Thermometry in noble gas dielectric barrier discharges at atmospheric pressure using optical emission spectroscopy

We measure the temperature, T, of dielectric barrier discharges (DBD), in noble gases using optical emission spectroscopy (OES), by analysing rotational bands in the emission spectra of the first negative system (FNS) of N₂⁺. This has the advantage that rotational structure can be fully resolved even with a spectrograph of average performance, and that the rotational temperature, T_rot (~ T_gas) can then be determined from a conventional Boltzmann plot. Ionization of N2 occurs mainly via Penning transfer from metastable excited states of He (ca. 20 eV) or Ne (ca. 16.6 eV). Using two glass-walled DBD chambers of very different volumes (0.1 and 20 liters), we have studied atmospheric-pressure discharges in flowing helium (He) or neon (Ne) containing traces of nitrogen. Discharges were excited by audio-frequency (10 kHz) high voltage (HV) using a needle as the HV electrode and a dielectric (alumina)-covered planar grounded counter-electrode. OE spectra were acquired with a 0.5 m focal length spectrograph, coupled to an intensified charge coupled device (ICCD) detector. Using the (0-0) R-branch of the FNS N₂⁺ (B²Σ_u⁺ - X²Σ_g⁺) bands near a wavelength of 391.4 nm, we have measured axial (inter-electrode) distributions of T_rot for the two different reactor volumes in both He and Ne. T_rot values were found to be highest at the needle electrode, of about 450 K and 740 K for He and Ne, respectively; in He, T_rot dropped to a minimum of about 405 K at the mid-gap position in the small chamber, and ~ 360 K near the planar electrode in the large chamber. We conclude that temperatures in noble gas discharges depend critically on thermal conductivities of the particular gases (K_He = 1.9; K_Ne = 0.6, both in mW.cm^-1.K^-1) and on other experimental factors that influe- - nce heat transfer.