Raman scattering medium effects

This was first investigated by Huang n). Long-wave optical phonons are those with small k values. A polar phonon is an infrared-active phonon. Polar phonons therefore can only be observed in the Raman effect for crystals having no center of symmetry in the elementary cell. For centro-symmetric crystals the rule of mutual exclusion applies infrared-active phonons are forbidden in Raman scattering and vice versa. The elementary cells of NaCl and LiF have a center of symmetry, but GaP has none. The following considerations may therefore be applied to GaP as an example. This crystal has two atoms in the elementary cell and is cubic. It can be treated as an optically isotropic medium. 

The Raman effect can be seen, from a classical point of view, as the result of the modulation due to vibrational motions in the electric field-induced oscillating dipole moment. Such a modulation has the frequency of molecular vibrations, whereas the dipole moment oscillations have the frequency of the external electric field. Thus, the dynamic aspects of Raman scattering are to be described in terms of two time scales. One is connected to the vibrational motions of the nuclei, the other to the oscillation of the radiation electric field (which gives rise to oscillations in the solute electronic density). In the presence of a solvent medium, both the mentioned time scales give rise to nonequilibrium effects in the solvent response, being much faster than the time scale of the solvent inertial response. 

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. 

The phenomenon of surface-enhanced infrared absorption (SEIRA) spectroscopy involves the intensity enhancement of vibrational bands of adsorbates that usually bond through contain carboxylic acid or thiol groups onto thin nanoparticulate metallic films that have been deposited on an appropriate substrate. SEIRA spectra obey the surface selection rule in the same way as reflection-absorption spectra of thin films on smooth metal substrates. When the metal nanoparticles become in close contact, i.e., start to exceed the percolation limit, the bands in the adsorbate spectra start to assume a dispersive shape. Unlike surface-enhanced Raman scattering, which is usually only observed with silver, gold and, albeit less frequently, copper, SEIRA is observed with most metals, including platinum and even zinc. The mechanism of SEIRA is still being discussed but the enhancement and shape of the bands is best modeled by the Bruggeman representation of effective medium theory with plasmonic mechanism pla dng a relatively minor role. At the end of this report, three applications of SEIRA, namely spectroelectrochemical measurements, the fabrication of sensors, and biochemical applications, are discussed. 

Much more important for gas phase spectroscopy than the hyper-Raman effect are the various coherent Raman effects, so we shall develop the theory of coherent Raman scattering in rather more detail. The usual starting point is the bulk polarization of the medium expressed as a function of the electric field vectors of the various light waves present simultaneously in the medium (SI)... 

A theoretical approach of such Boson peaks is developed, for example, in Refs. 55 and 56 (see Fig. 37). One important characteristic of the Boson peaks is that their spectral profile is universal in the sense that it does not depend on the composition of the glass, the polarization of the light, or the temperature [57], as shown in Fig. 38 in reduced intensity and reduced wave-vector scales. However, these low-lying peaks may not be due purely to acousticlike states localized optical states, medium-range-order effects, and so forth may coexist in the same frequency range and be revealed by Raman scattering. 

Owing to the coherence, we need to consider the macroscopic evolution of the field in a medium that shows a macroscopic polarization induced by the field-matter interaction. This will be done in three steps. First, the polarization induced by an arbitrary field will be calculated and expanded in power series in the field, the coefficients of the expansion being the material susceptibilities (frequency domain) or response function (time domain) of wth-order. Nonlinear Raman effects appear at third order in this expansion. Second, the perturbation theory derivation of the third-order nonlinear susceptibility in terms of molecular eigenstates and transition moments will be outlined, leading to a connection with the spontaneous Raman scattering tensor components. Last, the interaction of the initial field distribution with the created polarization will be evaluated and the signal expression obtained for the relevant techniques of Table 1. 

Figure 4.2 Two common interactions between light and matter, (a) At certain angles, light is refracted when passing from one medium to another (at dashed line), changing direction and velocity, (b) When light hits a particle, elastic collisions result in changes in direction and phase (Rayleigh scattering), whereas inelastic collisions result in frequency changes (Raman effect).

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