Energy levels and characteristic features in photoionization of Cl III using the R-matrix method

We report study of Cl III for its large number of fine structure bound levels, 890, with $n\leq$10 and $l\leq$9, and 1/2 $\leq j \leq$ 19/2 of even and odd parities with spectroscopic designations and photoionization cross sections ($\sigma_{PI}$) of the levels revealing various characteristic features. Various resonant structures and the shapes of the background, and their interference in $\sigma_{PI}$ are illustrated for the ground, excited equivalent electron, low and high lying excited levels with single valence electron, and effects of fine structure couplings. $\sigma_{PI}$ of the ground level shows Rydberg series of resonances on smooth background, and of equivalent electron levels producing strong closely spaced Rydberg series of resonances belonging to the low lying core ion excitation thresholds. They will impact applications in low temperature plasma. For the single valence electron excited levels, we find that $\sigma_{PI}$ of low lying excited states are dominated by Rydberg series of resonances and of high lying excited states exhibit prominent broad feature of Seaton resonances. Partial photoionization cross sections of the ground level for leaving the core ion in the ground and various excited levels are also presented for applications in plasma modeling. The study was carried out in relativistic Breit-Pauli R-matrix method using a large close coupling wave function expansion of 45 levels of the core ion configurations $3s^23p^2$, $3s3p^3$, $3s^23p3d$, $3s^23p4s$, $3s3p^23d$, and $3p^4$ with closed 1s, 2s, 2p orbitals. They belong to the optimized set of 13 configurations of Cl~IV, $3s^23p^2$, $3s3p^3$, $3s^23p3d$, $3s^23p4s$, $3s^23p4p$, $3s^23p4d$, $3s3p^23d$, $3s3p^24s$, $3s3p^24p$, $3p^4$, $3p^33d$, $3p^34s$, $3p^34p$. Cl III energies are in good agreement with measured values. $\sigma_{PI}$ features of low lying levels were benchmark with observation carried out at Advanced Light Source at LBNL with very good agreement. The present set of high accuracy data should complete for any practical applications.