Absorption and emission of polarized light

Electron microscopy revealed that the LB film had a liquid-crystalline-like structure. This means that many polymer chains were oriented parallel to the substrate plane and exhibited a preferential orientation of their backbones along the dipping direction. Absorption spectra recorded with the incident light polarized either parallel or perpendicular to the dipping direction show a maximum at 330 nm (3.76 eV) in both cases, but A and A l, the absorbances parallel and 

Chart 1.6 Chemical structure of poly(2,5-di-isopentyloxy-/3-phenylene), DPOPP. 

Chart 1.7 Chemical structure of poly(vinyl cinnamate). 

The creation of anisotropy is treated in some detail in Section 4.4, which deals with the trans-cis isomerization of azobenzene compounds. 

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The technique of fluorescence depolarization uses in principle fluorescent dyes covalently bound to proteins. The label is excited by polarized light the fluorescence is also polarized to a degree that is inversely related to the amount of Brownian motion occurring during the interval between absorption and emission of the light. The information about the amount of motion helps to form an idea on the size and shape of the protein being investigated. 

Substituting (2.7) into Eq. (2.2), we obtain the angular dependence G(0, ip) of the probability of absorption and emission of light with polarization vector E ... 

In concluding this chapter, we can say that the description presented thus permits us to connect the probability of absorption and emission of light of any polarization with the orientation of the angular momentum of the molecule. This may serve as a basis for more detailed analysis of the processes of creating an anisotropic distribution of angular momenta both on the upper, as well as on the lower, level of an optical transition (Chapter 3), including the effect of an external field (Chapters 4, 5)... 

The fluorescence depolarization technique for mobility and ordering is based on the fact that the probability of absorption and emission is directional. Light polarized along a certain axis will preferably excite molecules oriented with their transition dipole moment in the same direction. The probability varies with cos 0, where 0 is the angle between the transition dipole moment and the electric field vector of the light. Emission of a photon obeys the same cos 0 (28) rule. That means that a molecule oriented with its transition dipole moment along the Z-axis will be likely to emit a photon with the same polarization. In the depolarization technique, polarizers are used to quantify the intensity of the parallel (ly) and perpendicular (Ij.) components to the original direction of polarization. 

Since the electric dipole moment operator is a vector operator, the electric dipole transition moment will also be a vector quantity. The probability of an electric dipole transition is given by the square of the scalar product between the transition moment vector in the molecule and the electric field vector of the light, and is therefore proportional to the squared cosine of the angle between these two vectors. Thus, an orientational dependence results for the absorption and emission of linearly polarized light. The orientation of the transition moment with respect to the molecular system of axes is... 

Fluorescence polarization has been discussed by a number of authors in somewhat different ways, which will be generalized here. The fluorescence intensity observed under an analyzer with electric vector Q from polymeric materials excited by polarized light with electric vector P is given by the product of two types of the second rank tensors of optical anisotropies, absorption and emission tensors in equation (47), where K is an instrumental constant, Pj and Qp are direction cosines of the polarization directions of the polarizer and analyzer, and A-,j and Ep, are the second rank tensors for the anisotropic absorption and emission of the bulk materials, with respect to the specimen coordinates O-X1X2X3. (In equation (47) and hereafter, an asterisk indicates the summation convention.)... 

The propagation of light in multiple scattering media is quantified usually on the level of radiative transfer or particle diffusion. Scattering, absorption, and emission are considered as independent statistical processes, and the consequences of wave character are either ignored, like polarization, or added as an additional parameter, like the phase function P(ji n) that describes the angular distribution of scattered... 

Crystals have two distinct advantages over amorphous samples 1) the inhomogeneous broadening is reduced and 2) the complexes are oriented, and by using polarized light the principal components of the transition moment for the various absorption and emission lines can be determined. The sharp line... 

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