Time-resolved CRDS measurements of the N₂(A³Σ _u⁺) density produced by nanosecond discharges in atmospheric pressure nitrogen and air

Cavity ring-down spectroscopy (CRDS) is used to measure the number density of N₂(N₃(A³Σ_u⁺) metastables produced by nanosecond repetitively pulsed discharges in nitrogen and air preheated at 1000 K and atmospheric pressure. The densities of N ₂(A) are inferred from the absorbance of the Q₁(22) and Q₃(16) lines of the (2 ← 0) vibrational band of the first positive system (B³Π_g - N₂(A ³Σ_u⁺)ofN₂ at 769.945 nm. The procedure for determining the temporal evolution of the density of metastable from the measured ring down signals is presented. The maximum number densities are in the range of 10¹⁴-10¹⁵ molecules cm^-3 for air and nitrogen discharges, respectively. In nitrogen, the decay of the N₂(A) density is shown to be a second-order process with a rate coefficient of 1.1 × 10^-9 cm³ s^-1 at 1600 K with a factor of 2 uncertainty. In air, the decay is estimated to be 1 order of magnitude faster than that in nitrogen owing to quenching by atomic and molecular oxygen. Furthermore, the rotational temperature is determined by comparison of CRDS measurements and simulations of several rotational lines of the (2 ← 0) band of the first positive system of N₂ between 769.8 and 770.7 nm. The rotational and vibrational temperatures are also determined by comparison of optical emission measurements and simulations of the second positive system of N₂ between 365 and 385 nm. In these CRDS measurements, we achieved a temporal resolution down to 50 ns.