Time-resolved populations of N2(A3Σu+,v) in nanosecond pulse discharge plasmas

Abstract

Absolute time-resolved populations of N2 in the excited A3Σu+ electronic state that are generated in a repetitive ns pulse discharge in nitrogen have been measured by Cavity Ring Down Spectroscopy (CRDS) and Tunable Diode Laser Spectroscopy (TDLAS). CRDS measurements of N2(A3Σu+,v = 0–2) populations are made in the discharge afterglow at pressures of 22 and 39 Torr. The data reduction procedure takes into account the linewidth of the pulsed laser source, which is comparable with the absorption linewidth and results in a non-single exponential ring down decay. Peak N2(A3Σu+,v = 0–2) populations after a 10-pulse ns discharge burst are in the range of 1012–1013 cm−3. In the afterglow, these populations exhibit a relatively slow decay with the characteristic time of approximately 500 μs, most likely due to the quenching by N atoms. TDLAS data have been taken at a higher pressure of 132 Torr. Absolute time-resolved N2(A3Σu+,v = 0,1) number densities are measured during ns pulse discharge bursts up to 5 pulses long and in the afterglow, peaking at 6·1012 cm−3 and 3·1013 cm−3, respectively. The results indicate that N2(A3Σu+) is generated after every discharge pulse on a 20–50 μs time scale, much longer compared to the discharge pulse duration of ~100 ns, and subsequently decays between the pulses. The decay rate increases during the discharge burst. In the afterglow, N2(A3Σu+,v = 0,1) population decay significantly more rapidly compared to the low-pressure CRDS conditions, with the characteristic time of approximately 160 μs after a 5-pulse discharge burst.