Raman Result Research Articles

ESR Study of the Local-Order of Chalcogenide Glasses

ESR study of as-quenched and annealed Ge-S glasses was performed at various temperatures from 292 K to 408 K. The electron spin density in Ge 10 S 90 is found to increase with temperature. At 408 K, which is just below the glass transition temperature ( T g =423 k), it reaches twice as much as that at T =292 K. The experimental facts together with a previous Raman result can be explained by a very short-chain crossing mechanism which arises from the instability of S–S bonds. In this model the ESR intensity can be regarded to reflect the medium range order of the glassy state. Observed annealing effects are also discussed.

ChemInform Abstract: RAMAN AND BRILLOUIN‐RAYLEIGH SCATTERING STUDIES OF THE DISPLACIVE PHASE TRANSITION IN MALONONITRILE AT 294.7 K

A comprehensive temperature dependent inelastic light scattering study of a malononitrile single crystal using Raman and Rayleigh–Brillouin spectroscopic techniques has been carried out, with a special emphasis on the phase transition at 294.7 K, observed by Zussman and Alexander. Optical soft modes associated with the phase transition have not been observed, despite the fact that internal vibrations involving the CH2 scissoring motion and the symmetric C–N stretching vibrations are affected by the phase transition. The librational frequencies have been assigned for the first time, using the present Raman result and the low temperature heat capacity data. Among the three librational modes, the libration about the c axis, corresponding to νL(Bg, c), shows anomalies in both the frequency and the linewidth data. The temperature dependent Rayleigh–Brillouin scattering spectra have displayed the softening of the lowest frequency transverse acoustic mode at 294.7 K. A combination of the data on the librational ...

Raman and Brillouin–Rayleigh scattering studies of the displacive phase transition in malononitrile at 294.7 K

A comprehensive temperature dependent inelastic light scattering study of a malononitrile single crystal using Raman and Rayleigh–Brillouin spectroscopic techniques has been carried out, with a special emphasis on the phase transition at 294.7 K, observed by Zussman and Alexander. Optical soft modes associated with the phase transition have not been observed, despite the fact that internal vibrations involving the CH2 scissoring motion and the symmetric C–N stretching vibrations are affected by the phase transition. The librational frequencies have been assigned for the first time, using the present Raman result and the low temperature heat capacity data. Among the three librational modes, the libration about the c axis, corresponding to νL(Bg, c), shows anomalies in both the frequency and the linewidth data. The temperature dependent Rayleigh–Brillouin scattering spectra have displayed the softening of the lowest frequency transverse acoustic mode at 294.7 K. A combination of the data on the librational mode, νL(Bg, c), with the Brillouin–Rayleigh results suggests the importance of coupling of the transverse acoustic mode to the νL(Bg, c) libration in malononitrile single crystal. Evidence for the coupling is also manifest in the line shape of the depolarized Rayleigh scattering spectra in various crystal orientations. The linewidth and intensity of the central Rayleigh peak as well as the linewidth of the longitudinal acoustic phonon were also found to be affected by the phase transition.

Valence Bond Theory of Organic Charge-Transfer Salts

Abstract A valence bond (VB) description is introduced for organic charge-transfer (CT) salts based on mixed stacks… D+γA−γD+γA−γ … of donors (D) and acceptors (A). Extrapolating exact results for finite rings shows that the magnetic gap ΔEm is finite for γ γ c as expected in the paramagnetic ionic limit γ= 1. The small finite ΔEm in the “ionic” TMPD-TCNQ complex indicates incomplete CT, in agreement with the resonant Raman result of γ∼ 0.7. The extension of diagrammatic VB techniques to partly-filled segregated ion-radical stacks like … A −γ A −γ A −γ A −γ or … D +γ D +γ D +γ D +γ… is discussed.

Laser Raman scattering of neurotoxins isolated from the venoms of sea snakes Lapemis hardwickii and Enhydrina schistosa.

The venoms of sea snakes (family: Hydrophiidae) contain potent neurotoxins which bind to the acetylcholine receptor in the neuromuscular junction. A major toxin was isolated from the venoms of the sea snakes Lapemis hardwickii and Enhydrina schistosa according to previously published methods. These pure toxins were studied by laser Raman spectroscopy to elucidate further the structure-function relationship to the neurotoxin. Spectra were obtained from the powder, aqueous solution, and deuterated derivatives of each toxin. The peptide backbone conformation of these neurotoxins was found to be of "anti-parallel beta configuration," as distinct amide I and III bands appeared at 1672 and 1240 cm-1, respectively. No indication of alpha helical structure in the neurotoxins was observed from amide I and III bands. This was further confirmed by the spectra of the neurotoxins after deuterium exchange. The peaks due to a single tyrosine residue appeared at 644, 834, and 846 cm-1. The intensity ratios of the toxin from L. hardwickii venom were 0.92, 1.0, and 0.84 at 644, 834, and 846 cm-1. It is concluded that the tyrosine residue is involved in some unusual intramolecular interactions and not readily accessible to water molecules. Similar results were obtained for the toxin of Enhydrina schistosa (common sea snake). The fact that only 50% of the tyrosine molecule is modified by nitration is in complete agreement with laser Raman result. The lack of a sharp Raman line at 1361 cm-1 suggested that the single tryptophan residue may be "exposed." The previous demonstration that the tryptophan residue can be modified readily with different reagents confirms these results. A relatively symmetrical disulfide peak at 512 cm-1 indicates that the geometry of the C-C-S-S-C-C linkage is nearly identical for all four disulfide bonds in the molecule. The absence of phenylalanine was established by the lack of a phenylalanine peak in the laser Raman spectra and by amino acid analysis.

Kohn-Luttinger Interference Effect and Location of the Conduction-Band Minima in6H SiC

Recent Raman-scattering results place the conduction-band minima of $6H \mathrm{SiC}$ on the $M\ensuremath{-}L$ symmetry line, leaving undetermined the parameter ${k}_{z}$ that is required to fix their positions exactly. This parameter could be established by electron-nuclear-double-resonance (ENDOR) measurements, as a similar parameter is for Si, but the present ENDOR results for $6H \mathrm{SiC}$ are insufficient. The Raman result determines the planar part of the donor Kohn-Luttinger interference pattern, and it is found that the highly symmetric pattern concentrates the donor-electron density on lattice sites of planes like the donor plane. Because of the Bloch portion of the wave function, this leads to binding-energy differences for the three inequivalent nitrogen donors, for each donor has a distinct set of neighboring like planes. Whether or not a like plane is favorably placed for a binding-energy enhancement depends on the axial interference factor, which is a function of ${k}_{z}$. Thus, an interpretation of the donor binding energies is used to determine ${k}_{z}$. The suggested ${k}_{z}$ is one for which $6H \mathrm{SiC}$ has six conduction-band minima.