Variations of the gas temperature in He/N2 barrier discharges

In order to obtain the gas temperature in the microdischarge areas and in the ambient gas of dielectric barrier discharges in helium/nitrogen mixtures we have studied the rotational temperature of the emission of molecular nitrogen ions. In these mixtures the excited state N-2(+)(B(2)Sigma (+)(u)) is formed mainly by Penning ionization. We have calculated the rotational spectra of the transition N-2(+)(B(2)Sigma (+)(u) --> X(2)Sigma (+)(g)) using a known reaction scheme for discharges in helium/nitrogen mixtures (Pouvesle et al 1982 J. Chem. Phys. 77 817) by averaging over the temporal variation of the gas temperature after the ignition of a microdischarge. We obtained this variation by solving the Navier-Stokes equations for this case. It is shown, that under our experimental conditions (p(He) = 38.5-156 kPa. p(N2) = 40-6000 Pa) the rotational population of the nitrogen ions corresponds to the equilibrium distribution at the temperature of the surrounding gas. Experimentally we obtained the rotational temperatures of molecular nitrogen ions by high-resolution optical emission spectroscopy. For the investigated range of nitrogen and helium partial pressures we fitted to these temperature values the calculated ones and obtained from these fits estimates of the following parameters of the microdischarges: 310 +/- 10 K as the ambient gas temperature and 28 +/- 1 mum as the microdischarge radius r.