Raman mechanism

Chemical Enhancement Compared to electromagnetic enhancement, aspects of the contribution of chemical enhancement [27-29], are less known. Chemical enhancement has other synonyms such as charge transfer (CT) and the short-range effect. It is associated with the electronic structure (state) of the metal and the adsorbate, which can be explained by a resonance-like Raman mechanism (vide infra). As most of the electrochemical SERS systems involve the formation of strong or weak chemical bonds for the adsorbate with the electrode surface, it is... 

Benzene, toluene, 2-and 4-picoline Hydrated and dehydrated silica T, Raman Mechanisms of sorption, models of volatile aromatic pollutant interactions 994... 

For completeness, it is mentioned that the two processes of sir, according to the direct and the Raman mechanism, do not fit the experimental data. Moreover, an... 

Consider Raman transitions between thennalized molecular eigenstate g (ground) and molecular eigenstate/ (final). The quantum mechanical expression for responding to colours and j is the famous (thennalized) Kramers-Heisenbergequation [29]... 

The metal substrate evidently affords a huge ( 10 and even as high as 10 [84, 85]) increase in the cross-section for Raman scattering of the adsorbate. There are two broad classes of mechanisms which are said to contribute to this enhancenient [, and Ml- The first is based on electromagnetic effects and the second on cheniicaT effects. Of these two classes the fomier is better understood and, for the most part, the specific mechanisms are agreed upon the latter is more complicated and is less well understood. SERS enhancenient can take place in either physisorbed or chemisorbed situations, with the chemisorbed case typically characterized by larger Raman frequency shifts from the bulk phase. 

Of great interest to physical chemists and chemical physicists are the broadening mechanisms of Raman lines in the condensed phase. Characterization of tliese mechanisms provides infomiation about the microscopic dynamical behaviour of material. The line broadening is due to the interaction between the Raman active chromophore and its environment. 

The second excitation mechanism, impact scattering, involves a short range interaction between the electron and the molecule (put simply, a collision) which scatters the electrons over a wide range of angles. The usefiil feature of impact scattering is that all vibrations may be excited and not only the dipole active ones. As in Raman spectroscopy, the electron may also take an amount of energy hv away from excited molecules and leave the surface with an energy equal to Eq + hv. 

Tokmakoff A, Lang M J, Jordanides X J and Fleming G R 1998 The intermolecular interaction mechanisms in liquid CS2 at 295 and 165 K probed with two-dimensional Raman spectroscopy Chem. Phys. 233 231-42... 

The mechanism for Stokes and anti-Stokes vibrational Raman transitions is analogous to that for rotational transitions, illustrated in Figure 5.16. As shown in Figure 6.3, intense monochromatic radiation may take the molecule from the u = 0 state to a virtual state Vq. Then it may return to u = 0 in a Rayleigh scattering process or to u = 1 in a Stokes Raman transition. Alternatively, it may go from the v = state to the virtual state Fj and return to V = (Rayleigh) or to u = 0 (Raman anti-Stokes). Flowever, in many molecules at normal... 

One effect of mechanical anharmonicity is to modify the Au = t infrared and Raman selection rule to Au = 1, 2, 3,. .., but the overtone transitions with Au = 2, 3,... are usually weak compared with those with Au = t. Since electrical anharmonicity also has this effect both types of anharmonicity may contribute to overtone intensities. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

The mechanisms of lead corrosion in sulfuric acid have been studied and good reviews of the Hterature are available (27—30). The main techniques used in lead corrosion studies have been electrochemical measurements, x-ray diffraction, and electron microscopy. More recendy, laser Raman spectroscopy and photoelectrochemistry have been used to gain new insight into the corrosion process (30,31). 

Thus far we have discussed the direct mechanism of dissipation, when the reaction coordinate is coupled directly to the continuous spectrum of the bath degrees of freedom. For chemical reactions this situation is rather rare, since low-frequency acoustic phonon modes have much larger wavelengths than the size of the reaction complex, and so they cannot cause a considerable relative displacement of the reactants. The direct mechanism may play an essential role in long-distance electron transfer in dielectric media, when the reorganization energy is created by displacement of equilibrium positions of low-frequency polarization phonons. Another cause of friction may be anharmonicity of solids which leads to multiphonon processes. In particular, the Raman processes may provide small energy losses. 

Some materials undergo transitions from one crystal structure to another as a function of temperature and pressure. Sets of Raman spectra, collected at various temperatures or pressures through the transition often provide useftil information on the mechanism of the phase change first or second order, order/disorder, soft mode, etc. 

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

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