Low-temperature Raman Spectra Research Articles

Low-temperature structures and Raman spectra of crystalline modifications of selenium difluoride dioxide

Low-temperature Raman spectra of solid SeO2F2 indicated the existence of two crystalline modifications between 174 and 100 K, the crystal and molecular structures of which have been determined by single-crystal X-ray diffraction techniques at 150 and 120 K. At about 130 K the higher-temperature tetragonal α form undergoes a first-order phase transformation to give the lower-temperature monoclinic β form. In both modifications the SeO2F2 molecules have similar distorted tetrahedral geometries: the weighted averages of the bond parameters are Se–O 1.57(1), Se–F 1.66(1)A, O–Se–O 126.7(9), F–Se–F 94(2) and O–Se–F 107.9(2)°. The molecules are only very weakly associated through intermolecular Se–O ⋯ Se contacts: in the α form each selenium atom is co-ordinated by two oxygen atoms from two different neighbouring molecules, while the oxygen atoms have contacts to two other molecules forming a three-dimensional network. In the β form these Se–O ⋯ Se contacts give rise to a structure consisting of corrugated layers of SeO2F2 molecules. Raman spectra of both modifications are consistent with their crystal structures and a factor-group correlation analysis confirmed the observed splittings of the Se–F stretching modes.

Second-order Raman spectra of SiC: Experimental and theoretical results from ab initio phonon calculations.

We report experimental and theoretical low-temperature second-order Raman spectra of 3C-SiC. Similar to the spectra of other group-IV and III-V semiconductors the ${\mathrm{\ensuremath{\Gamma}}}_{1}$ spectrum is strong, and the ${\mathrm{\ensuremath{\Gamma}}}_{12}$ and ${\mathrm{\ensuremath{\Gamma}}}_{15}$ spectra are very weak. The theoretical Raman spectra have been calculated using ab initio phonon eigensolutions and phenomenological polarizability coefficients. Good agreement between theory and experiment has been obtained. The ${\mathrm{\ensuremath{\Gamma}}}_{1}$ spectrum exhibits three distinct peaks at 1302, 1400, and 1619 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which occur in the gaps of the overtone density of states. This exemplifies the importance of taking into account the coupling matrix elements rather than simply relying on the one-to-one correspondence between the overtone density of states and the ${\mathrm{\ensuremath{\Gamma}}}_{1}$ Raman spectrum, which is commonly done for ZnS-type semiconductors.

Schwingungsspektren und Normalkoordinatenanalyse der 35Cl/37Cl-isotopierten Chloro-Iodo-Osmate(IV), [OsClnI6-n]2-, n = 0-6 / Vibrational Spectra and Normal Coordinate Analysis of the 35Cl/37Cl Labelled Chloro-Iodo-Osmates(IV), [OsClnI6-n]2-, n = 0-6

All components of the two series [Os35ClnI6-n]2- and [Os37ClnI6-n]2-, n = 0-6, including the pairs of geometric isomers for n = 2, 3, 4, have been prepared and isolated by ion exchange chromatography on DEAE cellulose. The highly resolved low temperature (80 K) IR and Raman spectra of the pure isotopomers show distinct isotopic shifts for different vibrational modes up to 10 cm-1. Normal coordinate analyses have been performed, based on a general valence force field. Due to the stronger trans influence of I as compared to Cl in all asymmetric Cl˙—Os—I′ axes the Os—I′ bonds are strengthened and the Os—Cl˙ bonds are weakened, as indicated by valence force constants for Os—I′ approximately 9% higher, for Os—Cl˙ 7% lower, as compared with the values calculated for symmetric axes.

Raman scattering in electronically excited C 60

The low-temperature Raman spectrum of 2a 0-fcc single crystal C 60 recorded using low-irradiance 514 nm excitation shows only a single peak in the region of the 1468 cm −1 out-of-phase ring mode. For higher irradiances a new broad peak appears in the spectrum at a somewhat lower frequency. Simultaneously, an increase of the luminescence is observed. As the irradiance increases the new peak gains intensity and shifts to lower frequencies, whereas the orginal peak slowly disappears. It is argued that this peak originates from electronically excited C 60. A simple electronic four-level model is proposed to explain the observed effects.

X-ray and Raman study of the low temperature NH4MnF3 structure; evidence of librational motion of the NH4+ ion

The structure of the low temperature phase of NH4MnF3 has been determined by means of X-ray powder diffraction using the Rietveld method. The structure is orthorhombic with space group Pnma, Z=4. The unit cell dimensions at T=83 K are a=5.952(1) AA, b=8.541(1) AA and c=5.949(1) AA. The low temperature Raman spectra have been interpreted consistently with the space group. The librational modes of the ammonium ion have been identified from the temperature dependence of the Raman lines. A model for the orientations of the NH4+ ions is proposed such that the minimum energy is achieved when each H atom points towards one of the four nearest F atoms. The depolarization of some of the Raman peaks in tentatively explained as an effect of orientational disorder of the NH4+ ions in the low temperature phase.

