Relaxation direct current electric field

Apart from purely electronic effects, an asymmetric nuclear relaxation in the electric field can also contribute to the first hyperpolarizability in processes that are partly induced by a static field, such as the Pockels effect [55, 56], and much attention is currently devoted to the study of the vibrational hyperpolarizability, can be deduced from experimental data in two different ways [57, 58], and a review of the theoretical calculations of p, is given in Refs. [59] and [60]. The numerical value of the static P is often similar to that of static electronic hyperpolarizabilities, and this was rationalized with a two-state valence-bond charge transfer model. Recent ab-initio computational tests have shown, however, that this model is not always adequate and that a direct correlation between static electronic and vibrational hyperpolarizabilities does not exist [61]. 

If it is assumed that the field direction inside and outside the field disturbance region remains unchanged, the response of the electrons can be studied by using a kinetic approach similar to that as applied above for the relaxation studies in unifonn fields. Unlike the case with these relaxation problems, in the current case the isotropic distribution related to the homogeneous state in the undisturbed electric field is used sufficiently far from the field disturbance region as a boundary value for the isotropic distrihution ( ,z) when solving the parabolic equation, Eq. (54). 

Conventional CE in continuous electric fields cannot separate DNAs above 40 kb, due to reptation and loss of resolution. One has to resort to pulsed-field gel electrophoresis. This technique makes use of periodically varied direction and strength of an electric field (the net field direction is forward). The DNA molecules not only migrate in the direction of the field but also stretch out lengthwise. The time required for relaxation is directly proportional to the molecule length. Longer molecules relax less than shorter ones when the current is changed. DNA molecules are forced to reorient and are separated according to their sizes. 

In many practical situations, the current is alternating (ac)—that is, an applied voltage or electric field changes direction with time, as indicated in Figure 18.23a. Consider a dielectric material that is subject to polarization by an ac electric field. With each direction reversal, the dipoles attempt to reorient with the field, as illustrated in Figure 18.33, in a process requiring some finite time. For each polarization type, some minimum reorientation time exists that depends on the ease with which the particular dipoles are capable of reahgmnent. relaxation frequency The relaxation frequency is taken as the reciprocal of this minimum reorientation time. 

---