Ultraviolet transition probabilities in N II.

Abstract

Oscillator strengths have been calculated for the ultraviolet transition array 2${\mathit{p}}^{2\mathrm{\ensuremath{-}}}$2p3s in the N ii spectrum. Recent astrophysical, experimental, and theoretical works have called attention to two interesting features of these lines. First, the intercombination line at 748 \AA{} is exceptionally strong; second, the branching ratio between the $^{1}$D and $^{1}$S decay modes of the $^{1}$P\ifmmode^\circ\else\textdegree\fi{} level is unusually large. Both these effects reflect the strong interaction of 2${\mathit{s}}^{2}$2p3s with the ``plunging'' configuration 2s2${\mathit{p}}^{3}$; unlike other ions in the carbon isoelectronic sequence, these configurations are overlapping for N ii. In the present work multiconfiguration Hartree-Fock (MCHF) wave functions have been used to compute oscillator strengths for all the lines in the 2${\mathit{p}}^{2\mathrm{\ensuremath{-}}}$2p3s transition array. The MCHF results give wavelengths which are within 1% of the observed wavelengths; the length and velocity forms of the oscillator strengths agree with each other to within 7%; and the predicted mean lifetime of the 2p3s $^{1}$P\ifmmode^\circ\else\textdegree\fi{} level agrees with a recent beam-foil--laser result to 4% [Y. Baudinet-Robinet et al., Phys. Rev. A 42, 1080 (1990)].