MODEL OF SELF-TRAPPED EXCITONS IN ALKALI-HALIDES

We have carried out an ab initio many-electron variational calculation of the adiabatic potential-energy surface (APES) for the lowest triplet state of the self-trapped exciton (STE) in LiCl, NaCl, and KCl. Calculations of the H center in these crystals show that the [111] orientation is favored, in agreement with experimental results for NaCl but not for KCl, in which it is oriented along a [110] direction (no measurement exists for LiCl), and hence most detailed calculations for STE's are carried out for NaCl. It is found that the APES minimum for each crystal occurs when the Cl2- molecular ion is displaced along its molecular axis from its symmetrical position (D2h) nearly halfway to the nearest halogen lattice point. The calculated transition energies for the optical absorption and luminescence at this configuration agree with the experimental values for the triplet STE, although the calculated stretching vibration frequency of the Cl2- Molecular ion in NaCl is much smaller than that for the H center, contradictory to recent resonant Raman studies. Other minima are found at the nearest F-H pair configuration, in which the Cl2- Molecular ion is reoriented by 90-degrees from the initial orientation and next-nearest F-H pair. Extremely small luminescence energy at these configurations excludes the possibility that they are the candidates for the luminescent state of the STE. It is found that, after the displacement of the Cl2- molecular ion beyond the first minimum of the APES towards the nearest F-H pair configuration, the total energy is lowered by reorientation, inducing an anomaly on the APES. The results of a recent experimental investigation, including existence of several types of relaxed configuration of the STE in alkali halides, the stretching vibration frequency, and the femtosecond oscillation on APES, are discussed on the basis of the results of the calculation.