Absorption Spectrum and Quantum States of the Praseodymium Ion. II. Anhydrous Praseodymium Fluoride in Films

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Abstract
The absorption spectra of anhydrous praseodymium fluoride were examined to confirm term designations derived from the spectra of anhydrous praseodymium chloride. In the fluoride, praseodymium ion is at the center of a micro‐field of C2V symmetry and in the chloride it is in a field of C3h symmetry. There was complete consistency in the interpretations of the spectra of the two compounds. A new phenomenon appeared in the spectrum of the fluoride, however. The selection rules prohibiting some of the possible transitions of an ion in a field of C2V symmetry valid at the temperature of liquid nitrogen were lifted at the temperature of liquid helium, where in fact all possible transitions occurred. There is evidence against any gross crystallographic transformation. It is surmised that the positions of energy minimum for the ions lie slightly off the symmetric positions possibly due to higher order interactions such as occurs in the case of barium titanate.