Showing papers on "Infrared spectroscopy published in 1981"

Vibrational interaction between molecules adsorbed on a metal surface: The dipole-dipole interaction

Abstract: We have developed a theory for the properties of vibrational excitations in molecules adsorbed on a metal surface. The coherent potential approximation (CPA) is used in the treatment of the vibrational interaction between the molecules. We show, by interpreting infrared spectra of substitutionally disordered systems consisting of isotopic mixtures of CO on Cu(100), that the molecules interact mainly through their dipole fields. We also show that in interpreting the integrated absorptance in infrared spectroscopy or the relative loss intensity in electron-energy-loss spectroscopy it is necessary to take into account the screening due to the electronic polarizability of the adsorbed molecules. A simplified version of the CPA result is used for a discussion of the absorption spectra of partial monolayers of one isotope. With the assumption that the CO molecules are randomly distributed, comparison between theory and experiment indicates that the dipole-dipole interaction alone is responsible for the coverage-dependent frequency shift for CO adsorbed on a transition metal [Ru(001)], whereas there is an almost equally large counteracting chemical shift on a noble metal [Cu(100)]. The meaning and origin of the dynamical dipole moment of adsorbed CO molecules are discussed. We find that the increase of the dynamical dipole moment (by a factor 2-3) upon adsorption probably is due to charge oscillations between CO $2{\ensuremath{\pi}}^{*}$ molecular orbitals and the metal. Finally, we outline how the theory developed here can be applied to a fundamental step in photosynthesis.

Raman and IR absorption spectroscopic studies on α, β, and amorphous Si3N4

Abstract: Vibrational excitations of α- and β-Si 3 N 4 were studied by Raman and infrared absorption spectroscopies. Group theory was employed to interpret the observed spectra of crystalline α- and β-Si 3 N 4 . Both Raman and IR spectra of amorphous Si 3 N 4 exhibit two broad peaks centered at about 400 and 900 cm −1 . Amorphous Si 3 N 4 was found to be of the non-molecular, random-network type.

Site distribution studies of Rh supported on Al2O3—An infrared study of chemisorbed CO

Abstract: Using infrared spectroscopy, chemisorbed CO has been employed as a surface probe to observe differences in Rh site distributions on Al2O3‐supported Rh. It has been found that higher loadings of Rh lead to Rh site distributions resembling those found on Rh(111) single crystals, whereas low Rh loadings produce isolated Rh sites preferentially. The isolated Rh sites yield infrared spectra characteristic of a species Rh(CO)2. Chemisorption of oxygen onto the CO‐covered surface effectively eliminates crystalline Rh sites, leaving only the isolated Rh sites which maintain their bonding to CO as seen by infrared spectroscopy. Estimates of the infrared extinction coefficient for Rh(CO)2 have been made. The results are consistent with data from other laboratories which indicate that the Rh is in a Rhd+ oxidation state.

Infrared energy levels and intensities of carbon dioxide—II

Abstract: Energy levels and rotational constants are provided for all carbon dioxide infrared absorption transitions of significance for terrestrial atmospheric transmission. Constants have been recalculated using new FTS measurements in the 4.3 μm region at elevated temperature, new FTS measurements in the 15, 5, and 2 μm regions, precise laser measurements around 10 μm, and high resolution grating spectrometer measurements of isotopically enriched samples. Band centers, band strengths, and coriolis coupling constants are given for some 560 bands.

The thermotropic phase behavior of ascorbyl palmitate: an infrared spectroscopic study

Abstract: The infrared spectra of aqueous potassium ascorbyl palmitate were studied as a function of temperature using Fourier transform infrared techniques. From a light scattering experiment the Krafft point of 0.1 M potassium ascorbyl palmitate was determined to be 48 °C. The temperature-induced changes in infrared spectral parameters such as frequency and bandwidth characterize this Krafft point as a phase transition from a conformationally ordered, poorly hydrated solid phase, to an isotropic micellar phase. The phase transition of this "pseudosoap" occurs over a temperature range of about 10 °C, reflecting the progressive hydration of the solid upon micellization, a behavior typical of surfactants such as soaps.

