Electron-impact excitation of C3+ and O5+:: the effects of coupling to the target continuum states

We have carried out extensive R-matrix close-coupling calculations of the electron-impact excitation of C3+ and O5+. We have determined effective collision strengths for transitions between the lowest nine terms and cross sections between the ground state and selected excited terms of these ions from 41-state R-matrix with pseudo-states (RMPS) calculations that employ basis sets consisting of nine physical states and 32 pseudo-states. In order to investigate the dependence of electron-impact excitation on coupling of the bound states to the target continuum states and also the highly excited bound states included in these RMPS calculations, we have compared these results with those determined from our 9-state and 13-state R-matrix calculations without pseudostates. As one would expect, this additional coupling is in general less important in O5+ than in C3+ however, these effects vary significantly with the type of transition and are complicated by the resonance contributions. This makes it difficult to draw any general conclusions regarding their dependence on ionization stage. These results complement earlier work on the Li-like ions Be+ and B2+ and provide improved sets of excitation data for these ions.