Atomic data from the Iron Project XLV. Relativistic transition   probabilities for carbon-like Ar XIII and Fe XXI using Breit-Pauli R-matrix   method
  The Breit-Pauli R-matrix method developed under the Iron Project has been used to obtain extensive sets of oscillator strengths and transition probabilities for dipole allowed and intercombination fine structure transitions in carbon like ions, Ar XIII and Fe XXI. The complete set consists of 1274 fine structure bound energy levels and 198,259 oscillator strengths for Ar XIII, and 1611 bound levels and 300,079 oscillator strengths for Fe XXI. These correspond to levels of total angular momenta of 0 <= J <= 7 of even and odd parities formed from total spin of 2S+1=5,3,1, and orbital angular momenta 0 <= L <= 9 with n <= 10, 0 <= l <= 9 for each ion. The relativistic effects are included in the Breit-Pauli approximation. The close coupling wavefunction expansion for each ion is represented by the lowest 15 fine structure levels of target configuations, 2s2.2p, 2s.2p2 and 2p3. The energy levels are identified spectroscopically using a newly developed identification procedure. The procedure also makes a correspondence between the fine strucure energy levels and LS terms. This provides the check for completeness of the calculated levels. Comparison is made of the present energies and the f-values with the available observed and theoretical values. Present transition probabilitis agree very well with the relativistic atomic structure calculations of Mendoza et al. for the intercombination transitions, 2s.2p3(5So)2 - 2s2.2p2(3P){1,2},(1D)2. This further indicates that the importance of the neglected Breit interaction decreases with ion charge and constrains the uncertainty in the present calculations to within 15% even for the weak transitions. 
Sultana N. Nahar (Department of Astronomy, The Ohio State University)