Raman Scattering Technique Research Articles

The influence of molecular weight on the Resonant Raman Scattering of polyacetylene

For the first time, polyacetylene, (CH) x, films of known number average molecular weights, M n, have been examined using Resonance Raman Scattering techniques. (CH) x samples of 500, 5300, 10500, and 100,000 Daltons were synthesized using the classical titanium tetra-n-butoxide/triethyl aluminum catalyst/cocatalyst system. After thermal isomerization, modeling of the RRS spectra using the Brivio, Mulazzi model indicate that the 100,000 Dalton polymer is composed principally of long trans conjugated segments. In contrast, polymers of 10500 M n and below are seen to contain significantly larger fractions of short trans conjugated segments. For samples below 10500, no clear relationship between actual polymer molecular weight and G, the ratio of long to short segments, is observed. These results suggest that not until very long chains are obtained can samples containing a large fraction of long conjugated segments be obtained. Ambient, short term oxidation of 100,000 M n polymer shows an increase in satellite band intensities at ω c−c and ω c=c, where ω is frequency, which corresponds to a decrease in G. Low level chain oxidation or doping is shown to preferentially occur within long conjugated segments.

Time- and Space-Resolved Study of a Dressed Polariton: The Polariton Fermi Resonance in Ammonium Chloride

Using a time- and space-resolved coherent anti-Stokes Raman-scattering technique we study the dephasing of a polariton state that crosses and strongly couples with a two-phonon band. Polariton damping is found to be dominated by a direct and phonon-assisted disintegration of the polariton into the two-phonon continuum.

Time-resolved Raman scattering in GaAs quantum wells.

We employ a picosecond--time-resolved Raman-scattering technique to investigate the dynamics of two-dimensional electron-hole plasma confined to a multiple--quantum-well structure composed of layers of GaAs and (GaAl)As. The lifetime and the intersubband scattering time of the electrons photoexcited on the lowest electronic subbands are determined separately. Calculation of the rate of intersubband scattering by longitudinal-acoustical phonons successfully accounts for the observation of relatively long-lived electrons on the second subband of a 215-A\r{} multiple--quantum-well structure.

Observation of increase in Raman intensity of surface phonon polaritons on rough surfaces of ZnTe

An increase in Raman intensity of surface phonon polaritons has been observed in free-standing thin slabs (thickness ∼ 7 μm) of single- crystal ZnTe with rough surfaces by a conventional Raman-scattering technique at room temperature. The samples with various root-mean- square roughness heights have been prepared by making use of final polishing powders with different mean-grit sizes, and the surface roughness has been confirmed by measuring the intensities of the diffuse scattering of the laser light. The increase of the Raman intensity is proportional to the mean-square height of the surface roughness, and the shift of the dispersion relation is also proportional to it. The frequency shift is explained by a perturbation theory of the surface-roughness-induced scattering.

Raman scattering from the upper and lower modes of surface phonon polaritons in ZnTe

The upper and lower modes of surface phonon polaritons have been observed in free-standing thin slabs (∼7 μm) of single-crystal ZnTe by a conventional Raman-scattering technique at room temperature. The experimental dispersion relations were measured in various scattering angles (outside the sample) from 0.9° to 2.0° and agree with the theoretical dispersion relations. The main factors enabling us to observe the two surface modes are the appropriate selections of the material with a small attenuation factor for surface phonon polaritons and of the thickness of the sample slabs.

High-purity ZnSe obtained by metalorganic chemical vapor deposition epitaxy

High-quality homoepitaxial layers of ZnSe have been obtained by the metalorganic method. Optical measurements demonstrate the good purity of these samples. Resonant electronic Raman-scattering experiments have allowed the identification of some residual donors and shown that most of them originate in the substrate. A shallow donor, with an ionization energy of about 30.4 meV, not yet mentioned in the literature, has been observed and is believed to be iodine. Excitation Spectroscopy and Selective Pair Luminescence techniques have shown the existence of a new shallow acceptor associated to a donor-acceptor pair transitions band peaking at 2.75 eV, the ionization energy of which would be about 56 meV. The resonant electronic Raman-scattering technique, used here for the first time to investigate donors, appears much more sensitive than classical luminescence to characterize the semiconductor materials.

