The two limiting cases of dipole-allowed excitation transfer rates between molecules, $w\sim \frac{1}{R^3}$ (Perrin) and $w\sim \frac{1}{R^6}$ (Förster), where $R$ is the separation, may be obtained as well-defined limits of a single function $w\left(R\right)\mathrm{}$. The demonstration is based on a model which allows the interaction strengths and relaxation rates to be compared explicitly and which has a generalization describing the situation in real molecules.

• Received 25 July 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.803