Low‐temperature Raman spectrum of phase V ammonium nitrate

AbstractRaman spectra of polycrystalline normal and three 15N isotopic ammonium nitrates (15NH4NO3, NH4 15NO3 and 15NH4 15NO3) were recorded at various temperatures between 10 and 300 K. Isotopic shifts of both internal and external modes of the ammonium ion relative to isotopic normal NH4NO3 were observed. Various components of the hetero‐ionic coupled internal mode (ν3 of the nitrate ion with ν4′ of the ammonium ion) were assigned with more certainty on the basis of the 15N isotopic shifts. Detailed studies of the temperature dependence of one internal mode (ν1) of the NO3 ‐ ion and several external modes were made. The frequency and half band width versus temperature plots for both the internal mode and the external modes of 15NH4NO3 indicate only one crystal structure (phase V) between 10 and 257 K. Comparison of the frequency shifts between various isotopic compounds in the external mode region permitted a more precise assignment of the lattice modes associated with the NH4+ and NO3− ions to be made.

Raman scattering by 3d 6 electrons of Fe 2+ in zinc blende diluted magnetic semiconductors

The electronic transitions inside the 5 E multiplet of Fe 2+ have been studied by resonant Raman scattering in the diluted magnetic semiconductors Cd 1- x Fe x Te. The experements are performed in a quasi backscattering geometry on single crystals oriented in the (1 1 0) plane. The low temperature Raman spectra display anti-Stokes and Stokes lines at 18.8 cm −1 in Cd 1- x Fe x Te and at 18.2 cm −1 in Zn 1- x Fe x Te. The observed Raman shifts correspond to the excitat of the magnetic dipole transition Γ 1 → Γ 4 induced by the exchange coupling. The line position as well as the polarization features are in good agreement with the theory. Moreover an additional Raman line with a Stokes shift ω L0 + ω Γ 1Γ 4 is clearly observedd at 188.5 cm −1 in Cd 1- x Fe x Te.

Single-crystal Raman spectroscopy of the α alums CsM(SO4)2· 12H2O (M = Co or Ir) between 275 and 1200 cm–1

Low-temperature polarised single-crystal Raman spectra of CsM(SO4)2·12H2O (M = Co or Ir) have been recorded between 275 and 1200 cm–1. A full assignment of the spectra has been completed, including all the Raman-active internal modes of the tervalent hexaaqua cation [ν1(MO6) 548 (Co) and 554 (Ir), ν2(MO6) 509 (Co) and 540 (Ir) and ν5(MO6) 332 av. (Co) and 314 av. cm–1(Ir)]. The relationship between the wavenumber of the ν1(MO6) mode and the metal–oxygen bond length is significantly different for the tervalent cations of the second- and third-row transition metals than for those of first-row transition metals and Group 13 metals. Solution Raman spectra of [Ir(OH2)6]3+ have enabled determination of ν1(MO6)aq(536 cm–1); the relationship between this value and that obtained at low temperature in the solid state closely follows that found previously for the chromium(III) and rhodium(III) aqua ions. The external modes of water co-ordinated to the tervalent cation are highly sensitive to the alum type, the metal–ligand bond length, and the stereochemistry of water co-ordination and provide additional insight into the metal–water interaction. This, in turn, casts light on the assignment of the external modes to particular molecular motions.

Phase transitions in adamantane derivatives: 1-fluoroadamantane

The solid-state phase behaviour of 1-fluoroadamantane (1-C10H15F) has been investigated by differential scanning calorimetry, and by infrared and Raman spectroscopy. The phase transition occurs at 227 K on cooling and at 231 K upon heating, with average ΔHt and ΔSt of 1.65 kJ mol−1 and 7.3 J K−1 mol−1. The transition was also apparent in the low-temperature IR and Raman spectra by a sudden line narrowing, and characteristic spectral features of each phase were identified. Both phases I and II were disordered, and further cooling of phase II resulted in the formation of a glass. Attempts to induce a glass transition (predicted to occur at ~90 K) by slow heating were unsuccessful. Vibrational spectroscopic studies at high pressures showed a phase transition at 3.3 ± 1.0 kbar, upon compression. The phases formed under pressure were the same as those induced by lowering temperature. Key words: high-pressure infrared and Raman spectroscopy, phase transition, plastic crystal, orientational disorder.

