Raman active phonon spectra

The IR and Raman-active phonon spectra exhibit pronounced maxima in the ranges of the intraicosahedral phonons of crystalline icosahedral boron-rich structures and in particular of the covalent intericosahedral B—B bonds known from boron-rich solids. This confirms that the external bonds of the icosahedra are largely covalent, similar to those of P-rhombohedral boron. Therefore a certain relationship between the electronic properties seems possible. 

Fig. 15. Symmetry analysis of the CEF transitions and the Raman-active phonons of NdBj for the (110) face at 77 K. The unpolarized spectrum is shown at the top.

The most intense feature in the Raman microscopy spectrum of the ash specimen is at 475 cm Actinide oxides are known to crystallize at high temperature with a fluorite (CaFs) structure and space group Fm3m(Ol). This structure is predicted to possess a simple vibrational structure with one IR active phonon of T symmetry (302 cm for PUO2) and one Raman active phonon of T2g symmetry (478 cm for PUO2) at = 0 [88]. Therefore, the 475-cm Raman band can be assigned to the T2 mode of PUO2. 

Isotope superlattices of nonpolar semiconductors gave an insight on how the coherent optical phonon wavepackets are created [49]. High-order coherent confined optical phonons were observed in 70Ge/74Ge isotope superlattices. Comparison with the calculated spectrum based on a planar force-constant model and a bond polarizability approach indicated that the coherent phonon amplitudes are determined solely by the degree of the atomic displacement, and that only the Raman active odd-number-order modes are observable. 

Free carriers change Raman spectra, either by single particle contribution to the spectrum, or by phonon- plasmon interaction. In addition, interference of electronic transition continua with single phonon excitations may lead to Fano line shapes, as mentioned in the introduction. The Fano effect is encountered in p-doped Si crystals, as shown in Fig. 4.8-19. The shown lines correspond to the respective Raman active mode at 520 cm for crystals with 4 different carrier concentrations, excited with a red laser. The continuous line is calculated according to Eq. 4.8-6. Antiresonance on the low frequency side and line enhancement on the high frequency side are a consequence of the positive value of Q. A reverse type of behavior is possible in the case of a negative Q. 

As we will show in the Section 3, the presence of the Kohn anomaly can be easily detected by Raman spectroscopy because the phonon affected by the Kohn-anomaly at F is Raman-active (called G peak. Section 3). Consequently, everything that affects the shape of the Fermi surface will also produce a change in the Raman spectrum and in particular on the G peak. 

The Raman scattering in La3Se4 at 300 K up to 500 cm and at 10 K up to —1000 cm is compared with that of Sm3S4 and Sm2S3. Nine Raman active modes are expected. These are difficult to resolve, since some of them appear only as shoulders in three broad bands between 150 and 400 cm or are hidden in the Rayleigh tail of the spectrum. Two weak excitations are observed below 150 cm L The spectral features are classified as first-order phonon scattering, Morke et al. [23]. 

As a general rule the Raman active doubly degenerate phonons of E2 species have never been observed in any polymer and this should be particularly the case of polyconjugated polymers. The attention should be focussed at the 10 modes, -dependent, some of which should be strong in the Raman spectrum, according to the theories presented in this paper. The A modes should coincide neither with A2 nor with the modes. IR, Rcunan frequency coincidences should occur for the E species. E are predicted to be extremely weak in the Raman spectra, but could be recognized in the IR spectrum of a stretch-oriented sample since they should show a dichroism when the spectra are recorded in polarized light. On the contrary the 9A2 modes should show // dichroism. 

Raman spectra are usually represented by the intensity of Stokes lines versus the shifted frequencies 12,. Figure 1.15 shows, as an example, the Raman spectrum of a lithium niobate (LiNbOs) crystal. The energies (given in wavenumber units, cm ) of the different phonons involved are indicated above the corresponding peaks. Particular emphasis will be given to those of higher energy, called effective phonons (883 cm for lithium niobate), as they actively participate in the nonradiative de-excitation processes of trivalent rare earth ions in crystals (see Section 6.3). 

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). 

This uitrasonio-opticai technique (or haif-opticai technique [89]) was aiso a hyphenated technique in terms of energy sources viz. thermai and opticai for phonon and photon production, respectiveiy). Thermai surface phonons restrict practical application of the technique owing to their iow scattering efficiency, which results in overly long data collection times (typicaiiy severai hours for a singie spectrum, even with advanced multipass interferometers). Similar to active Raman spectroscopy, coherent acoustic phonons are assumed to be excited by two narrow-line frequency tunable laser beams at different frequencies or by laser pulses of short duration compared to the acoustic period. 

Different models have been used to derive the particle size from Raman spectra As an example, we shah briefly explain the phonon confinement model (PCM). The scattering of one photon by n phonons is governed by the momentum conservation. Only vibrations from the center of the Brillouin zone (BZC) should therefore be active in one phonon process (first-order Raman spectrum) and this is actually the case in large and flawless crystals, where... 

A Raman spectrum also shows overtones and combination modes. In the case of graphene a prominent feature is given by the second order of the D peak, historically named G since it is the second most prominent band always observed in the Raman spectrum of graphite and more recently renamed as 2D peak. Since this peak originates from a process where momentum conservation is satisfied by two phonons with opposite wave-vectors, it does not require any defect for its activation, and it is thus always visible in the Raman spectrum. 

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