UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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Conference: Bucharest University Faculty of Physics 2001 Meeting


Section: Optics and Spectroscopy


Title:
PROCESSES OF NITROGEN BANDS EXCITATION IN A REMOTE THERMAL EXPANDING PLASMA GENERATED IN AR/N2


Authors:
A. de Graaf, B. Mitu*, G. Dinescu*, M.C.M. van de Sanden


Affiliation:
Department of Applied Physics, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands

*National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG-36 Magurele, Bucharest, 76900 Romania


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Keywords:


Abstract:
In this work results concerning a thermal arc plasma expanding in a low pressure environment (0.25 mbar) are presented. Plasma is DC generated between three cathodes and a nozzle in a wall stabilized arc, at high current (30-90 A), high flow rates (50-100 sccm) and expands in a vacuumed vessel. The expanding plasma is in a recombining state, the excitation by chemical reactions and energy transfer between species being dominant in comparison with the excitation induced by electron collision. The emission spectroscopy technique has been used to unravel the energy transfer processes. The intense emission lines and bands appearing in the spectral range 360-450 nm have been studied. Depending on experimental conditions they were: Ar lines, N2+(FNS: B2??u - X2??g+), N2 (SPS: C3??u-B3??g). In order to discern the importance of excitation transfer from Ar species to nitrogen species experiments were performed with nitrogen added directly in the arc channel or injected in expansion. Our study indicates that the lines emission originates in the decay of excited Ar atoms created by the triple recombination of Ar+ emanating from arc. The low rate of this process allows the Ar+ to survive in high concentration in expansion and explains the Ar lines emission at large distances from the arc. The excitation transfer from Arm to the molecular nitrogen is the main reaction that sustains the SPS emission. The charge transfer reactions of Ar+/N+ with molecular nitrogen produce nitrogen ions in fundamental or directly in excited states. The production of excited molecular ions sustains the FNS emission. The V-E excitation transfer from vibrationally excited nitrogen to the molecular ion in the fundamental state contributes also to FNS excitation, this contribution being stronger at larger arc distances.