Observation and analysis of the optical absorption and fluorescence spectra of green-colored Np:CaF2

Abstract

The optical absorption spectra seen at room and cryogenic temperatures in green-colored Np:CaF2 crystals have been attributed to Np3+ substitution at Ca2+ [000] sites with three distinct modes of charge compensation and correspondingly lowered local symmetries. These are: A, C3v symmetry associated with the presence of an F− compensator at empty [(1/2)(1/2)(1/2)]-type sites; B, C3v symmetry related to the replacement of a nn F− at a [(1/4)(1/4)(1/4)]-type position by O2−; C, C4v symmetry attributed to the presence of an F− compensator at empty [00(1/2)]-type sites. A two-parameter fit to the five lowest excited multiplet centers yielded the following F2 and ζ values (in cm−1): A, 214.5 and 2032; B, 212.7 and 2001; C, 219.1 and 2083. The multiplet splittings of each of these three spectra have been fit with crystal potential (’’crystal-field’’) parameters. Fluorescence, consisting of several broad bands near 1.4 μm and four lines near 2.5 μm, was associated with the Np3+ type-C sites. At liquid He temperature, the 2.5 μm spectrum consisted of just one sharp line at 3929 cm−1. Vibrational effects were largely absent, both in absorption and fluorescence.