‘‘Giant resonances’’ in luminescence soft-x-ray excitation spectra of phosphors

Effects of the 4d→4f ‘‘giant resonances’’ of rare-earth constituents in phosphor host lattices were observed for the first time by means of luminescence excitation spectroscopy. Sintered powder samples of rare-earth (La,Gd) oxysulfides activated with terbium or europium were excited using monochromatic synchrotron radiation with photon energies between 100 and 200 eV. Whereas the total-photoelectron-yield spectra showed an enhancement of photoemission at the excitation energies corresponding to the 4d→4f giant resonances of lanthanum or gadolinium, a drop in luminescence intensity was observed at these energies. This apparent discrepancy with expectations from optically detected extended x-ray-absorption fine-structure measurements is discussed.