Abstract

Most resonances of molecules, such as optical, infra-red or electron spin resonance transitions are shifted and broadened in solids, compared to the spectra of the isolated molecule in the gaseous phase. Inhomogeneous or statistical broadening reflects the variety of surroundings felt by the many molecules probes in a sample while homogeneous broadening, a dynamical effect, is caused for example by the elastic Interaction of the electronic degrees of freedom with intermolecular phonons, or structural rearrangement by tunneling relaxation (two level systems). Homogeneous linewidths at low temperatures are much larger in amorphous materials than in crystals, with quite different temperature dependencies, reflecting the kind and density of modes coupled to the electronic transition. Hence, homogeneous broadening can add to our understanding of the differences between crystals and glasses, and has been studied intensively.

© 1992 Optical Society of America