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

Intermediate-coupling collision strengths are calculated for all transitions between the states 3s² ¹{S}₀$, 3s3p(${}³P₀,1,2, ¹{P}₁$), 3${p}²$(${}¹D₂, ³{P}₀$,1,2, ${}¹S₀), 3s3d(³{D}₁$,2,3, ${}¹D₂), and 3s4s(³{S}₁$, ${}¹S₀). Calculations are carried out in a ten-state distorted-wave approximation. Resonance effects are considered by using multichannel quantum-defect theory, and relativistic effects in the target Hamiltonian are taken into account in the Breit-Pauli formulation. The important 3s² ¹{S}₀$--3s3p ${}³P₁ 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-A{} (resonance) and 417.3-A{} (intercombination) lines for Fe xv in the Sun, but will reduce the discrepancy for this ratio for other Mg-like ions observed in tokamak plasmas.