Emission-line ratios for [N II] in gaseous nebulae and a comparison between theory and observation

R-matrix calculations of electron impact excitation rates among the 2s(2)2p (2) P-3, D-1, S-1, and 2s2p(3) S-5 levels of N II are presented. These resu lts are used in conjunction with other recent calculations of electron impa ct excitation rates and Einstein A-coefficients for N II to derive the emis sion-line ratio : ratio diagrams and where (R-1, R-2) (R-1, R-3), where R-1 = I(5756.2 Angstrom)/I(6549.9 + 6585.2 Angstrom), R-2 = I(2143.5 Angstrom) /I(6549.9 + 6585.2 Angstrom), and R-3 = I(2139.7 Angstrom)/I(6549.9 + 658.2 Angstrom), for a range of electron temperatures (T-e = 5000-20,000 K) and electron densities (N-e = 10(2)-10(7) cm(-3)) appropriate to gaseous nebula e. These diagrams should, in principle, allow the simultaneous determinatio n of T-e and N-e from measurements of the [N II] lines in a spectrum. Plasm a parameters deduced for a sample of gaseous nebulae, using observational d ata obtained from ground-based telescopes plus the International Ultraviole t Explorer and Hubble Space Telescope satellites, are found to show general ly excellent internal consistency and to be in good agreement with the valu es of T-e and N-e estimated from other line ratios. These results provide o bservational support for the accuracy of the theoretical ratios and hence t he atomic data adopted in their derivation. Theoretical ratios are also pre sented for the infrared line pair R-4 = I(122 mum)/I(205 mum), and the usef ulness of R-4 as an electron density diagnostic is briefly discussed.