2001ApJS..132..103K

R-matrix calculations of electron impact excitation rates among the 2s²2p^{2 3}P, ¹D, ¹S, and 2s2p^{3 5}S levels of N II are presented. These results are used in conjunction with other recent calculations of electron impact excitation rates and Einstein A-coefficients for N II to derive the emission-line ratio: ratio diagrams (R₁, R₂) and (R₁, R₃), where R₁=I(5756.2 Å)/I(6549.9+6585.2 Å), R₂=I(2143.5 Å)/I(6549.9+6585.2 Å), and R₃=I(2139.7 Å)/I(6549.9+6585.2 Å), for a range of electron temperatures (T_e=5000-20,000 K) and electron densities (N_e=10²-10⁷ cm^–3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of T_e and N_e from measurements of the [N II] lines in a spectrum. Plasma parameters deduced for a sample of gaseous nebulae, using observational data obtained from ground-based telescopes plus the International Ultraviolet Explorer and Hubble Space Telescope satellites, are found to show generally excellent internal consistency and to be in good agreement with the values of T_e and N_e estimated from other line ratios. These results provide observational support for the accuracy of the theoretical ratios and hence the atomic data adopted in their derivation. Theoretical ratios are also presented for the infrared line pair R₄=I(122 µm)/I(205 µm), and the usefulness of R₄ as an electron density diagnostic is briefly discussed.