On the conformational analysis of methyl cyanoacetate

Ab initio calculations with full geometry optimization at the 4-21G level show that methyl cyanoacetate, NCCH 2COOCH 3, can exist as the cis (τ=0°) or as the gauche (τ= 145°) conformer, where τ measures the (N)CC(H 2)CO dihedral angle. The cis is calculated to be 3.92 kJ mol −1 more stable than the gauche. Mid infrared spectra of the vapor and solid (77 K) phases and of solutions in liquid argon (80 K) along with Raman spectra of the liquid phase of the undeuterated molecule and of NCCH 2COOCD 3 are reported and assigned. Asymmetric top contour simulations show that in the vapor phase the gauche form is the more stable of the two conformers observed. Low temperature Raman spectra of the liquid phase show that the enthalpy difference between the conformers is 2.46±0.2 kJ mol −1, favoring the cis conformer.

Raman scattering resonant with excitons in CdZnTe/ZnTe multiple quantum wells

We have studied CdZnTe/ZnTe multiple quantum wells which have alloy wells and strained layers. Low-temperature Raman spectra reveal a strain-induced shift of the longitudinal optic phonon energy consistent with a strain coherently shared between well and barrier layers. In resonant Raman profiles, we observe strong incoming and outgoing resonances at the n=1 heavy hole exciton transition that are well correlated with features in the photoluminescence and excitation spectra. Scattering by the CdTe-like phonon indicates an energy range for exciton localization by alloy fluctuations. The behavior of the ZnTe-like phonon near resonance is interpreted with exciton-polariton relaxation.

Evidence of defect-induced polarization clusters in nominally pure KTaO3 from low-temperature Raman and hyper-Raman spectra

A comparison is made between low-temperature Raman and hyper-Raman spectra of six single crystals of KTaO3 grown as pure according to the present state of the art by top-seeded solution and spontaneous-nucleation techniques. All samples differ in four spectral features: (a) first-order Raman intensity, (b) hyper-Rayleigh intensity, (c) width of the soft-mode hyper-Raman line, (d) soft-mode frequency. While (a), (b) and (c) are approximately linear functions of each other, (d) turns out to vary with the square of (a) or (b). No correlation is found between my of the four quantities under study and the photoluminescence at 687 run recently assigned to Ta3+ near oxygen vacancies. The results are interpreted in terms of polarization clusters formed by the host crystal in response to static or quasi-static defects of the random local-field type. Since the radius of a single polarization cluster reaches the magnitude of about five lattice constants, the specific defect structure at the centre is screened, so that the observed relationships between (a), (b), (c) and (d) become rather general.

Structure of the primary electron donor in photosystem I: a resonance Raman study

Low-temperature resonance Raman (RR) spectra have been obtained at resonance with the Soret transition of chlorophyll a in photosystem I particles containing large amounts either of the triplet state of P700 or of its radical cation state. Subtracting these spectra from those of resting reaction centers yielded RR spectra of P700 in its neutral, ground state. These spectra arise from two distinct chlorophyll a molecules differing by the strengths of the bonding interactions assumed by their keto carbonyl groups, the stretching frequencies of which are found at 1655 and 1675 cm-1. The present results rule out previous hypotheses that P700 might have consisted of a single, chemically modified chlorophyll a molecule. Neither of the bonding interactions assumed by the keto carbonyls of the P700 chlorophylls most probably involves chlorophyll-chlorophyll bridging through water molecules, as surmised in the so-called special pair models, but likely consists of H bonds with distinct protein sites. The magnesium atoms of the two P700 chlorophylls are 5-coordinated. Hence, the structural model of P700 provided by the present data is qualitatively the same, in terms of bonding interactions, as that currently accepted for the bacterial primary donor.

Synthesis and characterization of new fluorosulfinate: NH 4SO 2F

Sulfur dioxide reacts with dry ammonium fluoride to give ammonium fluorosulfinate. The infrared and low temperature Raman spectra of the new salt are described.

