Steady-State vs. Pulsed Excitation Recombination Kinetics: Simulations and Experiments on Pores, Powders and Random Media

Abstract

The non-classical kinetics of exciton recombination in restricted geometries provides the foundation for a new experimental technique of probing the exciton dynamics and the sample topology. The phosphorescence and delayed fluorescence decays exhibit a dramatic dependence on the duration of the excitation. The comparison of pulsed and steady-state excitation provides information on the local topology of the sample and on the average hopping time of the exciton and the exciton diffusion length. This is possible because the distribution of the exciton population is non-Poissonian under steady-state excitation conditions. In addition, the pulse-created distribution also loses its Poissonian character with time. The experimental systems are: 1) Isotopic mixed naphthalene crystals above and below percolation; 2) Naphthalene crystalline powder; 3) Naphthalene embedded into porous glass. Except for the mixed crystals above the percolation concentration, all samples exhibit the non-classical effects. The interpretation is aided by Monte-Carlo simulations on: 1) Cubic lattice; 2) Three-dimensional percolation clusters; 3) Three-dimensional islands; 4) The fractal Menger “sponge”; 5) Square lattice: 6) Two-dimensional percolation clusters; 7) Sierpinski “carpet” and “gasket”; 8) One-dimensional lattice; 9) One-dimensional islands. The porous glass data are only consistent with the one-dimensional island model.