Matrix isolation spectroscopy

Abstract: 1 The history of matrix isolation spectroscopy- Section A - Techniques- 2 Infrared and Raman matrix isolation spectroscopy- 3 Electronic spectroscopy of matrix isolated solutes- 4 Magnetic circular dichroism - matrix isolation spectroscopy- 5 Electron spin resonance studies of radicals trapped in rare-gas matrices- 6 Moessbauer spectroscopy on matrix-isolated species- 7 Time and frequency resolved vibrational spectroscopy of matrix isolated molecules: Population and phase relaxation processes- 8 Stable molecules- 9 Generation and trapping of unstable solutes in low temperature matrices- 10 The characterisation of high temperature molecules using matrix isolation and vibrational spectroscopy- 11 High pressure studies- 12 Non-traditional matrix isolation: adducts- Section B - Matrix Effects- 13 Interpretation of infrared and Raman spectra of trapped molecular impurities from interaction potential calculations- 14 Matrix induced changes in the electronic spectra of isolated atoms and molecules- 15 Matrix effects studied by electron spin resonance spectroscopy- 16 Molecular motion in matrices- 17 Vibrational band intensities in matrices- Section C - Applications- 18 Matrix isolation spectroscopy of metal atoms and small clusters- 19 Vibrational spectra of matrix isolated gaseous ternary oxides- 20 Matrix isolation spectra (IR, Raman) of transition metal compounds- 21 Metal carbonyls - structure, photochemistry, and IR lasers- 22 Matrix isolation vibrational spectroscopy on organic molecules- 23 Conformational isomerism studied by matrix isolation vibrational spectroscopy- 24 Hydrogen bonding in matrices- Author Index

Conducting complexes of polyphenylene sulfides

Abstract: Poly(p‐phenylene) sulfide, poly(m‐phenylene) sulfide, and the newly synthesized polymer poly(thio‐2,8‐dibenzothiophenediyl) have been treated with strong electron acceptors (AsF5, SbF5) to form conducting complexes with p‐type electronic conductivities up to 3 S/cm. Near IR to UV absorption spectra and temperature‐dependent conductivity measurements suggest a localization of charge carriers even at high doping levels. Elemental analysis and IR spectroscopy demonstrate that heavy exposure to AsF5 causes substantial changes in the backbone structure of these polymers. The dopant appears to predominantly induce the formation of carbon–carbon bonds bridging the sulfur linkages to form thiophene rings. This chemical modification enhances the conductivity of the complex and, in the case of poly(m‐phenylene), is shown to be an actual prerequisite for achieving high conductivity.

Effect of heat treatment on the performance of carbon-supported transition-metal chelates in the electrochemical reduction of oxygen

Abstract: A study has been made of the changes occurring in carbon-supported metalloporphyrins and metallo-phthalocyanines during heat treatment, which improves both the stability and the activity of these materials in the electrochemical reduction of oxygen in acid electrolyte. A variety of physical techniques was applied, including X-ray photoelectron spectroscopy (X.p.s.), Fourier-transform infrared spectroscopy, electron spin resonance and Mossbauer spectrometry.During the heat treatment a bond can be formed between the carbon surface and both the central metal ion and the macrocyclic ligand, which thereby loses all its peripheral substituents while retaining its inner part. In some cases some of the chelate molecules are destroyed. The active sites formed in the more active catalysts consist of an isolated metal ion surrounded by the thermally modified ligand. A relation between redox potential and O2-reduction activity of the same form as that for untreated catalysts can be used to explain the observed enhancements in activity.

Phase Transitions and Nonplanar Conformers in Crystalline n-Alkanes.

Abstract: Crystals of n-alkanes show a remarkable series of solid-solid phase transitions. In the odd n-alkanes C(25), C(27), and C(29) a previously unknown transition is found by both calorimetry and infrared spectroscopy. The ubiquitous presence of nonplanar conformations of the chains is shown by infrared spectroscopy. The nonplanar conformers constitute approximately half the molecules in the highest temperature solid phase of C(29).

