Molecular orbital theory for heavy-metal luminescent centers: Application to the La2O3:Bi phosphor

A detailed semiempirical self-consistent charge and configuration molecular orbital theory is constructed for the problem of optical electronic states in phosphors doped by heavy metal ions. A cluster model is assumed for the luminescent center and all Madelung potential effects are included within the LCAO–MO framework. Expressions for energies of lower singly and doubly excited configurations are fomulated. Total energies for the cluster are obtained including spin–orbit and limited configuration interaction effects. The procedures are applied to the case of La2O3:Bi and reasonable agreement is found with presently available excitation spectra, thermoluminescence, and decay time data. The results confirm the presence of metastable levels and suggest a new interpretation of the luminescent processes in which the aspherical electronic delocalization about the Bi center plays a significant role.