Low-temperature Raman Spectra Research Articles

Kristallstrukturen, Schwingungsspektren und Normalkoordinatenanalyse von cis-(n-Bu4N)2[ReBr4(NCS)(SCN)], trans-(n-Bu4N)2[ReBr4(NCS)(SCN)] und trans-(n-Bu4N)2[ReBr4(NCSe)(SeCN)

The crystal structures of cis-(n-Bu4N)2[ReBr4(NCS)(SCN)] (1) (monoclinic, space group P21/n, a = 11.203(3), b = 11.738(5), c = 35.218(7) Å, β = 93.434(4)°, Z = 4), trans-(n-Bu4N)2[ReBr4(NCS)(SCN)] (2) (monoclinic, space group P21/n, a = 11.644(7), b = 13.695(3), c = 29.028(8) Å, β = 95.96(4)° Z = 4) and trans-(n-Bu4N)2[ReBr4(NCSe)(SeCN)] (3) (monoclinic, space group P21/n, a = 11.894(2), b = 13.737(2), c = 28.869(7) Å, β = 96.98(10)° Z = 4) have been determined by single crystal X-ray diffraction analysis. Based on these molecular parameters the low temperature (10 K) IR and Raman spectra of the (n-Bu4N) salts have been assigned by normal coordinate analysis. The valence force constants are fd(ReN) = 1.70 (1), 1.75 (2) and 1.75 (3), fd(ReS) = 1.32 (1) and 1.37 (2) and fd(ReSe) = 1.20 mdyn/Å (3).

Kristallstrukturen und Normalkoordinatenanalyse von trans-(n-Bu4N)2 [ReCl4IX], X = NCS, NCSe / Crystal Structures and Normal Coordinate Analyses of trans-(n-Bu4 N)2[ReCl4 IX], X = NCS, NCSe

The crystal structures of trans-(n-Bu4N)2[ReCl4I(NCS)] (triclinic, space group P1, a = 11.268(3), b = 11.696(2), c = 18.109(3) A, a = 98.68(2) β = 106.40(2), γ= 97.58(2)°, Z = 2)and trans-(n-Bu4N)2 [ReCl4I(NCSe)] (orthorhombic, space group P212121 a = 11.839(2), b = 12.2679( 10), c = 31.136(5) A, Z = 4) have been determined by single crystal X-ray diffraction analysis. Based on the molecular parameters of the X-ray determinations the low temperature (10 K) IR and Raman spectra o f the (n-Bu4N) salts have been assigned by normal coordinate analysis. The valence force constants are fd(ReN) = 1.69(NCS) and 1.65(NCSe) mdyn/Å

Low temperature Raman study of dimethylacetylene

On the basis of low temperature Raman spectra from 10 to 170K of polycrystalline dimethylacetylene (DMA), two stable and two metastable crystalline phases were detected. One can be identified with a previously reported metastable phase [N. Prasad, R. Kopelman, Chem. Phys. Lett. 20 (1973) 513–516] and can be observed from 10 to 75K, when it transforms into another metastable phase. This one, in turn, transforms at 115K into the stable C2/m phase. To enable better assignment of the observed internal modes, especially those involving methyl group, matrix isolation of DMA at 10K was performed. In the recorded Raman spectra at matrix ratios of 1:100, 1:50 and 1:25, the strongest bands observed are of a1s symmetry. Besides them, we were able to detect bands at 218 and 1040cm−1 of e1d symmetry, which are only infrared active when no intermolecular interactions are present. The Raman spectrum of DMA as a thin solid film was also recorded. The bands’ positions are found not to deviate significantly in comparison with the spectra of the matrix isolated sample, and thanks to the better signal to noise ratio, several additional bands were detected. Among those is the ν8 at 1147cm−1 of a4s symmetry.

Kristallstrukturen, Schwingungsspektren und Normalkoordinatenanalyse von cis- und trans-[ReCl4 (NCSe)(SeCN)]2- / Crystal Structures, Vibrational Spectra, and Normal Coordinate Analysis of cis- and trans-[ReCl4 (NCSe)(SeCN)]2-

The crystal structures of cis-(n-Bu4N)2[ReCl4(NCSe)(SeCN)] (monoclinic, space group P21/n a = 10.794(3), b = 11.687(3), c = 35.716(4) Å, β = 96.97(1)°, Z = 4) and trans- (CH2Py2)[ReCl4(NCSe)(SeCN)] (monoclinic, space group P21/c, a = 8.348(2), b = 7.518(1), c - 31.295(7) Å, ß = 97.12(2)°, Z = 4) have been determined by single crystal X-ray diffraction analysis. Based on the molecular parameters of the X-ray determinations the low temperature (10 K) IR and Raman spectra of the (n-Bu4N) salts have been assigned by normal coordinate analysis. The valence force constants are fd(ReN) = 1.68 and fd(ReSe) =1.15 mdyn/Å.

