  A detailed analysis on the effect of spherical impenetrable confinement on
the structural properties of two-electron ions in S-states have been done. The
energy values of 1sns [n = 2-4] (3Se) states of helium-like ions (Z = 2-5) are
estimated within the framework of Ritz variational method by using explicitly
correlated Hylleraas-type basis sets. The correlated wave functions used here
are consistent with the finite boundary conditions due to spherical
confinement. A comparative study between the singlet and triplet states
originating from a particular electronic configuration shows incidental
degeneracy and the subsequent level-crossing phenomena. The thermodynamic
pressure felt by the ion inside the sphere pushes the energy levels towards
continuum. Critical pressures for the transition to strong confinement regime
(where the singly excited two-electron energy levels cross the corresponding
one-electron threshold) as well as for the complete destabilization are also
estimated.
