1 Introduction

Nitrogen is among the most abundant elements in the universe. For the estimation of stellar chemical abundances the determination of radiative transition probabilities is of primary interest. Knowledge of the spectrum of singly ionised nitrogen is clearly required for the understanding of the associated plasmas. Moreover, it is frequently used in laboratory studies and in industrial applications. Although during the last decade, much theoretical effort has been devoted to advanced atomic structure calculation which provided such radiation data for NII (part of the Opacity Project), some accurate experimental data are still required (a) to establish with confidence the uncertainties of advanced calculations and (b) to settle especially the question to what degree LS coupling is applicable for the individual line strengths in this spectrum. According to Wiese et al. (1996) there is a lack of experimental data which are accurate enough to sensitively test the calculated results. Recently Musielok et al. 1996, have reported A_ki values for a fairly large number of lines (52) of the 3s-3p and 3p-3d transition arrays of NII with a wall-stabilized arc source. One of the aims of the present work is just to provide in only one experiment a great amount of data. These will be compared with the most recent literature. Furthermore, this experiment gives A_ki values for 13 NII spectral lines not collected in previous works, at least to the authors's knowledge.

All the results were obtained from measurements performed in a linear discharge lamp, where a mixture of helium and nitrogen was prepared so that self-absorption effects were negligible. The plasma source employed provides not only all kind of interferometric and spectroscopic registers with great reproducibility in different discharges, but also makes it possible in a broad range of electron density and temperature values. This allows us to acquire some reliable spectra for weak or non-isolated lines, otherwise very difficult to obtain.

In the present work absolute A_ki values have been obtained from relative intensity measurements taking as reference values the A_kidata available in the last NIST critical compilation (Wiese et al. 1996). In this way, we have determined NII excitation temperatures from Boltzmann-plots, and also temperatures given from NII/NI intensities ratios. The very good agreement between the two methods suggests that the plasma is well described by a partial local thermodynamic equilibrium (pLTE) model. The absence of self-absorption and the spectral calibration have been very carefully taken into account. The number of measurements (15) performed for each line along the plasma life, and its very controlled features, allows us to obtain a very good A_ki value by the mean value, and its uncertainty by the standard deviation (usually around 10%).