Well-defined Absorption Bands Research Articles

Synthesis and crystal-chemistry of Na(NaMg)Mg5Si8O22(OH)2, aP21/mamphibole

In the present work, we characterize the amphibole Na(NaMg)Mg5Si8O22(OH)2 synthesized at 0.4 GPa and 750, 800, and 850 °C, and 0.5 GPa, 900 °C. Experiments at 800 and 900 °C yielded crystals suitable for single-crystal data collection. Structure refinement shows that synthetic Na(NaMg)Mg5Si8O22(OH)2 has P 21/ m symmetry at room T . The two non-equivalent tetrahedral double-chains differ in their degree of stretching and kinking. The infrared spectrum of synthetic Na(NaMg)Mg5Si8O22(OH)2 has two well-defined absorption bands at 3742 and 3715 cm−1 which can be assigned to O-H bands associated with the two independent anion sites (O3A and O3B) in the structure. The higher frequency band is assigned to the shorter O3B-H2 bond, and the lower frequency band is assigned to the longer O3A-H1 bond. The broader shape of the 3743 cm−1 band is consistent with a stronger interaction of the H2 atom with ANa, which is confirmed by structure refinement. Increasing T of synthesis causes a progressive departure from the ideal stoichiometry via the A□1 BMg1 ANa−1 BNa−1 substitution, as confirmed by EMPA, structure refinement, and FTIR spectroscopy.

Silica gels with tailored, gold nanorod-driven optical functionalities

The incorporation of metal nanoparticles with various size and shapes provides silica gels with optical functionalities arising from the characteristic optical properties of the starting nanoparticle dispersions. The uniform distribution of the preformed nanoparticles and their mutual separation within the gels are key factors to maintain single-particle properties. These requirements are achieved by coating the nanoparticles with thin silica shells, which greatly enhance their stability and ensure separation between metal nanoparticle cores. The great advantage of this procedure is its ability to use nanoparticles with tailored shapes and thus to expand the possible functionalities to be implemented. We show the incorporation of gold nanorods with varying aspect ratio, which allows to have well-defined absorption bands even in the near infrared.

Artificial neural network analysis of laboratory and in situ spectra for the estimation of macronutrients in soils of Lop Buri (Thailand)

Reflectance spectrometry is an emerging and non-destructive detection technique bearing fast, cheap, and accurate results compared with conventional assessments. Most field and laboratory-based spectrometers are restricted to VNIR (visible–near-infrared). However, soils fail to show well-defined narrow absorption bands in this region. This obstructs the use of curve feature as a diagnostic criterion for soil nutrient predictions. In this paper artificial neural network (ANN) is implemented to estimate soil organic matter, phosphorous, and potassium from the VNIR spectrum (400–1100 nm). Macronutrients were modelled from 41 bare soil reflectances of Lop Buri province, Thailand. Neurons were trained from 7 bandwidth categories derived from laboratory-based StellarNet spectroradiometer and in situ photometer. Satisfactory results were attained and compared across different synthesised bandwidths. Models exhibited slightly better estimates from the laboratory than in situ spectra, and from narrower than broader bandwidths. Widening bandwidth corresponds with attenuated predictive powers, coupled with rising errors. Cross validation of models yielded acceptable correlations. The strength of models confirmed the capability of ANN to estimate macronutrients by solving difficulties incurred from high cross-channel correlations prevailing in conventional statistical techniques.

Synthesis and thermal decomposition of nitrate-free boehmite nanocrystals by supercritical hydrothermal conditions

Single-phase boehmite nanocrystals that are free of nitrate contamination could be prepared at supercritical hydrothermal conditions at 400 °C and 35 MPa for 30 min. XRD analysis showed that the single phase boehmite was formed in the absence of alkali species. Two well-defined and strong absorption bands observed at 3300 and 3092 cm −1 in the present infrared spectra showed that the boehmite by supercritical hydrothermal conditions was highly crystallized. Furthermore, IR spectra confirmed that the boehmite samples were free of NO 3 −-related impurities in contrast to boehmite produced by hydrothermal conditions as reported by Music et al. [Mater. Lett., 40 (1999) 269] and Mishra et al. [Mater. Lett., 42 (2000) 38]. At a temperature of 725 °C, the present boehmite powders were decomposed into 10 nm γ-Al 2O 3. Further heating of the samples to 1250 °C led to the formation of single-phase α-Al 2O 3.

