VUV spectroscopy of wide bandgap materials

Abstract

In this paper we will present VUV spectroscopy experiments performed at the Superlumi station of Hasylab, DESY, Hamburg, on samples of BaF 2 crystals activated with Ce and BaF 2, (Ba,La)F 2 crystals activated with Er. The results of these experiments include time resolved luminescence and luminescence excitation spectra obtained under wavelength selective VUV and UV excitation by pulsed synchrotron radiation. We will reveal the information provided by the VUV/UV excitation spectra of the Ce 3+ 5d → 4f as well as Er 3+ 4f n −15d → 4f n and 4f n → 4f n emissions on energy transfer mechanisms from the fluoride host to the rare earth ion. We will demonstrate that the fast energy transfer channels involve bound excitons while the generation of free electrons and holes leads to slower processes dependant on hole and/or electron trapping. We will demonstrate that differences between the excitation spectra of the 5d → 4f emission in Ce and 4f 105d → 4f 11 emission in Er activated BaF 2 are generated by the coupling of the 4f → 5d transition to the 4f 10 core of the Er 3+ ion. We will also identify the additional band, absent for Ce, which is due to the exchange split high spin (HS) state of the 4f 105d configuration responsible for the slow decay of the excited Er 3+ ions in BaF 2 and (Ba,La)F 2. Finally we will provide evidence and explain why the dominant VUV 4f 105d → 4f 11 Er 3+ emission in BaF 2 is spin-forbidden and slow while in the mixed (Ba,La)F 2 crystals it is spin-allowed and fast.