Polarisation high frequency

The high-frequency polarisation of a molecule consists almost entirely of the slight rearrangement of the electron clouds forming the bonds between the atoms. It is the polarisabilities of individual bonds that are assumed to be additive, to a good approximation. If the molecules are randomly oriented, then so are all the bonds of a particular type. The sum of the polarisabilities of all the bonds of this type must therefore be isotropic, so the polarisability for an individual bond can be replaced by a scalar average value called the mean value or spherical part of the tensor. These scalar values can be added together arithmetically for the various bonds within the molecule to obtain the spherical part of the polarisability for the molecule. This is what has so far been called a. ... 

When dipoles are directly attached to the chain their movement will obviously depend on the ability of chain segments to move. Thus the dipole polarisation effect will be much less below the glass transition temperature, than above it Figure 6.4). For this reason unplasticised PVC, poly(ethylene terephthalate) and the bis-phenol A polycarbonates are better high-frequency insulators at room temperature, which is below the glass temperature of each of these polymers, than would be expected in polymers of similar polarity but with the polar groups in the side chains. 

Impedance Some of the errors arising from the use of linear polarisation resistance led to interest and development in a.c. systems.An early development used a fixed a.c. frequency and a commercial instrument was produced in the UK. Inaccuracies still occurred, however, and were due to the electrode impedance which is fequency dependent. Electrode reactions have a capacitance component, in addition to resistance, resulting in a requirement to measure the impedance. However, the total impedance comprises values for the reaction, solution, diffusion and capacitance. Measurements at different frequency are more reliable, particularly where high solution resistances occur. Simplifications for industrial monitoring have been developed consisting of two measurements, i.e. at a high (10 kHz) and low frequency (0-1 Hz). The high-frequency measurement can identify the... 

At these high frequencies, the retarding effect of the ion-atmosphere on the movement of a central ion is greatly decreased and conductance tends to be increased. The capacitance effect is related to the absorption of energy due to induced polarisation and the continuous re-alignment of electrically unsymmetrical molecules in the oscillating field. With electrolyte solutions of low dielectric constant, it is the conductance which is mainly affected, whilst in solutions of low conductance and high dielectric constant, the effect is mostly in relation to capacitance. 

Poluektov et al. used high-frequency time-resolved spin-polarised EPR spectroscopy of radical pairs to characterise quantitatively isotopically labelled quinine exchange in the PS I reaction centre of proteins.91 Intra-subunit interactions in the Fe-S cluster of PS I of Synechocystis sp. PCC 6803 were studied by using mutations and following the changes in stabilisation of the cluster by EPR spectroscopy.92... 

Long-wavelength refractive index [40,41] suggests a high frequency dielectric constant ex = a2 = 8.4, a value supported by recent measurement [42] and ab initio calculation [43], Ionic polarisability makes a significant contribution at low frequencies where, although measurement is presently inhibited by a high conductivity, s0 15 is recommended [43],... 

The polarisation itself is, just as the molecular polarisability is, the result of deformation polarisation Pd and the orientation polarisation Ps. Accordingly, the total polarisation is equal to P = Pd + Pa. Because of the resistance to motion of the atom groups in the dielectric, there is a delay between changes in the electric field and changes in the polarisation. The deformation polarisation takes place instantaneously (more precisely in a time of the order of 10-14 s) on the application of an electric field. There are two limiting values of e Erxi at short times or high frequencies and es at long times or low frequencies. This means that we have for the deformation polarisation... 

Orientation of molecules is much too slow to contribute to polarisation at these high frequencies. Then substituting (2.34) in (2.33) gives a quantity that is usually called the molar refraction of the material ... 

Fig. 7.6 Above zero-field splitting (Bq = 0) of a triplet state for non-spherical symmetry. s = energy, D, = fine-structure constants, lhf>. Ey>, and 7z> = spin functions, x, y,z = polarisation of the magnetic high-frequency field Bi cosfflt for the three ESR transitions with quantum energies D-E, D+E and 2 in zero field (zero-field resonances). Below the three zero-field resonance lines from naphthalene molecules in a biphenyl crystal at T= 83 K. After [5].

Which of the following contributes to the polarisability of a solid in a very-high-frequency electric field ... 

In section 9.2.2 above it is shown that the refractive index of a medium is related to the high-frequency deformational polarisability of its mole-... 

The question arises, therefore, whether an optimum frequency exists beyond that defined by the power-supply circuitry at high frequencies and the effects of interfacial polarisation at low frequencies. 

At high frequencies of the a.c. field, the total polarisation of the entire sample is switched very fast and the ground, antiferroelectric state may be bypassed. Then the switching occurs between the two ferroelectric states as in an SSFLC cell. With increasing frequency (for example, from 100 Hz to 10 kHz) the double hysteresis loop is substituted by a single loop typical of ferroelectrics as shown in Fig. 13.16 by the solid and the dashed curves. 

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