Overtone spectra of crystalline CaWO4: vibrational exciton density-of-states functions

The first and second overtone regions of crystalline CaWO 4 are observed by direct infrared absorption. The two band systems reveal the entire vibrational density-of-states, i.e., all k states contribute to the band shape. The first overtone region contains fine structure but in the second overtone region, each combination (overtone) band is made up of only one broad peak. This is interpreted as resulting from the different ways of forming two-way versus three-way combinations of the fundamental k states. The shapes of the overtone bands are used to provide information on the structure of the fundamental exciton bands and on the relative magnitude of interactions along certain directions in the crystal.