Molecular properties internal electric fields

As soon as we consider the molecular nature of a material, we realise that the internal electric field will vary from point to point as a consequence of the interaction of fields from the dipoles which are induced on each molecule by the applied field, although the space-average electric field over a volume large in comparison with molecular size (this is equivalent to the classical electric field based on a continuum model) may still be uniform. The field acting on an individual polarisable entity like an atom or molecule is called the local field Eh, and it is an important concept in linking observable bulk behaviour of a material with the properties of its constituent atoms or molecules. 

Almost simultaneously and independently, two research groups started systematic IR spectroscopic work on the adsorption of homonudear diatomic molecules in zeolites, viz. the groups of Foerster [587,588] and Cohen de Lara [216, 589-593]. Their studies provided valuable information both about the properties of the sorbents, e.g., the internal electric fields the sorbate molecules, e.g., their modes of motion and mobility and the interaction between adsorbate and zeolite, e.g.,the effect of adsorption on the molecular bonds. Mostly, A-type zeolites were employed as hosts. In fact, some early studies were also carried out with X-andY-typezeolites [594,595]. 

The purpose of this Chapter is to describe the dielectric properties of liquid crystals, and relate them to the relevant molecular properties. In order to do this, account must be taken of the orientational order of liquid crystal molecules, their number density and any interactions between molecules which influence molecular properties. Dielectric properties measure the response of a charge-free system to an applied electric field, and are a probe of molecular polarizability and dipole moment. Interactions between dipoles are of long range, and cannot be discounted in the molecular interpretation of the dielectric properties of condensed fluids, and so the theories for these properties are more complicated than for magnetic or optical properties. The dielectric behavior of liquid crystals reflects the collective response of mesogens as well as their molecular properties, and there is a coupling between the macroscopic polarization and the molecular response through the internal electric field. Consequently, the molecular description of the dielectric properties of liquid crystals phases requires the specification of the internal electric field in anisotropic media which is difficult. 

Fluctuations in the dielectric properties near the interface lead to scattering of the EW as well as changes in the intensity of the internally reflected wave. Changes in optical absorption can be detected in the internally reflected beam and lead to the well-known technique of attenuated total reflectance spectroscopy (ATR). Changes in the real part of the dielectric function lead to scattering, which is the main topic of this review. Polarization of the incident beam is important. For s polarization (electric field vector perpendicular to the plane defined by the incident and reflected beams or parallel to the interface), there is no electric held component normal to the interface, and the electric field is continuous across the interface. For p polarization (electric field vector parallel to the plane defined by the incident and reflected beams), there is a finite electric field component normal to the interface. In macroscopic electrodynamics this normal component is discontinuous across the interface, and the discontinuity is related to the induced surface charge at the interface. Such discontinuity is unphysical on the molecular scale [4], and the macroscopic formalism may have to be re-examined if it is applied to molecules within a few A of the interface. 

In comparing solution calculations, however, there are still differences in the choice of the quantity that is used to characterise the response of the solution. A distinction is made between the property of a solvated pNA molecule and the effective hyperpolarizability of pNA in solution. In the former case the modifications of the pNA molecule have been calculated in the solution without any applied electric field present. In the second case the applied electric field has been present while the structure of the pNA and its immediate surroundings have been optimized. In both cases the calculated molecular quantity has to be used with internal field factors to recover the macroscopic response—but the choice of field factors may be different in the two cases. 

As the molecular-scale heterogeneity of the active layer greatly influences the power conversion efficiency, it is fundamentally important to identify and control the structural and optical properties and their relation to the function of a photovoltaic device. Ellipsometry is especially useful in determining the complex index of refraction and layer thickness, as well as structural details in thin-film geometry [69-72]. This information is needed to calculate the internal optical electric field distribution and the resulting photocurrent action spectra with respect to the efficiency of thin-film devices [66]. 

The characteristics of optical and electro-optical liquid crystal devices are determined by the refractive indices of the materials, thus an understanding of the relationship between refractive indices and molecular properties is necessary for the design of improved liquid crystal materials. In developing a molecular theory for any electrical or optical property, the problem of the internal or local electric field has to be addressed. This arises because the field experienced by a molecule in a condensed phase differs from that applied across the macroscopic sample. The internal field has... 

---