Can anions possess bound doubly-excited electronic states?

Anions play an important role in many fields of chemistry. Many molecules possess stable anions, but these anions often do not have stable electronic excited states and the anion loses its excess electron once excited. All the known stable valence excited states of anions are singly-excited states, i.e., valence doubly-excited states have not been reported. As excited states are relevant for numerous applications, and constitute basic properties, we searched for valence doubly-excited states which are stable, i.e., exhibit energies below that of the ground state of the respective neutral molecule. We concentrated on two promising prototype candidates, the anions of the smallest endocircular carbon ring Li@C₁₂ and of the smallest endohedral fullerene Li@C₂₀. By employing accurate state-of-the-art many-electron quantum chemistry methods, we investigated the low-lying excited states of these anions and found that they possess several low-lying stable singly-excited states and, in particular, a stable doubly-excited state each. It is noteworthy that the found doubly-excited state of Li@C₁₂^- possesses a cumulenic carbon ring in sharp contrast to the ground and singly-excited states. The findings shed light on how to design anions with stable valence singly- and doubly-excited states. Possible applications are mentioned.