Spatial density profiles of the N₂metastable state A³Σ_u⁺, the B³Π_gand C³Π_ustates in an asymmetric rf discharge

Abstract

A laser-induced fluorescence technique, in combination with optical emission spectroscopy, was applied to investigate the spatial distribution of the number densities of neutral species in an asymmetric low-pressure 27.12 MHz discharge in pure nitrogen. The primary targets of this investigation were the three nitrogen triplet states C 3 u , B 3 g and the metastable A 3 + u each in their lowest vibrational levels v = 0, 1, 2 and additionally v = 8 of A 3 + u which can be populated very efficiently in the plasma sheath. The absolute number densities of all these states were determined in a Rayleigh scattering experiment, the results of which have been published elsewhere (Krames B, Glenewinkel-Meyer T and Meichsner J 2001 J. Appl. Phys. 89 3115). The spatial profiles as a function of the discharge parameters were studied by varying the gas pressure between 10 and 100 Pa and the effective rf voltage up to 260 V. These profiles of the different molecular states vary markedly in the plasma bulk as well as in the sheath near the powered electrode. A model, which includes the production, diffusion and quenching rates of contributing species and also fluorescence cascade processes, explains these varieties. A comparison with the experimental data at 50 Pa confirms the results of the model calculations.

Laser-induced fluorescence spectroscopy of and absolute density calibration by Rayleigh scattering in capacitively coupled rf discharges

“…In contrast to the optical emission, the A 3 Σ + u , v = 0 density profiles are still symmetric, whereas the absolute A 3 Σ + u , v = 0 density decreases with increasing self-bias voltage. This is a big difference to the measurements at lower pressures (and larger gaps) in [6] in a capacitively coupled rf discharge and [8] in an inductively coupled rf discharge, where the density increases with the self-bias voltage resp. rf power.…”

“…But this cannot explain the large decrease in figure 10. Reconsidering the results in [6], the increase is also related to an increase of the B 3 Π g density. This is not the case in our measurements, because the mean optical emission from B 3 Π g , v = 3 to A 3 Σ + u , v = 1 remains nearly constant.…”

“…Therefore, the optical emission from B 3 Π g , v = 3 to A 3 Σ + u , v = 1 and the densities of A 3 Σ + u , v = 0 are plotted again for different rf power in figure 10. Firstly, as it is expected, the asymmetric character of the optical emission profiles increases with the self-bias voltage and the maximum shifts toward the powered electrode, because electron impact excitation is the most important excitation process, located at the sheath edge [6]. In contrast to the optical emission, the A 3 Σ + u , v = 0 density profiles are still symmetric, whereas the absolute A 3 Σ + u , v = 0 density decreases with increasing self-bias voltage.…”

“…Examples for the LIF application to low pressure nitrogen discharges can be found in [3,4,5,6]. They differ in particular in the absolute density calibration, which can be done straight forward by calculation [4] or by a comparison with scattered Rayleigh light [5].…”

Detection of metastable excited molecules N2(A3Σu+) in an atmospheric pressure nitrogen discharge by Raman scattering

“…) reported in the past includes absorption spectroscopy [10,11], laser absorption spectroscopy [12], laser intercavity absorption spectroscopy [8,13], and laser-induced fluorescence [14,15]. However, all of these reports are limited to the low pressure range of P < 1 kPa.…”