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 2s22p2 and 2s2p3, along with 14 n= 3 states with configuration 2s22p3l (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 2s22p23P, 1D, 1S and 2s2p35S levels for electron temperatures in the range log Te(K) = 2.0−5.1, appropriate for astrophysical applications.