Polarisation analysis of bimolecular excitations mediated by energy transfer: A common theoretical framework for fluorescence migration and sequential Raman scattering

When polarised light interacts sequentially with two independently mobile molecules or chromophores, radiation of a longer wavelength and a changed polarisation state is commonly detected. One well-known example is the case where energy migrates from a molecule initially excited by input radiation to another which fluoresces. Another such process is sequential Raman scattering by two separate molecules. In this paper a theoretical framework is developed that formally establishes the link between these two process. The theory encompasses the most general case; the intermolecular energy transfer can be either radiative or non-radiative, and the transition dipole moments, for the upward and downward processes within each chromophore, can be either parallel or non-parallel. For the Raman processes, resonance features are also accommodated. Results for the fluorescence anisotropy and Raman depolarisation ratio are explicitly presented as well as more general rate equations applicable to arbitrary polarisation conditions.