IR spectra of powder hematite; effects of particle size and shape

Abstract: Hematites obtained by heating goethite gave different IR absorption spectra depending on the temperature of formation. Hematites formed between 250-600~ consisted of lath-like crystals (average size 0-4 x 0-08 #m) and showed, in accordance with theoretical predictions, very similar IR spectra whose absorption bands could all be assigned to surface mode vibrations. However, significantly different IR spectra were given by hematites formed between 700-950~ the differences being correlated with variations in the size and shape of the particles. Differences observed in the IR spectra of powder hematite do not therefore justify new names for the mineral, as have been proposed in the literature. The IR absorption frequencies of many inorganic powders have frequently been interpreted as lattice vibration modes with no regard to the actual size and shape of the particles. Lattice mode frequencies are independent of particle size and shape only if the dimensions of a polar crystal are very large compared with the phonon wavelength (Ruppin & Englman, 1970). For smaller crystals, e.g. clay minerals, the departure from bulk-mode frequencies may become quite important (Farmer ,~ Russell, 1966). Crystalline iron oxide and oxyhydroxide products of pedogenesis differ markedly in particle size as well as in extent of isomorphous substitution (Schwertmann & Taylor, 1977). Since both effects greatly influence the IR spectra they need to be thoroughly understood before a species identification is made on the basis of IR evidence. The effect of isomorphous substitutions on the IR spectra of goethite have recently been described by Mendelovici et al. (1979). During the course of an investigation into the thermal and mechanical transformation of goethite, it was noted that IR spectra of the hematite products differed markedly depending on the temperature at which they were formed and the length of time for which they were ground. Similar observations have been recently reported and explained either in terms ofcrystallinity and crystal defects (Yariv & Mendelovici, 1979) or the presence of OH absorption bands (Wolska, 1981). However, similar differences in the IR spectra of natural hematites had been previously interpreted by Estep (1977) as due to the dependence oflR optical phonon frequencies on crystallite size and shape. The purpose of this investigation is to demonstrate that particle size and shape strongly affect the IR spectra of powder hematite and that they could be the underlying reason for the reported differences. EXPERIMENTAL PROCEDURE

Identification of electron traps in thermal silicon dioxide films

Abstract: The infrared absorption of thermal SiO2 has been measured using the attenuated total reflectance technique. The samples were subjected to various water diffusion and annealing treatments. Electron trapping was also measured in similar samples. The infrared measurements show the presence of SiH, SiOH, and H2O groups in the SiO2 films, particularly after H2O diffusion. Examination of both the infrared absorption results and the electron trapping results show that the electron trap with a cross section of 1×10−17 cm2 is associated with SiOH groups, and that the electron trap with a cross section of 2×10−18 cm2 is associated with H2O.

Analysis of CaOSiO2 and CaOSiO2CaF2 glasses by Raman spectroscopy

Abstract: Raman spectra of CaOSiO2 and CaOSiO2CaF2 glasses were measured to analyze three states of oxygen, i.e. bridging, non-bridging and free oxygen, in a similar way to the previous work on PbOSiO2 glasses. The stretching vibrational modes of the SiO bond in CaOSiO2 and CaOSiO2CaF2 glasses corresponded to the bands at 880, 920, 975 and 1050 cm−1. It is suggested from the comparison of the Raman spectra of the glasses with the Raman and infrared absorption spectra of crystalline silicates that these bands would arise from the SiO4 tetrahedron with four, three, two and one non-bridging oxygens, respectively. The fractions of bridging, non-bridging and free oxygens were calculated from the intensities of the four Raman bands and the composition of the glasses. They were in agreement with those obtained from the thermodynamical model for the CaOSiO2 glasses. When the content of CaF2 was smaller than 15–20 mol.% and the CaO SiO 2 ratio was smaller than unity, CaF2 contributed to the breakage of some SiO bonds.

Properties of polycrystalline silicon prepared by chemical transport in hydrogen plasma at temperatures between 80 and 400 degrees C

Abstract: Polycrystalline silicon films have been prepared using chemical transport in a low-pressure plasma in a temperature range 80-400 degrees C and at deposition rates up to approximately 3 AA s-1. Their Raman spectra show several features which are correlated with X-ray diffraction measurements and attributed to the presence of crystalline and amorphous-like components. Optical absorption, infrared spectra, preliminary data on dark conductivity and some further properties are reported.

