Exciplexes (excited complex) are a special kind of molecules, that can only exist if one of the atoms in the molecule is in an excited state. The first ones observed were alkali-helium (molecules with helium and alkali metals, such as Lithium, Sodium, etc.) exciplexes in 1995. Since then, physicists have discovered exciplexes in liquid helium, cold helium gas, and on helium nanodroplets. Now physicists have found a new one, which contains one caesium atom, and seven helium atoms, and a simpler one with two helium atoms (D Nettels et al).

Dumbbell Caesium, surrounded by helium (Courtesy: PhysicsWeb)Researchers (Peter Moroshkin and colleagues at the University of Fribourg) say that this molecule may only exist at very low temperature and high pressure. These molecules are good candidates for studying fundamental quantum physics and quantum chemistry. Similar exciplexes might exist for Rubidium-Helium as well.