Kerr effect, orientational

Fig. 8. Principle of the magnetooptical read-out of domain patterns by the polar Kerr effect. The polarisation plane of the incoming laser beam is rotated clock- or counterclockwise according to the orientation (up or down) of the magnetic moments.

Combined dipole moment and Kerr effect studies are regularly used by Russian workers for the conformational analysis of phosphorus heterocyc1es.135 230 In a study of the interaction of phenol with phosphoryl groups the Kerr effect was used to evaluate not only the extent of hydrogen bonding but also the influence of changes in polarity and polarisation upon stability constants.231 In a similar study the orientation of the aryl groups of 1,3,5-triazaphosphorines (82) were shown to be less coplanar than biphenyl in the gas phase. 2 3 2... 

When a strong static electric field is applied across a medium, its dielectric and optical properties become anisotropic. When a low frequency analyzing electric field is used to probe the anisotropy, it is called the nonlinear dielectric effect (NLDE) or dielectric saturation (17). It is the low frequency analogue of the Kerr effect. The interactions which cause the NLDE are similar to those of EFLS. For a single flexible polar molecule, the external field will influence the molecule in two ways firstly, it will interact with the total dipole moment and orient it, secondly, it will perturb the equilibrium conformation of the molecule to favor the conformations with the larger dipole moment. Thus, the orientation by the field will cause a decrease while the polarization of the molecule will cause an... 

Dsecs. From their measurements the authors concluded that this optical Kerr effect is due mainly to the orientation of anisotro-pically polarized molecules and probably not caused by electron phenomena... 

This outline of the response theory has for simplicity been limited to molecules with axial symmetry of y and Aa and to the field on, field off cases, but can be extended in both respects without basic difficulties. Detailed comparisons with experiment have not yet been made, but it already is clear that Kerr effect relaxation data can now provide more valuable and better defined information about orientational dynamics of biopolymers and other molecules than was previously possible. With the increasing accuracy and time resolution of digital methods, it should be possible to study not only slow overall rotations of large molecules (microseconds or longer) but small conformational effects and small molecule reorientations on nano and picosecond time scales. Moreover, one can anticipate the possibilities, for simple problems at least, of extending response theory to other quadratic and higher order effects of strong electric fields on observable responses. 

Figure 1. Intensity profile of optical Kerr effect of NB at 25°C vs. time. The zero time is arbitrary and the peak transmission of the Kerr effect is about 10%. The rise time is 5.3 ps and the decay time is 15.2 ps. This decay time corresponds to a molecular orientation time of 30.4 ps (6).

Figure 38 shows the XRD pattern (a) and hysteresis loop (b) of FePt C double-layered nanocomposite thin-film medium. The soft underlayer FeCoNi (111) peak and the Zl0 FePt (001) and (002) peaks are shown only in the XRD pattern. This means that the preferred crystal orientation of Tl0 FePt C nanocomposite film is successfully obtained on this SUL by nonepitaxial growth. The polar-Kerr measurement shows a square loop that is only sensitive to the top layer the Kerr effect data shown in this loop give the coercivity Hc = 8.5 kOe, nucleation field Hn = 5.65 kOe, remanence ratio S = 1, and loop slope ( at Hc) a = 3.3, respectively. 

As we see, the parameter 1 results from Langevin reorientation of the polarizability ellipsoid and is always positive. The second of the above parameters, 2, corresponds to Bom s term in the Kerr effect and can be positive or negative, depending on the electric structure of the molecule. The third, the Debye parameter 3, has no counterpart in other phenomena of molecular orientation, and is specific to the non-linear dielectric behaviour of dipolar substances. 

As frequency increases, all dispersion curves decrease to virtually zero. This indicates that EB is produced by the dipolar-orientational mechanism whereas the anisotropy of the dielectric polarizability of the macromolecules is virtually imper-ceptile in the Kerr effect. The displacement of curves towards higher frequencies with... 

However, this mechanism of motion does not provide any great contribution to the Kerr effect since the dispersion curves of EB fall to virtually zero (Fig. 60). This difference may be interpreted by the proportionality of the orientational EB of a ripd-chain polymer to the square of the number of monomer units in segment whereas increment Ae/c related to the orientational mechanism is proportional to S (see Sects. 5.8 and 5.9). Hence, in the case of dielectric polarization the part played by the deformational mechanism as cmnpared to the orientational mechanian can be more important than in the case of EB. 

The display of intramolecular motion in rigid-chain polymers can also be observed if the kinetics of the Kerr effect is studied for a series of fractions of relatively high molecular we t. In fact, as already mentioned (p. 171), the kinetics of the behavior of a polar chain molecule in the electric fidd is determined by relative values of relaxation times of its orientation tq and deformation tj. Since tq increases with M proportionally to M(rjl, and tj is independent of M, it might be expected that at relatively high M the inequality tq > rj will hold and, hence, the polarization and the anisotropy of the solutmn in the electric field will follow the deformational mechanism. 

The approach developed may also be extended to treat all the other averages (P (cos i)))(t) characterizing orientational relaxation in fluids [43]. In particular, the evaluation of the average of the second-order Legendre polynomial (/Tfcos 0))(t) (e.g., this quantity describes the dynamic Kerr effect [8]) is given in Appendix III. 

Note that according to Eqs. (77) and (78) the index change induced by the reorientation of anisotropic chromophores is quadratic in the poling held Ep and is therefore an orientational Kerr effect. The index change has also a quadratic dependence on the dipole moment. Equation (78) shows that the refractive index increases for an optical held polarized along the direction of the poling held and decreases for a polarization in a direction perpendicular to it. 

The ability of anisotropic and anisometric particles to assume some co-orientation in external force fields is not only responsible for significant changes in scattering properties but also causes birefringence (double refraction), i.e., the average refractive indexes of two beams polarized in perpendicular planes happen to be different. The specific orientation of particles and birefringecne may be caused by the action of electric field (Kerr effect), magnetic field (Cotton-Mouton effect), or in the case of anisotropic particles by flow of medium (Maxwell effect) [25]. 

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