Broad Raman Band Research Articles

Vibrational Spectra and Structural Order in the Lithium-Rare Earth Sulfides

Raman spectra have been measured on a series of sulfide compounds LiLnS2, where Ln is one of the rare earths between Pr and Yb. Compounds of the smaller rare earths have the ordered αNaFeO2 structure and give two-band Raman spectra in agreement with group theoretical calculations. Compounds of the larger rare earths have a disordered NaCl structure which, contrary to the NaCl selection rules, give an intense broad Raman band spanning the wavenumber region between the bands of the ordered structure. The broad band is interpreted as a density of states function with contributions of phonons from throughout the Brillouin zone. The massive breakdown of the k = 0 selection rule occurs because of the charge mismatch between monovalent and trivalent ions distributed over the crystallographically equivalent sites of the NaCl structure.

Raman scattering from diamond particles

Raman spectra of diamond powders with size less than 2 μm have been measured as a function of the particle size. The Raman line was found to become more asymmetric with some tailing towards lower Raman shifts, broader, and weaker with decreasing particle size. The observed result can be explained by a phonon confinement effect rather than by a strain effect. This work predicts that it is very difficult to detect Raman spectra of diamond particles with size less than ∼50 Å. A broad Raman band, whose intensity becomes stronger with decreasing particle size, was observed around 600 cm−1 in the spectra of diamond powders with particle size less than 2 μm. We hypothesize that the broad band arises from transverse acoustic phonons near the Brillouin zone boundary because of the relaxation in the wave vector selection rule.

Raman scattering in the green phase RE 2BaCuO 5 compounds, where RE  Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu

We have investigated the Raman scattering in RE 2BaCuO 5, where RE  Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, in the 50–1500 cm −1 region. We have also recorded the electronic spectra of some of these RE 2BaCuO 5. From these studies, we concluded that the broad Raman band located at about 1100 – 1177 cm −1 was an overtone band, and its origin was not due to electronic Raman scattering as has been previously attributed.

Properties of diamond membranes for x-ray lithography

Microwave plasma chemical vapor deposition was used to deposit diamond films on polished silicon and then the substrate was back-etched to form taut free-standing membranes up to 75 mm diam. Raman spectroscopy and x-ray diffraction verify that the films are diamond. Transmission electron microscopy and x-ray diffraction reveal that the films are fine-grained polycrystalline diamond with an average grain size between 15 and 110 nm. The films contain microcrystalline graphite and nondiamond carbon as indicated by the broad Raman bands at 1355 and 1560 cm−1, respectively. The measured x-ray transmission of a 3.25-μm-thick membrane at the Cu Kα line is 99.2% while the optical transmission at the He-Ne laser line is 58% for a 1-μm membrane, uncorrected for reflection losses from both surfaces, absorption, and scatter. The transmission in the ir is at the theoretical limit, 71%. The surface roughness of a typical 3-μm-thick membrane is about 30 nm on the side away from the silicon substrate and 20 nm on the side adjacent to the substrate. The surface roughness scales inversely with methane fraction in the supply gas due to decreasing grain size. The typical membrane stress is 0.03–0.15 GPa. The films contain between 0.8% and 7.0% hydrogen, as determined by 15N nuclear reaction analysis. No correlation was observed between hydrogen content in the film and deposition temperature; however, a direct correlation was observed with methane fraction. The average in-plane distortion is 70 nm for a 1.5-μm-thick diamond mask with 0.2-μm-thick tungsten absorber. No measurable out-of-plane distortion was observed for evaporated gold absorber metallization while tungsten produced 0.8 μm distortion.

The origin of two new Raman bands in the surface‐enhanced Raman spectra of pyridine adsorbed on colloidal silver particles

AbstractTwo new broad and strong Raman bands in the surface‐enhanced Raman spectrum of pyridine adsorbed on colloidal silver particles were observed, one near 612 cm−1 and the other near 924 cm−1. It is concluded that these bands are caused by a boron compound in silver sol.

