Abstract
Infrared absorption from the fundamental local mode vibrations of substitutional H- ions (U centres) paired with Sr2+ and Ba2+ impurities in calcium fluoride has been observed. The spectra include absorption peaks near 965 cm-1 which are shown to be due to two distinct types of centre, depending on whether the hydride ions occupy first or second neighbour fluorine ion sites to the alkaline earth impurity. The results are compared with those obtained previously for hydride ions paired with divalent rare earth ions in the same host lattice and with cation impurities in various mixed alkali halide crystals.