Hund's rule in the (1s2s)1,3S states of the two-electron Debye atom

Abstract

We present an investigation of the (1s2s)1,3S excited states of the two-electron atom immersed in a plasma modeled by the Debye or screened Coulomb potential. Three variants of the Debye atom are considered. The validity of Hund's multiplicity rule is confirmed, and the contribution of the interparticle repulsion energy to the singlet-triplet splitting is examined. The feature that this system shares with the unscreened two-electron atom as well as with the confined two-electron atom and the two-electron quantum dot is that the triplet wave function is contracted relative to that of the singlet. This feature affects both the behavior of the 2s-electron ionization energies and the relative magnitudes of the interparticle repulsion energies in the singlet vs. the triplet. Debye screening of the one-body attraction effectively reduces the nuclear charge, enhancing the reversal of the relative magnitudes of the triplet vs. singlet interparticle repulsion energies. Debye screening of the interparticle repulsion acts in an opposite way.