Knowledge Bank

An extensive set of oscillator strengths (f), line strengths (S) and radiative decay rates (A) for the allowed and forbidden transitions in Fe XVIII is presented. The results include 1174 fine structure levels of total angular momenta J = 1/2–17/2 of even and odd parities with 2 ≤ n ≤ 10, 0 ≤ l ≤ 9, orbital angular momenta, 0 ≤ L ≤ 11, and spin multiplicities 2, 4. These yield about 171 500 transitions. The allowed electric dipole (E1) transitions, 141 869 in total, are obtained in the close coupling approximation using the relativistic Breit-Pauli R-matrix (BPRM) method. The forbidden electric quadrupole (E2), octupole (E3), and magnetic dipole (M1), quadrupole (M2) transitions, 29 682 in total, are obtained from configuration interaction Breit-Pauli atomic structure calculations using the code SUPERSTRUCTURE. The close coupling wavefunction expansion for Fe XVIII includes 10 levels of configurations 2s^2 2p^4, 2s^2 p^5, and 2p^6 in the BRPM calculations. All fine structure levels have been identified spectroscopically. Comparison of the present energies with the available observed energies shows very good agreement. The lifetime of level 2s 2p^6 ^2S calculated from the present radiative decay rate agrees very well with the single measurement. The present decay rate for the well known E2 line arising from 2p^5(^2P°_3/2 − ^2P°_1/2) transition within the ground state term also agrees almost exactly with NIST tabulated value. However, varying degree of agreement is found with previously calculated values for some transitions.