Energy levels, radiative rates and electron impact excitation rates for transitions in He-like Cl XVI, K XVIII, Ca XIX and Sc XX.

We report calculations of energy levels, radiative rates and electron impact excitation cross<br/>sections and rates for transitions in He-like Cl XVI, K XVIII, Ca XIX and Sc XX. The grasp<br/>(general-purpose relativistic atomic structure package) is adopted for calculating energy levels<br/>and radiative rates. To determine the collision strengths and subsequently the excitation rates,<br/>the Dirac atomic R-matrix code (darc) is used. Oscillator strengths, radiative rates and line<br/>strengths are reported for all E1, E2, M1 and M2 transitions among the lowest 49 levels of<br/>each ion. Collision strengths are averaged over a Maxwellian velocity distribution and the<br/>effective collision strengths obtained listed over a wide temperature range up to 107.4 K.<br/>Comparisons are made with similar data obtained from the flexible atomic code (fac) to<br/>highlight the importance of resonances, included in calculations with darc, in the<br/>determination of effective collision strengths. Discrepancies between the collision strengths<br/>from darc and fac, particularly for forbidden transitions, are also discussed. Additionally,<br/>theoretical lifetimes are listed for all the 49 levels of the above four ions.