Both the axial density and temperature profiles of ground-state nitrogen atoms have been measured in a microwave discharge and its afterglow in the presence of the so-called short-lived afterglow by means of two-photon absorption laser-induced fluorescence (TALIF). The temperature is obtained from the Doppler broadening of the spectral profile, after deconvolution with the laser profile. The N atom temperature decreases from about 1400 K in the end of the discharge zone to about 300 K in the downstream part of the afterglow. The sharp temperature decrease immediately behind the discharge zone can reasonably be explained by heat transfer to the flow tube wall. The absolute N atom density is obtained by calibrating the fluorescence yield with a TALIF signal from krypton atoms. The N density increases from 1.5×10²¹ m^-3 in the discharge zone to about 3.5×10²¹ m^-3 in the late afterglow. However, the N atom flux is conserved along the flow tube, indicating negligible consumption or production of N atoms in the short-lived afterglow.

A spectroscopic study involving N₂(B³ Pi _g, v'=11 to A³ Sigma _u⁺, v"=7), N₂(C³ Pi _u, v'=0 to B³ Pi _g. v"=0) and N₂⁺(B² Sigma _u⁺, v'=0 to X² Sigma _g⁺v"=0) transitions is presented for description of the characteristics of the so-called short-lived afterglow (SLA) created by a nitrogen microwave discharge (433 MHz) at pressures in the range 1.5-10 hPa. The spatial investigation along the flow system reveals strong intensity gradients but an approximate constancy, in SLA, of intensity ratios of the first transition to the second and third one. Criteria drawn notably from these observations are proposed in order to estimate the extension of the main regions encountered in the afterglow. The maximum intensity of emissions in SLA, their location and the gas temperature are studied versus the transmitted microwave power, the gas velocity and the pressure. On the basis of the presented results, a kinetic model is built to describe the birth of SLA and its bulk features. The main role played by the vibrationally excited ground state molecule is discussed. A basic consequence is the strong interdependence of the vibrational energy and the temperature profile for occurrence of the maximum of intensity in SLA.

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.