Reorientation process

Reverse saturable absorption is an increase in the absorption coefficient of a material that is proportional to pump intensity. This phenomenon typically involves the population of a strongly absorbing excited state and is the basis of optical limiters or sensor protection elements. A variety of electronic and molecular reorientation processes can give rise to reverse saturable absorption many materials exhibit this phenomenon, including fuUerenes, phthalocyanine compounds (qv), and organometaUic complexes. 

It is also well known that alkyl groups can be tran.sferred intramolecularly from one position to another on the same ring and intermoiccularly from one aromatic ring to another through dealkylation reactions catalyzed by Lewis acid. The intramolecular alkyl-transfer is called reorientation or isomerization and the intermolecular alkyl transfer is referred to as disproportionation. Reorientation processes arc normally faster than disproportionation. 

Molecular Motions and Dynamic Structures. Molecular motions are of quite general occurrence in the solid state for molecules of high symmetry (22,23). If the motion does not introduce disorder into the crystal lattice (as, for example, the in-plane reorientation of benzene which occurs by 60° jumps between equivalent sites) it is not detected by diffraction measurements which will find a seemingly static lattice. Such molecular motions may be detected by wide-line proton NMR spectroscopy and quantified by relaxation-time measurements which yield activation barriers for the reorientation process. In addition, in some cases, the molecular reorientation may be coupled with a chemical exchange process as, for example, in the case of many fluxional organometallic molecules. ... 

Figure 15 A lamellar block copolymer phase is reoriented through external shear. The initial phase has the direction of the lamellae parallel to the shear gradient direction. The most stable state would be to orient the director parallel to the shear and shear gradient direction. However, the reorientation process gets stuck before true equilibrium is reached. The stuck orientation is relatively stable, because the lamellae have to be broken up before they can further align with respect to the shear flow. Reprinted with permission from Ref. 56.

A second type of behavior existing in the PLZT s is the linear (Pockels) effect which is generally found in high coercive field, tetragonal materials (composition 3), This effect is so named because of the linear relationship between An and electric field. The truly linear, nonhysteretic character of this effect has been found to be intrinsic to the material and not due to domain reorientation processes which occur in the quadratic and memory materials. The linear materials possess permanent remanent polarization however, in this case the material is switched to its saturation remanence, and it remains in that state. Optical information is extracted from the ceramic by the action of an electric field which causes linear changes in the birefringence, but in no case is there polarization reversal in the material. 

The dipole autocorrelation function, , defined previously. The full-time dependence of this function for liquid carbon monoxide has been successfully determined experimentally from Fourier inversion of infrared band shapes.2,15 In fact, this was one of the reasons this system was studied. This function has also been successfully evaluated in terms of models of the molecular reorientation process.58 s memory function, KD(t), is defined by... 

This section presents a fundamental development of Sections V and VI. Here a linear dielectric response of liquid H20 is investigated in terms of two processes characterized by two correlation times. One process involves reorientation of a single polar molecule, and the second one involves a cooperative process, namely, damped vibrations of H-bonded molecules. For the studies of the reorientation process the hat-curved model is employed, which was considered in detail in Section V. In this model a hat-like intermolecular potential comprises a flat bottom and parabolic walls followed by a constant potential. For the studies of vibration process two variants are employed. 

Experiments described in this section are suited to investigate ultraslow motion with correlation times in the millisecond-to-second range. Here, the NMR spectra are given by their rigid-lattice limit and one correlates the probability to find given NMR frequencies at two different times separated by the so-called mixing time tm [11,72]. A two-dimensional (2D) spectrum results, being a function of two NMR frequencies at t = 0 and t = tm, respectively. Since the NMR frequency reflects the orientation of the molecule, 2D spectra provide a visual representation of the reorientational process. Time- and frequency-domain... 

Fig. 41 (top)], which are straightforwardly related to the jump angles [11,72]. In contrast, the dynamics involved in the a-process results in complete randomization of the molecular orientation so that the NMR frequencies at tm — 0 and tm xa> respectively, are uncorrelated and a box-like 2D spectrum with off-diagonal intensity spread all over the frequency plane is observed [cf. Fig. 41 (bottom)]. Information about the course of the reorientation process is available when 2D spectra are measured for different mixing times tm [72,94]. 

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