Thermally induced disorder

Cohen S R, Naaman R and Sagiv J 1986 Thermally induced disorder in organized organic monolayers on solid substrates J. Phys. Chem. 90 3054-6... 

Therefore the lack of an observable bleach can only be explained by the cancellation of all contributions to the pump-probe signal, which is the case for a perfect harmonic state. It can be shown that the anharmonicity of a vibrational exciton is a direct measure of its degree of delocalization [5]. Thus, we conclude that the free exciton state is almost perfectly delocalized at 90 K. As temperature increases, a bleach signal starts to be observed, pointing to a non-complete cancellation of the different contributions of the total pump-probe signal. Apparently, thermally induced disorder (Anderson localization) starts to localize the free exciton. The anharmonicity of the self-trapped state (1650 cm 1), on the other hand, originates from nonlinear interaction between the amide I mode and the phonon system of the crystal. It... 

Dipole rotation refers to the alignment, by effect of the electric field, of molecules in the sample that have permanent or induced dipole moments. As the electric field of microwave energy increases, it aligns polarized molecules. As the field decreases, thermally induced disorder is restored. In fact, applied microwave fields cause molecules, on average, to temporarily spend very slightly more time pointing in one direction than in others. Associated with that very small fraction of preferred orientation there is another very small fraction of molecular order imposed and hence a tiny bit of energy. When the... 

Figure 1 (A) Aligned polarized molecules in the electromagnetic field. (B) Thermally induced disorder when the electromagnetic field is removed. (Adapted with permission from Kingston HM and Jassie LB (1998) Introduction to Microwave Sample Preparation, Theory and Practice, ACS Professional Reference Book Series, p. 11, Washington DC American Chemical Society.)...

Point defects, static disorder, and thermally induced displacements lead to an increase of the background intensity between the spots. Depending on the correlation between the scatters, the background is either homogeneous (no correlation) or... 

Spin-crossover phase transition of a manganese(IU) complex [Mn(taa)] was studied by variable-temperature laser Raman spectroscopy and it was found that the vibrational contribution in the transition entropy is not dominant in contrast to the cases of ordinary iron spin-crossover systems. The discovery of a dynamic disorder in the HS phase by means of dielectric measurements provided an alternative entropy source to explain the thermally induced spin-crossover transition. This dynamic disorder was attributed to the reorienting distortion dipoles accompanying the E e Jahn-Teller effect in HS manganese(III) ions. 

Thermally-induced network vibrations broaden the absorption edge and shift the band gap of semiconductors. The thermal disorder couples to the optical transition through the deformation potential, which describes how the electronic energy varies with the displacement of the atoms. The bond strain in an amorphous material is also a displacement of atoms from their ideal position, and can be described by a similar approach. The description of static disorder in terms of frozen phonons is a helpful concept which goes back 20 years. Amorphous materials, of course, also have the additional disordering of the real phonon vibrations. 

Fig. 15. Schematic of polar and alignment disorder as measured by XRD and NMR. Upper line polar disorder with random up or down orientations of the direction of the molecules with equal probabilities to restore the inversion symmetry for X-ray, the two directions are possible with equal probabilities on each site NMR cannot measure polar disorder, all sites are equivalent. Lower line alignment disorder of the molecules characterized by a long correlation time as compared to the inverse Larmor frequency coL, and librational disorder represented by ellipses of thermal-induced rotations (with angular amplitude possibly larger than the corresponding disalignment) around the mean alignment direction of the molecules. The frequencies of librations 1 jx is much larger than the Larmor frequency. For X-ray, both orientational disorders are mixed up with a preponderant contribution of the high-frequency librations for NMR, only alignment disorder remains.

Schertel A, Hahner G, Grunze M and Wdll C 1996 Near edge x-ray adsorption fine structure investigation of the orientation and thermally induced order-disorder transition in thin organic films containing long chain hydrocarbons J. Vac. Sci. Technol. A 14 1801-6... 

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