been quantitatively described as the sum of two separate Boltzmann distributions. The metastable species produced in a helium hollow cathode lamp and a rare gas flowing afterglow have been studied using laser techniques. In the rare gas flowing afterglow, the metastable species re spatially separated from the active discharge. The optogalvanic transients obtained by laser pumping the 388.9 nm atomic helium transition in a helium afterglow are very similar to those obtained in the hollow cathode lamp. Transient optogalvanic signals have also been observed in neon and argon flowing afterglows when the laser was tuned to transitions originating in the metastable lower states of these atoms. Different mechanisms for the origin of the optogalvanic effect in the hollow cathode lamp and the flowing afterglow are discussed