3 July 1998 Ž . Chemical Physics Letters 290 1998 379–384 q ž 3 q / Evidence for charge exchange between N and N A S in a 2 u low-temperature nitrogen plasma G.J.H. Brussaard a,1 , E. Aldea b , M.C.M. van de Sanden a, ) , G. Dinescu b , D.C. Schram a a Department of Physics, EindhoÕen UniÕersity of Technology, P.O. Box 513, 5600 MB EindhoÕen, The Netherlands b Institute of Physics and Technology of Radiation DeÕices, Low Temperature Plasma Department, P.O. Box MG-36, 76900 Bucharest, Romania Received 6 February 1998; in final form 30 April 1998 Abstract q Ž 2 q . q Ž 2 q . The emission from the first negative system, N B S ™N X S qhn , is studied in the flowing nitrogen 2 u 2 g afterglow of a DC arc plasma. Investigation of the spectrum shows overpopulation of the vibrational levels 6 and 7 of the q Ž 2 q . excited molecular ion, N B S . Selective excitation of these levels is explained by a charge exchange reaction between 2 u Ž 3 q . atomic ions in the ground state and metastable molecules in the N A S state. The emitted intensity of the first negative 2 u system is shown to be linear with electron density n for n )2 =10 16 m y3 , a higher-order dependence exists below this e e q Ž 2 q . q value. This is consistent with population of N B S by atomic ions, N . q 1998 Elsevier Science B.V. All rights 2 u reserved. Nitrogen plasmas are used in a wide variety of applications ranging from deposition of nitrides to the study of re-entry phenomena of spacecraft. As the most abundant constituent of air, nitrogen also plays an important role in many open atmospheric plasmas. This has led to extensive study of the phenomena in nitrogen plasmas and the modelling of reactions taking place. Especially the so-called long- lived or red afterglow of nitrogen molecules has fascinated scientists since the early days of plasma wx science 1 . Another phenomenon is the appearance of the short-lived or pink afterglow. The latter is the focus of the research presented in this Letter. ) Corresponding author. E-mail: m.c.m.v.d.sanden@phys.tue.nl 1 E-mail: g.j.h.brussaard@phys.tue.nl Kinetic modelling has proven to be a powerful tool in the study of the aforementioned phenomena. Especially the modelling of neutral molecule densi- ties and atomic densities in the afterglow of DC discharges has provided detailed insight into the w x kinetics of the nitrogen plasma 2–6 . However, only few publications include results on the modelling of excited molecular ions. This could elucidate the mat- ter further, since one of the most prominent features in the spectra of most nitrogen plasmas is the emis- sion from molecular ions, the so-called first negative w q Ž 2 q . q Ž 2 q . x system N B S ™N X S q hn . One of 2 u 2 g the features studied in detail is the creation of ions in the afterglow. It is suggested that excited neutral w x molecules react to generate ions and electrons 7–9 . The subsequent excitation of these molecular ions is not well understood and possibly depends on the 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 98 00544-2