Additional Absorption Band Research Articles

Operando ATR-FTIR analysis of liquid-phase catalytic reactions: can heterogeneous catalysts be observed?

The applicability of a React-IR™ system, containing a diamond attenuated total reflection (ATR) crystal, in heterogeneous liquid-phase catalytic reactions was evaluated by analysis of the catalytic esterification of 1-octanol and hexanoic acid over a Nafion/silica catalyst in an open reflux configuration at atmospheric pressure. The reaction was performed in either cumene (at 427 K) or n-decane (at 447 K). The concentration profiles of the esterification reaction, as determined by this real-time in situ IR spectroscopic technique, are in qualitative and quantitative agreement with those determined by conventional off-line GC analysis. Interestingly, besides the bands assigned to the ester, alcohol, and acid, an additional strong and broad absorption band was observed at around 1100 cm −1 in the spectra during the esterification reaction in cumene (at 427 K). It was assessed by variation of the reaction mixture that this band is a result of the reaction of silica with octanol, yielding Si–O–R functionalities. The relevance of this reaction for the kinetics of the studied catalytic esterification, is discussed. More importantly, the contribution of solid particles and leached species to the 1100 cm −1 band is evaluated. Strong indications exist that solid catalyst particles are contributing to the spectra, implying that on-line analysis of intermediate species adsorbed on heterogeneous catalysts is in specific cases possible using the React-IR™ technique.

Absorption of light by exchange-coupled ions in a 2D antiferromagnet

The optical absorption spectra of Rb2MnxCd1−xCl4 crystals are experimentally studied in the vicinity of a magnon sideband of the exciton band at a manganese content x ranging from 1.0 to 0.4. Additional absorption bands are observed with an increase in the magnetic structural disorder upon replacement of manganese ions by cadmium ions. An analysis of the evolution of the additional absorption bands in a magnetic field during the spin-flop phase transition and the change in the intensity with variations in the manganese content x demonstrates that these bands are associated with the excitation of the exchange-coupled pairs of manganese ions located in different environments in a plane square lattice. The phase boundary between the antiferromagnetic and spin-flop phases is constructed using the results of optical measurements. The manganese content corresponding to the magnetic percolation point is evaluated.

Valency states of Yb, Eu, Dy and Ti ions in Li2B4O7 glasses

Abstract Absorption and emission spectra of Eu and Dy, Yb and Ti ions in Li2B4O7 glasses grown in oxygen and hydrogen gas atmospheres were measured for valency states and lattice-sites analysis. For the Li2B4O7 glass doped with Eu2+, Eu3+ and Dy3+ ions which were grown in oxidizing and reducing atmospheres, absorption and emission bands due to these ions were investigated before and after γ-irradiation. For the Yb3+-doped Li2B4O7 glass, a weak, broad band was observed near the sharp 976.3 nm absorption band. The origin of this band is discussed in comparison with other glasses. Moreover, irradiation experiments using γ-rays were also performed in order to investigate the possibility of valency change of Yb ions. It was found that Ti4+ ions, which are produced under oxidizing atmosphere, change to Ti3+ ions after γ-irradiation with a dose of 105 Gy. An additional absorption band observed at about ∼500 nm is due to the Ti3+ ions accompanied by charge-compensating vacancy and does not give any emission.

Comparative Studies on Energy Relaxation Dynamics of Directly Linked ZnII Porphyrin Dimers with Different Dihedral Angles

Photophysical properties of the strapped ZnII porphyrin dimers (Sn, the strap chain between the porphyrins changes; −O−(CH2)n−O−, n = 1, 2, 3, 4, 8, and 10) and directly linked ZnII porphyrin dimer (Z2) have been investigated by the steady-state and time-resolved spectroscopic methods. The reduced dihedral angle in Sn gives rise to additional absorption bands around 400 nm as well as broad fluorescence in the near-infrared region. The fluorescence lifetimes of Sn become gradually, but not so significantly, shorter as the strap length becomes shorter, indicating that the lifetimes of the lowest excited singlet states of Sn are not affected much despite their structural distortions such as mean plane deviations and the existence of two conformers. Fluorescence excitation and viscosity-dependent fluorescence spectra suggest that S2 exists as two conformers, A and B conformer, which has been also confirmed by X-ray data. The B conformer with a shorter interporphyrin distance has stronger interporphyrin interactions than the A conformer, which was revealed by the fluorescence excitation and fluorescence spectra. The A conformer is responsible for the shorter wavelength fluorescence at ca. 660 nm, whereas the B conformer gives rise to the near-infrared fluorescence. The photoinduced near-infrared absorption band was suggested to be characteristic of S2, which was not observed in Z2. The conformational dynamics of S2 with a time constant of 15 ps was observed in the transient absorption and fluorescence upconversion measurements. From the comparative studies of Z2 and Sn, we have demonstrated that the orientation of the two porphyrin macrocycles has a considerable effect on their photophysical properties.

