COHERENT EXCITON PERCOLATION: ISOTOPIC MIXED $^{1}B_{2u}$ NAPHTHALENE

Author Institution: Department of Chemistry, The University of Michigan,A model for ``coherent” exciton percolation in molecular mixed crystals, defining a critical concentration for an exciton insulator-to-conductor transition is proposed. The fluorescence spectra of the $^{1}B_{2{u}}$ exciton in the binary alloy of naphthalene $C_{10}H_{8}/C_{10}D_{8}$ is studied at $2^{\circ} K$ and the exciton flow in the crystal is monitored by the emission from betamethylnaphthalene, a third component of $10^{-3}$ mole fraction concentration, which behaves as a supertrap for the exciton. The $^{1}B_{2{u}}$ Frenkel exciton exhibits effective percolation at a concentration of 0.50 mole fraction which is close to 0.59, the theoretical upper limit for site percolation in a two dimensional square lattice. The observed data are analyzed and compared to the results of various lattice percolation models. The introduction of out-of-plane and next-nearest-neighbor interactions through “Monte Carlo” calculations is discussed as a means of better representing the true topology and behavior of this system