First-order phonon Raman scattering

To evaluate the crystallinity of the films, Raman spectroscopy is used. A typical Raman spectrum is presented in Fig. 4. Of the crystalline diamond, a narrow peak at a frequency of 1332 cur1 is characteristic, which is caused by the first-order phonon scattering by the crystal lattice. The non-diamond carbon is represented in the spectrum by two diffuse bands at ca. 1350 and 1550 cm-1. When comparing the peaks height, one should keep in mind that the Raman signal is 50 times more sensitive to the non-diamond carbon than to the crystalline diamond [20], In the high-quality diamond films used as electrodes, the non-diamond carbon component rarely exceeds 1%. Raman spectroscopy data have been corroborated by the independent impedance spectroscopy measurements (see below). According to [21], the inner layer of a diamond film is enriched with the admixture of non-diamond carbon as compared to its outer layer. 

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]. 

Inelastic scattering of radiation in solids is typified by the Raman effect, which involves the creation or annihilation of phonons or magnons. If a single phonon is involved, the scattering event is referred to as the first-order Raman effect in second-order Raman effect two phonons are involved. The polarizability associated with a phonon mode can be represented as a power series of the phonon amplitude, u, as follows ... 

Table 3.2 summarizes the optical phonons of crystals with wurtzite structure and the scattering configurations, in which the optical phonons are predicted to produce backward signal in first-order Raman scattering. 

In Raman scattering, the excitation light couples to changes in the polarizability and first order transitions are allowed in crystalline silicon. (See Lannin (1984) for a discussion of Raman scattering in amorphous silicon.) The scattering intensity, as a function of the phonon frequency, co, is approximately... 

The second-order Raman spectra from 1,700 to 3,300 cm are relatively weak and broad compared to the first order Raman spectra. There are many papers devoted to the second-order Raman processes in graphite [49, 59-62] and carbon nanotubes [8, 34, 63]. The interpretation of the spectral behavior of the second-order bands in terms of DR have been done [3, 64, 65]. The corresponding phonon is involved in an inter-valley scattering process of in-plane modes (LO or TO) around the K-points in the two-dimensional (2D) Brillouine zone (BZ, Fig. 7.4) [66, 67]. 

The key result is the appearance of first order Raman scattering (forbidden in centrosymmetric paraelectric phase) by optical phonon modes (TO2, LO3, TO4, and LO4) in all the film systems. These phonon peaks are weak in the nearly stoichiometric MBE-grown film, and are the strongest in the non-stoichiometric (Sro.9Ti03 x) film with intentionally introduced large Sr deficiency. The nominally stoichiometric PLD-grown SrTiOs films also exhibit strong first order Raman peaks nearly identical behavior was observed in both 50- and 1000-nm-thick films. The temperature evolution of Raman spectra (Eig. 21.13b) clearly indicates... 

Sirenko AA, Akimov lA, Pox JR, Clark AM, Li H-C, Si W, Xi XX (1999) Observation of the first-order Raman scattering in SrTi03 thin films. Phys Rev Lett 82 4500 Akimov lA, Sirenko AA, Clark AM, Hao J-H, Xi XX (2000) Electric-field induced soft-mode hardening in SrTiOa films. Phys Rev Lett 84 4625 99. Tenne DA, Clark AM, James AR, Chen K, Xi XX (2001) Soft phonon modes in Bao.5Sro.5Ti03 thin films studied by Raman spectroscopy. Appl Phys Lett 79 3836-3838... 

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... 

The spectral function leading to a quasi-first-order Raman spectrum close to the critical point of a displacive system contains basically three contributions from 1) the soft mode, 2) the central peak and 3) higher order Raman scattering arising from processes involving hard and soft phonons... 

In Ih phase, the selection rules for D6h are violated due to the proton disorder. So the observed band shape represents mostly the phonon density of states (DOS). As shown in Fig.2 (b), observed Raman spectra in the librational region in Ih phase surprisingly agrees with results of the neutron scattering (IINS) and MD calculation. In XI phase, Raman spectra show mostly the first order scattering around the T -point but qualitative agreement with neutron and MD studies is also seen in XI phases. 

Infrared Absorption is a single-photon process. Here, also, kiR = K 0 applies. Thus, infrared absorption detects only phonons at the F point of the first BZ. In this case, we have oo = L2, where ho) is the quantum energy of the infrared radiation. The frequencies or the wavenumbers of the optical phonons in molecular crystals are of the order of 3 THz or 100 cm" thus the wavelengths of infrared absorption are of the order of 100 /xm. Infrared spectroscopy of phonons in molecular crystals is therefore in fact far-infrared spectroscopy. The symmetry selection rules are complementary to those for Raman scattering for vibrations with u and g states w g transitions are allowed and g g transitions are forbidden. 

IR absorption and Raman scattering Vibrational spectroscopy (VS) is very important in the context of HMFG, since it is one of the most powerful techniques for the study of glass structure and also because the main applications of these glasses derive from their IR transparency. The principal objective of this section is to discuss the nature of the fundamental (first-order or one-phonon) vibrational modes of representative HMFG and some of their structural implications. 

Substitution of the cation in the solid solution system Smi R Se by, e.g., Y or La reduces the lattice parameter and the 4f-5d excitation gap, without yielding the transition into the metallic intermediate-valence phase (Gronau 1979). In fig. 3 we show the polarized Raman spectra of Smj jjY Se at 80K for x = 0, 0.25, 0.50, 0.75 and 1.0, obtained by Giintherodt et al. (1981a). For the sake of completeness, Sm, jLag jSe has also been included. For the latter sample one observes below 200 cm first-order defect-induced Raman scattering from acoustic and optical phonons which is absent in pure SmSe. The / = 1 peak of Smo gjLap osSe has drastically broadened compared to that of pure SmSe at... 

Raman scattering has been performed by Stiisser et al. (1982) for the inter-mediate-valence phases of the solid solution systems Smj R S (R = Y, La, Pr, Gd, Tb, Dy, Tm 0.15cation mass has been observed in between the gap of the acoustic- and optical-phonon branches for all Sm concentrated (x < 0.5) intermediate-valence phases. The Raman spectra of Smi R S with R = Y, Pr, Gd, Dy in the upper jpart of fig. 45 show at 300 K a maximum of the scattering intensity near 200 cm . This gap mode for x <0.50 is due to first-order scattering as demonstrated by its temperature dependence (fig. 45... 

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