Electronic structure and luminescent properties of Cu+ and Ag+ impurity centers in NaCl

Fluorescence studies of Cu⁺ and Ag⁺ impurity centers embedded in NaCl are reported. The multiple scattering X_α method is used to describe the electronic structure of these ions in NaCl matrix. It is shown that taking into account the external lattice potential is necessary to obtain good results which can be compared to experiment. This has been done through the computation of an embedded NaCl cluster potential. Moreover, in NaCl:Ag⁺, it is also necessary to include relativistic corrections in order to obtain a reliable energy diagram. This is easily done by use of the Wood Hamiltonian, which allows the self-consistent inclusion of Darwin and mass–velocity terms in MS X_α codes. A good quantitative agreement with experiment is finally obtained for the optical excitations and the emission mechanism of the luminescent centers. The metal–chlorine distance is predicted slightly larger than the NaCl bulk value and the harmonic force constant of the a_1g vibrational breathing deformation of the cluster is calculated and found in good agreement within experiment. For the first time the Stoke shift of such systems is evaluated.