On the saturation of Tb phosphors under cathode-ray excitation. II. Upconversion processes in the excited-activator bath

A physical model based on resonant electric dipole interaction between excited activators is presented that may be used for gaining insight into the second-order energy-loss process in cathode-ray-irradiated Tb phosphors. It is shown that resonant up-conversion of the 5DJ excitons (J=3,4) to highly excited 4f and 5d states may be of equal importance. The derivation of the rate constants for this second-order energy-loss process is given and experimental values for the 5D4 -5D4 and 5D3 -5D4 interaction are presented for Tb-doped YAG, YAGaG, LaOBr, and Y2SiO5. The values depend on previously determined optical cross sections for excited-state absorption. Based on this model we explain well-known differences in saturation behavior between these Tb-doped phosphors under cathode-ray excitation.