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Vacuum ultra-violet absorption spectra and far-infrared spectra (10 to 500 cm('-1)) were obtained for SrF(,2):RE('+3) and BaF(,2):RE('+3) system. Vacuum ultra-violet spectra were obtained at liquid nitrogen and liquid helium temperatures. Far-infrared spectra were obtained at room temperature only and also included the CaF(,2):RE('+3) system. The rare earth impurities consisted of Ce, Pr, Nd, Dy, Ho and Er. Ce is omitted from the ultra-violet work. The concentrations of the dopants varied from 0.005% atomic to 0.1% atomic. We find in the present work that SrF(,2):RE('+3) and BaF(,2):RE('+3) systems possess vibrational modes with frequencies of 438 cm('-1) and 405 cm('-1), respectively. These frequencies are considerably higher than the maximum frequencies in their corresponding density of states curves and are designated as belonging to local vibrational modes. These frequencies are similar to the 485 cm('-1) frequency of the CaF(,2):RE('+3) system and support the view that the 485 cm('-1) frequency belongs to a local mode. An analysis of the ultra-violet data shows that the 438 cm('-1) and 405 cm('-1) frequencies are independent of the RE('+3) impurity in either host. The data shows that these two frequencies are also independent of the symmetry (charge compensation) of the samples. Different concentrations of dysprosium indicate that the frequencies are independent of the concentration of the same RE('+3) impurity within a given host as well. To obtain far-infrared data necessitated the development of a suitable operational system. A major portion of the present work was concerned with this development and low temperature far-infrared work was reserved for future studies. Far-infrared transmission (absorption) and reflection studies were initiated in an attempt to observe the local mode frequencies of the CaF(,2):RE('+3), SrF(,2):RE('+3), and BaF(,2):RE('+3) systems. All samples exhibited very little transmittance in the regions of interest. Spectra of samples with the same host are quite similar. Spectra of samples with different hosts possess the same general features but are shifted in wavenumber from host to host. Any absorption due to the RE('+3) impurities could not be reliably determined from any of the absorption spectra. Reflection spectra revealed the Reststrahlen bands of the host crystals. Source: Dissertation Abstracts International, Volume: 43-03, Section: B, page: 0660. Thesis (Ph.D.)--University of Windsor (Canada), 1982.