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The influence of intermolecular coupling on spontaneous emission

The spontaneous fluorescence emission rate (the Einstein rate) from an isolated molecule A is changed by non-identical neighbours of species B, as in vapour phase mixtures, liquid solutions or solids. In this paper the underlying pairwise A–B interactions are analysed in their effect on the rate of emission by A. The effects are reported for all separation distances using non-relativistic quantum electrodynamics with full account of retardation. The Einstein rate for molecule A in isolation is taken as standard. For short A–B distances the two larger changes are due to a reduction in the transition moment after wavefunction renormalization and to the change in emission frequency, i.e., the spectral shift, usually a red shift. Other important changes include some emission by the ‘passive’ molecule B, and a change in the emission rate of A through its coupling to B, even before the change in transition moment is brought in. A novel feature is that the emission by B induced by one-photon exchange between A a...