Luminescence Decay by Energy Migration and Transfer: Observation of Diffusion-Limited Relaxation

The time dependence of the luminescence decay following pulsed excitation is examined for a system in which excited paramagnetic ions decay by a combination of intrinsic relaxation processes, direct energy transfer to acceptor ions, and energy migration. Three limiting cases are considered: direct energy transfer in the absence of diffusion, rapid diffusion, and diffusion-limited relaxation. Investigations of the transient europium fluorescence from a chromiumdoped europium phosphate glass, where excited Eu³⁺ ions form the donor system and Cr³⁺ impurities act as energy acceptors, are reported which illustrate these limiting cases. By varying the temperature and Cr³⁺ concentration, transient fluorescence behavior characteristic of diffusion-limited relaxation is studied. The data are analyzed to determine the probability for direct Eu³⁺ {→} Cr³⁺ energy transfer, the critical transfer distance for energy exchange, and the diffusion constant D for energy migration through the europium system. D varies with temperature as levels having greater probabilities for resonant Eu³⁺-Eu³⁺ energy transfer become thermally populated. In the diffusion-limited relaxation case the decay rate is predicted to be proportional to D3/4; this 3/4 power law is verified.