2004A&A...413..779N

An extensive set of oscillator strengths (f), line strengths (S) and radiative decay rates (A) for dipole allowed, intercombination and forbidden transitions in Fe XX is presented.
Result. include 1792 bound fine structure levels of total angular momenta J=1/2-19/2 of even and odd parities, with 2≤n≤10, 0≤l≤14, orbital angular momenta, 0≤L≤14, and total spin multiplicities 2S+1=2,4,6, yielding to about 3.8x10⁵ allowed and 13,874 forbidden transitions in Fe XX. These results far exceed the available data in literature. The existing data compiled by the National Institute for Standards and Technology (NIST) are available for little over one hundred transitions, and the previously calculated data under the Opacity Project includes 75590 LS multiplets. The present ab initio calculations are carried out in the close coupling approximation using the relativistic Breit-Pauli R-matrix method for allowed and intercombination E1 transitions. The wavefunction expansion includes 20 fine structure levels of configurations 2s²2p², 2s2p³, and 2p⁴ 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.