Stress splitting of infrared absorption bands due to U-centers

Abstract

We are reporting on measurements of the splitting under uniaxial stress of the U center local mode absorption bands in KCl and KI. These infrared bands consist of two different components: Firstly, the main band which corresponds to direct excitation of the threefold degenerate first vibrational level of the local mode; and secondly, sidebands to the main band, which arise from transitions to a final state in which a local mode quantum is excited and another phonon is either created or destroyed. This combined absorption may be caused by the anharmonic coupling or nonlinear dipole moment coupling of the two vibrations involved.(1)(2) Main band splittings under stress were measured in KCl and KI. They give the magnitude of the separate contributions of cubic, tetragonal, and shear strain to the shift of the local mode frequency. On the basis of a most simple model, this information may be used to obtain the contributions to the sidebands of phonons of the same symmetries. The behavior under stress of a sharp sideband peak at 93.7 cm-1 separation from the main band in KI was also observed, and the symmetry of the 93.7 cm-1 phonon directly deduced from the splitting pattern.