Electroinduced structural changes in manganese dioxide + manganese hydroxide films as characterized by real-time surface-enhanced Raman spectroscopy

Structural changes occurring during reduction of thin (ca. 10 nm) MnO 2 films and oxidation of similar Mn(OH) 2 films electrodeposited onto electrochemically roughened gold have been examined using surface-enhanced Raman spectroscopy (SERS). Sequences of SER spectra were obtained during potential excursions under galvanostatic and linear-sweep voltammetric conditions by using multichannel Raman detection. The electrochemical redox behavior of the MnO 2 films is comparable to that for thicker layers as well as for MnO 2 battery electrodes. The MnO 2 film exhibits a broad asymmetric Raman band at 585 cm −1, due to Mn-O lattice vibrations. Upon initiating film reduction, this band narrows and a sharp feature at 670 cm −1 appears, also due to a metal-oxide vibration; neither band is shifted upon substituting D 2O for H 2O electrolyte. Further reduction yields a Mn-OH vibrational band (identified from the deuterium isotope shift) first at 415 cm −1, eventually being replaced by a broader Mn-OH feature at 470 cm −1. These latter features are consistent with the formation of MnOOH and Mn(OH) 2, respectively. The 670 cm −1 band is attributed tentatively to an intermediate Mn(IV)-O-Mn(III) species en route to MnOOH. Reoxidation of this film yields comparable electro-chemical and SERS responses to oxidation of Mn(OH) 2films. The 415 cm −1 Mn-OH feature reappears during Mn(OH) 2 oxidation and MnO 2 is eventually formed as evidenced by the appearance of the 585 cm −1 Mn-O band. However, the 670 cm −1 feature is not observed during film oxidation, an Mn-OH band at 500 cm −1 appearing in concert with the 415 cm −1 band instead. This is attributed to the formation of two distinct MnOOH phases during Mn(OH) 2oxidation. The virtues of SERS for elucidating redox-induced structural changes within oxide and other thin surface films are pointed out.

Crystal and solution structures of the B-DNA dodecamer d(CGCAAATTTGCG) probed by Raman spectroscopy: heterogeneity in the crystal structure does not persist in the solution structure

The self-complementary dodecamer d(CGCAAATTTGCG) crystallizes as a double helix of the B form and manifests a Raman spectrum with features not observed in Raman spectra of either DNA solutions or wet DNA fibers. A number of Raman bands are assigned to specific nucleoside sugar and phosphodiester conformations associated with this model B-DNA crystal structure. The Raman bands proposed as markers of the crystalline B-DNA structure are compared and contrasted with previously proposed markers of Z-DNA and A-DNA crystals. The results indicate that the three canonical forms of DNA can be readily distinguished by Raman spectroscopy. However, unlike Z-DNA and A-DNA, which retain their characteristic Raman fingerprints in aqueous solution, the B-DNA Raman spectrum is not completely conserved between crystal and solution states. The Raman spectra reveal greater heterogeneity of nucleoside conformations (sugar puckers) in the DNA molecules of the crystal structure than in those of the solution structure. The results are consistent with conversion of one-third of the dG residues from the C2'-endo/anti conformation in the solution structure to another conformation, deduced to be C1'-exo/anti, in the crystal. The dodecamer crystal also exhibits unusually broad Raman bands at 790 and 820 cm-1, associated with the geometry of the phosphodiester backbone and indicating a wider range of (alpha, zeta) backbone torsion angles in the crystal than in the solution structure. The results suggest that backbone torsion angles in the CGC and GCG sequences, which flank the central AAATTT sequence, are significantly different for crystal and solution structures, the former containing the greater diversity.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of incommensurability on the phonon modes in the one-dimensional superionic conductor K-hollandite

Polarized Raman and infrared spectra of the one dimensional (1-D) superionic conductor ( K 2 x Mg x Ti( 8− x ) O 16; x = 0.77) have been measured over the phonon frequency region 10–1000 cm −1 as a function of temperature and pressure. The majority of the IR and Raman active modes predicted by group theory for the (Ti, Mg)O 6 framework were observed. The frequencies and their IR and Raman scattering cross-sections for the incommensurate lattice of K + ions were calculated using a one dimensional linear chain model. This model assumes Coulomb interaction between nearest neighbors that are located in a sinusoidal potential due to the framework lattice. Several broad Raman bands were attributed to amplitudon type modes from the one dimensional incommensurate K + ion sublattice. The IR active phason modes could not be identified unambiguously due to the underlying TiO framework vibrations which are known to possess large anharmonicity and oscillator strenghts.

