  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.
