Electron-impact excitation of ions in the magnesium sequence: Fe xv

Intermediate-coupling collision strengths are calculated for all transitions between the states 3$s^2$ ${}^1$ $S_0$, 3s3p(${}^3$ $P_0$,1,2, ${}^1$ $P_1$), 3$p^2$(${}^1$ $D_2$, ${}^3$ $P_0$,1,2, ${}^1$ $S_0$), 3s3d(${}^3$ $D_1$,2,3, ${}^1$ $D_2$), and 3s4s(${}^3$ $S_1$, ${}^1$ $S_0$). Calculations are carried out in a ten-state distorted-wave approximation. Resonance effects are considered by using multichannel quantum-defect theory, and relativistic effects in the target Hamiltonian are taken into account in the Breit-Pauli formulation. The important 3$s^2$ ${}^1$ $S_0$–3s3p ${}^3$ $P_1$ transition is found to be subject to large resonance enhancement, as previously shown by other workers for less highly charged Mg-like ions. Term coupling among the target states also affects several transitions considerably. Present results are compared with previous calculations; some significant differences are noted. The new results suggest a serious discrepancy between calculated and observed relative intensities of the 284.2-Å (resonance) and 417.3-Å (intercombination) lines for Fe x v in the Sun, but will reduce the discrepancy for this ratio for other Mg-like ions observed in tokamak plasmas.