The hopping motion of the self-trapped exciton in NaCl

Abstract

The decay kinetics and the yield of the π luminescence from the lowest triplet state of the self-trapped exciton have been studied in NaCl containing Li + ions. It is found that the π luminescence band which is observed at 6K is replaced by a luminescence band peaked at 3.34 eV above 77K. The 3.34 eV luminescence band is ascribed to the recombination of the relaxed exciton trapped by a Li + ion, (V k e) Li. The decay of the π luminescence induced by an electron pulse and the time change of the luminescence from (V k e) Li are explained in terms of the characteristic equation of the diffusion-limited reaction of the lowest triplet self-trapped excitons with the Li + ions. From the analysis of the dependence of the decay rate of the π luminescence on temperature and on the Li + concentration, we found the diffusion constant D of the lowest triplet self-trapped exciton in NaCl to be given by D = D 0e −E a kT with D 0 = 2.13 × 10 −3 cm 2 s and E 0 = 0.13 eV. The present result can be regarded as the first clear experimental evidence for the hopping diffusion of the self-trapped exciton in alkali halides. The obtained values of E a and D 0 are discussed using the small polaron theory. The effect of the anharmonicity on the hopping of the self-trapped excitons is suggested to be significant.