Intracavity laser absorption spectroscopy applied to measure the absolute density and temperature of N₂(A³Σ⁺_<i>u</i>) metastable molecules in a flowing N₂microwave discharge

Abstract

The first absolute density measurement of N2(A 3Σ+u;v = 0) metastable molecules in the short-lived nitrogen afterglow of a 440 Pa microwave discharge is achieved by using a intracavity laser absorption spectroscopy technique. The results obtained show that this technique is very well suited for the diagnostics of N2(A 3Σ+u) in the nitrogen afterglow. This is a first important experimental step with the aim of probing regions where the densities are expected to be the most significant. The densities measured in the discharge zone and in the short-lived afterglow maximum emission zone are about 5×1017 and 6×1016 molecule m-3, respectively. The gas temperature is also deduced both from the Doppler profile of the spectrally resolved rotational lines and from the rotational distribution in the N2(A 3Σ+u;v = 0) state. These temperatures are about 1000 K in the discharge zone and 530 K in the short-lived afterglow maximum emission zone, respectively. The relatively high density of metastable molecules in the short-lived afterglow maximum emission zone reveals the important role played by these species in the short-lived nitrogen afterglow and can give a new insight into the mechanisms involved in the so-called nitrogen short-lived afterglow emission. We also conclude that this high density of N2(A 3Σ+u) in the short-lived afterglow results from an equilibrium between some local production mechanisms and destruction by reaction with N atoms.