Linear optical effects, theory

Other Work on Water-Related Systems. Sonoda et al.61 have simulated a time-resolved optical Kerr effect experiment. In this model, which uses molecular dynamics to represent the behaviour of the extended medium, the principle intermolecular effects are generated by the dipole-induced-dipole (DID) mechanism, but the effect of the second order molecular response is also include through terms involving the static molecular / tensor, calculated by an MP2 method. Weber et al.6S have applied ab initio linear scaling response theory to water clusters. Skaf and Vechi69 have used MP2/6-311 ++ G(d,p) calculation of the a and y tensors of water and dimethylsulfoxide (DMSO) to carry out a molecular dynamics simulation of DMSO/Water mixtures. Frediani et al.70 have used a new development of the polarizable continuum model to study the polarizability of halides at the water/air interface. 

Absorbing systems the Borrmann effect The Borrmann effect is the anomalous increase in the transmitted X-ray intensity when a crystal is set for Bragg reflexion. An analogous optical effect in absorbing cholesteric media in the vicinity of the reflexion band has been predicted and confirmed experimentally. The origin of the effect can be readily understood by extending the dynamical theory to include absorption. However, in contrast to the X-ray case, the polarization of the wave field and the linear dichroism play an essential part. 

The MO measurements provide information about the angular distribution of molecules in the x, y, and z film coordinates. To extract MO data from IR spectra, the general selection rule equation (1.27) is invoked, which states that the absorption of linearly polarized radiation depends upon the orientation of the TDM of the given mode relative to the local electric field vector. If the TDM vector is distributed anisotropically in the sample, the macroscopic result is selective absorption of linearly polarized radiation propagating in different directions, as described by an anisotropic permittivity tensor e. Thus, it is the anisotropic optical constants of the ultrathin film (or their ratios) that are measured and then correlated with the MO parameters. Unlike for thick samples, this problem is complicated by optical effects in the IR spectra of ultrathin films, so that optical theory (Sections 1.5-1.7) must be considered, in addition to the statistical formulas that establish the connection between the principal values of the permittivity tensor s and the MO parameters. In fact, a thorough study of the MO in ultrathin films requires judicious selection not only of the theoretical model for extracting MO data from the IR spectra (this section) but also of the optimum experimental technique and conditions [angle(s) of incidence] for these measurements (Section 3.11.5). 

As expected, for polyenes even lowest level of theory allows for an adequate description of the correlation effects in contrast to the polyacene case. However, despite a low level of accounting for the electron correlation by the cue(2)-CCSD method for polyacenes, this approach gives a qualitatively correct dependence of the values of the optical and non-linear optical properties on the system size (see the next section). We attribute this behavior to the size-extensivity of the CCSD method. 

The linear optical properties (UV-visible and Raman) of PDA crystals have been thoroughly characterized and are reasonably well-understood. The lowest energy optical transition is typically located at about 2.0 eV and is excitonic in origin. Distortion of the backbone due to deliberately induced disorder or strain caused by side group interactions shifts this transition to higher energies. Crystal strain (e.g. polymer chains in the monomer lattice) can shift the transition either way. More work/ theory and experiment, is needed to sort out and understand these effects. 

R. Williams, Optical-Rotary Power and Linear Electro-Optic Effect in Nematic Liquid Crystals of p-Azoxyanisole, J. Chem. Phys., Vol. 50, p. 1324 (1969) and D. Meyerhofer, A. Sussman, and R. Williams, Electro-Optic and Hydrodynamic Properties of Nematic Liquid Films with Free Surfaces, J. Appl. Phys., Vol. 43, p. 3685 (1972). J. S. Smart, Effective Field Theories of Magnetism, W. B. Saunders Co., Philadelphia, Pa. (1966). 

Detailed theories dealing with these quantum mechanical, nonlinear, or transient optical effects are given in Chapters 8 and 10. As an example consider the expression for the (linear) molecular polarizability given in Equation (10.28). Note that the refractive indices of an excited molecule are completely different fiom those associated with a molecule in the ground state these differences are dne to the lact that a totally different set of dipole matrix elements dy and frequency denominators (oo, — co,) are involved. 

The linear optical properties, such as extinction and scattering by small spherical metal particles, were explained theoretically by the groundbreaking woik of Mie in 1908 [226], Within this theory, metallic NPs with a diameter of 2 nm are characterized by the SPR effect as well. However, experimental data [223,227] show that very small particles with a diameter of l-2nm do not display this phenomenon, since the electrons are located in discrete energy levels [89,228-230],... 

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