Isotropic medium reorientation

Dynamic fluorescence anisotropy is based on rotational reorientation of the excited dipole of a probe molecule, and its correlation time(s) should depend on local environments around the molecule. For a dye molecule in an isotropic medium, three-dimensional rotational reorientation of the excited dipole takes place freely [10]. At a water/oil interface, on the other hand, the out-of-plane motion of a probe molecule should be frozen when the dye is adsorbed on a sharp water/oil interface (i.e., two-dimensional in respect to the molecular size of a probe), while such a motion will be allowed for a relatively thick water/oil interface (i.e., three-dimensional) [11,12]. Thus, by observing rotational freedom of a dye molecule (i.e., excited dipole), one can discuss the thickness of a water/oil interface the correlation time(s) provides information about the chemi-cal/physical characteristics of the interface, including the dynamical behavioiu of the interfacial structure. Dynamic fluorescence anisotropy measurements are thus expected... 

A number of mechanisms are known to contribute to spin-lattice relaxation in a molecule dipole-di-pole, spin-rotation, quadrupolar, scalar, and CSA, each associated with a corresponding relaxation rate. Analytical interest focuses only on the intemu-clear C-H dipole-dipole relaxation with a rate I IDD = 1/Tidd = (f//1.988)(l/Ti), which is proportional to the inverse sixth power of the carbon-proton distance rca, and depends on the rotational and internal molecular motion. For medium-sized molecules of nearly spherical shape (isotropic molecular reorientation) in solvents of low viscosity the condition of extreme narrowing holds, i.e. ct)cTc l for all Larmor frequencies ft)c, and therefore Ri=ANtc. The correlation time %c is a measure of the velocity of the molecule s rotational diffusion jumps. For N nearest protons within a distance rca the quantity A takes the form A = jrca... 

A well-known nonlinear process taking place in the liquid state of anisotropic molecules is the optical-field induced birefringence (optical Kerr effect ). This nonlinearity results from the reorientation of the molecules in the electric field of a light beam. In the isotropic phase the optical field perturbs the orientational distribution of the molecules. In the perturbed state more molecules are aligned parallel to the electric field than perpendicularly to it and as a consequence the medium becomes birefringent. On the other hand in liquid crystals the orientational distribution of the molecules is inherently anisotropic. The optical field, just as a d.c. electric or magnetic field, induces a collective rotation of the molecules. This process can be described as a reorientation of the director. 

Laser-induced molecular reorientation is a common cause of optical nonlinearity in a fluid medium. In this respect, liquid crystals are often strongly nonlinear because of their large molecular anisotropy and strong correlation between molecules. The nonlinear optical properties of liquid crystals in the isotropic phase have already been studied quite extensively by a number of researchers in the past decade, This is, however, not true for liquid crystals in the mesophases. 

To the extent that molecular reorientation may be considered as approximatively isotropic the overall rotation is characterized by a single correlation time For small - or medium - size mole-... 

Suppose the other extreme in which molecules in a liquid reorient isotropically in small angular steps is examined. It is necessary to consider the Debye equation for rotation of a rigid sphere of radius a in a medium of viscosity rj [7.16]... 

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