Color centers in lead molybdate crystals

Color center formation in PbMoO4 crystals, originating from nonstoichiometric batch composition, BaO and Bi2O3 additions, high-temperature annealing of crystals, and UV illumination, is discussed. Additional optical absorption bands are identified. The photochromism of the crystals is shown to be due to charge exchange, Pb2+ + Mo6+ ⇄ Pb3+ + Mo5+, of the centers.

Effect of Mn Doping in ZnO Thin Films Deposited by Pulsed Laser Deposition

AbstractMn doped ZnO thin films were grown using pulsed laser deposition technique on (001) Al2O3 substrates. The x-ray diffraction data confirmed highly c-axis oriented and hexagonal structure of ZnO and Mn doped ZnO thin films. However, an unidentified secondary peak was observed at very close to ZnO (002) peak. Micro Raman spectra of ceramics as well as thin films showed disorder induced Raman bands besides standard wurtzite ZnO modes. The intensity of several LO modes of ZnO was increased with the increase of Mn concentration. Optical absorption data showed an additional absorption band towards lower than bandgap energy (3 eV) with the increase in Mn content. The incorporation of Mn in ZnO thin films reduced the value of resistivities and mobilities with an increase in carrier concentrations.

The effect of organic ligands on the crystallinity of calcium phosphate

Calcium phosphate phases precipitated under critical supersaturation were identified and studied in detail using X-ray powder diffraction, electron probe microanalysis, infrared spectroscopy (IR) and transmission electron microscopy. These synthetic calcium phosphates formed by spontaneous precipitation at pH 7, 25°C and 0.1 M ionic strength (NaCl as the background electrolyte). The combination of several methods allowed detailed characterisation of the calcium phosphates. The purpose of the work was to assess the influence of carboxylate ligands, specifically acetate and citrate, on the quality of the calcium phosphate precipitate. All precipitates were identified as non-stoichiometric, calcium-deficient hydroxylapatites (HAPs), containing carbonate, HPO 4 2−, sodium and chloride impurities. No other phases were found to be present in any of the precipitates. The presence of citrate resulted in a decrease in crystal size and a higher degree of apatite lattice imperfection in the precipitated HAP. Furthermore, IR spectroscopy showed a higher amount of carbonate present in that HAP, compared with the ones formed in the control and acetate experiments. An additional absorption band, in the infrared spectrum of the HAP formed in the presence of citrate, was observed at 1570 cm −1; this is interpreted as carboxyl groups bound to HAP.

Character of tightly bound excitons in small argon clusters: Insights from size-dependent energy shifts

The structure of the first electronically excited states of small ${\mathrm{Ar}}_{\overline{M}}$ clusters $(\overline{M}<~100)$ embedded in large ${\mathrm{Ne}}_{\overline{N}}$ $(\overline{N}=7500)$ clusters is investigated using fluorescence excitation spectroscopy. In the energy range of the characteristic absorption of Ar clusters (11.5--12.9 eV), surface excitons of Ar clusters embedded in Ne disappear, while additional absorption bands appear. They are assigned to excitons at the interface between the Ar cluster and the Ne host cluster. The observed energy shift of all absorption bands is proportional to the logarithm of the cluster radius. This can be understood in the Frenkel-exciton model, taking the resonant excitation transfer into account.

Electronic structure and charge dynamics of the Heusler alloyFe2TiSnprobed by infrared and optical spectroscopy

We report on the electrodynamics of a Heusler alloy Fe2TiSn probed over four decades in energy: from the far infrared to the ultraviolet. Our results do not support the suggestion of Kondo-lattice behavior inferred from specific heat measurements. Instead, we find a conventional Drude-like response of free carriers, with two additional absorption bands centered at around 0.1 and 0.87 eV. The latter feature can be interpreted as excitations across a pseudogap, in accord with band structure calculations.

Terahertz dielectric properties of polystyrene foam

We have performed terahertz time-domain spectroscopy measurements on three types of polystyrene foam. We find that between 0.2 and 4 THz, the extinction of this material is low and that it has a remarkably low refractive index of 1.017 to 1.022 in this range, with little variation as a function of frequency. In foam produced with HCFC 142b gas (1-chloro-1,1-difluoroethane) as the blowing agent, we find an additional absorption band at 0.5 THz caused by rotational transitions in this gas. The low extinction and refractive index make polystyrene foam a very suitable material to be used as a dichroic filter that blocks the near-IR and transmits THz radiation with small losses of less than 1.5 cm-1 for frequencies of <4 THz, and as a substrate for THz imaging.

