Excitation process of Ce3+ and Eu2+ ions doped in SrGa2S4 crystals under the condition of multiplication of electronic excitations

Excitation spectra for 5d-4f photoluminescence (PL) bands of Ce3+ and Eu2+ ions doped in SrGa2S4 crystals have been measured at room temperature in the energy region up to 25 eV. These spectra allow us to investigate the excitation process of luminescent ions under host excitation with vacuum ultraviolet (VUV) photons. A gentle increase in intensity is commonly observed at around 16.5 eV in the excitation spectra. Optical constant spectra were obtained from measurements of reflectivity and absorption edge spectra of undoped SrGa2S4 crystals. These data were compared with the results of a relativistic DV-Xα calculation, to clarify the electronic structure of SrGa2S4. Ultraviolet photoelectron spectroscopy (UPS) has been carried out under the excitation with photons in the 12-20 eV range. The detailed analysis of UPS spectra reveals that the multiplication of electronic excitations (MEEs) occurs above 16.5 eV. The excitation process of Ce3+ and Eu2+ ions is discussed together with the processes of MEEs.