Energy transfer mechanism switching in disordered solids

The excitation decay of the long wavelength portion of the inhomogeneously broadened O,O band of the transition due to triplet-triplet energy transfer in 1-bromo-4-chloronaphthalene (orientationally disordered solid) at 4.2 K is analyzed in terms of one-dimensional exchange and 3 dimensional dipole-dipole mechanisms. The fit to a dipole-dipole mechanism is observed at long decay times and very long excitation wavelength (i.e., very low acceptor concentrations). As the excitation wavelength decreases (i.e., as the acceptor concentration increases), the one-dimensional exchange mechanism, which describes only the early portion of the decay at the longest wavelength, begins to dominate in describing the full decay range.