  When atoms or molecules exist in the form of stable negative ions, they play
a crucial role in the gas phase chemistry. Determining the existence of such an
ion, its internal energy and its stability are necessary prerequisites to
analyze the role of this ion in a particular medium. Experimental evidence of
the existence of a negative methane ion CH$_4^-$ has been provided over a
period of 50 years. However, quantum chemistry had not been able to explain its
existence, and a detailed recent study has shown that the experimentally
observed species cannot be described by the attachement of an electron in the
ground state of CH$_4^-$. Here we describe CH$_4^-$ as being a metastable
species in its lowest quartet spin state and we find that this species is a
CH$_2^-$-:H$_2$ exciplex with three open shells, lying 5.8 eV above the methane
singlet ground state but slightly below the dissociation fragments. The
formation of charged exciplexes is a novel mechanism to explain small molecular
anions with implications in a plethora of basic and applied research fields.