Halogenated Carboxonium Salts: Preparation and Structural Analysis.

We have studied the preparation and isolation of the halogenated alkylidene oxonium salts CH(3)OCHX(+)MF(6)(-) (X = Cl, F; M = As, Sb). Addition of dichloromethylmethylether at -78 degrees C to the superacidic media HF/MF(5) resulted in the formation of the chlorine compounds CH(3)OCHCl(+)MF(6)(-), whereas at -65 degrees C the fluorinated salts CH(3)OCHF(+)MF(6)(-) are formed by a chlorine/fluorine exchange. The salts are characterized by their low-temperature IR and Raman spectra. Additionally, the crystal structures of the hexafluoroantimonates are presented. The structures of the cations turned out to be different in some points. The salt CH(3)OCHF(+)SbF(6)(-) crystallizes in the monoclinic space group P2(1)/c with 4 molecules in the unit cell and a = 9.942(2), b = 7.454(2), and c = 10.297(3) Å; beta = 111.27(2) degrees. The salt CH(3)OCHCl(+)SbF(6)(-) crystallizes in the monoclinic space group P2(1)/c with 4 formula in the unit cell and a = 5.970(3), b = 12.019(5), and c = 10.994(5) Å; beta = 92.59(3) degrees.

Disorder-induced phonon modes, built-in electric fields and structural properties of CdTe/GaAs heterostructures grown by MBE

We present the results of an optical characterization of CdTe/GaAs(100) heterostructures grown by molecular beam epitaxy prepared following different protocols for substrate preparation: (i) As-decapping and, (ii) cycles of annealing and sputtering. Their structural, vibrational and electronic properties have been studied by experiments of Raman and photoreflectance spectroscopies. The line shape of the low temperature Raman spectra presented the transverse and longitudinal optic modes as well as a broad feature around 162 . This feature has been interpreted in terms of disorder-induced phonons corresponding to maxima in the one phonon density of states of CdTe. We did not find any evidence of phonons from an interfacial GaTe layer or Te oxide compounds. The value of the CdTe band gap energy was obtained from fittings of the PR spectra around the critical point and also from the corresponding analysis of the Franz-Keldysh oscillations present in the spectra. This procedure allowed us to determine the magnitude of the electric field at both the CdTe/GaAs interface and CdTe surface.

Light scattering from electronic and magnetic excitations in

We present the Raman scattering spectra of single crystal. In the high-temperature phase, besides phonon modes, there are three broad continuum-like features observed at energies centred at 160, 603 and which are described as an indication of d-d electronic transitions of V ions. Due to the magnetic ordering at T < 34 K, appearing as a consequence of charge ordering, these paramagnetic bands split into several excitons with energies of 65, 106, 200, 2000 and . The assigning of the lowest-energy mode at as the one-magnon mode, as well as the onset of the magnetic continuum at , is done on the basis of Monte Carlo calculation of excited states of a Heisenberg dimerized Hamiltonian without frustration. A good agreement between the measured and calculated magnetic excitation energies for and J = 455 K has been found. Other lines in the low-temperature Raman spectra represent phonons becoming Raman active due to the Brillouin-zone-folding effect.

Characterisation of LaGdO 3 by X-ray powder diffraction and raman spectroscopy

LaGdO 3 is prepared via a reverse-strike coprecipitation method and sintered at temperatures of 1350–1450°C for 6 h. Raman spectroscopy of the sintered materials revealed profiles similar to that observed for pure B-type Gd 2O 3. Low temperature (−196°C) Raman spectra showed a total of 18 bands of a possible 21 allowed modes predicted for the monoclinic B-type mixed rare earth oxide with C2/ m space group. These bands were assigned to modes in comparison to the band assignments for Gd 2O 3 as follows; 71 ( B g ), 82 ( A g ), 98 ( B g ), 109 ( A g ), 124 ( A g ), 158 ( A g ), 182 ( A g ), 219 ( A g ), 242 ( A g ), 279 ( A g ), 315 ( B g ), 366 ( A g ), 390 ( B g ), 419 [ B g +( A g ?)], 436 ( A g ), 458 ( A g ), 560 ( A g ) and 575 ( A g ) cm −1. Powder X-ray diffraction patterns revealed a monoclinic pattern for materials sintered at all three temperatures; 1350, 1400 and 1450°C.