Energy-level Assessment for Small Silver Clusters in AgBr Emulsion from Combined Experimental Data of Light Absorption, Spectral Sensitivity and Photobleaching

The combination of spectroscopic information concerning the light absorption, spectral sensitivity, and photobleaching involving the reduction sensitization centers allowed reasonable positioning of their electronic energy levels relative to the band edges of silver halides. A well-defined absorption band centered at 455 nm was observed both for the reduction sensitization centers produced by dimethylamine borane (DMAB) and for the hydrogen hypersensitization centers by using a diffuse transmittance spectroscopy method. We conclude that the common type of reduction sensitization centers, which must be predominantly hole trapping, have their HOMO (highest occupied molecular orbital) levels ∼1.9 eV below the conduction band edge. Additional centers produced at the high level of DMAB sensitization have a higher HOMO level ∼1.6 eV below the conduction band edge, which is identical with that previously ascribed to the subimage center. The dielectric polarization of the surrounding medium critically affects these energy levels. Possible limitations of the spectroscopic information as well as its advantages are also discussed.

A new sandwich-type diphthalocyanine as a potential optical transducer for NO 2 detection

Bis(phthalocyaninato)titanium(IV) (Ti(Pc) 2), a sandwich-type metal diphthalocyanine, is used as a chemical transducer for the optical detection of nitrogen dioxide. The optical properties of this chemical transducer after its immobilization on controlled pore glass (CPG) by a casting from a Ti(Pc) 2 solution in α-chloronaphthalene are described. This compound is characterized by three well-defined absorption bands in the visible region ( λ=506, 675 and 720 nm) which undergo spectral changes in the presence of nitrogen dioxide. Two different optodes were manufactured and characterized. A plastic flat reflector is used as a solid support placed in front of an optical fibre bundle, and Ti(Pc) 2 is immobilized on CPG that was previously embedded in the plastic reflector either by casting or by sublimation. The characterization of the developed optodes in terms of sensitivity, reversibility and response time is described.

SEM, EDS, and FTIR Examination of Archaeological Mineralized Plant Fibers

The chemical and p ysical structures of archaeological mineralized plant fibers are studied using energy x-ray dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and FTIR microspectrometry, and compared with two contemporary bast fibers. The outer surfaces of the fibers and the inner surfaces of the lumens are heavily en crusted with copper. Fresh-fractured cross sections of the mineralized fibers display small bead-like copper inclusions embedded within the fiber structure. In comparison with modern bast fibers, the mineralized fibers display an absence of hemicellulosic absorption bands (1740 cm-1), greater intensity, and sharper and well-defined indi vidual absorption bands in the 1200-1500 cm-1 region rather than a broad overlap of absorption found in the modern fibers. The mineralized fibers also display a lower crystallinity ratio ( 1372/2900 cm-1) than those of their counterparts.

Detection of two interstellar absorption bands coincident with spectral features of C60+

MORE than a hundred well-defined absorption bands, arising from diffuse gas in the interstellar medium, have been observed in the visible and near-infrared spectra of stars 1–4. The identity of the species responsible for these bands has remained unclear, although many possibilities have been suggested5,6. Carbon-based molecules ubiquitous in the interstellar medium have been widely favoured as potential carriers of some of the diffuse interstellar bands7–10,29; in particular, C60+ has been thought to be a promising candidate9,29. Here we present the results of a search for C60+ in the near-infrared spectra of seven stars, based on recent laboratory measurements of the absorption spectrum of this species11–13. We find two diffuse bands that are coincident (within 0.1%) with laboratory measurements on C60+ in a Ne matrix11. From this observation and the total absorption, we estimate that 0.3–0.9% of interstellar carbon is in the form of C60+. The molecule is very stable, which should allow it to survive in the interstellar medium for a long time14, but the inhibition of C60+ formation by hydrogen probably limits its abundance.

Nature of the absorption bands in Zn 1−xCo xS

To study the nature of the absorption bands in Zn 1− x Co x S single crystals, optical absorption measurements were made on samples with cobalt composition x = 0.03 and 0.05 as a function of temperature in the wavelength region between 400 and 3000 nm. Each spectrum showed three well-defined absorption bands below the fundamental absorption gap, corresponding to transitions from the fundamental 4A 2( 4F) ground state to the 4T 1( 4F), 4T 2( 4F) and 4T 1(P) excited states. Besides these main bands, the infrared region has a series of small peaks. In the visible region, just below the onset of the band edge, there is a well-defined structure related to electronic transitions from the 3d shell of the cobalt ion.

