Raman-active Vibrational Modes Research Articles

Vibrational dynamics associated with multiple elastic configurations in KI: Ag +

A systematic experimental investigation of the fcc lattice-defect system KI: Ag + shows that the thermally activated disappearance of the strengths of the IR and Raman active vibrational modes and the appearance of rf dispersion and frequency-shifted UV electronic transitions are associated with the Ag + ion moving from an on-center to an off-center configuration. The possibility that an impurity/intrinsic-localized-mode complex could dissociate to produce the rapid temperature dependence has been tested but the absence of a concentration dependence for the normalized vibrational mode strengths with temperature appears to rule out this trapping model. A jump-rotational-diffusion model can describe the observed results in terms of two elastic configurations separated from each other by a large energy barrier. The impurity jumps between the configurations with temperature dependent dwell times. The physics behind multiple elastic configurations existing at a single point defect site is still unclear.

Raman self-conversion of femtosecond laser pulses and generation of single-cycle radiation

Stimulated Raman scattering of light in the regime of laser pulse durations shorter than the period of Raman-active vibrational mode $2\ensuremath{\pi}/{\ensuremath{\Omega}}_{R}$ is investigated. Using full equations for the field and the medium polarization, we obtain an insight into the stage of frequency conversion where the slowly varying amplitude approximation is inapplicable. It is found that the laser pulse experiences a continuous downshift of its spectrum, resulting in the formation of an intense single-cycle electromagnetic pulse. The role of the competing processes is analyzed.

Low-energy Raman-active phonons of multiwalled carbon nanotubes

We performed spatially resolved measurements on multiwalled carbon nanotubes. The first-order spectra do not reveal any characteristic, fingerprint-like signal as it is known from singlewalled nanotubes. Instead, we observed at special positions on the sample Raman-active vibrational modes in the low-energy region, which, as we show, differ from those of singlewalled nanotubes and graphite. Possible explanations for the origin of these modes will be given.

Theoretical Study of Raman and Infrared Active Vibrational Modes in Highly Compressed Solid Hydrogen

The Raman and infrared active vibrational modes in solid hydrogen at mbar pressures have been studied, using first-principles methods, and assuming hcp lattices of the molecular centers and several patterns of molecular orientations. The midlying phonon mode whose frequency reaches about $1900{\mathrm{cm}}^{\ensuremath{-}1}$ at 200 GPa and the frequency difference between the in-phase and out-of-phase vibronic modes are in good agreement with the experiments, which suggests that the lattice of the solid hydrogen is hcp even beyond the phase transition at 150 GPa, and the molecules are canted from the $c$ axis.

Intra- and inter-molecular modes reflected in femtosecond time-resolved optical heterodyne-detected optical Kerr effect signal of molecules in liquids

We present a theoretical work on the femtosecond time-resolved optical heterodyne-detected optical Kerr effect (OHD-OKE) measurement of molecules in liquids to clarify the role of intra- and inter-molecular low-frequency modes. An expression for the signal of the OHD-OKE measurement is derived on the basis of the double space representation. The theory is applied to the system of neat chloroform at room temperature. A Raman active vibrational mode with a frequency of 262cm −1 is considered, together with rotational degrees of freedom or Raman active inter-molecular vibrational modes. All the vibrational modes are assumed to be harmonic. For simplicity, a free and diffusive rotor model is adopted. A quantum mechanical treatment is also used to take into account rovibrational dephasings. The reorientational relaxation is treated in the Debye-Einstein model. Effects of the rovibrational dephasing appear in the OHD-OKE signal in a subpicosecond time regime, but intra-molecular quantum beats disappear in the early time region compared with the experimental signal. The effects of the reorientational relaxation appear in a picosecond time regime. Inclusion of inter-molecular vibrational modes qualitatively explains the OHD-OKE measurement, although calculations by the present model are only approximate.

Raman scattering from small spherical particles.

Raman coupling coefficients are calculated for the acoustic vibrations of a small dielectric sphere. The mean components of the strain tensor are calculated for the symmetric and quadrupolar Raman-active vibrational modes, within a continuum approximation which considers a vibrating homogeneous sphere. The Raman coupling coefficient depends on the crystalline structure and on the microscopic scattering mechanism. For cubic Bravais lattices and for a dipole-induced dipole scattering mechanism, the coupling coefficient of the symmetric vibrations vanishes. The Raman intensity of the inner modes is found to be small with respect to that of surface modes. The scaling law, which gives the Raman coupling coefficient as a function of the particle size, has been derived. The coupling of the sphere with a surrounding elastic medium has been considered and found to cause shift and broadening of the lines. This effect can alter significantly the estimated mean value and distribution of particle sizes.

