Iron Project. LIV. Fe XX

The wavefunction expansion includes 20 fine structure levels of configurations 2s ^2^2p^2^, 2s2p^3^, and 2p^4^ of the core ion Fe XXI. Computed energy levels are identified spectroscopically using a newly developed procedure based on quantum defects and channel contributions. All 55 observed levels have been identified and are in agreement to much less than 1% with most of the calculated values.

Coefficients for radiative decays for the forbidden E2, E3, M1 transitions are obtained through atomic structure calculations including relativistic terms in the Breit-Pauli approximation using the code SUPERSTRUCTURE. The theoretical transition probabilities are compared with available values, with varying degree of agreement. Erratum: A bug was detected and corrected in early 2007 in one of the stages called RECUPD (Storey, Berrington, Werner, 2007, private Iron Project communications) of the Breit-Pauli R-matrix (BPRM) codes of the Iron Project.

It was related to the the phase factor of dipole matrix elements of the Iron Project (Berrington et al., 1995, Comput. Phys. Commun. 92, 290) and affected certain transition probabilities.

The new results on energies and transition probabilities for Fe XX (Nahar, 2004, J/A+A/413/779, this paper) and Fe XVIII (Nahar, 2006, J/A+A/457/721) using the corrected code are replacing the earlier results at CDS. It should be noted that stage RECUPD in BPRM codes does not exist in the LS coupling R-matrix code from the Opacity Project (Berrington et al., 1987, J. Phys. B 20, 6379) and hence does not affect the non-relativistic results.

Results for other ions, obtained using BPRM codes, did not indicate the bug effect when compared with existing results. However, I am repeating the calculations for these ions and will replace the earlier results. The E1 transition probabilities for Fe XX have been recomputed.

The new files are: table3a, table3b -> fine structure energy levels of Fe XVIII table4a, table4b -> complete table of E1 fine structure transitions in Fe XVIII table5 -> table of fine structure transitions in Fe XVIII using observed energies In paper LXII (Fe XVIII), J/A+A/457/721, files were also corrected.