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ABSTRACT:We have obtained the bound 1s 2 1 S, 1s2s 1,3 S, and 1s2p 1,3 P states energies of helium atom in dense plasma environments in accurate variation calculations. A screened Coulomb potential to represent the Debye model is used for the interaction between the charged particles. A correlated wave function consisting of a generalized exponential expansion has been used to take care of the correlation effect. The 1s 2 1 S, 1s2s 1,3 S, and 1s2p 1,3 P states energies along with the ionization potential, the energy splitting between the 1s2s 3 S, and 1s2s 1 S states, transition energies between the ground state and low-excited states of He estimated for various Debye lengths, are reported. The results show high degree of accuracy even under strong plasma conditions.

Two S-wave resonances in the positron-helium system are calculated using the stabilization method. A model potential for helium is used to represent the He + ion core and the outer electron. Hylleraas-type wavefunctions are used to represent the correlation effects between the outer electron, the positron and the He + core. Resonance energies and widths for two resonances lying below the Ps (n = 2) threshold are reported.

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Resonance states of C4+, O6+, Ne8+ ions interacting with screened Coulomb potentials have been investigated using the stabilization method. We employ highly correlated exponential basis functions for S-, P-state calculations and employ CI-type basis functions for D-state calculations. We report a total of seven S-wave resonances of which four belong to 2sns (n = 2, 3, 4, 5) series and three belong to the 2pnp (n = 2, 3, 4) series. In addition, a total of eight P-wave resonances of which three belong to 2snp+ (n = 2, 3, 4), three belong to the 2snp− (n = 3, 4, 5) series, two belong to the 2pnd (n = 3, 4) and the lowest D-wave resonance for each systems and each screening parameters below the n = 2 thresholds of the respective subsystems are reported. In the unscreened case, we have found some resonance states that have not been reported before. In the screened case our reported results are new and show some interesting behaviour with and without electron–electron screening.