Comparative spectroscopic studies of Cs- and K-doped polyacetylene.

In this paper, we report combined NMR and resonance-Raman-scattering (RRS) results on (CH${)}_{\mathit{x}}$ doped with Cs at different dopant concentrations. With $^{133}\mathrm{Cs}$ NMR, a resonance line is shifted at \ensuremath{\approxeq}30 ppm at low concentration (\ensuremath{\approxeq}5%) attributed to a diluted phase, whereas a second line at \ensuremath{\approxeq}-300 ppm is recorded for higher concentrations, attributed to an ordered stage-1 phase. With RRS, usual results are obtained until a concentration level y=8%, with the observation of two doping-induced Raman-active vibrational modes at 1272 and 1576 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. For higher concentrations (y=9.5%), an additional doping-induced Raman mode is recorded at \ensuremath{\approxeq}1518 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. These features are in total agreement with the NMR results, provided distinct perturbations, corresponding to the two phases, are taken into account to explain Raman data. These features in the case of Cs-doped (CH${)}_{\mathit{x}}$, differ significantly from previous results obtained in highly n-doped systems like (${\mathrm{CHK}}_{\mathit{y}}$${)}_{\mathit{x}}$.

Infrared and Raman evidence for dimers and polymers in RbC60.

The infrared- and Raman-active vibrational modes of ${\mathrm{C}}_{60}$ were measured in the various structural states of ${\mathrm{RbC}}_{60}$. According to earlier studies, ${\mathrm{RbC}}_{60}$ has an fcc structure at temperatures above \ensuremath{\sim}100 \ifmmode^\circ\else\textdegree\fi{}C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled (``quenched''). We show that the spectra obtained in the polymer state are consistent with each ${\mathrm{C}}_{60}$ molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with ${\mathrm{Rb}}_{2}$(${\mathrm{C}}_{60}$${)}_{2}$, a dimerization of ${\mathrm{C}}_{60}$.

FT-Raman spectroscopy of wool—I. Preliminary studies

Abstract The FT-Raman spectrum of wool has been obtained using near-IR excitation. No significant fluorescence was observed and the spectra could be obtained routinely. No sample damage was observed for laser powers up to 400 mW. Several methods of sample presentation for both wet and dry wool were investigated and the optimum data collection conditions were determined for each. Dry dense plugs of wool fibers (∼0.16 g cm −3 ) were found to provide the best spectra. Due to improvements in band resolution and signal-to-noise ratio, several previously unobserved spectral features in the wool spectrum have become apparent. The assignment of the Raman-active vibrational modes of wool are reviewed and updated to include these features. Raman spectra obtained from chlorinated and untreated wool samples did not exhibit any significant differences. In contrast, the ATR spectra obtained from these samples exhibited significant differences in the SO stretching region.

Raman spectra of n-doped trans-polyacetylene systems: Experiments and theory.

We present Raman-scattering spectra of n-doped trans-(CH${)}_{\mathit{x}}$, trans-${(}^{13}$CH${)}_{\mathit{x}}$, and trans-(CD${)}_{\mathit{x}}$ systems. Experimental data have been obtained by using different excitation wavelengths (${\ensuremath{\lambda}}_{\mathit{L}}$) from the near uv (351.1 nm) to the near ir (1064 nm). For ${\ensuremath{\lambda}}_{\mathit{L}}$=457.9 nm, doping-induced bands are localized at \ensuremath{\approxeq}1270 and 1602 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in trans-(CH${)}_{\mathit{x}}$,\ensuremath{\approxeq}1240 and 1550 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in trans-${(}^{13}$CH${)}_{\mathit{x}}$, and 1564 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in trans-(CD${)}_{\mathit{x}}$. The peak position and the shape of these bands change slightly when ${\ensuremath{\lambda}}_{\mathit{L}}$ is moved to the red and near-ir range. Also, additional bands appear in the frequency range 1000--1120 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in trans-(CH${)}_{\mathit{x}}$ and around 880 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in trans-(CD${)}_{\mathit{x}}$. The interpretation of these features is given by using the perturbed-Green-function method. In the framework of this model, we have been able to calculate the Raman-active doping-induced vibrational modes by considering the perturbation due to the trapped charges on the lattice dynamics of the conjugated segments in all the samples studied. The calculated modes are found in excellent agreement with experimental data.

