Bottleneck in multiphonon nonradiative transitions

The nonradiative decay process of rare-earth ions in solids has been considered up to now to be independent of excitation intensity. In this paper we show that the multiphonon nonradiative decay probabilities of rare-earth ions in a germanate glass are reduced at high excitation state densities for the larger energy gaps. The classical exponential energy gap law is shown to ''rotate'' at higher excitation around the 3.2-phonon point. The observed effect is described in terms of a spatial saturation of the accepting mode term with an effective diffusion length ranging between 45 and 20 Angstrom for excited state density from 2 x 10(17) to 8 x 10(18) cm(-3) at an active ion concentration of 2.5 x 10(19) cm(-3).