2006A&A...457..721N

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 yeild about 171500 transitions. The allowed electric dipole (E1) transitions, 141869 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, 29682 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 2s²2p⁴, 2s2p⁵, and 2p⁶ 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 2s2p^{6 2}S 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 2p⁵(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.