Measurement of rotational temperatures in high-pressure microhollowcathode (MHC) and capillary plasma electrode (CPE) discharges

We report the results of rotational temperature (TR) measurements in high-pressure (400 Torr) microhollow cathode (MHC) discharges in Ne with a trace admixture of N₂ using the unresolved N₂ second positive band and the N₂⁺ first negative band. Signičantly different values for TR were obtained for respectively N₂ and N ₂⁺ . The rotational temperatures obtained from the analysis of the N₂ band system are slightly above room temperature, increase with increasing discharge current, and may be interpreted as the gas kinetic temperature in the MHC discharge plasma. By contrast, the analysis of the N₂⁺ band system yielded rotational temperatures of more than 900 K, which are the result of collisional reaction processes leading to the formation of rotationally excited N₂⁺ ions. The effective lifetime of these ions is comparable to the rotational relaxation time, so that the ions retain much of their rotational excitation prior to emission and are not in thermal equilibrium with the bulk gas. Thus, the TR values obtained for N₂⁺ cannot be equated with the gas kinetic temperature in the plasma. We also carried out a rotational analysis of the unresolved N₂ second positive system emitted by an atmospheric-pressure capillary plasma electrode (CPE) discharge in ambient air and measured a temperature of 545 K. This temperature may be close to the gas kinetic temperature as the rotational analysis utilized N₂ emissions from inside the capillary which is the region of highest plasma density and highest gas temperature in a CPE discharge.