Absorption transition dipole

Even in situations in which the molecules of interest are randomly oriented (e.g. in solution) the orientational distribution of emitting molecules may not be isotropic, due to the fact that the incident (excitation) beam photoselects molecules based upon the relative orientation of the absorption transition dipoles, p abs> with respect to the incident polarization vector, if [7,9,10]. The probability of absorption is proportional to I jfabs if I thus, for example, molecules oriented such that if... 

Upon excitation, the fluorophore absorbs only photons whose electric vectors are orientated parallel to its absorption transition dipole. Any angular... 

This result has also been derived by Gordon The extension to the general case, with emission and absorption transition dipoles at angje a, gives the result... 

Hi) UV/visible spectroscopy. In comparison with IR and fluorescence spectroscopic techniques, UV/visible spectroscopy is only occasionally used for characterizing monolayers. It can be applied if the monolayer contains molecules with 7i-electron systems of which the electron transitions are in the UV/visible part of the spectrum. By measuring polarized transmission spectra or reflection-absorption spectra at different angles of incidence, the second order parameter of the absorption transition dipole moment in the chromophoric groups caii be determined. In the case of a reflection-absorption configuration, the underlying theory is similar to that of IRRAS, i.e. based upon calculation of the reflection and transmission coefficients in a stratified-layer system and extended to account for the anisotropic nature of monolayers ). 

FRET experiments involve complementary pairs of fluorophores that are matched to allow coupling to take place between the fluorescence emission transition dipole moment of a given donor fluorophore and the absorption transition dipole moment of a corresponding acceptor fluorophore, provided that donor and acceptor are sufficiently close in space. This... 

The steady-state fluorescence anisotropy/polarization method is also simple and relies on the fact that the probe molecule will tumble rapidly in solution when free, but will have restricted motion upon binding to a macromolecule. Optical excitation of the probe by polarized light will result in preferential absorption by those molecules whose absorption transition dipole is parallel to the electric field vector direction of the light. The subsequent fluorescence will be partially polarized. The definitions of anisotropy (r) and polarization (P) are [188]... 

Anistropy measurements are based on the photose-lective excitation of fluorophores by plane-polarized light. In an isotropic medium, the fluorophores are randomly oriented. Upon excitation with polarized light, those fluorophores whose absorption transition dipole is aligned parallel to the electric vector of the excitation, will be preferentially excited. If the molecule rotates and tumbles out of this plane during the excited state, light is emitted in a different plane from the excitation light. The intensity of the emitted light can be monitored in vertical and horizontal planes and thus, fluorescence anisotropy (r) and polarization ( ) are defined by ... 

As already explained, the probability of photon absorption by a given molecule depends on a number of factors (see the optical selection rules). If polarized light is employed [61], it also depends on the orientation of the absorption transition dipole moment, with respect to the polarization plane of the excitation light (described by the angle (p). Molecules with their absorption dipole moment parallel to the polarization plane of the excitation light are excited preferentially, while those oriented perpendicularly are not excited at all. For a general orientation with angle (j), the dipole moment can be decomposed into parallel and perpendicular components, /ipcos, and /ipsin, respectively, and the excitation probability is proportional to (cos ). ... 

From a theoretical point of view, the experimentally accessible time-resolved anisotropy, r(f), represents the autocorrelation function of orientations of the emission transition dipole moment at time t and the absorption transition dipole moment fiA(t = 0) at the instant of excitation, t = 0, and can be expressed as... 

The observed anisotropy will be decreased if the emission and absorption transition dipoles are not parallel, or if the molecule rotates before it fluoresces. If the transition dipole for emission makes an angle with respect to the transition... 

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