Kristallstrukturen, Schwingungsspektren und Normalkoordinatenanalyse von cis-[ReCl4X2]2-, X = NCS, NCSe / Crystal Structures, Vibrational Spectra and Normal Coordinate Analysis of cis-[ReCl4X2]2-, X = NCS, NCSe

The crystal structures of cis-(n-Bu4N)2[ReCl4(NCS)2] (triclinic, space group P1̅, a = 11,245( 1), b = 20.174(3), c = 21.320(8) Å, a =109.06(2), β = 96.46(2), γ = 98.22(5)°, Z = 4) and cis-(Ph4P)2[ReCl4(NCSe)2]·2CH2Cl2 (triclinic, space group P1̅, a = 10.341(2), b = 13.436(3), c = 19.616(4) Å, α = 92.70(2), β = 92.02(2), γ = 89.99( 1)°, Z= 2) have been determined by single crystal X-ray diffraction analysis. Both ambidentate ligands NCS and NCSe are bonded via the N atom. Using the molecular parameters of the X-ray determinations the low temperature (10 K) IR and Raman spectra of the (n-Bu4N) salts have been assigned by normal coordinate analysis based on a modified valence force field. The valence force constants fd(ReN) are 1,78(NCS) and 1,79(NCSe) mdyn/ Å.

Raman scattering and photoluminescence properties of MBE-grown GaN on sapphire

We have comprehensively investigated low temperature Raman and photoluminescence (PL) spectra of gas source MBE-grown GaN layers on sapphire. Strong and sharp Raman peaks are observed in the low frequency region which are enhanced by resonant excitation of yellow luminescence transitions. They are attributed to the electronic transitions related to shallow donor levels in hexagonal GaN. It is proposed that a low frequency Raman peak at 94 cm−1 may be caused by pseudo-local vibration modes related to defects involved in yellow luminescence transitions. The PL spectra are measured in dependence on temperature and excitation intensity. The correlation with residual impurities in GaN is discussed.

Defect-induced Raman scattering in resonance with yellow luminescence transitions in hexagonal GaN on a sapphire substrate

Strong defect-specific low-frequency peaks are detected in low-temperature Raman spectra of hexagonal GaN grown by molecular beam epitaxy on sapphire substrate. The intensity of these peaks is found to be enhanced by excitation in resonance with yellow luminescence transitions. The validity of the assignment to electronic Raman scattering (ERS), as proposed before for their counterparts in cubic GaN on GaAs [M. Ramsteiner, J. Menniger, O. Brandt, H. Yang, and K. H. Ploog, Appl. Phys. Lett. 69, 1276 (1996)], is confirmed. Our results imply that the observed ERS peaks are related to shallow donors which are not necessarily hydrogenic. One Raman peak at very low frequency (11.7 meV) is alternatively explained by a pseudo-localized vibrational mode.

Schwingungsspektren und Normalkoordinatenanalyse von Hexathiocyanatoplatinat(IV), Kristallstruktur von [Py2CH2][Pt(SCN)6] / Vibrational Spectra and Normal Coordinate Analysis of Hexathiocyanatoplatinate(IV), Crystal Structure of [Py2CH2][Pt(SCN)6

By treatment of [PtCl6]2- with an excess of SCN– in aqueous solution [Pt(SCN)6]2- is formed. The X-ray structure determination on a single crystal of [Py2CH2][Pt(SCN)6] (orthor-hombic, space group Pna21, a = 15.5084(9), b = 11.8444(11), c = 13.2830(9)Å, Z = 4) shows, that the thiocyanate groups are exclusively S-coordinated with average Pt-S distances of 2.378 Å and Pt-S-C angles in the range from 103.1° to 108.1°. Based on the molecular parameters of the X-ray determination the low temperature (10 K) IR and Raman spectra have been assigned by normal coordinate analysis. The valence force constant is fd(PtS) = 1.62 mdyn/Å.

