Electronic structure of the relaxed exciton trapped by an alkali impurity in KBr

The relaxed exciton trapped by a foreign alkali ion (V K e) A in KBr has been studied through optical absorption and emission measurements by means of pulse radiolysis techniques as well as conventional X-ray excitation. The decay times of the pi -polarised 2.9 eV emission (V K e) Na and 2.75 eV emission of (V K e) Li are found to have a constant value of 2.5*10 -5 and 1.5*10 -5 s, respectively, below 60K. Transient optical absorption bands of (V K e) A are observed at 1.65 and 1.5 eV for (V K e) Na and 1.75 and 1.6 eV for (V K e) Li , features of which are very similar to those of the self-trapped exciton. The initial state of the emission of (V K e) A is thus identified unambiguously as the lowest triplet state. It is also found that the formation of (V K e) Na decreases above 30K. The shifts of transition energies and changes in decay rates of (V K e) A from those of the pure self-trapped exciton are discussed in terms of perturbation of a nearby foreign alkali ion.