Highly localised excitons in sodium halides: A linear combination of Gaussian-type orbitals X_α model cluster study

A cluster model that takes relaxation and Madelung effects into account is found suitable for the investigation of strongly localised excitations of alkali halide crystals in the soft-X-ray range. Application to NaCl and NaF confirms that the structures below the photoconduction threshold are due to excitonic quasi-atomic ‘inner-well’ states but, based on these cluster calculations, new assignments are given for all excitations except the first excited state. For NaCl, the two peaks assigned to Na 3d-type final states which split in the octahedral environment show an ordering e_g<t_2g at variance with the well known level ordering in transition-metal complexes. This is attributed to their rather diffuse character which leads to a dominant pi -type metal-ligand interaction. The hitherto postulated 'inversion' of the Na 3p and 3d final states is not supported by the cluster calculations.