2s and 2p energy inversion in a doubly-core-excited four-electron system

Abstract

It is commonly believed that an atomic system with a 2s electron gives a lower energy than that of a system with a 2p electron. The author shows that in a doubly-core-excited four-electron system, the energy of the 2s22p2 3P state is substantially higher than that of the 2s2p3 5S 0one. In the case of Be I, it is higher by 0.48 eV. The energies for the 2s2p3 5S0 and 2s22p2 3P states are calculated for Z=3 to 10. Relativistic corrections are included. The reason for this 2s-2p inversion is the exchange interaction, which is particularly important in the formation of a bound state or resonance in a negative ion.