Raman susceptibility

Raman Susceptibility Careful measurements of Raman intensity in the compound 123 near x = xopt, the composition for the highest Tc, show [13]... 

This effect is related with the Stimulated Raman scattering process. A strong beam (pump) incident on a Raman active medium induces a change of the refractive index, which in turn influences the propagation of a weaker beam (probe). A typical value for Raman susceptibility is Re(x 5 ) 10 esu, with a time response 10 s. 

Variable temperature/RH Raman or FTIR for heat/moisture-induced molecular demixing Spectral monitoring of crystallization during storage, etc., LF Raman susceptibility presents superior sensitivity to the phonon peaks from lattice vibration thus detect nanocrystaUites (30 A) even in low API content (2-5 %) MIR region highly susceptible to the interference from moisture peaks thus poses hurdle for hygroscopic samples... 

Thirdly, the collision-induced Raman resonances can expand the application of Raman spectroscopy to situations where the conventional Raman susceptibility vanishes. Andrews et al., for example, have demonstrated that sharp vibrational transitions in an initially unpopulated excited electronic state of a molecule can be observed as extra four-wave mixing resonances, even though they cannot be observed in absorption because of rapid dephasing of the electronic transition. In these experiments pentacene molecules were doped in a benzoic acid crystal, and phonon scattering rather than collisions provided the dephasing mechanism. CARS of equally-populated ground state rotational levels in molecules would also become possible by observing collision-induced resonances. 

In the work by Boyd, the Stokes susceptibility (sometimes called the Raman susceptibility) is explicitly derived for a multilevel system. However, in practice, it is easier and quantitatively correct to relate the gain constant to the scattering cross section as indicated in Chapter 5. The reader is reminded of the role played by the population difference Ni - N2 in x, which could reverse sign depending on Ni and N2. 

There are different paths to achieving surface specificity. One can exploit optical susceptibilities and resonances that are nonzero only at the surface or only for the molecular species of interest adsorbed on the surface. Examples include the use of second-order nonlinear mixing processes such as second harmonic generation7-9 for which the nonlinear susceptibility tensor is nonzero only where inversion symmetry is broken. Spectroscopic techniques with very high selectivity for molecular resonances such as surface-enhanced infrared or Raman spectroscopy10-12 may also be used. 

Most of the four above-mentioned properties for Raman spectra can be explained by using a simple classical model. When the crystal is subjected to the oscillating electric field = fioc " of the incident electromagnetic radiation, it becomes polarized. In the linear approximation, the induced electric polarization in any specific direction is given by Pj = XjkEk, where Xjk is the susceptibility tensor. As for other physical properties of the crystal, the susceptibility becomes altered because the atoms in the solid are vibrating periodically around equilibrium positions. Thus, for a particular... 

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