Molecular motion, bands

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

Marabella L. J. Molecular motion and band shapes in liquids, Appl. Spectr. Rev., 7, 313-55 (1974). 

Breuillard C., Ouillon R. Infrared and Raman band shapes and dynamics of molecular motions for N20 in solutions v3 band in CCL and liquid SF6. Mol. Phys. 33, 747-57 (1977). 

Figure 1.15 shows the orientation function vs. elongation for seven of the absorption bands measured. Each molecular motion in its own way responds to the applied strain. The three NXL bands (720, 740, 780 cm i) increase steadily with stretching but each exhibits a different slope. Two of the XL bands (625 and 633 cm ) increase in nearly parallel fashion but the 553 and 516 cm bands exhibit a plateau at elongations X > 1.6. The orientation function rises more steeply for the XL phase than for the NXL, indicating pronounced alignment of crystallites into the draw direction. The plateau indicates that a maximum of orientation is reached that is specific to the XL phase. NoncrystaUine material... 

One of the most interesting applications of Femtochemistry is the stroboscopic measuring of observables related to molecular motion, for instance the vibrational periods or the breaking of a bond [1], Because femtosecond laser fields are broadband, a wave packet is created by the coherent excitation of many vibrational states, which subsequently evolves in the electronic potential following mostly a classical trajectory. This behavior is to be contrasted to narrow band selective excitation, where perhaps only two (the initial and the final) states participate in the superposition, following typically a very non-classical evolution. In this case one usually is not interested in the evolution of other observables than the populations. 

Since frequencies for EPR spectroscopy are -100 times higher than those for NMR spectroscopy, correlation times (Chapter 3) must be less than 10-9 s if sharp spectra are to be obtained. Sharp bands may sometimes be obtained for solutions, but samples are often frozen to eliminate molecular motion spectra are taken at very low temperatures. For spin labels in lipid bilayers, both the bandwidth and shape are sensitively dependent upon molecular motion, which may be either random or restricted. Computer simulations are often used to match observed band shapes under varying conditions with those predicted by theories of motional broadening of lines. Among the many spin-labeled compounds that have been incorporated into lipid bilayers are the following ... 

If the reduced spectral densities do indeed mirror the pure singlemolecule contribution, then at least for symmetric-top liquids, which should have only one basic type of librational mode, it does not seem that the two observed bands can represent two distinct types of molecular motions. Similarly, the reduced spectral densities of liquids composed of less symmetric molecules also can often be fit to the same two types of bands, despite the existence of multiple possible librational modes. 

The broad-band dielectric study of highly filled PDMS is complementary to the NMR study of molecular motions in filled PDMS. The dielectric experiments were performed in the frequency range of 10" -10 Hz [27], A combined analysis of the dielectric spectra both for filled PDMS and the pure components of the mixtures was used to assign the dielectric losses to motions of adsorbed and non-adsorbed PDMS chain units. As discussed above, the interpretation of the results is based on a two-phase model assiuning the exchange of chain units at the surface of Aerosil between adsorbed and non-adsorbed states. 

The mid-IR range of the electromagnetic spectrum, 4000 cm" (2.5 pm) to 625 cm" (16 pm), encompasses absorption bands corresponding to the fundamental vibrations of most functional groups. Each band in an IR spectrum can be characterized by the wavenumber (or frequency ) of its absorption maximum, in addition to its intensity and bandwidth. The wavenumber at which an absorbance occurs is characteristic of the underlying molecular motion and consequently of the atoms participating in the chemical bond and on their conformation and immediate environment. The bandwidth, on the other hand, is related to the rates of motion of the molecule, and increases as motional rates increase. 

Time Domain Spectroscopy (T.D.S.).—Transient dielectric methods for the study of molecular motions occurring in less than 10 s are a relatively recent addition to the chemist s armoury. The availability of tunnd diode pulse-geam ators and wide-band sampling oscilloscopes led to the devdop-ment in the 1960 s of pulse reflection techniques known as time domain reilectometry (Ld.r.). The value of these methods was soon recognized in the fields of electronic and communication engineering for the qualitative analysis of transmission line systems and by 1965 had been used for... 

The need for an account of inertial effects in relaxation at hi frequencies has been known for a considerable time. - In dilute gases a description in terms of free rotation perturbed by collision is substantially developed to account for the shapes of i.r. and Raman lines. In crystals at sufficiently low temperature, description in terms of bands of regular oscillations perturbed by anharmonicity provides a regular sequence of approximations. In dense gases and in liquids - it is necessary to face simultaneously a complex pattern of molecular motions and a difficult question of the delayed electrical interaction which gives rise to the internal... 

T. Zerda, J. Schroeder, and J. Jonas. Raman band shape and dynamics of molecular motion of SFj in the supercritical dense fluid region. J. Chem. Phys., 75 1612-1622 (1981),... 

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