Poster presented by K. M. Aggarwal and F. P. Keenan at 11th International Conference on Atomic Processes in Plasmas, Queen's University Belfast, 19-22 July 2011

1. ELECTRON IMPACT EXCITATION OF Cl XVI
K. M. Aggarwal and F. P. Keenan
Astrophysics Research Centre, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, Northern Ireland
Introduction
Emission lines from several ionisation stages of chlorine, including He-like Cl XVI, have been observed
in 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 of
particular interest as these are useful for the determination of electron densities and temperatures in the
solar corona and transition region. Emission lines of Cl XVI have also been measured in lasing and
fusion plasmas. However, to analyse observations, atomic data are required for a variety of
parameters, such as energy levels, radiative rates (A- values), and excitation rates or equivalently the
effective 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 are
also available for some transitions on the NIST website, but there is paucity for accurate collisional
atomic 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), magnetic
dipole (M1), and magnetic quadrupole (M2), because these are also required for plasma modelling.
Calculations
For the determination of wavefunctions we employ the fully relativistic GRASP code, and for the
calculations of Ω, the Dirac atomic R-matrix (DARC) code of PH Norrington and IP Grant. Collision
strengths and effective collision strengths are calculated for all 1176 transitions among the 49 levels of
the n ≤ 5 configurations over a wide energy (temperature) range up to 580 Ryd (107.2 K), suitable for
applications in astrophysical and other plasmas. Additionally, parallel calculations have also been
performed with the Flexible Atomic Code (FAC) of Gu, so that all atomic parameters can be rigorously
assessed 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.