Effective collision strengths for transitions in singly ionized nitrogen

Abstract

The R-matrix method has been used to calculate effective collision strengths for the electron-impact excitation of the carbon-like ion N II. The lowest 23 LS target states are included in the expansion of the total wavefunction, consisting of the nine n = 2 states with configurations 2 s 22p 2 and 2s2p 3 , along with 14 n = 3 states with configuration 2s 2 2p3l (l= s,p,d). The fine-structure collision strengths have been obtained by transforming to a jj-coupling scheme using Saraph's JAJOM program, and have been determined at a sufficiently fine energy mesh to delineate properly the resonance structure. Effective collision strengths are obtained by averaging the electron collision strengths over a Maxwellian distribution of velocities. Results are presented for transitions among the 2s 2 2p 2 3 P, 1 D, 1 S and 2s2p 3 5 S levels for electron temperatures in the range log T e (K) = 2.0-5.1, appropriate for astrophysical applications.