Symmetry-specific electron-phonon coupling for electronic states near the Fermi energy of metallic polyaniline: resonant Raman scattering

The resonance Raman excitation profiles of the vibrational modes of the emeraldine salt form of polyaniline protonated with camphor sulfonic acid are remarkably and selectively enhanced, indicating improved structural and electronic order. We demonstrate that the electronic states near the Fermi energy ( E F) of metallic polyaniline interact with a specific optical phonon mode. The 1598 cm −1 Raman-active vibrational mode (A g symmetry) exhibits a distinct resonance enhancement associated with the mid-infrared (mid-IR) absorption in metallic PANI. Since the mid-IR oscillator strength results from the intraband free-carrier Drude absorption, the symmetry of the electronic wavefunctions near E F matches the vibrational pattern of the 1598 cm −1 normal mode. In contrast, the same mode shows no resonance enhancement with the interband absorption at 2.6 eV, implying that the symmetry of the bond relaxation of the excited state is orthogonal to the vibrational pattern of the 1598 cm −1 mode. By contrast, the resonance Raman excitation profile of the 1626 cm −1 mode is in strong resonance with the 2.6 eV absorption, but this mode is not as strongly in resonance with the free-carrier Drude absorption.

Rb1C60: Linear Polymer Chains and Dimers

ABSTRACTThe infrared- and Raman-active vibrational modes of C60 were measured in the various structural states of Rb1C60. According to earlier studies, Rb1 C60 has an f cc structure at temperatures above ∼ 100°C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled (“quenched”). We show that the spectra obtained in the polymer state are consistent with each C60 molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with Rb2(C60)2, a dimerization of C60.

14] Raman and resonance Raman spectroscopy

Publisher Summary Because of its high content of structural information and its applicability to a wide variety of samples, Raman spectroscopy has become an important and widely used method for studying biological macromolecules. Raman spectroscopy is a form of vibrational spectroscopy in which a sample is interrogated with an intense light beam and the spectrum of Raman active vibrational modes obtained through analysis of the inelastically scattered photons. Resonance Raman spectroscopy is a specialized form of Raman spectroscopy in which the wavelength of the interrogating light beam lies under an electronic absorption band of the sample. This produces a resonance enhancement in the scattering intensity of only those bands that are coupled to the excited electronic transition. This resonance effect minimizes the interference from bands due to nonabsorbing portions of the sample, allowing one to zoom in on and selectively probe chromophoric sites. This chapter describes the Raman and resonance Raman techniques with an emphasis on their application to the study of metalloproteins. First, it discusses the underlying theoretical basis for these techniques to present the relationship with other forms of spectroscopy and to facilitate an understanding of subsequent sections on spectral interpretation. Next, the instrumentation, procedures, and methods used to obtain Raman and resonance Raman spectra are discussed. The interpretation of Raman spectra is described by considering several recent examples of studies of metalloproteins.

Raman study of vibrational properties in solid C60.

Raman scattering has been performed on pure polycrystalline ${\mathrm{C}}_{60}$ powder over a frequency range of 20--2500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The phonon Raman lines appearing only between 250 and 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ can be assigned to all the Raman-active vibrational modes (2${\mathit{A}}_{\mathit{g}}$+8${\mathit{H}}_{\mathit{g}}$) predicted for the isolated ${\mathrm{C}}_{60}$ molecules, which suggests only minor effects of intermolecular coupling on the properties of molecular vibrations. A comparison of the spectra taken at room temperature and low temperature (150 K) shows no significant change of the vibrational Raman modes after an orientational ordering transition.

Amorphization in Gd2Ti2O7 and CaZrTi2O7 irradiated with 3 MeV argon ions

Irradiation of Gd2Ti2O7 and CaZrTi2O7 with 3 MeV Ar+ ions results in the simultaneous expansion of the unit-cell volume and radiation-induced amorphization. The damage cross sections for amorphization, which were determined hy X-ray diffraction, are 0.023 nm2 and 0.014 nm2, respectively. These damage cross sections are relatively low and suggest some inherent resistance of these structures to amorphization, in agreement with previous studies. Transmission electron microscopy and electron diffraction confirmed the amorphous character of both materials at high fluences. Raman spectroscopy indicated a decrease in scattered intensity with ion fluence for the Raman-active vibrational modes of Gd2Ti2O7 and CaZrTi2O7. In the case of Gd2Ti2O7, increases in both linewidth and wavenumher with ion fluence also were observed.

