Aligning molecules with polarized

Preparing aligned molecules with polarized radiation... 

When the photon energy is swept through a core level ionization potential, the absorption profile has resonance structure superimposed to the atomic profile. This structure is due to dipole transitions from the core level to unfilled molecular orbitals. Because the features are caused by dipole transitions between states of defined symmetry, the polarization dependence of these structures can be used to determine the orientation of transition dipoles with respect to the substrate. Therefore, NEXAFS spectroscopy can be used to determine the orientation of aligned molecules with or without the requirement of long range order. 

Molecules with polar bonds are attracted to one another because they can align themselves in such a way that the positive end of one dipole is adjacent to the negative end of... 

In the second type of interaction contributing to van der Waals forces, a molecule with a permanent dipole moment polarizes a neighboring non-polar molecule. The two molecules then align with each other. To calculate the van der Waals interaction between the two molecules, let us first assume that the first molecule has a permanent dipole with a moment u and is separated from a polarizable molecule (dielectric constant ) by a distance r and oriented at some angle 0 to the axis of separation. The dipole is also oriented at some angle from the axis defining the separation between the two molecules. Overall, the picture would be very similar to Fig. 6 used for dipole-dipole interaction except that the interaction is induced as opposed to permanent. 

The values of P range from + to — (0 = 0 or 90°). This equation would be considerably simpler if only those molecules with their transition moments parallel to the electric vector were capable of absorption or if the molecules were perfectly aligned, that is, 0 = 0. Then the angle between the two transition moments could be directly determined from the observed degree of polarization P. 

Figure 1.10 Polarization of molecules in an electric field. In the absence of an applied electrical field (a), molecules are aligned randomly, with no net dipole. When the field is applied (b), the solvent molecules are polarized and align themselves to reduce the strength of the field...

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... 

In 1912, Debye suggested that the high dielectric constants of water, ethanol, and other highly polar molecules are due to the presence of permanent dipoles within each individual molecule. There is a tendency for the molecules to align themselves with their dipole axes in the direction of the applied electric field. 

Two polarization mechanisms are possible. If the molecules possess a permanent electric dipole moment pbp rm, each molecule can align its moment with the field direction by reorientation, producing a macroscopic dipole moment. Even if perm = 0 in the field-free limit, each molecule can achieve a field-dependent dipole moment pind by induction. The induced dipole moment is proportional to field strength, pind = a , where a is the electric polarizability of the molecule. In both cases, work must be performed on the system to achieve the macroscopic polarization. Molecules with large permanent dipole moments correspond to high k. 

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