Energy transfer in luminescent materials: A random walk problem with variable step length

We study the incoherent energy transfer in molecular crystals, in which the interactions are due to exchange and multipolar long-range forces. For the triangular, square and hexagonal two-dimensional lattices we determine numerically the number of distinct sites Rn visited in random-walks of n steps; this complements our former results for three-dimensional lattices. From the average and the variance of Rn we evaluate the excitation decay caused by randomly distributed traps.