On the influence of metastable reactions on rotational temperatures in dielectric barrier discharges in He-N-2 mixtures

In dielectric barrier discharges in helium-nitrogen mixtures of 1 bar the r otational 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 differe nce comes from two different classes of excitation processes: 'fast' reacti ons mainly of He metastables and ions during or immediately after the activ e microdischarge, i.e. in a hot environment, and 'slow' reactions of the me tastable N-2(A (3)Sigma (+)(u)) state. Chemical reactions in general determ ine the effective lifetimes of the metastables. The long-living nitrogen me tastables diffuse far away from the microdischarge area and react in the (c ool) ambient gas. For the various discharge conditions we evaluated, from a bsolutely measured line intensities, the concentrations of N-2(A (3)Sigma ( +)(u)), NO, and OH (formed as a consequence of surface reactions) as functi ons of the average discharge power and estimated the electron density in th e discharge. The variation of the measured rotational temperature of the fi rst negative system with p(N2) can be attributed to changes of the gas heat ing and, consequently, the gas dynamics in the microdischarges.