Absorption-line-shape model for F-center-CN - Molecule defect pairs in CsCl

A systematic study of the temperature-dependent absorption spectra of FH (CN-) defect pairs in a CsCl host crystal is presented. At the lowest temperatures, two distinct absorption peaks are observed. As the temperature is increased, the separation of the peaks decreases, the relative oscillator strengths change, and the peaks broaden. These experimental results are explained by a simple model which considers that the nearly degenerate 2s and 2pz electronic states of the F center are strongly mixed by the intrinsic and displacement dipole moments of the CN- molecule. From this study, one obtains important parameters such as coupling constants, effective phonon mode frequencies participating in the absorption process, and a parameter describing the degree of 2s-2pz mixing, which are crucial for testing the physical models proposed for the electronic to vibrational energy-transfer process in this defect system.