Raman process

This basic instrumentation, here described within the context of spontaneous Raman scahering, may be generalized to most of the other Raman processes that are discussed. Specific details can be found in the citations. 

Wang 0-S 1975 The stimulated Raman process Quantum Electronics vol 1A, ed H Rabin and 0 L Tang (New York Academic) pp 447-72... 

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. 

In the opposite case, PA < 1, the Raman process with the rate constant proportional to is... 

Hyper-Raman spectroscopy is not a surface-specific technique while SFG vibrational spectroscopy can selectively probe surfaces and interfaces, although both methods are based on the second-order nonlinear process. The vibrational SFG is a combination process of IR absorption and Raman scattering and, hence, only accessible to IR/Raman-active modes, which appear only in non-centrosymmetric molecules. Conversely, the hyper-Raman process does not require such broken centrosymmetry. Energy diagrams for IR, Raman, hyper-Raman, and vibrational SFG processes are summarized in Figure 5.17. 

At higher temperatures, the two-phonon (Raman) processes may be predominant. In such a process, a phonon with energy hcOq is annihilated and a phonon with energy HcOr is created. The energy difference TicOq — ha>r is taken up in a transition of the electronic spin. In the Debye approximation for the phonon spectrum, this gives rise to a relaxation rate given by... 

If optical phonons are responsible for the Raman processes, the Einstein model for the phonon spectrum is more appropriate. In this case, one finds... 

This model also explains the unexpected magnetic field dependence of the relaxation time in a temperature range in which Raman processes are normally expected to be predominant [32]. 

I2H2O as a function of the reciprocal temperature. The points are data obtained from fits of the Mdssbauer spectra (Fig. 6.6). The broken curve is a fit to the Einstein model for a Raman process. The dotted curve corresponds to a contribution from a direct process due to interactions between the electronic spins and low-energy phonons associated with critical fluctuations near the phase transition temperature. (Reprinted with permission from [32] copyright 1979 by the Institute of Physics)... 

The temperature dependence of the MRD profile for the protein-water systems where the protein is magnetically a solid, is remarkably weak. The relaxation rate is proportional to IjT, which is consistent with Eq. (4) that was derived on the assumption that the relaxation process is a direct spin-phonon coupling rather than an indirect or Raman process. If it were a Raman process, there would be no magnetic field dependence of the relaxation rate therefore, the temperature dependence provides good evidence in support of the theoretical foundations of Eq. (6). 

The most serious problem associated with conventional Raman spectroscopy is the minuteness of the effect. Even in the most favorable cases, only on the order of 10 of the incident intensity is converted into signal. With the development of lasers and the improvements in detection sensitivity, Raman spectroscopy has made considerable progress in recent years. Still, the lack of sensitivity has restricted Raman use. Nevertheless, the magnitude of the Raman effect can be greatly increased using nonlinear Raman processes. If... 

An induced Raman process exists which is related to collision-induced absorption like the Raman spectra of ordinary molecules are related to the infrared rotovibrational spectra. 

We note that the theory of collision-induced, nonlinear Raman processes exists [212, 211] but experimental investigations are virtually non-existent. 

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