Analysis of reaction products in the oxidation reactions of hydrocarbons by means of matrix-isolation FTIR spectroscopy

The application of matrix isolation-FTIR spectroscopy for the product analysis of reaction systems in atmospheric chemistry is described. A mixture of Ar and O2 with a molar composition of 80% and 20%, respectively, is used as carrier gas. The Ar/O2 matrix deposited at 7±1 K exhibits an O2-doublet absorption band centred at 1550 cm−1 and 1551.5 cm−1 which can be used as an internal reference for calibration purposes. The reaction products isolated in the Ar/O2 matrix give sharp and well-defined absorption bands which are suitable for quantitative analysis. Various procedures and associated problems in the calibration are discussed in detail. The matrix isolation-FTIR technique was applied to the ozonolysis of simple alkenes and the photooxidation of biacetyl.

Applications of FTIR spectroscopy in structural studies of cells and bacteria

In this report FTIR spectra of cells and bacteria, and related recent work are reviewed and presented. The data may be used for diagnostic purposes and for probing nuclei, membranes and proteins. The monitoring of environmental samples is linked to characteristic bands and band ratios of well-defined absorption bands.

Photomodulation spectroscopy of defects in hydrogenated amorphous silicon germanium alloys

The photomodulation of the sub-bandgap optical absorption spectrum was measured for a series of amorphous hydrogenated silicon germanium (a-Si 1−xGe x:H) alloy specimens. For atomic germanium concentration exceeding 0.10 a single well-defined defect absorption band was observed which was associated with the germanium-related D-center found by electron spin resonance. The absorption peak was displaced by 0.4 eV from the optical gap as the gap varied from 1.65 to 1.0 eV.

Study of Alkoxide Silica Gels by Infrared Spectroscopy

The infrared spectra of silica gels made from alkoxides are different from those made from fumed silica, and from the infrared spectra of other amorphous silicas in which the SiO2 network is relatively complete. One important feature of these spectra is the occurrence of an absorption band at 960 cm−1 due to the vibration of dangling -Si-OH bonds in the alkoxide gels, which is not present in many other silicas. There also are less well-defined absorption bands due to structural defects for the alkoxide gels, but their interpretation is speculative. Dried gels made with more concentrated reactants contain more defects, and heat treatment reduces the concentration of dangling bonds, especially above 900°C. For gels containing fluorine, two absorption bands appear at 932 and 980 cm−1, and these are interpreted as arising from terminal -SiF and -SiF2 groups, respectively.

Optical and elastic properties of linear metal–chain compounds: The 1,2-benzoquinonedioximates (bqd) of nickel, palladium, and platinum, M(bqd)2

Polarized single crystal reflectance spectra of the linear metal–chain complexes Pt(bqd)2, α-Pd(bqd)2, β-Pd(bqd)2, Pd(bqd)2I0.55, Ni(bqd)2, and Ni(bqd)2I0.5 (bqd−=ion of 1,2-benzoquinonedioxime, bqdH) have been recorded at room temperature. Well-defined sharp absorption bands polarized parallel to the metal atom chains are observed in the frequency region from 0.5 to 2.5 eV, and are most reasonably identified as the interband electronic transitions between the ndz2 valence and the (n+1) pz conduction bands in a direct optical excitation process (n=3, 4, 5). The elastic behavior (Young’s modulus and internal friction) examined using a vibrating reed technique in the temperature range 17–300 K reveals, in the case of α-Pd(bqd)2, a peculiar phase transformation occurring along the metal atom chains below 100 K. From powder x-ray diffraction patterns recorded at ambient and at liquid nitrogen temperatures, this phase transition is interpreted as a lattice distortion resulting in the formation of stable chains of dimerized neutral metal complexes.

