Actinometry Measurement of Dissociation Degrees of Nitrogen and Oxygen in N₂–O₂Microwave Discharge Plasma

Abstract

The dissociation degrees of N2 and O2 are examined in a nitrogen–oxygen mixed microwave discharge plasma in a cylindrical quartz tube of 26 mm inner diameter with a discharge pressure of 0.5–1.0 Torr and a microwave power of 600 W by the actinometry method. We measured the electron temperature and density with a Langmuir double probe, while the vibrational and rotational temperatures of the first and second positive bands of N2 were measured by optical emission spectroscopy. Even when the line intensity of atomic nitrogen was weak and partly coincided with the high-intensity band spectrum of the first positive system due to its small dissociation degree, the actinometry method was found to be feasible when the first positive band spectrum, calculated as a function of the rotational and vibrational temperatures, was subtracted from that observed experimentally. It was found that the dissociation degrees of both N2 and O2 increase with the molar ratio of nitrogen in the mixed N2–O2 discharge gas for the same total discharge pressure. The experimental results are discussed by comparison with a simple numerical model based on chemical kinetics in the plasma. It was found that the dissociation of oxygen molecules is enhanced by the collision with excited nitrogen molecules, particularly those with metastable states, whereas that of nitrogen is suppressed by an admixture of oxygen molecules due to the chemical quenching processes of nitrogen atoms.

Progresses in Experimental Study of N2 Plasma Diagnostics by Optical Emission Spectroscopy

“…For example, the transition P 33 belongs to an a ↔ a transition and Q 33 to an s ↔ s transition. (Ichikawa et al, 2010).…”

“…The Franck-Condon and the Hönl-London factors used are specified elsewhere (Sakamoto et al, 2007). , whereas the broken lines are those newly included in our study (Sakamoto et al, 2007;Ichikawa et al, 2010). Figure 7 is an example of comparison between the 1PS spectrum calculated on the basis of the scheme indicated in Fig.…”

“…this section, we describe the actinometry method with the help of 1PS subtraction to measure the dissociation degree of nitrogen (Ichikawa et al, 2010). We also experimentally examined the effect on the nitrogen dissociation degree of admixture of rare gases.…”

“…we treat the N 2 -rare gas mixture plasmas to examine the dissociation degree that must depend on the gas mixture ratio. In this subsection, we describe the basic principle of actinometry including atomic and molecular processes of the N 2 -gas mixture discharge plasmas, which was applied to N 2 -O 2 mixed discharge (Ichikawa et al, 2010). Regarding the population and depopulation of these states, we must consider the following reactions due to collisional relaxation by the ground-state molecules of N 2 for about a 1-Torr discharge to deduce the number density of nitrogen atoms.…”

“…These processes are generally negligible for actinometry measurement in the present microwave discharge plasmas, which are categorized into ionizing plasmas. Further discussion was described elsewhere (Ichikawa et al, 2010).…”