On the influence of metastable reactions on rotational temperatures in dielectric barrier discharges in He-N2 mixtures

In dielectric barrier discharges in helium-nitrogen mixtures of 1 bar the rotational temperature of the first negative system (transitions N-2(+)(B -->X)) depends on the nitrogen partial pressure P-N2 and increases up to 600 K at high p(N2) while that of the second positive system (transitions N-2(C -->B)) is equal to 310 +/- 10 K for all discharge conditions. This difference comes from two different classes of excitation processes: 'fast' reactions mainly of He metastables and ions during or immediately after the active microdischarge, i.e. in a hot environment, and 'slow' reactions of the metastable N-2(A (3)Sigma (+)(u)) state. Chemical reactions in general determine the effective lifetimes of the metastables. The long-living nitrogen metastables diffuse far away from the microdischarge area and react in the (cool) ambient gas. For the various discharge conditions we evaluated, from absolutely measured line intensities, the concentrations of N-2(A (3)Sigma (+)(u)), NO, and OH (formed as a consequence of surface reactions) as functions of the average discharge power and estimated the electron density in the discharge. The variation of the measured rotational temperature of the first negative system with p(N2) can be attributed to changes of the gas heating and, consequently, the gas dynamics in the microdischarges.