Abstract
The R-matrix method is used to obtain collision strengths for the electron impact excitation of the oxygen-like Ca XIII ion for all transitions between the 16 target states of even and odd parity. The wavefunctions for these states are generated using the configuration interaction method. The calculated values of the target state energies agree with the experimental values within 7%. The initial and final state wavefunctions for the composite (electron+ion) system are obtained by retaining these states in the R-matrix basis function expansion. In the low partial wave region (total angular momentum L<or=9) the standard full exchange R-matrix method is used for the scattering calculation. Beyond this, a no-exchange R-matrix method is applied to calculate collision strengths for a sufficiently large number of partial waves (up to total angular momentum L=30). The Burgess sum rule is used to determine the high partial wave (L>30) contribution to the cross sections for dipole allowed transitions. However, it is seen that collision strengths for almost all the transitions, including the dipole allowed, converge below L=30. The calculations are carried out in LS coupling and the results are compared with those obtained in the distorted-wave approximation along with other results, where available.
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