Vacuum ultraviolet spectroscopy of Ce3+-doped SrMgF4 with superlattice structure

X-ray diffraction of Ce3+-doped SrMgF4 (SMF:Ce) crystals shows a superlattice structure, reflecting the distribution of Ce3+ polyhedra centres observed in optical experiments. Optical absorption bands and fluorescence bands from the Ce3+ polyhedra centres overlap in the vacuum ultraviolet (VUV) and ultraviolet (UV) regions, respectively, so that wide pumping and tuning ranges are expected for laser operation. The SMF: Ce crystals, as well as the isomorphous BaMgF4, are candidates for a tunable laser gain material with nonlinear properties. The optical absorption, excitation, and fluorescence bands observed in the SMF: Ce crystals at low temperatures are ascribed to five distinct fluorescent centres. Three centres have well-known Ce3+ optical characters, for example, fluorescence with double peaks separated by 2000 cm(-1) and five resolved absorption/excitation bands. These centres are assigned to Ce3+-polyhedra classified by weak and strong crystal fields as a consequence of the superlattice structure. The other two fluorescence bands observed in the visible region have 1.5-2 times larger linewidths than those of the former three bands. These bands are interpreted as optical transitions from complexes consisting of Ce3+ and one or two electrons trapped at a vacancy of the nearest neighbour F-ligand ions