Microscopic and macroscopic aspects of energy transfer in rare earth systems

Recent developments in condensed matter physics are reviewed. Special attention is given to the elucidation of the microscopic processes responsible for the diffusion of optical energy and the newly developed methods which probe these interactions. The atomic interactions are related to laboratory observables. The methods and work described here concern only those processes that are noncoherent in terms of the phase correlation of wavefunctions in the excitation transfer process.