Raman effect, definition

The definition of the depolarisation ratio, pi, is illustrated in Fig. 1 for linearly-polarised incident radiation and a 90° scattering geometry. In the normal Raman effect it is well known that the measurement of p may identify the symmetry of the vibrational mode responsible for a given Raman band pi < 3/4 (pi = 0 in cubic or higher symmetries) for totally symmetric modes and Pi= 3/4 for non-totally symmetric modes. In the resonance Raman effect, the value of P, and its dependence on the exciting frequency, may be more informative. This is because the symmetries of... 

These second harmonic processes are still spontaneous, just like the conventional Raman effect, which means that there is no definite phase relationship between the Raman waves scattered from separate molecules the scattering is incoherent. Furthermore, the hyper-Raman intensity is several orders of magnitude weaker than conventional Raman, so measurements are very difficult. 

It can be seen from Figures 3.7 and 3.8 that the calculations reproduce very well not only the experimental spectra but also the experimentally observed isotopic shifts indicating a high reliability of the computational method. According to this comparison, definite attribution can be made for even the difficult Raman bands that cannot be assigned based solely on the experimental results. It is, however, necessary to mention at this point that the calculated Raman spectrum provided directly by the ab initio computations correspond to the normal Raman spectrum with the band intensity determined by the polarizability of the correlating vibration. Since the intensity pattern exhibited by the experimentally recorded resonance Raman spectrum is due to the resonance enhancement effect of a particular chromophore, with no consideration of this effect, the calculated intensity pattern may, in many... 

The EPR results discussed in this section are suggestive rather than definitive for the existence of O4 on oxide surfaces. However, it is clear from the preceding discussion that IR spectroscopy has proved to be a powerful technique to study O4 as a matrix-isolated species and the use of IR, Raman, and optical absorption together with EPR is likely to prove a very effective approach in elucidating the nature and properties of these complex oxygen ions on the surface. 

The half-widths of the second harmonic bands 2vd and 2vg are 56 and 64 cm, respectively (Fig. 7.9a, b). This is practically the same frequency shift as observed for the maxima of these bands relative to similar bands observed in the single crystalline graphite sample (Fig. 7.8a, b). The half-widths observed for the first-order vibration bands v and Vg are 56 and 21 cm correspondingly. The too narrow width of the observed tone bands does not allow for a more definite conclusion as concerning the appearance of cooperative effects in the vibration modes of the SWCNT. Nevertheless, a significant role of nonlinear interactions of their excitations shows the importance of their wave properties for the Raman spectra interpretation. 

Plasmonic nanostructures that are materials consisting of noble metal nanoparticles with sizes of 1-100 nm are known as specific substrates for surface enhanced Raman scattering and luminescence enhancement [1-4]. These effects are stimulated by the localized surface plasmon absorption (LSPA) and may be controlled by the change of metal nanoparticle sizes, their concentration and a substrate choice [5]. New opportunities for surface-enhanced effect realization and optimization are now discussed in connection with bimetallic nanostructures [6]. At the technological aspect one of the simplest types of a binary nanostructure is a stratified system made of two different monolayers, each is consisted of definite metal nanoparticles. The LSPA properties of these binary close-packed planar nanostructures are the subject of the paper. 

The influence of SOj on SCR of NO by hydrocarbons may be reversible [20, 21] or irreversible [22, 23]. Activities and selectivities of the main and/or the deactivation reactions might be negatively affected. However, most of the studies thus far were not sufficiently definitive to reveal the cause/effect relationships for SCR catalyst deactivation by SOj. In the present study, sulfur tolerances of Cu- and H- exchanged mordenite catalysts for NO removal reaction were examined to elucidate the cause of the loss of NO removal activity of the catalysts. Deactivation by SOj of Cuj sites was also investigated by various catalyst characterization methods including TGA, TPSR, Raman and XAS. 

However, direct evidence for these suggestions is currently not available, and the circumstantial evidence is neither overwhelming nor definitive. The recent development of in situ, real time monitoring of MW-accelerated reactions by Raman spectroscopy will perhaps provide answers to the important questions. Loupy has proposed that the non-thermal effects depend on the reaction medium, that is, solvent-free or polar or nonpolar solvent, the polarity of the transition state, and the position of the transition state along the reaction coordinate. He has suggested that specific MW effects are more apparent in apolar solvents and those reactions which have late transition states. 

In view of the severity of the complications which can be experienced in assessing, by diffraction methods, the degree of order in a short hydrogen bond, it is important to state that vibrational spectroscopy (Raman, IR and neutron inelastic scattering) can provide definitive evidence as to the local effective symmetry of the bond see, for example. Chapters 21, 23 and 24. Indeed, in situations of this type, an ideal requirement would be that a mutually consistent structural and vibrational description be achieved before our diffraction-obtained structural picture can be accepted unequivocally. 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

---