Fine-structure energy levels and radiative rates in Si-like chlorine

Excitation energies and radiative rates for electric dipole (E1) transitions among the 86 fine-structure levels belonging to the configurations (1s22s22p6)3s23p2, 3s3p3, 3s23p3d, 3p4, 3s23p4s, 3s23p4p, 3s3p2(2S)4s, 3s3p2(2P)4s, 3s3p2(4P)4s, 3s3p2(2D)4s, 3s23p4d and 3s23p4f of Cl IV are calculated using extensive configuration-interaction (CI) wave functions obtained with the CIV3 computer code of Hibbert. The relativistic effects in intermediate coupling are incorporated by means of the Breit-Pauli Hamiltonian. In order to keep the calculated energy splittings close to the energy values of the National Institute for Standards and Technology, we have made small adjustments to the diagonal elements of the Hamiltonian matrices. Our calculated energy levels, including their orderings, are in excellent agreement with the available NIST values. The mixing among several fine-structure levels is found to be very strong. From our radiative rates we have also calculated radiative lifetimes of the fine-structure levels. Significant differences between our calculated lifetimes and those from a sophisticated calculation for a few low lying levels are noted and discussed. In this calculation, we also predict new data for several fine-structure levels where no other theoretical and/or experimental results are available.