Excitation Cross Sections, Collision Strengths and Line Ratios of O~II

The forbidden [O~II] lines are one of primary density diagnostics in astrophysical sources. We have carried out relativistic Breit-Pauli R-matrix (BPRM) calculations for the non-dipole transitions of interest. The coupled channel target is represented by a 16-level wavefunction expansion dominated by configurations $2s^22p^3$, $2s2p^4$, $2s^22p^23s$. Resonances are delineated in detail to ascertain the effect on Maxwellian averaged collision strengths. Results for the forbidden transitions among the lowest 5 levels of the ground configuration $2s^22p^3(^4S^o_{3/2},^2D^o_{5/2,3/2}$, $^2P^o_{3/2,1/2})$ show that the fine structure collision strengths do not significantly depart from values obtained from a purely $LS -> LSJ$ transformation. Present results differ significantly with similar BPRM calculations by McLaughlin and Bell (1998), but are essentially in agreement earlier results of Pradhan (1976). The astrophysically important line ratio I(3729)/I(3726) derived from the present results is in agreement with the high and low-density limits expected on physical grounds, and with recent observations. The present excitation cross sections for the $^4S^o-^2D^o$ transition shows excellent agreement with measurements from a merged beam experiment (Zuo et al 1995).