Symmetry Raman scattering

Another related issue is the computation of the intensities of the peaks in the spectrum. Peak intensities depend on the probability that a particular wavelength photon will be absorbed or Raman-scattered. These probabilities can be computed from the wave function by computing the transition dipole moments. This gives relative peak intensities since the calculation does not include the density of the substance. Some types of transitions turn out to have a zero probability due to the molecules symmetry or the spin of the electrons. This is where spectroscopic selection rules come from. Ah initio methods are the preferred way of computing intensities. Although intensities can be computed using semiempirical methods, they tend to give rather poor accuracy results for many chemical systems. 

Abstract—Experimental and theoretical studies of the vibrational modes of carbon nanotubes are reviewed. The closing of a 2D graphene sheet into a tubule is found to lead to several new infrared (IR)- and Raman-active modes. The number of these modes is found to depend on the tubule symmetry and not on the diameter. Their diameter-dependent frequencies are calculated using a zone-folding model. Results of Raman scattering studies on arc-derived carbons containing nested or single-wall nanotubes are discussed. They are compared to theory and to that observed for other sp carbons also present in the sample. 

Surface-enhanced Raman scattering, 5, 787 Swartzite structure, 6,848 Symmetry, 1, 190 Synergism... 

Using Raman scattering, Steigmeier of RCA Zurich has found in one of our samples a weak structure at 1875 cm-1. This finding, which implies a lack of inversion symmetry, is consistent with our Si—H model. 

Unfortunately, the different selection rules that apply to resonant and normal Raman scattering were not taken into account in this spectral interpretation. In the following, it is shown that the conclusions and assignments mentioned above have to be modified on the basis of symmetry considerations as discussed by Ricchiardi et al. (41). 

The DFT calculations led to a structure of C3 , symmetry for the model compound S(NMe)3 (Figure 9 and Table 7) with 42 normal vibrations, whose irreducible representation is given by 8A (Ra) + ISff (Ra/IR) + 6A" (IR) + WE" (Ra). The brackets indicate activities in Raman scattering and/or infrared absorption. 

An illustrative example of the influence of symmetry on the number of vibrations is provided by the homologous series of cyclic sulfur allotropes S (n = 6-12). The IR absorptions for these ring systems are weak owing to the low polarity of S-S bonds. However, sulfur is a good Raman scatterer because S-S bonds are readily polarised. The various sulfur allotropes have different symmetries in addition to a different number of sulfur atoms and, consequently, each allotrope exhibits a characteristic Raman spectrum (Figure 3.8). ... 

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 connection of the 36 hydrogen atoms to the C60 cage lowers the molecular symmetry and activates Raman scattering from a variety of initially forbidden phonon modes (Bini et al. 1998). In addition, the appearance of the C-H stretching and bending modes and those related to various isomers of C6)0n%, results in a very rich Raman spectrum. The comparison of the phonon frequencies for live principal isomers of C60I f 6, obtained by molecular dynamics calculations, with experimentally observed phonon frequencies has led to the conclusion that the material prepared by the transfer hydrogenation method contains mainly two isomers, those with symmetries DM and S6 (Bini et al. 1998). 

Raman spectroscopy has been an excellent diagnostic tool in the study of the Ceo molecule and its salts, because of the high symmetry these substances possess. In the case of C6o- which has a center of symmetry, the gerade modes are observed only in the Raman spectrum. Furthermore, the materials are excellent Raman scatterers. 

For the intramolecular modes, Raman scattering allows us to observe many modes of g symmetry,47 among which the most intense are the completely symmetric modes of the aromatic skeleton 390 cm "1 for bending of C-C-C angles, and 1400 cm "1 for stretching of the C-C bonds (breathing mode). 

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