K-Shell Photoabsorption Studies of the Carbon Isonuclear Sequence

K-shell photoabsorption cross sections for the isonuclear C i –C iv ions have been computed using the R -matrix method. Above the K-shell threshold, the present results are in good agreement with the independent-particle results of Reilman & Manson. Below threshold, we also compute the strong 1 s → np absorption resonances with the inclusion of important spectator Auger broadening effects. For the lowest 1 s → 2 p , 3 p resonances, comparisons to available C ii , C iii , and C iv experimental results show good agreement in general for the resonance strengths and positions. Our results also provide detailed information on the C i K-shell photoabsorption cross section including the strong resonance features, since very limited laboratory experimental data exist. The resultant R -matrix cross sections are then used to model the Chandra X-ray absorption spectrum of the blazar Mkn 421.