ELECTRON IMPACT EXCITATION OF Cl XVI K. M. Aggarwal and F. P. Keenan Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN, Northern IrelandIntroductionEmission lines from several ionisation stages of chlorine, including He-like Cl XVI, have been observedin solar plasmas. The n =2 lines of He-like ions in the x-ray region (namely, the resonance w: 1s2 1S0 -1s2p 1Po1, intercombination x and y: 1s2 1S0 - 1s2p 3Po2,1, and forbidden z: 1s2 1S0 - 1s2p 3S1) are ofparticular interest as these are useful for the determination of electron densities and temperatures in thesolar corona and transition region. Emission lines of Cl XVI have also been measured in lasing andfusion plasmas. However, to analyse observations, atomic data are required for a variety ofparameters, such as energy levels, radiative rates (A- values), and excitation rates or equivalently theeffective collision strengths (γ), which are obtained from the electron impact collision strengths (Ω).Additionally, atomic data for Cl XVI are highly required for the study and modelling of fusion plasmas.Experimentally, only energy levels are available for Cl XVI on the NIST website. Similarly, A- values arealso available for some transitions on the NIST website, but there is paucity for accurate collisionalatomic data for Cl XVI. Therefore here we report a complete set of results (namely energy levels,radiative rates, and effective collision strengths) for all transitions among the lowest 49 levels of Cl XVI.These levels belong to the 1s2, 1s2l, 1s3l, 1s4l, and 1s5l configurations. Finally, we also report the A-values for four types of transitions, namely electric dipole (E1), electric quadrupole (E2), magneticdipole (M1), and magnetic quadrupole (M2), because these are also required for plasma modelling.CalculationsFor the determination of wavefunctions we employ the fully relativistic GRASP code, and for thecalculations of Ω, the Dirac atomic R-matrix (DARC) code of PH Norrington and IP Grant. Collisionstrengths and effective collision strengths are calculated for all 1176 transitions among the 49 levels ofthe n ≤ 5 configurations over a wide energy (temperature) range up to 580 Ryd (107.2 K), suitable forapplications in astrophysical and other plasmas. Additionally, parallel calculations have also beenperformed with the Flexible Atomic Code (FAC) of Gu, so that all atomic parameters can be rigorouslyassessed for accuracy.Results & Conclusions Energy levels and their lifetimes are listed in Table 1. Measurements of many energy levels are not available and some measurements are non-degenerate. GRASP energies are lower than NIST by about 0.2 Ryd. FAC energies are higher than GRASP by about 0.1 Ryd. GRASP and NIST orderings are the same but of FAC differ particularly for the n = 5 levels. Inclusion of n=6 levels makes no appreciable difference either to energies or to their orderings. GRASP and FAC energy levels agree within 0.1 Ryd and orderings are also the same. GRASP and FAC f- values agree within 20% for all transitions. Partial collision strengths are shown in Figs. 1-3. Ω values from DARC and FAC are compared in Figs. 4 and 5. Ω values from FAC are anomalous for many transitions. At Te=105.8 K, γ values from DARC and FAC differ by over 20% for about 40% transitions, and generally ΥD > γF. At Te=107.2 K, γ values from DARC and FAC differ by over 20% for about 43% transitions, and generally ΥF > γD, mainly because ΩF are anomalous.