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

Abstract

The primary x-ray diagnostic lines in He-like ions are mainlyexcited by electron impact from the ground level to the n = 2 levels, but at hightemperatures n≥2levels are also excited. In order to describe the atomic processes morecompletely, collision strengths are computed for O VII including for the firsttime all of the following: (i) relativistic fine structure, (ii) levels up ton = 4, and (iii)radiation damping of autoionizing resonances. The calculations are carried out using theBreit–Pauli R-matrixmethod with a 31-level eigenfunction expansion. Resonance structuresin collision strengths are delineated in detail up to the n = 4thresholds. For highly charged He-like ions radiation damping ofautoionizing resonances is known to be significant in certain energy ranges.We investigate this effect in detail and find that while resonances arediscernibly damped radiatively as the series limit n → ∞is approached from below, the overall effect on effective cross sectionsand rate coefficients is found to be very small. Collision strengthsfor the principal lines important in x-ray plasma diagnostics,w, x, yand z,corresponding to the four transitions to the ground level 1s2(1S0) ← 1s2p(1P1o), 1s2p(3P2o), 1s2p(3P1o), 1s2s(3S1),are explicitly shown. Significant differences are found with previous works for severaltransitions. The contribution from the resonances converging to the levels from thecomplex n = 4is found to be significant for some transitions and can increase the rate by a factorof four. This work is carried out as part of the Iron Project-RmaX Network.