Chemical Bond Mechanism for Helium Revealed by Electronic Excitation

Helium chemistry is notoriously very impervious. It is therefore certainly no surprise that, for example, beryllium and helium atoms, in their ground state, do not bind. Full configuration-interaction calculations show that the same turns out to be true, save for a long-range shallow attraction, for the Be⁺ + He system. However, quite astonishingly, when one electron is re-added to Be⁺ in an excited 2p_π or 3s orbital (Be ¹P or ¹S), a bound adduct with He is formed, at an interatomic separation as short as 1.5 Å. Understanding why this happens reveals an unsuspected chemical mechanism that stabilizes helium compounds at the molecular level.