Absorption, luminescence, and electron paramagnetic resonance of molybdenum ions in CdWO 4

Abstract

A single crystal of cadmium tungstate (CdWO 4) containing approximately 200 ppm of molybdenum was grown by the Czochralski method and then characterized in a series of optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), and electron paramagnetic resonance (EPR) experiments. The Mo 6+ ions substitute for W 6+ ions and serve as recombination sites for electrons and holes when the crystal is exposed to ionizing radiation. A charge-transfer absorption band for the Mo 6+ ions was observed near 320 nm at 10 K. The PL experiments, performed at low temperature with 325 nm excitation, showed a Mo-associated emission peaking near 680 nm. A direct correlation of the 680 nm emission and the 320 nm absorption band was established by the PLE data. When these doped CdWO 4 crystals are exposed at low temperature either to light that is near or above the band gap or to X-rays, the Mo 6+ ions can trap an electron and form stable Mo 5+ ions. The EPR spectrum of the Mo 5+ ions was observed at temperatures near 15 K, and a complete set of parameters describing the g matrix was obtained from an angular dependence study.