1 Introduction

With the aim of calculating accurate astrophysical opacities, extensive set of oscillator strengths (f-values) for bound - bound transitions and photoionization cross sections for bound - free transitions were obtained under the Opacity Project (The Opacity Project 1995, 1996; Seaton et al.
citeSeaton1994). These data are available through the OP database, the TOPbase (Cunto et al. 1993). Determination of opacities requires transition probabilities or oscillator strengths for all transitions of all ionization stages of the constituent elements in the plasma as obtained under the OP. However, the OP f-values were obtained in LS coupling whereas transitions among fine structure levels are often needed in various astrophysical models as well as in experimental or observational spectral analysis. For example, electron densities in solar flares can be determined from the fine structure transitions in Fe XXI (e.g. Pallavicini et al. 1977). Transition probabilies for fine structure levels were obtained for a number of Fe ions, such as Fe II (Nahar 1995), Fe III (Nahar & Pradhan 1996) and Fe XIII (Nahar 1999) through algebraic transformation of LS multiplets calculated in close coupling approximation. Hence, no relativistic effects were included for these transitions.

The Breit-Pauli R-matrix (BPRM) (Hummer et al. 1993; Berrington et al.
citeBerrington1995) method includes relativistic effects in the Breit-Pauli approximation (Scott & Burke 1980; Scott & Taylor 1982). It enables the calculations of both the dipole allowed ( $\Delta S=0$) and the intercombination ( $\Delta S\neq 0$) transitions, in contrast to LS calculations where only dipole allowed transitions could be included. Incorporation of the relativistic effects in the close coupling R-matrix method yields a large number of fine structure transition probabilities with higher accuracy.

However, a major obtstable in the BPRM calculations has been the spectroscopic identification of the large number of fine structure energy levels being calculated. These are obtained as the eigenvalues of the BP Hamiltonian labeled only by the total angular momentum and parity, i.e. by $J\pi$, which is insufficient for unique identification. Except for large scale model calculations, complete identification of levels is needed for various diagnostics and spectrocopic applications. A new procedure is developed to identify these levels by a complete set of quantum numbers through analysis of collision channels (Nahar & Pradhan 2000). The procedure also makes a correspondence between the fine structure levels and their proper LS terms.

BPRM method has been applied for large scale computations of f-values for a few iron ions, Fe V (Nahar & Pradhan 2000; Nahar et al 2000), Fe XXIV and Fe XXV (Nahar & Pradhan 1999). The f-values of Fe XXIV and Fe XXV are found to be very accurate. They agree within a few percent with the measured and other very accurate calculations available for a few transitions. The present work reports oscillator strengths for two carbon like ions, Ar XIII and Fe XXI. Both the dipole allowed and the intercombination transitions are considered. The accuracy and completeness of the results are discussed. While samples of fine structure energy and oscillator strengths are presented herein, the complete tables will be available electronically.