Abstract
R-matrix calculations of electron impact excitation rates among the 2s22p2 3P, 1D, 1S, and 2s2p3 5S 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 A)/I(6549.9 + 6585.2 A), R2 = I(2143.5 A)/I(6549.9 + 6585.2 A), and R3 = I(2139.7 A)/I(6549.9 + 6585.2 A), for a range of electron temperatures (Te = 5000-20,000 K) and electron densities (Ne = 102-107 cm-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne 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 Te and Ne 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.
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