Absorption dipole moment

The emission dipole moment must not be orthogonal to the acceptor absorption dipole moment (see Chapter 1). 

Simons (28) have shown experimentally that P of OH fluorescence produced by process 3 near 130 nm is negative, suggesting that the absorption dipole moment is in the molecular plane and the OH fluorescence dipole moment is perpendicular to the internuclear axis. This agrees with the assignment of the 0 transition BlA - and the observed OH fluorescence. The... 

To visualize the depolarization fields we may consider the following idea assume we find a molecule with its absorption dipole moment in the plane of the sample. We now adjust the polarization of the excitation beam such that the fluorescence is maximized. We may assume that this happens if the electric field vector in the focus is parallel to the dipole moment. Now, if we turn the incoming polarization by 90° the dominant electric field component will not be able to excite the molecule and the presence of other field components should become visible as weak, but distinctly non-circular spots. Figure 6 shows the result of such an experiment. In Fig. 6(b) the polarization has been turned by 90° as compared to (a) as indicated by the white arrows. The bright spots become dim and their symmetry changes to a four-lobed structure. These weak structures can be made visible if the excitation intensity is increased by a factor of five [see Figs. 6(c) and (d)]. 

The FRET efficiency depends on various parameters such as the distance between the donor and the acceptor, the spectral overlap of the donor emission spectrum and the acceptor absorption spectrum as well as the relative orientation of the donor emission dipole moment and the acceptor absorption dipole moment. The efficiency of the energy transfer process varies in proportion to the inverse sixth power of the distance separating the donor and acceptor molecules. Therefore, FRET measurements can be utilized as molecular ruler to determine the distances between molecules labeled with an appropriate Donor and Acceptor fluorochrome if they are within 10 nm of each other. 

Figure 28. Simulations of the excitation efficiency of a dye molecule by an aperture probe for various orientations with respect to the tip axis (z axis). The upper part sketches the geometrical arrangement with a section along the tip axis (left), a view of the aperture from below (right), and the filed lines (dotted). Depending on the orientation of the absorption dipole moment, the center of the excitation efficiency deviates from the location of the molecule by up to one-half an aperture diameter associated with a significant intensity decrease. (Adopted from [92].)...

Several techniques have been developed over the last 15 years to visually probe the morphology of surfactant monolayers at the air-water interface. In fluorescence microscopy, a small amount of fluorescently labeled surfactant molecules is added to a monolayer due to steric effects these tagged molecules tend to partition into less-ordered phases, which results in a visual contrast between coexisting phases [25-29]. Fluorescence microscopy has been used to determine domain sizes and shapes during phase transitions [25,28,30]. Polarized fluorescence microscopy (PFM) provides additional information on the lipid hydrocarbon chain ordering within condensed monolayers, especially in areas where the lipid hydrocarbon chains are tilted with respect to the surface normal [31,32]. The interaction of the electric field vector of the polarized light with the absorption dipole moment of the... 

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 ). ... 

Szabo proposed an interesting model-free formula for the time-resolved anisotropy in a macroscopically isotropic system [112]. He expressed r(f) as the autocorrelation function of orientations of the emission dipole moment at time t and absorption dipole moment at time t = 0 in a form suitable for general treatment of various systems, and particularly those with possible internal rotation ... 

Figure 1 Schematic of the interaction of polarized light with a fluorophore. The angle 6 represents the angle between the plane of incoming polarized light (electronic vector) and the absorption dipole moment, and the angle a represents the angle between the absorption and emission dipoles in the absence of molecular rotation.

Discrete and well-defined absorption dipole moments. 

All chromophores possess a well-defined absorption dipole moment, which determines the polarization that the excitation light must have in order to be absorbed by the chromophore. 

Hie frill 3D orientation of single molecules can be obtained by other techniques that can determine the absorption or emission transition dipole moment. Annular illumination is a technique that can determine the orientation of the absorption dipole moment, while the 3D orientation of the emission dipole moment can be revealed using defocused wide-field imaging, ° introducing aberrations or by direcdy recording the emission pattern in the objective s back focal plane. In... 

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