Helium atom embedded in non-ideal classical plasmas: Doubly excited singlet S states

The influence of the non-ideality (NI) of the classical plasmas on the doubly excited singlet S states of the helium atom (He) embedded in the plasma has been investigated theoretically. A pseudopotential containing the Debye length and the non-ideality parameter (NIP) as its characteristics is used to represent the screened interaction potentials in the plasma. Using a large wavefunction within the framework of the stabilization method, it has been possible to recognize six doubly excited singlet S states (five lying below the He+(2S) excitation threshold and one lying below the He+(3S) excitation threshold) for the plasma-free case. The energies and the autoionization life-times of those states are computed by fitting the density of states to the Lorentzian form. Convergence of the computed results is corroborated by increasing the number of terms in the employed wavefunction. For the plasma-free case, these results are in excellent agreement with the established results in the literature. A comprehensive analysis has been made on changes induced on those doubly excited states by varying NI over a wide range. It has been observed that the energies of the states gradually approach the corresponding threshold energies with the increasing NI of the plasma, whereas the change in the life-times (alternatively the widths of the states) of the states shows distinctive features depending on the angular momentum of an individual electron.