Growth of (110)-oriented epitaxial RBa 2Cu 3O 7− δ thin films and characterization with Raman spectroscopy

High-quality superconducting (110) and (103)/(013)-oriented SmBa 2Cu 3O 7−δ thin films, with T c ≈ 90 K, have been grown on SrTiO 3 (110) substrates using pulsed laser deposition. The (110) orientatioon was obtained using a PrBa 2Cu 3O 7−σ buffer layer. The structure of the films has been investigated with atomic force microscopy (AFM) and Raman spectroscopy. AFM revealed a granular stripe-like surface morphology. The orientation of the films was determined from Raman measurements by comparing the intensity of the O(4) A g mode, the lineshape of the Ba A g mode and the intensity of the O(2)−O(3) out of phase A g mode taken in different polarization geometries. Faint B 3 g , B 3 g and oxygen defect modes of the SmBa 2Cu 3O 7−σ system were also resolved in the low temperature Raman spectra of the (110)-oriented film. A method for assessing the phase of the complex Raman tensor components of the A 1 g -like modes has been devised. It is illustrated for the case of the 114 cm −1 Ba A 1 g phonon: at 514.5 nm laser energy a phase difference of (2 ± 0.2) rad is found between the xx (yy) and zz Raman tensor components.

Lattice dynamics of fluoride scheelites: I. Raman and infrared study of and ( and Yb)

IR and Raman active modes have been measured in several fluoride scheelites, namely the laser matrix (studied as a reference) and four compounds (Ln = Ho, Er, Tm and Yb). The observed phonons have been assigned in accordance with the various irreducible representations of the point group of the crystal. In three modes never observed before have been measured. The polarized experimental IR reflection spectra have been analysed through a `four-parameters' model to give the mode frequencies and dampings. For the whole set of the compounds, the frequency dependence of the dielectric function, optical indices and absorption coefficient have been deduced from the IR study. The electronic Raman effect has been shown in the low-temperature Raman spectra for Ln= Tm, Ho and Yb. The corresponding measured wavenumbers are compared with the calculated energies between crystal-field levels of L SJ states, each being associated to an irreducible representation of the ion-site group .

Characterization of ferrous FixL-nitric oxide adducts by resonance Raman spectroscopy.

Resonance Raman spectra of the nitric oxide adducts of the ferrous forms of two soluble truncations of Rhizobium meliloti FixL, FixL* and FixLN, are reported. At room temperature, four isotope sensitive vibrations are observed for both ferrous FixL*-NO and ferrous FixLN-NO. For FixL*-NO, they are observed at 558, 525, 450, and 1675 cm(-1) and are assigned to v(Fe-NO) of a six-coordinate nitrosyl adduct, v(Fe-NO) of a five-coordinate nitrosyl adduct, delta(Fe-NO) of a six-coordinate nitrosyl adduct, and v(N-O) of a five-coordinate nitrosyl adduct, respectively. Similar frequencies are observed for the FixLN-NO isotope sensitive bands. On the basis of the frequencies and spectral separation of the v(Fe-NO) and delta(Fe-NO) modes, the Fe-N-O unit is concluded to have a bent geometry similar to those observed for the nitrosyl adducts of ferrous hemoglobin and myoglobin. Both proteins can be converted to predominantly five-coordinate nitrosyl adducts by lowering the temperature. In low-temperature resonance Raman spectra of FixL*-NO and FixLN-NO, the 558 cm(-1) bands are significantly decreased in intensity and v(Fe-NO)5-c (the Fe-NO stretching vibration for the five-coordinate nitrosyl adduct) is observed at 529 and 526 cm(-1), respectively. Analysis of the v3 and v8 vibrations for these nitrosyl adducts also supports the presence of both five- and six-coordinate nitrosyl adducts of FixL* and FixLN at room temperature and the conversion to predominantly five-coordinate nitrosyl adducts at low temperatures. This temperature-dependent interconversion is reversible. The possible physiological relevance of the thermally accessible five-coordinate state is discussed. The width of v(Fe-NO)6-c at half-height is 1.3 times broader in FixLN-NO than in FixL*-NO, suggesting that the Fe-N-O geometry is more homogeneous in FixL*-NO. In low-temperature spectra of FixLN-NO, a second v(N-O)5-c band is observed, indicating that more than one conformation is attainable in the five-coordinate FixLN-NO. This second v(N-O)5-c is not observed for five-coordinate FixL*-NO, further suggesting a more conformationally restricted nitrosyl heme in FixL*. These variations in the vibrations involving the Fe-NO unit indicate that the kinase domain influences the heme structure. The spectral differences are discussed in terms of the interdomain interactions that result in ligation-dependent mediation of the kinase activity.

