TIME-RESOLVED POPULATIONS OF N2(A3Σu+,v) IN NANOSECOND PULSE DISCHARGE PLASMAS

Abstract

Absolute time-resolved populations of N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$) excited electronic state generated in a repetitive ns pulse discharge in nitrogen have been measured by Cavity Ring Down Spectroscopy (CRDS) and Tunable Diode Laser Spectroscopy (TDLAS). CRDS measurements of N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$,v=0-2) populations are made in the discharge afterglow at a pressures of 10-40 Torr. The data reduction procedure takes into account the linewidth of the pulsed laser source, comparable with the absorption linewidth and resulting in a non-single exponential ring down decay. Peak N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$,v=0,1) populations after a 10-pulse ns discharge burst are 1.5x10$^{11}$ cm$^{-3}$. In the afterglow, these populations exhibit a relatively slow decay with the characteristic time of approximately 500 $\mu$s, most likely due to the quenching by the N2 molecules in the ground electronic state. TDLAS data have been taken at a higher pressure of 130 Torr. Absolute time-resolved N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$,v=0,1) number densities are measured during ns pulse discharge bursts up to 25 pulses long and in the afterglow, peaking at 5x10$^{12}$ cm$^{-3}$ and 3x10$^{13}$ cm$^{-3}$. The results indicate that N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$) is generated after every discharge pulse on a 20-50 $\mu$s time scale, much longer compared to the discharge pulse duration, and decays between the pulses. The decay rate increases during the discharge burst. In the afterglow, N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$,v=0,1) populations decay significantly more rapidly compared to the low-pressure CRDS conditions, with the characteristic time of approximately 100 $\mu$s. The results demonstrate that both CRDS and TDLAS diagnostics can be used for time-resolved, absolute N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$) measurements in transient nonequilibrium plasmas and the afterglow, with the detection limit of $\approx$ 10$^{10}$ cm$^{-3}$. The data obtained using these two diagnostics are complementary, since TDLAS measurements can be used at the conditions when the N$_{2}$(A$^{3}$$\Sigma$$_{u}$$^{+}$) populations may be too high, or vary too rapidly for accurate CRDS measurements.