Non-luminescent disexcitation of F-center pairs by exchange-softened vibrational modes

The non-luminescent and spin-dependent disexcitation process observed in F-center pairs in alkali halides at low temperature is explained by a covalent bond within the pair. Exchange effects give a negative contribution to the lattice potential energy. If the pair separation is small, local modes become unstable and spontaneous lattice distortions bring back the pair in its ground state.