Assuming the low molecular reorientation approximation, the formulae for the third-order electric polarization induced in liquids composed of rigid noninteracting dipolar, symmetric-top molecules in spherical solvents were derived. Our medium is acted on by a strong external dc bias electric field superimposed on a weak ac electric field, and the classical SmoluchowskiDebye equation for rotational diffusion of the symmetric-top molecules is applied. In order to highlight the influence of the anisotropy of rotational diffusion tensor components and the orientation of permanent dipole moment of the molecule on the complex linear and nonlinear electric susceptibilities, we present three-dimensional plots of the linear and nonlinear dispersion and absorption spectra, for different values of the frequency of ac electric field.
IntroductionLinear and nonlinear dielectric relaxation phenomena in molecular liquids, although discovered many years ago, are still the basis for wide experimental and theoretical studies. The Smoluchowski-Debye model of rotational diffusion of noninteracting, spherical-top molecules in solutions has been successfully applied in description of linear dielectric relaxation [1][2][3], the optical Kerr effect [4][5][6], the light scattering [7,8], various phenomena connected with the third-order electric polarization in liquids [9][10][11] and especially the nonlinear dielectric effect (NDE) measured at the frequency of the probe field [12][13][14][15][16][17][18]. This last effect, named the Langevin saturation, is characterized by a negative contribution to the nonlinear electric susceptibility χ(−ω; ω, 0, 0), due to a decrease in the potential energy of the polar molecules in dc electric field.