Structure and water-oxidizing capabilities of dimeric ruthenium EDTA complex ions

Low-temperature resonance Raman (RR) spectra of the dimeric Ru{sup III}Ru{sup IV} ion formed by oxidation of Ru(edta)OH{sub 2}{sup {minus}} exhibited a prominent band at 433 cm{sup {minus}1}, which shifted to 431 cm{sup {minus}1} upon substitution of ({sup 18}O)-H{sub 2}O in the primary coordination sphere of the precursor ion. A second band, at 324 cm{sup {minus}1}, underwent a comparable isotope-dependent shift to lower energies. The RR spectra were insensitive to deuterium substitution. These features establish that the dimer is a {mu}-oxo ion, i.e. (Ru(edta)){sub 2}O{sup 3{minus}}; from the magnitude of the isotopic shifts, the Ru-O-Ru angle was estimated to be 165{degree}. The reduction potential for the Ru{sup IV}{sub 2}/Ru{sup III}Ru{sup IV} couple was acid-insensitive over the range pH 2-11, suggesting that the dimer does not contain coordinated H{sub 2}O in either oxidation state. Cyclic voltammographic results in neutral solution suggested that (Ru(edta)){sub 2}O{sup 2{minus}} was unstable with respect to decomposition. A reinvestigation of the water-oxidizing capabilities of this ion by polarographic, gas chromatographic, and mass spectrometric methods failed to reveal O{sub 2} among the gaseous products. Instead, copious quantities of CO{sub 2} were formed, the source of which was coordinated EDTA. 29 refs., 8 figs., 1 tab.

AsIAlCl, a Salt of the Last Missing Tetrahaloarsonium Ion

The As ion in the salt 1 displays Td symmetry, as shown unambiguously by the low temperature Raman spectrum of the highly unstable compound. Prepared at −95°C according to reaction (a), compound 1 thus completes the series of tetrahaloarsenic(v) cations

Vibrational spectra and conformational analysis of chloromethylchloroformate

Infrared spectra of vapour and low temperature amorphous solid phases and Raman spectra of vapour, liquid and low temperature amorphous solid phases of ClCOOCH 2Cl, ClCOOCD 2Cl and ClCOOCDHCl are reported. The spectra indicate the presence of the same single conformer in each of these phases. Asymmetric top contour simulation of some vapour phase IR bands shows the conformer has s- cis orientation of the ester function and gauche orientation of the chloromethyl grouping. This conclusion is substantiated by the study of CH and CD stretches in mid-IR and near-IR spectra of ClCOOCHDCl in different phases. The assignments are augmented by a normal coordinate analysis using a transferable simplified valence force field. The latter is used to assign the splittings observed in the d 1-derivative.

Darstellung und Schwingungsspektren der 35Cl/37Cl-markierten Fluoro-Chloro-Platinate(IV) / Preparation and Vibrational Spectra of 35Cl/37Cl Labelled Fluoro-Chloro-Platinates(IV)

All components of the two series Cs2[PtF„35Cl6-n] and Cs2[PtFn 37Cl6-n], including the pairs of geometric isomers for n = 2,3, 4, have been prepared and isolated by ion exchange chromatography on DEAE cellulose. The highly resolved low temperature (80 K) IR and Raman spectra of the pure isotopomers show distinctive isotopic shifts for different vibrational modes up to 10 cm-1. The excellent agreement of the observed values with Teller-Redlich calculations verifies the assignment of all stretching and especially of the narrow deformation bands. For transcomplexes containing 35Cl-Pt-37Cl axes, the lowered symmetry affects the restriction of the rule of mutual exclusion

Picosecond Raman investigations of interligand electron transfer in transition metal complexes

Two color picosecond Raman spectroscopy has been utilized to measure interligand electron transfer in the excited MLCT states of mixed ligand Ru(II) complexes. No contribution to electron transfer was found from hot vibrational levels produced by photoexcitation. For thermally relaxed vibrational levels the first direct measurement of the upper limit to the rate of interligand electron hopping is reported. It was found that the rate from the thermally equilibrated vibrational states was ≤2×106 s−1. Low temperature Raman spectra indicate that the slow rate of electron transfer creates a nonstatistical energy distribution between dissimilar ligands. The implications of these results are discussed within the framework of electron transfer theory.

Nachweis der isotopomeren closo-Hydrohexaborate 11Bn 10B6-nH6 2- , n = 0—6, durch Tieftemperatur-Raman-Spektroskopie / Detection of the Isotopomeric closo-Hydrohexaborates 11Bn 10B6-nH6 2- , n = 0—6, by Low Temperature Raman Spectroscopy

Abstract The low temperature Raman spectra (10 K) of Cs2B6H6 and its 11B and 10B enriched derivatives prepared for the first time show well resolved signals o f the B6-cage vibrations υ1 (A1g), υ3(Eg) and υ9(T2g) for all isotopomers 11B10B6-nH6 2 -, n = 0-6. The intensities o f the sharp peaks correlate exactly with the statistical abundance ratio. W hereas the frequency decreases by nearly equidistant steps of 7 cm-1 with increasing n values, the geometric isomers for n = 2, 3 and 4 are separated by only about 2 cm-1. The band assignments are verified by Teller-Redlich calculations.