A new model for the self-trapped exciton in alkali halides

The authors report a quantum mechanical study of the structural and spectroscopic properties of the self-trapped exciton in LiCl. Their method interfaces the quantum cluster calculations on a Li₁₀Cl₄ cluster with a Mott-Littleton treatment of the relaxation of the surrounding lattice. Calculations on the V_k centre give structures that are in very good agreement with Mott-Littleton calculations, and yield optical absorption energies that compare well with experimental values. Their study of the triplet ground state of the self-trapped exciton reveals a minimum energy structure of C_2v symmetry caused by a small of f-centre displacement (of approximately=0.07 AA) of the Cl₂^- (V_k) ions which comprise the hole component of the exciton. The electron is in a largely delocalized state around the hole with both components localized more on the Cl ion that is displaced towards the perfect lattice site. The magnitude of the off-centre displacement is much less than in the earlier studies of Song et al. (1990). A key feature in the success of these calculations was the self-consistent polarization of the surrounding lattice to the quantum cluster.