Electron impact excitation of helium-like oxygen up to n = 4 levels including radiation damping

The primary x-ray diagnostic lines in He-like ions are mainly excited by electron impact from the ground level to the n = 2 levels, but at high temperatures n≥2 levels are also excited. In order to describe the atomic processes more completely, collision strengths are computed for O VII including for the first time all of the following: (i) relativistic fine structure, (ii) levels up to n = 4, and (iii) radiation damping of autoionizing resonances. The calculations are carried out using the Breit–Pauli R-matrix method with a 31-level eigenfunction expansion. Resonance structures in collision strengths are delineated in detail up to the n = 4 thresholds. For highly charged He-like ions radiation damping of autoionizing resonances is known to be significant in certain energy ranges. We investigate this effect in detail and find that while resonances are discernibly damped radiatively as the series limit n → ∞ is approached from below, the overall effect on effective cross sections and rate coefficients is found to be very small. Collision strengths for the principal lines important in x-ray plasma diagnostics, w, x, y and z, corresponding to the four transitions to the ground level 1s2(1S 0) ← 1s2p(1P 1o), 1s2p(3P 2o), 1s2p(3P 1o), 1s2s(3S 1), are explicitly shown. Significant differences are found with previous works for several transitions. The contribution from the resonances converging to the levels from the complex n = 4 is found to be significant for some transitions and can increase the rate by a factor of four. This work is carried out as part of the Iron Project-RmaX Network.