レーザーラマン分光法

The basis of laser Raman spectroscopy and its recent application to the structural study of coals and related carbonaceous materials are briefly outlined along with experimental technique. In Raman scattering, skeletal vibrations such as condensed aromatic ring vibrations in coals appear strongly, differing from infrared absorption in that stretching vibrations of functional groups such as carbonyl and hydroxyl groups linked to the aromatic rings show strong absorption. All Raman spectra of raw coals of different rank exhibit two broad Raman bands around 1600cm-1 and 1380cm-1, which are very similar to those of PVC cokes and some resin chars heat treated at about 450°C. With increase in coal rank, both Raman bands increase gradually their intensity and become sharp.Similar trends can be also observed for carbonized coals when the carbonization temperature is increased. The half band widths and intensity ratio of the 1600cm-1 and 1360cm-1 bands for cokes from Miike coal show a remarkable change with carbonization temperature and, therefore, can be used as parameters for the structural characterization of cokes derived from various coals. A certain relationship is found among the intensity ratio of Raman bands, optical reflectance and microcrystallite size for some foreign coals.

Raman diagnosis of nucleic acid structure: sugar-puckering and glycosidic conformation in the guanosine moiety.

Observations of Raman spectra of various nucleic acids indicate that the guanine ring breathing frequency is sensitive to the internal rotation angle around the glycosidic bond and to the conformation of the five-membered ring of the ribose residue that is directly connected with the guanine residue in question. It is found that 682 cm-1 for C2'-endo-anti, at 665 cm-1 for C3'-endo-anti, and at 625 cm-1 for C3'-endo-syn. A DNA octamer d(GpGpApApTpTpCpC) shows, in its aqueous solution, a broad Raman band at 680 cm-1 with a tail at 670 cm-1. This fact suggests that the guanosine residues in this oligomer take primarily C2'-endo-anti conformation but an appreciable amount of fluctuation of the ribose ring structure towards C3'-endo is involved.

Raman and far-infrared studies of lattice modes of piezoelectric crystal of trioxane

Optically active lattice vibrations of trioxane (CH2O)3 crystal with the piezoelectric space group of R3c-C63v were investigated by means of Raman and far-infrared spectroscopy. As predicted by the group theory, one A1 [referred to as (A1)L, at 70 cm−1] and three E(EL1, EL2, and EL3 at 95, 90, and 65 cm−1, respectively) bands appeared with distinct dichroism in the polarized far-infrared spectra taken on a uniaxially oriented thin film cooled at liquid nitrogen temperature. Right-angle Raman scattering was measured on single-crystal specimens of various orientations at temperatures from 280 to 40 K. The lowest EL3 mode exhibited clear LO–TO doubling with a frequency gap of 6 cm−1, owing to the piezoelectric or polar character of the crystal. The angular dependence of the Raman frequency was also observed. The broad Raman band around 90 cm−1 was found to separate into two components of different polarization, assignable to the EL1 and EL2 modes. At a temperature as low as 60 K, one component (EL2) split further into a doublet due to the LO–TO splitting with a frequency gap of 4.1 cm−1.

The Raman spectra and the antiferroelectric phase transition of copper formate tetrahydrate

The Raman spectra of Cu(HCOO)24H2O and Cu(DCOO)24D2O have been compared in the temperature range 170-290K in connection with the paraelectric-antiferroelectric phase change at 235.3K. The transition is due to the ordering of the water layers of hydrogen atoms, the role of the water molecules in the transition is shown by the splitting of a broad Raman band, assigned to translational vibrations of H2O or D2O, into five lines below 235K. Librational modes of formate ions are assigned and it is shown that the doubling of the unit cell is accompanied by a splitting of the Raman lines assigned to the internal modes nu 2, nu 3 and nu 6 of the formate ion.