Energy levels of ground and singly excited states of two-electron atoms in dense quantum plasmas

The screening effect of dense quantum plasmas on the ground and singly excited states of two-electron atoms are investigated by employing the explicitly correlated Hylleraas configuration-interaction wave functions in the framework of Ritz variational principle. Exponential cosine screened Coulomb potential is used to model the electron-nucleus and interelectronic interactions in the atomic systems. Energy levels for the ground and 1snl L-1,L-3 (n <= 5 and 1 <= 4) singly excited states of H-, He, Li+, Be2+, and B3+ are reported for screening parameters varying from zero to the value nearing corresponding critical screening parameter. Convergence of our calculations are estimated by systematically increasing the number of basis functions. A thorough comparison with available theoretical predictions on the ground state energies at various screening parameters is made, while for the excited states a comparison with the well-established energies of unscreened atoms is performed. The variation of ionization energy and the validity of Hund's rule for two-electron systems in dense quantum plasmas are analyzed. (C) 2021 Elsevier Inc. All rights reserved.