Fluorescence polarization. Emission anisotropy

Polarisation de la lumiere Disposition des parties composant un rayon lumineux de sorte qu elles se comportent toutes de la meme fa on. 

Encyclopedic Methodique. Physique Monge, Cassini, Bertholon et al., 1822 

1) The absorption transition moment is not in a single direction for some molecules whose symmetry is (benzene), 

when a population of fluorophores is illuminated by a linearly polarized incident light, those whose transition moments are oriented in a direction close to that of the electric vector of the incident beam are preferentially excited. This is called photoselection. Because the distribution of excited fluorophores is anisotropic, the emitted fluorescence is also anisotropic. Any change in direction of the transition moment during the lifetime of the excited state will cause this anisotropy to decrease, i.e. will induce a partial (or total) depolarization of fluorescence. 

Fluorescence polarization measurements can thus provide useful information on molecular mobility, size, shape and flexibility of molecules, fluidity of a medium, and order parameters (e.g. in a lipid bi layer)2 . 

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Fluorescence polarization. Emission anisotropy Brownian diffusion equation for a spherical particle... 

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