Time resolved studies of naphthalene mixed crystals. Fractal and Euclidian behaviors of the migration kinetics

Time resolved experiments have been used in order to study the migration of triplet excitations at low temperature (T<4.2 K) in napthalene D8 crystals doped with 5% to 20% of naphthalene H8 containing a small amount of betamethylnaphthalene. The analysis of the time dependence of the fusion of two triplet excitations leads to clearly deomonstrate the fractal behavior of the exploration at short time and the Euclidian behavior at longer times. The spectral dimension has been measured for the smallest studied concentrations (c<12%); it ranges between 1.33 and 1 which is in very good agreement with a 3D percolation model. The time T0 at which the crossover between the fractal and Euclidian exploration occurs, has been approximately determined for the concentrations. It turns out that the 20% sample does not exhibit this fractal behavior on the studied time scale; this is also in agreement with the percolation model. We have also demonstrated that the time dependence of the fusion of two excitations gives a better insight in the energy migration processes than the time dependence of the supertrapping processes.