Orientation anistropy

Figure 4.9 illustrates time-gated imaging of rotational correlation time. Briefly, excitation by linearly polarized radiation will excite fluorophores with dipole components parallel to the excitation polarization axis and so the fluorescence emission will be anisotropically polarized immediately after excitation, with more emission polarized parallel than perpendicular to the polarization axis (r0). Subsequently, however, collisions with solvent molecules will tend to randomize the fluorophore orientations and the emission anistropy will decrease with time (r(t)). The characteristic timescale over which the fluorescence anisotropy decreases can be described (in the simplest case of a spherical molecule) by an exponential decay with a time constant, 6, which is the rotational correlation time and is approximately proportional to the local solvent viscosity and to the size of the fluorophore. Provided that... 

The specific heat for uncoupled spins does not depend on the orientations of the anisotropy axes however, the corrections due to the dipolar coupling do, as can be seen in Figure 3.4. As for the linear susceptibility, the effect of dipolar interaction is stronger in the case of parallel anistropy than for random anistropy. 

Kundu et al. (2007) employed the citric acid sol-gel method to prepare a nanoscale ordered perovskite cobaltite, which consists of 90° ordered domains of fhe layered "112 LaBaCoiOg. This perovskife exhibifs a high magnefic anisofropy in contrast to the disordered and ordered phases. The authors explained that the locking of fhe cobalf spins due fo the existence of 90° oriented nanostructure domains is responsible for the observed anistropy. 

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 ... 

Byelousova, O.A. Khanchich and S.P. Papkov, "Anistropy Viscosity and Orientation In Liquid Poly-p-benzam1de with Displacement Deformation" (sic), Vysokomol. Soyed., 21, 1407-1414 (1979) [English translation by Ralph McElroy Co., 2102 Rio Grande, Austin, TX 78705]. 

The effect of orientation of a solid and its resultant anistropy is given in the review papers by Anderson [2] and Knappe [3]. Pressure effects generally can be handled [3] as in the equation... 

T e orientation process also produces an increase of the DC conductivity [20], with stretched films showing a room temperature conductivity parallel to the chains of 3 x 10" S/cm, similar to the value found for Shirakawa poly acetylene [47], and a temperature independent anistropy of about 40. Above 200K the conductivity appears activated with an energy of activation of 0.4eV. This temperature dei ndence will be determined by the most difficult hops these are likely to be between chains. Thus, the temperature independent anisotropy can be explained simply by the smaller number of interchain hops in the... 

Commonly known as the Kerr Effect, this is the best known electro-optic phenomenon. Although initially studied in glasses by John Kerr (J) in 1875, who considered the birefringence to be related to electrically induced strain in the material, it is now used widely to follow the alignment due to orientation and deformation of macro-particles in solution and suspension (2—4). It owes its origin to anistropy of the refractive indices associated with the major geometric axes of the molecules. 

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