Knowledge Bank

The infrared spectra of crystalline $CaWO4$ and $SO2$ are observed by a Fourier Transform spectrometer. In the former the first and second overtones reveal the entire vibrational density-of states, i.e., all $\backslash boldmathK$ states contribute of the band shape. This is in contrast to the latter in which the binary combinations (overtones) can be described as one-site excitons giving normal factor-group splitting. i.e., only $\backslash boldmathK$ = 0 States contribute to the spectrum. The higher combinations (overtones) of crystalline $SO2$ show a single sharp line, indicating a dramatic reduction in exciton band width. It is suggested that in crystalline $CaWO4$ the strong dipole-dipole interactions between $WO4=$ ions are responsible for the deviation from the one-site exciton limit. The shape of its overtone bands are used to provide information on the structure of the fundamental exction bands and on the relative magnitudes of interaction along certain directions in the crystal.