Relativistic photoionization cross sections for C II

The first measurement of highly resolved photoionization cross sections of the ground state of C II was reported recently by Kjeldsen et al. [Astrophys. J. 524, L143 (1999)]. The observed resonance features showed good agreement with the theoretical calculations in the close-coupling approximation [S. Nahar, Astrophys. J. Suppl. Ser. 101, 423 (1995)]. However, there were several observed resonances that were missing in the theoretical predictions. The earlier theoretical calculation was carried out in LS coupling where the relativistic effects were not included. Present paper reports photoionization cross sections including relativistic effects in the Breit-Pauli R-matrix (BPRM) approximation. The configuration interaction eigenfunction expansion for the core ion C III consists of 20 fine-structure levels dominated by the configurations from 1s²2s² to 1s²2s3d. Detailed features in the calculated cross sections exhibit the missing resonances due to fine structure. The results provide a benchmark for the accuracy of BPRM photoionization cross sections, as needed for the recent and ongoing experiments for high-resolution photoionization cross sections at synchrotron-radiation sources.