Radiation-induced orientation polarization

The radiation-induced orientation polarization of dipolar molecules or groups consumes energy on a molecular scale (internally dissipated as heat) to overcome internal constraints induced by the external electric field. Electrically nonconducting materials capable of undergoing dielectric relaxation are commonly denoted as dielectrics. 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

So far we have discussed various studies on the relaxation of the population bPo of excited state rovibrational levels in elastic and inelastic collisions. To this end the intensity of fluorescence was measured in one or the other way. If an analysis of the state of polarization of the radiation is performed, one may obtain information on the behavior of alignment and orientation of the molecular angular momenta in elastic and inelastic collisions. If we register, under collisional conditions, the polarization properties of a directly laser-induced rovibrational level of the molecule, then, according to (2.35) and (2.39), it is possible to determine the rates... 

Suppose that a tetrahedral molecule such as CCI4 is irradiated by plane polarized light (E ). Then, the induced dipole (Section 1.7) also oscillates in the same yz-plane. If the molecule is performing the totally symmetric vibration, the polarizability ellipsoid is always sphere-like namely, the molecule is polarized equally in every direction. Under such a circumstance, I (Iy) = 0 since the oscillating dipole emitting the radiation is confined to the xz-plane. Thus, pp = 0. Such a vibration is called polarized (abbreviated as p). In liquids and solutions, molecules take random orientations. Yet this conclusion holds since the polarizability ellipsoid is spherical throughout the totally symmetric vibration. 

The definitions given above for An and Ca are in terms of the anisotropies in the optical properties over the wavelength range of visible light. Similar anisotropies can also exist or can be induced in other wavelength ranges. For example, infrared dichroism (the absorption of different amounts of polarized infrared radiation in the directions parallel and perpendicular to the direction of orientation) is useful in characterizing polymers by vibrational spectroscopy. 

The principle of the method is similar to that of the infrared method except that the polymer is illuminated by polarised radiation of a suitable wavelength to induce fluorescence and the emitted radiation is examined using an analyser. Information is thus obtained about molecular orientation with respect to two directions, the transmission directions of the polarizer and analyser. 

Since the jp-polarized radiation at cpi 0° has tangential and normal electric field components (1.4.4°), these components will interact with those dipoles within the film that are oriented parallel and perpendicnlar to the snrface, respectively, according to the selection mle (1.27). Fmthermore, under action of the external electric field normal to the surface, the dipoles experience an additional electric field induced by the polarized chemical bonds within the layer. It follows that in the jp-polarized spectra, the band near the longitudinal optical mode vlo will be observed, in addition to the band near vro resulting from the dipoles parallel to the film. In light of the discussion in Section 1.8.2, it is evident that because the dipole oscillations responsible for the vro band are directed along the surface, these oscillations will be quenched by a metal surface and, hence, be absent in p-polarized spectra of films on mefal substrates, as demonstrated in Fig. 3.2. 

Reagent properties. In which way is the reaction influenced by properties such as reagent quantum state, velocity or light polarization (which induces spatial orientation of the particle in resonance with the radiation) ... 

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