Polarisation studies on the Raman spectra of cubic crystals

Polarisation characters of the Raman lines of calcium fluoride (fluorspar) and potassium aluminium sulphate (alum) were investigated under the following conditions. Unpolarised light was incident normally on a face of the crystal making an angle 22.5° with a cubic face and the light scattered transversely along a cubic axis was analysed by a double image prism kept with its principal axes inclined at 45° to the vertical. Under these conditions the depolarisation factors of the Raman lines belonging to the totally symmetric (A), the doubly degenerate (E) and the triply degenerate (F) modes should be respectively =1, >1 and <1. The characteristic Raman line of CaF₂ at 322 cm^?1 exhibited a depolarisation value less than 1, showing thereby that the corresponding mode is a triply degenerate one (F). The Raman lines observed in the spectrum of K-alum were also classified and the results were compared with those given by previous investigators using standard crystal orientations.     

Evidence for intra-ionic anharmonicity in the Raman spectra of cubic ionic crystals

The importance of intra-ionic anharmonicity for Raman spectra in cubic ionic crystals is discussed. A calculation of the second order Raman spectra of of MgO assuming such an anharmonicity is presented and a satisfactory agreement with experiment is obtained.     

Polarisation of the IR luminescence of cubic ZnS single crystals

The polarization of the IR luminescence of oriented ZnS crystals has been measured. None of the three main emission bands shows measurable polarization after excitation with polarized or unpolarized light. This result supports centers with symmetryT _d because they do not require any selection rules for the polarization of optical dipole transitions.     

High-pressure Raman spectra of L-isoleucine crystals

Raman spectroscopy investigations of L-isoleucine crystals under high pressures have been carried out up to 7.3 GPa. From this study it was possible to observe modifications on bands associated to both rocking vibrations of r(NH₃⁺) and r(CO₂⁻) as well as to lattice modes at about 2.3 and 5.0 GPa. These modifications were correlated to either conformational change of molecules or to a solid–solid phase transition undergone by the crystals involving the hydrogen bonds that maintain the molecules held in the unit cell. A comparison with a few results on other amino acid crystals is also given.     

Scattering matrix for Raman effect in cubic crystals

The scattering matrix for Raman effect in cubic crystals for any general orientation of the crystal and any angle of scattering is given. The averages of the matrix over all possible orientations is also given and checks with the well known value of 6/7 for the depolarisation of degenerate lines for transverse scattering. The scattering matrix for backward scattering in terms of polar angles and for some special cases are also given.     

Second-order Raman spectra of CsCl single crystals

The second-order Raman spectra of CsCl single crystals have been measured at room temperature for different Raman polarizations and crystal orientations. With the help of the calculated selection rules for a CsCl-type lattice, observed nineteen peaks and shoulders were assigned by the critical point analysis based on the neutron scattering data. By comparing the observed Raman intensities with the thermally weighted two-phonon density-of-states calculated by using a simple shell model, it has turned out that though the measured spectra are dominantly determined by the two-phonon density-of-states, the effect of the q-dependence of the polarizability cannot entirely be neglected.     

Raman spectra of α-GaTe single crystals

Raman-active lattice vibrational modes of GaTe have been investigated at 300°K in the frequency range 13–300 cm^-1. The spectra of the Bridgman grown crystals were excited with the 1.06 μm line of the continuously operated YAG : Nd³⁺ laser. Fourteen Raman bands were observed. All phonon modes have been assigned on the basis of their observed polarisation dependence to an irreducible representation of the appropriate point group. The results are consistent with the existence of GaTe in monoclinic α-polytype (C³_2h).     

Raman spectra of lithium hydroxide single crystals

First order Raman frequencies, linewidths, intensities and assignments to the symmetry species of all modes predicted by group theory at 20°C and at 78 K are reported for the first time. Assignments of the fundamental frequencies to stretching vibrations or librations of the OH^? ions, internal vibrations of Li⁺ ions of the same layer or translational modes of the Li⁺ and OH^? layers are derived from factor group analysis and correlation methods. They correspond to the constructions of unit cell modes by Wickersheim[2].     

Two-phonon Raman spectra of LiH and LiD crystals

The experimental two-phonon Raman spectra of LiD and LiH are reported here. A deformation dipole model formulated elsewhere for LiD is used to compute the lattice dynamics of LiD and LiH required in the Raman intensity calculations. The use of the same model for LiH is justified by the good agreement between the peak positions of the temperature-weighted two-phonon density of states and the various experimental spectra. The Raman intensity calculations are carried out by treating the second-order expansion coefficients in the polarisability tensor as adjustable parameters. The need for the parameters associated with the next-nearest-neighbor ions is clearly demonstrated for all the spectra. A single set of six parameters for the T₂₈ spectra and thirteen parameters for the E₈ (or A₁₈) spectra is found to explain all the experimental spectra of LiD and LiH quite well. Since the polarisability of Li⁺ is very small, the need for the second-neighbor positive-positive parameters reflects on the extended and highly polarisable nature of the H^? or D^? ion.     

Raman spectra of photorefractive lithium niobate single crystals

We have conducted comparative studies of the Raman spectra of lithium niobate (LiNbO₃) crystals of different compositions for excitation in the visible and near IR regions. We have observed that the photorefractive effect is one of the factors leading to line broadening. For this reason, the linewidths may be greater upon Raman excitation in the visible region than for excitation in the near IR region. This may be explained by formation in the crystal, when illuminated by laser radiation in the visible region, of a three-dimensional sublattice of nanostructures and microstructures (with refractive index and other physical parameters different from the parameters of the host crystal) from which photorefractive light scattering occurs. Formation of nanostructures and microstructures makes an additional contribution (besides the contribution due to random and dynamic disorder in the arrangement of the structural units) to the broadening of the Raman lines in the visible region of the spectrum. Illumination of the crystal by radiation in the near IR region does not induce a sublattice of nanostructures and microstructures, due to a significantly smaller photorefractive effect.     

Clebsch-Gordan coefficients and two-phonon resonance Raman scattering in cubic crystals

The intensity of two-phonon resonance Raman scattering in cubic crystals is expressed in terms of Clebsch-Gordan coefficients. A plot of the scattering intensity against the direction angles of the polarization vectors of the incident and scattered radiation is obtained. Several useful polarization relations are obtained for the scattering intensities in experiments with different geometries.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 75–80, August, 1978.     

Raman spectra and phase composition of MnGeO3 crystals

MnGeO₃ single‐crystal samples have been synthesized by optical zonal melting and spontaneous crystallization. X‐ray crystal analysis showed the first sample to be a two‐phase one with phase ratio as follows: 17% – monoclinic C2/c, and 83% – orthorhombic Pbca; the phase ratio of the second sample was unknown. Raman spectra have been produced for these samples. Lattice dynamics has been simulated and polarization dependencies of lines' intensities have been analyzed to interpret experimental Raman spectra and to attribute lines to the spectra of monoclinic and orthorhombic phases. Copyright © 2015 John Wiley & Sons, Ltd.