Physics Letters A 359 (2006) 499–503 www.elsevier.com/locate/pla Effects of helium gas mixing on the production of active species in nitrogen plasma M.A. Naveed a , A. Qayyum b , Shujaat Ali a , M. Zakaullah a,∗ a Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan b Department of Physics, G.C. University, 54000 Lahore, Pakistan Received 8 March 2006; received in revised form 23 June 2006; accepted 1 July 2006 Available online 17 July 2006 Communicated by F. Porcelli Abstract Optical emission spectroscopy is used to investigate the effects of helium gas mixing on the electron temperature and the production of active species in nitrogen plasma generated by 50 Hz pulsed-DC power source. The electron temperature is determined from He–I line intensities, using Boltzmann’s plot method. The relative changes in the concentration of active species N 2 (C 3  u ) and N + 2 (B 2  + u ) are monitored by measuring the emission intensities of nitrogen (0–0) bands of the second positive and the first negative systems. It is found that the electron temperature can be raised considerably by mixing helium in nitrogen plasma, which in return plays a significant role in enhancing the concentration of active species through Penning effect of metastable states of the helium.  2006 Elsevier B.V. All rights reserved. PACS: 52.70.Kz; 52.80.Hc; 81.65.Lp Keywords: Glow discharge plasma; Optical emission spectroscopy; Penning effect Glow discharge nitrogen plasma is extensively used for ni- triding of numerous materials especially iron-based alloys to improve their surface properties including surface hardness, wear and corrosion resistance and fatigue strength [1,2]. Ad- dition of inert gases such as argon, neon and helium in nitrogen plasma, enhances the concentration of active species through Penning excitation and ionization processes. Helium has higher metastable energies than any other inert gas, thus making it a powerful Penning reagent for exciting plasma species through inelastic collisions [3]. Helium has also lower efficiency of cathode sputtering owing to its low mass compared to other inert gases [4] and thus can be added in nitrogen plasma for enhancing the production of active species without causing any considerable increase in the impurity level. The electron tem- * Corresponding author. E-mail addresses: mzakaullah@qau.edu.pk, zaka_qau_pk@yahoo.com (M. Zakaullah). perature is an important parameter of the plasma influencing the production of the active species by inelastic collisions. The addition of helium may provide a better control over the elec- tron temperature [5]. Therefore measurement of electron tem- perature at different percentages of helium in nitrogen plasma will help to know about particle collision processes, plasma reactions, and concentration of active species in the plasma. The most widely used optical method for measuring electron temperature, sensing atoms and molecules in plasma is optical emission spectroscopy (OES) owing to its non-perturbative na- ture [6,7]. The basic premise of this technique is that the emis- sion intensity of particular wavelength from an excited state is proportional to the concentration of species in that excited state [8]. This Letter reports the changes induced in electron tempera- ture and consequently the abundance of active [N 2 (C 3  u ) and N + 2 (B 2  + u )] species by varying the concentration of helium gas in the nitrogen plasma. A major concern is to enhance the concentration of active species, which are considered to be im- portant for the materials processing. 0375-9601/$ – see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2006.07.002