INVESTIGATION OF LS COUPLING IN NITROGEN II B.Blagojević, M.V.Popović and N.Konjević Institute of Physics, 11080 Belgrade, P.O.Box 68, Yugoslavia 1. INTRODUCTION LS or Russell-Saunders coupling is dominant for many transitions in the spectra of light elements. This is the case spin-orbit interaction in atomic Hamiltonian is more important than the electrostatic separation between levels of the same principal quantum number n. Since electrostatic separation increases as Z while the spin-orbit interaction grows as Z 4 α 2 where α is fine structure constant, the LS-coupling scheme becomes inappropriate at a certain point. Systematic failure of the LS-coupling approximation is expected from lower to higher elements of an isoelectronic sequence for nl-nl' transitions. The aim of this paper is to test predictions of LS-coupling theory for the transitions within 3s 3 P 0 -3p 3 D and 3p 3 D-3d 3 F 0 multiplets of singly ionized nitrogen. In addition we tested whether plasma electron density influences spontaneous emission coefficients as suggested by Chung et al [1]. 2. THEORY For LS coupling theory see e.g. Ref.[2]. Theoretical values calculated from multiconfiguration Dirac-Fock (MCDF) wave function of moderate accuracy are compared with our measured intensity ratios. For the case of pure LS coupling the relative line strength for a transition between levels J1 and J2 is proportional to the factor (Ref.[3], Appendix I) Values of the 6j symbol are given in Appendix D of Ref.[3]. The intensity ratio of two multiplet components is represented by [2] where I, λ and I’, λ’ are the total intensities and wavelengths of the two components, and E and E’ are the energies of the upper levels of the two components, respectively. ) 1 ( 1 ) 1 2 )( 1 2 ( 2 2 1 1 1 1 2 1 2       + + = L J J S L J J D line ) 2 ( exp ' 2 ' 4 ' '         −               = e line line kT E E D D I I λ λ