The relaxation dynamics of Ce³⁺ ions in phosphor materials relevant to solid state lighting has been investigated. Of particular interest was the temperature dependence of the emission efficiency in heavily doped yttrium aluminum garnet (YAG) materials, which show a decreased quenching temperature compared to low concentration samples. Using site se- lective spectroscopy, we demonstrate that the lowering of the quenching temperature in high concentration samples is due to cerium ions in distorted sites. Our results can be generalized that high concentrations of cerium and/or an addition of optically mute rare earth ions to broaden the emission spectrum will lead to a lowered quenching temperature. We studied also Na₂Gd₂O(BO₃)₂ doped with Ce³⁺, a system where Ce³⁺ ions occupy two distinct lattice sites. In this case it is shown that the luminescence quenching is due to photoionization.

Index words: Lifetime Measurements, Luminescencence, Level Crossing, Rare Earth Ions, Photoionization