Photo-Ionization of 3d-Ions in Fluoride-Phosphate Glasses

Interaction of radiation with glasses often modifies their properties (e.g., the refractive index). Most noticeable is a loss in transmission. Such phenomena can be exploited in photo-sensitive materials, but have to be avoided in other applications. To improve the understanding of defect generation processes, a systematic comparison of defect formation in (fluoride-)phosphate glasses doped with low concentrations of 3d ions was attempted. Samples doped with 10-5000ppm of Ti, V, Cr, Mn, Fe, Co, or Ni were irradiated in the UV range by excimer lasers. Defects, which generally form in the levels of several ppm, were characterized by optical and by electron spin resonance spectroscopy. V4+ was photo-oxidized to the empty valence shell d(0) ion. Co2+, Mn2+, and Fe2+ were all photo-oxidized to the trivalent state while Ni2+ was photo-reduced. The fully oxidized Ti4+ was also photo-reduced. Cr3+ showed photo-disproportionation into Cr2+ and Cr6+. Qualitative and quantitative changes in defect formation rates depend not only on the ion, but also on the radiation parameters, for example, the wavelength of the excimer lasers used (193, 248, 351nm) or the initial transmission of the glass samples at the chosen laser wavelength. Defect recovery was followed up to 10years after the irradiation experiments.