Effects of uniaxial stress on excitons inCu2O

We have investigated the effects of uniaxial stress up to 2.5 kbar on the first four dipole-forbidden states of the yellow exciton series in cuprous oxide. The linear and nonlinear splittings and shifts as well as the symmetries of the stress-split components were determined using the quadrupole-dipole Raman-scattering technique involving odd-parity phonons. We have observed the triply degenerate $1S$ quadrupole state split into a singlet and a doublet with the singlet energy increasing and the doublet energy decreasing with stress as first seen by Gross and Kaplyanskii. For the higher quadrupole $S$ states, however, the sense of the splitting was reversed from that of the $1S$, while the magnitude of the $3S$ splitting exceeded that of the $1S$ as previously noted by Agekyan and Stepanov. The quadratic stress coefficients for all components of the $1S$, $3D$, and $4S$ states were found to be equal while those for the $3S$ state were markedly different. A theoretical analysis was carried out for [001] stress using the effective-Hamiltonian formalism including both deformation of the bands and electron-hole exchange. The theory accounts satisfactorily for the general features of the $1S$, $3D$, and $4S$ states only if both the magnitude and sign of the exchange constant are considered to be dependent on the exciton state. For the $3S$, however, the nonlinear stress dependence cannot be reconciled with the theory for yellow excitons. These results suggest that the "$3S$ yellow exciton" may actually belong to the green-exciton series, or that interactions between yellow and green excitons with different principal quantum numbers may be significant although not included in the theory.

Raman observation of the ferroelectric phase transition in SnTe

The ferroelectric phase transition in SnTe from a cubic to a rhombohedral structure at liquid-helium temperatures is investigated by Raman-scattering techniques. Using a backscattering geometry, Raman peaks are observed which correspond to $q\ensuremath{\simeq}0$ longitudinal-optical (LO) phonons in bulk $p=2.2\ifmmode\times\else\texttimes\fi{}{10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ SnTe single crystals at 4\ifmmode^\circ\else\textdegree\fi{}K, as well as in $p=1.5\ifmmode\times\else\texttimes\fi{}{10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ SnTe single crystal at liquid-nitrogen temperatures. The dependence of the phase transition and the scattering spectrum on the carrier density is discussed.

Raman Scattering from a Nematic Liquid Crystal: Orientational Statistics

Raman-scattering techniques have been used to obtain a new quantitative measure of orientational statistics of individual molecules in a nematic liquid crystal. A marked discrepancy is observed between these measurements and the predictions of existing theories of nematic ordering.

Comparison of protein structure in crystals and in solution by laser Raman scattering: I. Lysozyme

The laser Raman-scattering technique was employed to examine the question of whether the structure of a globular protein is the same in crystals as in solution. Lysozyme was selected as a model system for this study. In the amide I and amide III regions we found a good agreement between the Raman spectra of lysozyme chloride crystals (in 100% relative humidity) and lysozyme solution (at pH 4.50), indicating that the main-chain conformation is the same between two phases. However, small but definite spectral differences were observed near 464, 622, 644, 934, 960, 978, 1032, 1129, and 1196 cm −1. Some of these spectral differences may be interpreted in terms of side-chain conformational changes. Additionally, we present Raman spectrum of lysozyme in the lyophilized form and compare it to those of crystals and solution. It was concluded that lyophilization caused conformational changes appreciably, both in the main chain and side chain.

Raman Scattering Techniques Applied to Problems in Solid State Physics

A discussion of the application of Raman scattering to solid state problems is followed by a description of the experimental technique. A recent experiment is described in which the thickness of the domain wall in the strongly coupled ferroelectric-ferroelastic material gadolinium molybdate is found to be within the limits 0.8-3 micro. The Raman scattering from part of the wall volume is characteristic of the high temperature paraelectric phase.