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 alkalihalides 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 distor-tions bring back the pair in its ground state. Alcali halide crystals with a high concentration of F- We consider a pair of F centers located at sites a centers show at low temperature a strong variation of and b (separation R). The first one is in its relaxed cx-the luminescent quantum yield under an applied static cited state and the second one in its ground state. A magnetic field [1, 21 which allows optical detection of local vibrational mode (frequency w, amplitude q electronic and nuclear magnetic resonances [3,4]. with <q2)~=~ = I) mixes to the electronic state an-These phenomena are explained by interaction of pairs other one ~ ~, having an energy higher by 6. Up to the of neighbouring centers, in which the electronic disex- second order, the q dependent wave function and its citation path (radiative or non-radiative) depends on energy take the form the symmetry of the two-spin wave function. Accord-ing to an hypothesis put forward by Markham et a!. — ~- w~q~) — wq~ 1+