LUMINESCENCE OF DIVALENT YTTERBIUM IN MAGNESIUM FLUORIDE-CRYSTALS

The luminescence properties of divalent ytterbium in magnesium fluoride are reported. Due to the large bandgap of MgF2 and the large crystal field splitting for the Yb2+ ion on the small Mg-Z+ site, the lowest component of the 4f(13)5d excited state is situated well below the conduction band in contrast to the situation in the alkaline earth fluorides. The emission band peaks at relatively long wavelength, viz. at 485 nm at 298 K. The quenching temperature is high (480 K). At low temperatures vibrational structure is observed in the excitation and emission spectra. The observation of zero-phonon lines in the excitation spectrum of the Yb2+ emission in MgF2 makes an accurate determination of the position of the different 4f(13)5d energy levels possible. Additional information on the position of the lowest excited states is obtained from temperature dependent decay time measurements. The energy difference between the two lowest excited states (about 6 cm(-1)) is smaller than usually observed for Yb2+. A comparison is made with the luminescence properties of MF(2):Yb2+ (M = Ca, Sr, Ba).