Induced polar alignment

Figure 3. Time dependence of harmonic generation (of polymer/guest induced polar alignment and relaxation) when the laser intensity is overly strong. The poling-field time dependence is depicted at top. (Reproducedfrom Ref. 23. Copyright 1983, American...

Polar alignment can be described physically through a knowledge of the orientational distribution function f ) (32). A related quantity that can be expressed as a single number is the order parameter. The order parameter describing the induced polar alignment G is given by... 

This concept has originally been named PASADENA (Parahydrogen And Synthesis Allow Dramatically Enhanced Nuclear Alignment) [6], but the spectroscopic method based on this phenomenon has subsequently also been called PHIP (Para-Hydrogen Induced Polarization) [7]. In this chapter the abbreviation PH IP will be used throughout. 

Orientation polarization can occur in materials composed of molecules that have permanent electric dipole moments. The permanent dipoles tend to become aligned with the apphed electric field, but entropy and thermal effects tend to counter this alignment. Thus, orientation polarization is highly temperature-dependent, unlike the forms of induced polarization which are nearly temperature-independent. In electric fields of moderate intensity, the orientation polarization is proportional to the local electric field, as for the other forms of polarization... 

Even if the liquid molecules have no net dipole, electrons in the molecule will be affected by an electric field. The ability of electrons to move in the presence of an electric field is called their polarizability. The movement of electrons results in an induced dipole, and the alignment of the induced dipoles with the electric field gives induced polarization. 

Figure 3. Different mechanisms for inducing polarization through (a) redistribution of electron density, (b) bond stretching, (c) alignment of dipoles, and (d) separation of ions.

The application of an electric field always causes some physical changes in the medium even if the liquid molecules are non-polar, the electrons in the molecule will be affected by the electric field. The movement of electrons within the molecule results in an induced dipole, ft, and the alignment of the induced dipoles with the electric field gives induced polarization. For example, if a positive charge is placed above the plane of a neutral benzene molecule, the average positions of the electrons will shift upward, giving the benzene molecule a dipole moment whose direction is perpendicular to the molecular plane. In summary, when a non-polar molecule is subjected to an electric field, the electrons in the molecule are displaced from their ordinary positions so that the electron clouds and nuclei are attracted in opposite directions and a dipole is induced thus the molecule temporarily has an induced dipole moment, ft. 

ALTADENA (Adiabatic Longitudinal Transport after Dissociation Engenders Net Alignment) [56]. All variants are nowadays known under the more generally acronym of PHIP (Parahydrogen Induced Polarization) [52]. The basic theory ofthe PHIP effect in an AX-spin system was given in the review paper of Bowers and Weitekamp [56]. 

In conductance, the ions absorb energy which is translated into heat and motion. In oscillometry, we have not only this aspect, but also the capacitance effect in which molecules absorb and return energy each frequency cycle owing to the induced polarization and alignment of electrically unsymmetrical molecules. 

The polar alignment that was induced in this system was shown to be stable for long periods of time. However, several complexities in the poling process were uncovered in this study. It was found that the induced SHG dropped precipitously as Tg was approached from below. This was speculated as being due to the establishment of a mono-... 

The monomers were then thermally polymerized and simultaneously irradiated with linearly polarized light. Two of the monomers were able to induce bulk alignment of the liquid crystals in direction perpendicular to the polarized light. Monomer 1 was effective in concentrations as low as 1%. It was also concluded from the experimental evidence that the photo-induced bulk alignment of the ferroelectric liquid crystals may take place by a mechanism that is different from one that takes place in achiral azopolymers. 

Minemoto S, Tanji H, Sakai H (2003) Polarizability anisotropies of rare gas van der Waals dimers studied by laser-induced molecular alignment. 1 Chem Phys 119 7737-7740 Yakshin MM (1948) On atomic polarization and bond refraction of complex compounds of platinum. Izvestia Sektora Platiny 21 146-156 (in Russian)... 

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