Abstract
The origin of differences in the rotational temperatures of various molecules and ions ( (B), OH(A) and N2(C)) is studied in helium atmospheric-pressure guided streamers. The rotational temperature of (B) is room temperature. It is estimated from the emission band of the first negative system at 391.4 nm, and it is governed by the temperature of N2(X) in the surrounding air. N2(X) is ionized by direct electron impact in the outer part of the plasma. (B) is deactivated by collisions with N2 and O2. The rotational temperature of OH(A), estimated from the OH band at 306.4 nm, is slightly higher than that of (B). OH(A) is excited by electron impact with H2O during the first 100 ns of the applied voltage pulse. Next, OH(A) is produced by electron impact with OH(X) created by the quenching of OH(A) by N2 and O2. H2O diffuses deeper than N2 into the plasma ring and the rotational temperature of OH(A) is slightly higher than that of (B). The rotational temperature of N2(C), estimated from the emission of the second positive system at 315.9 nm, is governed by its collisions with helium. The gas temperature of helium at the beginning of the pulse is predicted to be several hundred kelvin higher than room temperature.
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