F-Aggregate Centers in NaCl: Vibronic Structure and Symmetry Properties

Most color-center bands associated with $F$-aggregate centers in alkali halides show relatively strong 0-phonon lines and vibronic structure when observed at low temperature. The vibronic peak energies can in almost all cases be closely correlated on an empirical basis with zone-boundary critical-point energies of normal lattice phonon modes. However, without detailed knowledge of the defect geometries and electronic states involved, it is impossible to predict what the electron-phonon coupling should be. We present the observed vibronic spectrum for all the centers in NaCl which we have found to show fine structure, and discuss the apparent electron-phonon coupling seen. As a first step toward developing models for these centers, we also present results of stress splitting measurements on the 0-phonon lines in each case. The 0-phonon lines we have studied, their band assignments, and their observed symmetry as interpreted in terms of removal of orientational degeneracy, are as follows: 6329 Å, $R_2$, trigonal; 8375 Å, $N_1$, monoclinic I; 8681 Å, $N_2$, monoclinic I; 1.219 μ,?, monoclinic I; 1.306 μ, $M^{+}$, monoclinic I; 1.404 μ, impurity, rhombic I; 1.549 μ,?, rhombic I. In several cases the relative intensities of the line components can be analyzed to give the orientation of the dipole moment involved in the 0-phonon transition. Where it is possible to propose models by combining our stress measurements with the results of other experiments, we do so, with discussion as to the probable merit of each model.