Mechanism of N2 dissociation and kinetics of N(4S) atoms in pure nitrogen plasma

Abstract

This work deals with kinetics of the ground state nitrogen atoms N(4S) and N2 dissociation mechanism in pure nitrogen plasma. The experiment was carried out in positive column of DC glow discharge at a range of parameters p = 5 - 50 Torr, J = 20 - 100 mA. The use of axial homogeneous glow discharge allowed considering N(4S) balance for spatially uniform conditions controlled by only two terms: source (characterized by effective production rate kdisseff) and loss (characterized by effective loss time τNloss). Analysis of these parameters gains considerably better understanding of N2 dissociation mechanism in pure nitrogen plasma that was the main goal of the given work. So N/N2 dissociation rate as function of discharge parameters was obtained using two independent emission optical methods: actinometry on Ar atoms and N2 2+ band emission decay at discharge modulation. Measurements of N/N2 radial profiles allowed estimating N atom surface loss probability γNloss and correspondingly τNloss. It was revealed that γNloss depends on N(4S) concentration and thereby discharge conditions through the sorption balance of physisorbed N atoms. Simple phenomenological model taking into account basic surface processes provides γNloss data in good agreement with experiment. Finally, kdisseff was obtained as function of reduced electric field E/N and it was shown that even EEDF self-consistently calculated with accounting for N2 vibrational excitation is unable to provide observed values of kdisseff. Reasons of that fact are discussed in detail.