Selection rules for bimolecular photoabsorption

Bimolecular photoabsorption is a term describing processes in which two molecules or other chemically distinct entities undergo simultaneous excitation to electronic excited states, the energy being provided by the absorption of one or more laser photons. These processes have previously been referred to as synergistic, cooperative, or mean-frequency absorption. This paper introduces a new terminology for the description of such phenomena and provides an overview of the features associated with bimolecular processes involving the absorption of one, two or three photons from a single laser beam. Emphasis is placed on a detailed symmetry analysis based on irreducible tensors, leading to a comprehensive treatment of the selection rules for all the major molecular point groups. Finally, attention is drawn to some systems in which these effects have been detected, and others in which they might be expected to occur.