Longitudinal Optical Phonons in CaF2, SrF2, and BaF2

Off‐normal reflection spectra of thin films of CaF2, SrF2, and BaF2 deposited on a silver substrate have been obtained. They exhibit decreased reflectivity at the long wave limit of the longitudinal optical branch. The values so obtained are in agreement with the predictions by the Lyddane—Sachs—Teller relation. They are also in agreement with vibrational levels obtained from vibronic data for SrF2:Sm2+ and BaF2:Sm2+. The observed longitudinal optical phonons are at 466 cm—1 in CaF2, 366 cm—1 in SrF2, and 319 cm—1 in BaF2. The close agreement between vibronic data and direct observation of these vibrations is considered further evidence that a good model for these vibronic transitions must consider vibrations of a relatively large segment of crystal and not merely nearest neighbors.