Calculation of theFH(CN−)-center absorption bands in CsCl

The $F_H$-center electron energy levels in CsCl(${\mathrm{CN}}^{\mathrm{-}}$) are calculated with use of a pseudopotential formalism where the model potential incorporates the point-ion potential, ion-size effects, and the interaction with the ${\mathrm{CN}}^{\mathrm{-}}$ finite-size permanent electric dipole. Electronic polarization and a radial $A_1$ distortion of the nearest-neighbor cations are treated as perturbations. The presence of the ${\mathrm{CN}}^{\mathrm{-}}$ ion in the next-nearest-neighbor position to the vacancy reduces the point symmetry to $C_{4v}$. When the dipole moment is neglected, the splitting of the $F_H$(1) and $F_H$(2) bands is due to the ion-size effects which are found to be negligible. The calculated splitting is 0.28 eV compared to the experimental value of 0.27 eV when the dipole moment is pointing toward the vacancy. Two-thirds of the splitting is attributed to the dipole’s electric field gradient in the vacancy, and the remaining splitting is due to the difference in the localization of the two excited states. The electric field mixing of the S- and P-symmetry basis states is found to be negligible.