Identification and optical properties of axial lead centres in alkaline-earth fluorides

Axial Pb centres were produced by X-irradiation in Pb-doped CaF₂, SrF₂ and BaF₂, which show intense infrared emissions. One paramagnetic centre was identified by optically detected electron spin resonance (ESR) and electron-nuclear double resonance to be a Pb⁺ substituting for M²⁺ (M identical to Ca, Sr or Ba) next to an F^- vacancy along (111)(Pb⁺(1) centre). The absorption bands could be identified from the excitation spectrum of the infrared emission and from a combination of magneto-optical and ESR techniques. The emissions have quantum efficiencies near unity up to room temperature. The optical properties are explained by a crystal-field model analogous to that derived for the laser-active Tl⁰(1) centres in alkali halides. Laser action of the Pb⁺(1) centre is expected. Results on a Pb⁺(1)-Pb²⁺ centre and a diamagnetic Pb⁰(2) centre, which are produced simultaneously, are also reported.