Electron impact excitation of H-like, He-like and Li-like ions with Z ≤ 30

K. M. Aggarwal and F. P. Keenan (Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University, Belfast BT7 1NN, Northern Ireland, UK)

Atomic data, namely energy levels, radiative rates (A-values), and excitation rates or equivalently the effective collision strengths (Υ), which are obtained from the electron impact collision strengths (Ω), are required for a range of ions for the analysis and modelling of a variety of plasmas, such as solar, nebular, lasing, and fusion. Emission lines over a wide range of wavelengths from a range of H-like, He-like, and Li-like ions with Z  ≤ 30 have been observed in a variety of solar and astrophysical plasmas, and many of these are known to be highly useful for temperature and density diagnostics, as well as for the determination of chemical abundances.

Experimentally, most of the energy levels for many of the H-like, He-like, and Li-like ions with Z ≤ 30 are available on the NIST website. Corresponding theoretical results are also generally available with a high order of accuracy. The availability of radiative rates on the NIST website is restricted to a few transitions, but theoretical results are widely available, although some of those differ quite significantly among themselves, particularly for the weaker transitions. However, generating accurate A-values for the desired ions is not much of a problem with the computational resources available these days. Additionally, a variety of sophisticated atomic structure codes are available for the calculations of A-values, and as a result, highly reliable results are either already available in the literature or can be easily generated.

Similar results for collision strengths are also available for many of the above noted ions, but often either the available data are limited (to only a few transitions and/or to a few energies) or lack accuracy, particularly for the effective collision strengths. This is because calculations for Υ are computationally intensive, because resonances in the thresholds region need to be resolved in a fine energy mesh, in practice at thousands of energies, depending on the ion. The n ≤ 5 configurations of H-like, He-like, and Li-like ions generate a maximum of 49 levels. Therefore, with the computational resources available these days it should not be very difficult to generate the desired data. Subsequently, many calculations are available in the literature for many of the desired ions. However, the accuracy of the available results are often suspect. The major difficulty in generating accurate data is because of the near non-degeneracy among the levels of a state of these ions. As a result of this, collision strengths for many transitions, including forbidden, converge very slowly. Additionally, positioning of resonances can also be responsible for large variations among different calculations, particularly at lower temperatures. Therefore, in the recent past we have reported complete sets of results for all above noted atomic parameters for many of the H-like, He-like, and Li-like ions with Z ≤ 30. Work is in progress for a few more ions, and in this talk discrepancies with the other available results will be discussed, and a need will be emphasized for which ions calculations are particularly required.

Talk given at 3d CDAMOP, 14-16 December 2011, Delhi University, Delhi, India.

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One Response to Electron impact excitation of H-like, He-like and Li-like ions with Z ≤ 30

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