Electron impact excitation of Ca XX: collision strengths and rate coefficients

Collision strengths for all transitions belonging to states up to the n = 5 level of Ca XX are calculated in a wide electron energy range below 520 Ryd. These are obtained from the standard R-matrix program for all partial waves with L ≤ 12 and the no-exchange R-matrix program for 12<L ≤ 40. The resonances are delineated in the threshold energy region and collision strengths are tabulated at a few representative energies above threshold. These have also been integrated over a Maxwellian distribution of electron velocities to calculate the effective collision strengths. The results so obtained are tabulated for all transitions in an electron temperature range of log Te = 5.0-7.2 K. (author)