The polarized vibronic spectra of PrCl3and PrBr3, as well as Pr3+in LaCl3and LaBr3, were observed. The theory of vibronic transitions for the dilute salt is shown to predict a spectrum which closely resembles the vibronic spectrum of the pure salt, in agreement with experiment. Approximations valid when the optical vibrational branches of the crystal are flat leading to k=0 selection rules are shown to lead to agreement with the spectra fairly well, although slight violations in selection rules occur and can readily be explained. Tentative identifications of crystalline modes of vibration are assigned partly with the help of polarization data and vibronic selection rules, partly by consideration of frequency shift in comparing the chloride and bromide vibronic spectra, partly from Raman scattering data, and partly with the aid of the farhyphen;infrared absorption data of Buchanan, Murphy, and Caspers. The vibronic data in turn help refine the interpretation of the farhyphen;infrared results. The difference between longitudinal and transverse vibrating frequencies is discussed.
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