Surface electric field‐induced Raman spectroscopy from adsorbate and substrate modes

AbstractRaman spectroscopy due to electric fields at surfaces, which manifests itself as surface‐enhanced Raman scattering (SERS) from adsorbate modes, and as electric field‐induced Raman scattering (EFIRS) from substrate modes, is discussed. In SERS experiments with benzene adsorbed on small Ni/SiO2 and Pt/SiO2 clusters, using exciting radiation of different wavelengths, we observed not only very strong peaks of the normally Raman active vibrational modes but also normally infrared‐active and some inactive modes. Although the frequencies of the modes are only slightly shifted with respect to liquid benzene, the relative intensites of the Raman lines are considerably changed and in addition they are dependent on the excitation wavelength. The observation of metal‐carbon vibrations indicates a strong interaction between the adsorbate and the metal atoms. Therefore, in addition to the electric field, a metal‐molecule electronic charge transfer may be responsible for the enhancement mechanism.EFIRS is based on the symmetry‐forbidden scattering from the longitudinal optical (LO) substrate phonon. It allows the detection of the static electric field in the surface region of semiconductor substrates, which originates from electronic band bending. By EFIRS the band bending can be monitored at surfaces and interfaces for coverages from submonolayer up to more than 100 monolayers, depending on the overlayer material. Here, the Sb/GaAs(110) interface is treated as an example. Considerable band bending dynamics is observed for overlayer thicknesses up to 100 monolayers. Simultaneous analysis of the overlayer structure by Raman scattering from the Sb vibration modes allows the interpretation of the band bending dynamics.

Inelastic neutron scattering spectrum of the fullerene C 60

The vibrational spectrum of C 60 has been measured by inelastic neutron scattering at 20 K. All 14 infrared and Raman active vibrational modes can be identifeid together with many optically inactive and low-energy lattice modes. The spectrum extends smoothly to ≈ 200 MeV. Fairly good agreement with existing models of the vibrational properties of C 60 is found.

Electronic properties of polyacetylene prepared by the Durham ‘photoisomer’ route

We report here the properties of polyacetylene prepared via a precursor polymer, modified from the standard Durham precursor by photoisomerization of the monomer to 3,6-bis(trifluoromethyl)pentacyclo-[6.2.0.0 2,4,O 3,6,O 5,7]dec-9-ene. This modification gives a precursor with improved thermal stability, in that conversion to polyacetylene only occurs above 60°C. The conversion is highly exothermic, but the method is suitable for the formation of thin films. There is evidence from infra-red and X-ray photoelectron spectroscopy measurements for retention of fluorine in the thermally treated polymer, and we consider that this is probably due to isomerization of some of the side groups on the precursor polymer, rather than their elimination. Polyacetylene produced in the form of thin unoriented films is highly disordered, with a coherence length interchain order determined from X-ray scattering of only 1.8 nm. The electronic and vibrational structure of these films is much affected by the disorder, with the peak in the π-π∗ optical absorption band at 2.65 eV, and correspondingly high Raman-active vibrational modes. We compare also the properties of polymers prepared with different degrees of polymerization, achieved by addition of a chain transfer agent during polymerization of the monomer to form the precursor polymer. There is some evidence from the Raman spectra that the lower molecular weight precursors produce better ordered polyacetylene.

Raman spectrum of Na 2SO 4 (phase III)

The Raman spectrum of Na 2SO 4 in its metastable low temperature phase (space group D 2 h 17) has been measured at room temperature. All but two of the 18 Raman-active vibrational modes have been identified and assigned to symmetry species. The Raman spectrum of the metastable (phase III) form of Na 2SO 4 is very different from the corresponding spectrum of the stable (phase V) form consistent with large differences in their respective orthorhombic structures. The b axis of phase V coincides with the c axis of phase III and the a axis is likely to be common to both phases.

Ab initio valence force field calculations for quartz

We have derived valence force constants for the tetrahedral SiO4 unit and the inter-tetrahedral SiOSi linkage from previous ab initio molecular orbital calculations on H4SiO4 and H6Si2O7 using a split-valence polarized Gaussian basis set (6-31G*), and used these to calculate the infrared and Raman active vibrational modes of α-quartz. The calculation gives frequencies approximately 15% greater than experiment, as expected from harmonic force constants obtained at this level of Hartree-Fock theory, but the calculation gives the correct distribution of modes within each frequency range. Calculated 28–30 Si and 16–18 O isotope shifts and pressure shifts to 6 GPa are also in reasonable agreement with experiment. We have also used our ab initio force field to calculate the vibrational spectrum for β-quartz. The results suggest either that inclusion of a torsional force constant is important for determining the stability of this high temperature polymorph, or that the β-quartz has a disordered structure with lower symmetry (P62) domains, as suggested by earlier diffraction studies.