There are several possible reactions for both growth and decomposition of GaN layers in stable plasma conditions. A greater understanding of the plasma processes involved in the creation of the different N₂ species is required before many growth related problems can be resolved. A plasma source capable of selecting different active nitrogen species could be used to study the different reactions related to the growth process. In this paper, we present a spectroscopic study of a helicon plasma source. Five different nitrogen species in the plasma are monitored, namely: N II (atomic ion), N I (atomic neutral), (molecular ion) and two different excited states of the N₂ molecule as a function of the filling pressure. We show that by simply changing one plasma parameter (operating pressure), the plasma undergoes a transformation from an essentially atomic nature at low pressure to a dominantly molecular nature at higher pressure. The line intensity variations as a function of electron temperature are consistent with expectations based on the published formation rates for each species.