Tsomyk O. Rotational movement of polar surface groups and its manifestation in dielectric measurements

The dissertation is devoted to the theoretical study of the azimuthal jumping motion of an adsorbed polar molecule in a periodic n-well potential under the action of an external electric field. Starting from the perturbation theory of the Pauli equation with respect to the weak intensity of alternating field, we have derived explicit analytical expressions for the time dependence of the average dipole moment, as well as the frequency dependences of polarizability and the average angular velocity, the three quantities exhibiting conspicuous stochastic resonance. As shown, unidirectional rotation is possible only if the external alternating field simultaneously modulates minima and maxima of the potential. For a symmetric potential of hindered rotation, the average angular velocity (treated by the second-order perturbation theory with respect to the field intensity) does not vanish only at n = 2, i.e., when two azimuthal wells specify a selected axis in the system. Special attention is paid to a two-well asymmetric potential, with the asymmetry induced by local fields which arise from environmental inhomogeneities or from orientational ordering in the low-temperature region. As a result of the ordering, the dielectric loss spectrum exhibits specific features, namely, some additional peaks in the low-temperature region and a singular point with an infinite first derivative at the phase transition temperature. These measurements are sensitive to the structure of the local surrounding of the surface centre and give important information about it. It is therefore necessary to develop the models which enable description of polarizabilities for rotating polar surface centers. Another way to determine rotational motion of an adsorbed polar molecule is measuring the temperature-stimulated electric currents induced by stationary electric fields. A general solution of the relaxation equation has been derived which includes the equilibrium polarization. The analytical form of the solution for current density allows description of all thermally stimulated processes from the uniform point of view. It has been pointed out that microscopic consideration of the azimuthal jumping motion of a Brownian particle in a periodic n-well potential independently provides the relaxation equation