Photoreactivating enzyme from Escherichia coli: appearance of new absorption on binding to ultraviolet irradiated DNA

The photoreactivating enzyme, PRE, monomerizes pyrimidine dimers in DNA in a light requiring reaction (lambda greater than 300 nm). However, the purified PRE from E. coli has no well-defined absorption band for lambda greater than 300 nm. Using absorption difference spectroscopy, we show that when PRE is mixed with ultraviolet-irradiated DNA, new absorption appears in the spectral region required for catalysis. There is a concomitant decrease in the absorption of the mixture for wavelength less than 300 nm. The hyperchromicity for lambda greater than 300 nm is true absorption, not an artifact due to light scattering. Both the hyperchromicity (lambda greater than 300 nm) and hypochromicity (lambda less than 300 nm) can be reversed by irradiation of 365 nm with identical first-order kinetics. We estimate the molar extinction coefficient of the new absorption to be 6900 +/- 1400 at 350 nm. We conclude that the PRE from E. coli does not possess a distinct "chromophore" which by itself is entirely responsible for the absorption of photoreactivating light. Instead, new absorption results when PRE binds its substrate, dimer-containing DNA.

Crystallization of amorphous AgI studied by optical absorption of excitons

Exciton absorption spectra are reported for AgI measured during crystallization from the amorphous state. No well-defined absorption band could be observed in the disordered structure obtained by evaporating AgI onto a substrate at 80 K. An absorption band began to appear as the films were annealed to successively higher temperatures. The position of the absorption peak indicates that the crystallization of disordered AgI into a stable zinc blende structure occurs only after passing through an unstable wurtzite form which more closely resembles the amorphous structure. The activation energy for the initial crystallization process was found to be 2.0 × 10 −2 eV.

Optical Absorption of Localized Excitons in Cadmium Halide Crystals

Optical absorption spectra of CdBr 2 :I, CdCl 2 :I and CdCl 2 :Br have been measured at temperatures from 10 to 500 K. In every system, well-defined absorption bands due to doped halogen ions were observed on the low energy sides of intrinsic absorption edges of host crystals. Shapes of these bands are Gaussian around the band maxima and follow Urbach's rule in the low energy tail regions. These absorption can be attributed to the creation of excitons localized at doped halogen ions. Effective energies of lattice vibrations related to the central Gaussian part and the exponential tail part are estimated from the measured temperature dependence of half-widths and of Urbach's tails, respectively. Temperature dependence of these of these bands will be discussed using the Franck-Condon model and modified Urbach's rule.

Alkali Tin Paraperiodates (MSnIO6): Thermal Analyses, Electrical Conductivity, Infrared, and X-Ray Diffraction Studies

Crystalline sodium tin paraperiodate has been prepared; the X-ray powder diffraction data are indexed to the hexagonal system.The thermal analyses, TG and DTA of MSnIO6, where M = NH4, Na, K, Rb, Cs, reveal that the initial mode of decomposition is via partial evolution of I2 and O2.The infrared spectra of these compounds exhibit six well-defined absorption bands. These bands are located within the frequency region characteristic of I—O vibrational modes suggesting a lowering of the normal Oh symmetry for the [IO6]5− species.The electrical conductivity υs. temperature behavior for CsSnIO6 is reported.

Vacuum UV Absorption Spectra of Liquid Water and Ice

Fundamental absorption bands of water are studied in its various phases. Cubic ice has a well-defined absorption band at about 8.7 eV, while hexagonal and amorphous ices show only a gradual increase of absorption intensity toward the higher energy in the range from 7 to 10 eV. Absorption coefficient of liquid water is found to comply with Urbach rule at the low energy tail of the fundamental absorption.

Paramagnetic Faraday Rotation of EuSe

Faraday rotation and absorption measurements were made on a bulk single crystal and on evaporated polycrystalline films of EuSe. Bulk EuSe is ferromagnetic, transparent in the red, and exhibits an unusually large Verdet constant at room temperature: −9.7 min/Oe-cm at 6660 Å where the absorption is 173 cm−1. Measurements at 77°K show that the Verdet constant varies approximately inversely with temperature and thereby correlates with the magnetic susceptibility. These properties are due to the Eu+ + ion and are comparable to those we have observed in other divalent europium compounds such as Eu2SiO4. The evaporated films of EuSe exhibit a broad but well-defined absorption band at 4600 Å which is essentially duplicated in similarly prepared films of EuS and EuO at 5200 and 5800 Å, respectively. The wavelength dependence of the Faraday rotation of EuSe exhibits a rather complicated structure with two sign reversals in the region of the absorption band. At 6660 Å the Verdet constant measured at room temperature is −6.3 min/Oe-cm agreeing in sign and approximately in magnitude with the bulk value, whereas the maximum value of Verdet constant, +25 min/Oe-cm, occurs at 4330 Å.