Effect of ionizing radiation on the chemical composition, crystalline content and structure, and flow properties of polytetrafluoroethylene

Abstract: This article describes a study of ionizing radiation-induced changes in the chemical composition, crystalline content and structure, and flow properties in polytetrafluoroethylene (PTFE). Irradiations conducted in the presence of oxygen cause acid fluoride end groups to be formed, which on exposure to water vapor hydrolyze to form carboxylic acid end groups. Analyses by infrared (IR) spectroscopy indicate that when irradiated in a vacuum PTFE exhibits defect absorption bands which have been attributed to branch and crosslink formation. The crystalline content of PTFE which increases after exposure to radiation was monitored by IR spectroscopy, density, x-ray diffraction on unoriented samples, and differential scanning calorimetry (DSC) as the measurement probes. The melt viscosity of PTFE exposed to various radiation doses in air decreases dramatically after irradiation. Between 2.5 and 5 Mrd an increase in viscosity is attributed to the formation of branches and crosslinks. The effects of preirradiation crystallinity and postirradiation heat treatment were studied. A model is presented to explain the mechanism of the observed radiation effects.

Chemical bonding states in the amorphous SixC1–x: H system studied by X-ray photoemission spectroscopy and infrared absorption spectra

Abstract: XPS studies and infrared absorption measurements of the reactively sputtered (RS) amorphous SixC1–x: H alloy system have been made. The binding energy of the Si 2p core electrons decreases monotonically as the alloy composition x increases while the corresponding line-width remains almost constant. On the other hand, a curve of the C 1s core electron binding energy versus x has a kink at around x = 0·5∼0·6. Infrared absorption spectra reveal the existence of C–H, Si–H, Si–C bonds in the films. These results are discussed in terms of chemical bonding states.

Cure kinetics of a high epoxide/hydroxyl group‐ratio bisphenol a epoxy resin—anhydride system by infrared absorption spectroscopy

Abstract: An infrared absorption spectroscopy study of the curing kinetics of a low (1.12) epoxide/hydroxyl-group ratio bisphenol A epoxy resin—phthalic anhydride system is reported. A full infrared peak assignment to molecular vibrational modes is given for the range 400 to 4000 cm−1, and the optical density behavior of all peaks during reaction is discussed in detail. Proposed rival reaction mechanisms are considered and their respective kinetic behavior discussed. The reaction was found to follow consecutive-step addition esterification and simultaneous addition etherification, and epoxide—hydroxyl group and carboxylic acid dimer hydrogen bonding was found to occur. The reaction behavior supports a proposed hydroxyl group-limited inhomogeneous bulk reaction mechanism of a colloid type.

Low Temperature Air Oxidation of Caking Coals: Fourier Transform Infrared Studies

Abstract: Fourier transform infrared spectroscopy has been used to characterize the oxidation of a coking coal. The results demonstrate that the most important initial products of oxidation are carbonyl and carboxylic acid groups. Bands associated with carbon-oxygen single bonds, as in ethers or phenols, do not become prominent until the later stages of the oxidative process. Upon reaction with potassium in tetrahydrofuran a number of changes in the spectrum of both the oxidized and unoxidized coal become apparent. This reagent cannot be considered specific for cleavage of ether bonds, but can also lead to products usually associated with air oxidation.

Observation of Surface States on Ag(100) by Infrared and Visible Electroreflectance Spectroscopy

Abstract: By use of the new technique of electroreflectance in the infrared frequency range, a low-lying unoccupied surface state on the (100) single-crystal surface of Ag was detected. Another absorption feature, due to excitation into a higher surface state, has been seen around 3.1 eV. The positions of the two surface states are in good agreement with the calculated values. A study of the Stark shift of these states has revealed that the electric field near the metal surface has a much more complex structure than hitherto recognized.

Magnetic Properties and Moessbauer Spectra of Several Iron(III) Dicarboxylic Acid Complexes.

Abstract: : Polymeric iron (III) complexes of malonic, succinic, furmaric, and phthalic acids have been prepared and studied by variable-temperature (15-300 K) magnetic susceptibility, 57Fe Mossbauer spectroscopy, and infrared spectroscopy. In addition, properties of iron(III) acetate have been reinvestigated using these same techniques. The magnetic susceptibilities of these complexes have been described by a theoretical model which includes, in addition to intramodular exchange terms, a parameter describing intermolecular spin-exchange effects. As a consequence of this model it was not necessary to assume an isosceles triangular arrangement of the iron(III) ions in these materials in order to explain the magnetic data. Thus, the inclusion of an intertrimer exchange parameter, which varies from 1/2.1 cm for the acetate complex to 1/11.7 cm for the o-phthalate complex allowed for a complete description of the temperature-dependence of the magnetic susceptibilty of these compounds. Mossbauer and infrared spectroscopic studies were utilized to ascertain the correctness of oxidation state and structural assignments. (Author)