Dynamics in Glasses and Liquids: Optical Non-Linear Line Narrowing Experiments

Molecules in condensed phase systems are coupled to their environments through intermolecular interactions. These interactions influence the internal electronic and vibrational states of molecules. At any finite temperature, solvent molecules are in constant motion, even in a low temperature glass. Since intermolecular interactions are distance and orientation dependent, the motions of the solvent make the intermolecular interactions with solute molecules inherently time dependent. These time dependent interactions result in fluctuations in the solute's electronic and vibrational energy levels. Energy fluctuations are responsible for processes such as thermal activation of a chemical reaction.