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

Axial Pb centres were produced by X-irradiation in Pb-doped CaF 2 , SrF 2 and BaF 2 , 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 M 2+ (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 Tl 0 (1) centres in alkali halides. Laser action of the Pb + (1) centre is expected. Results on a Pb + (1)-Pb 2+ centre and a diamagnetic Pb 0 (2) centre, which are produced simultaneously, are also reported.