Low temperature raman spectra on tdae-c 60 single crystals

Low-temperature single crystal Raman spectra are presented for the organic ferromagnetic insulator TDAE-C 60. The mode intensities, lineshifts, linewidths and depolarization ratios are compared with alkali-doped fullerene materials and with pure C 60. A comparison of visible laser light excited spectra (E exc. = 2.41 eV) with infrared excited Raman measurements (E exc. = 1.17 eV) shows a selective resonance effect whereby in the visible, a frequency shift of only t he tangential modes is observed, while in the infraredwhich is resonant with the t m1u → t 1g electronic transition —also the radial modes are observed to shift. The effect is attributed to Raman resonance with different relaxed states of C 60 anion.

Raman and Infrared Spectra of Lithium Formate Monohydrate LiHCOO · H 2 O

Lithium formate monohydrate (FLMH) has been studied by Raman spectroscopy and infrared transmittance. The Raman spectra have been measured in the temperature range 10 to 490 K. The low temperature Raman spectra showed a temperature dependence of the intensities of the modes observed in the Z(XX)Y polarisation with a minimum at 220 K. The high temperature Raman spectra revealed an order-disorder phase transition related to the water molecules.

Raman spectra on TDAE-C60single crystals

Low-temperature single-crystal Raman spectra are presented for the organic ferromagnetic insulator TDAE-${\mathrm{C}}_{60}$. The mode intensities, line shifts, linewidths, and depolarization ratios are compared with alkali-doped fullerene materials and with pure ${\mathrm{C}}_{60}$. A comparison of visible laser light excited spectra (${\mathrm{E}}_{\mathrm{exc}}$=2.41 eV) with infrared excited Raman measurements (${\mathrm{E}}_{\mathrm{exc}}$=1.17 eV) shows a selective resonance effect whereby in the visible spectra, a frequency shift of only the tangential modes is observed, while in the infrared measurement, which is resonant with the ${\mathrm{t}}_{1\mathrm{u}}$-${\mathrm{t}}_{1\mathrm{g}}$ electronic transition, the radial modes are also observed to shift. The effect is attributed to Raman resonance with different relaxed states of ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$.

Pressure-Temperature Phase Diagram of the Spin-Peierls CompoundCuGeO3

Using Raman scattering we have mapped out the phase diagram of ${\mathrm{CuGeO}}_{3}$ at temperatures from 3 to 300 K and pressures up to 11 GPa. At 300 K and above 6 GPa we identify three high-pressure phases. Details of the phase transition behavior including reversibility depend on the shear strength of the pressure medium. The present results do not support the recent claim that the structural behavior is affected by intercalation. Low temperature (LT) Raman spectra show that the spin-Peierls transition temperature (14.3 K at $P\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$) increases to 25 K at 3 GPa. Above this pressure a new LT phase is observed. The phase stable above 6.6 GPa (hydrostatic conditions) exhibits, upon cooling, an additional Raman feature near $46{\mathrm{cm}}^{\ensuremath{-}1}$, which we tentatively assign to a singlet-triplet spin excitation.

Single-Crystal Raman Spectroscopy of the Rubidium Alums RbM(III)(SO(4))(2).12H(2)O (M(III) = Al, Ga, In, Ti, V, Cr, Fe) between 275 and 1200 cm(-)(1): Correlation between the Electronic Structure of the Tervalent Cation and Structural Abnormalities.

Low-temperature single-crystal Raman spectra for RbM(III)(SO(4))(2).12H(2)O (M(III) = Al, Ga, In, Ti, V, Cr, Fe) and RbM(III)(SO(4))(2).12D(2)O (M(III) = Al, V) have been collected and assigned in the range 275-1200 cm(-)(1). These results permit classification of the Ti and V rubidium sulfate alums to the beta modification, whereas the remaining tervalent cations give the expected alpha modification. The dimorphism of the rubidium sulfate alums is explained in terms of the electronic structure of the tervalent cation, where the observation of the beta modification is associated with unequal occupancy of the t(2g) (O(h)()) orbitals. For the rubidium vanadium alums the (3)E(g) <-- (3)A(g) electronic Raman (eR) transition permits quantification of the trigonal field splitting of the t(2g) (O(h)()) orbitals (ca. 1940 cm(-)(1)). The profile of the eR band is sensitive both to changes in temperature and to deuteration. Analysis of the eR band profile suggests a reduced spin-orbit splitting of the (3)E(g) manifold, this being ascribed to excited state Jahn-Teller (J-T) effects. The similarity of the Raman spectra of the cesium and rubidium titanium sulfate alums suggest that they exhibit closely related structural chemistry, with both subject to phase transitions below 80 K. The observation that modes of E(g) symmetry are coupled to the structural change is consistent with the interpretation that the trigonal field leaves an orbital doublet ground term for titanium(III), leading to a cooperative J-T effect.