Noble metal/titanium dioxide nanocermets for photoelectrochemical applications

Noble metal/TiO 2 nanocermets were deposited by rf sputtering from Ag:Ti and Au:Ti mosaic targets. Results of studies of the microstructure and the optical properties of Ag:TiO 2 and Au:TiO 2 thin films are discussed. Nanometric inclusions of Au and Ag in dielectric matrices create conditions for additional absorption band due to the surface plasmon resonance (SPR). The SPR occuring in the visible region of the light spectrum, at λ R=430–450 nm for Ag:TiO 2 and at λ R=580–630 nm for Au:TiO 2, improves the spectral characteristics of TiO 2 photoanodes. The presence of SPR is related to the concentration, size and distribution of the noble metal particles.

Behaviour of the water-soluble meso-tetra(4-methylpyridyl)porphine in mixed monolayers and in Langmuir–Blodgett films

The water-soluble tetracationic meso-tetra(4-methylpyridyl)porphine (PO2) was incorporated in mixed Langmuir monolayers (LM) with the anionic surfactant sodium hexadecylsulfate (SHS) and stearic acid (SA). The stability of the monolayer and the molecular packing of PO2 vary with the surface pressure and amount of stearic acid in the monolayer. The aggregation or tilting of PO2, occurring at the long transition during the compression run at the air–water interface, is reversible for all the ternary systems studied (PO2/SHS/SA). The organization of PO2 in LM, deduced from the π–A isotherms, is confronted with the spectroscopic data of LB monolayers and multilayers on silica substrates. At surface pressures below the long transition only one layer transfers onto the solid substrate whatever the number of vertical transfer strokes performed, while at high surface pressures LB films with several layers were formed. All absorption spectra of LB films exhibit a Soret maximum at λ1 ≈ 433 nm, which is ascribed to a nearly flat conformation of the PO2 ring. LB monolayers and multilayers of systems PO2/4SHS/4SA and PO2/4SHS/8SA, formed at high π(50 mN m−1), show an additional absorption band centred at λ2 ≈ 405 nm, which may account for a nonplanar conformation of PO2 with the pyridinium rings out of the porphine plane. The results suggest that the planar form of PO2 is imposed in the first layer by direct contact in a parallel orientation to the solid substrate, while the nonplanar form may coexist or even prevail in upper layers of LB films formed at high π.

Li2B4O7 glasses doped with Cr, Co, Eu and Dy

Abstract Absorption and emission spectra of Cr, Co, Eu and Dy ions in Li2B4O7 glasses melted in oxygen and hydrogen were measured for valency states analysis. It was stated that the presence of Cr6+ ion is limited by composition of the starting mixture and atmosphere of the melting and that this ion arises as Cr6+O− complex. Independently of the above factors in all the glasses there were present Cr3+ ions. Under γ-irradiation Cr6+–O− complex of 3d0 configuration can be disintegrated giving additional absorption band of Cr3+ and may be Cr4+ and higher valency centers. Corresponding photoluminescence is centered at about 430 nm. Absorption of Li2B4O7:Co glasses obtained in oxygen atmosphere reveals Co2+ and Co3+ ions which does not give any emission in the UV–VIS and near-infrared region of luminescence spectrum. The content and optical characteristics of Eu2+ and Eu3+-doped Li2B4O7 glasses are dependent of the growth atmosphere.

Raman investigation of single oxidized carbon nanotubes

AbstractThe oxidation process of single‐walled carbon nanotubes via nitric acid treatment was followed by IR‐, UV‐Vis‐NIR, and single bundle Raman spectroscopy. The introduction of functional, oxygen‐containing groups is revealed by an additional absorption band at 1725 cm−1, characteristic of carbonyl stretch vibrations. No significant shift of the optical absorption bands could be detected after oxidation. The combination of atomic force microscopy and confocal scanning resonance‐enhanced Raman microscopy was used to investigate thin bundles and, eventually, individual nanotubes in detail. These experiments enabled determination of the dependence of the Raman intensity of the G‐line (around 1590 cm−1) on the bundle height for both non‐oxidized and oxidized tubes. The Raman cross‐section of the oxidized tubes was found to be reduced by a factor of ˜4, compared to the pristine tubes. This observation is ascribed to all tubes within a bundle that are oxidized to the same degree.

Generation and healing behavior of radiation-induced optical absorption in fluoride phosphate glasses: The dependence on UV radiation sources and temperature

High purity fluoride phosphate (FP) glasses have a large transmission range from the vacuum ultraviolet to the infrared. They are attractive candidates for lens systems in microlithography equipment and excimer laser optics. Fluoride single crystals and vitreous silica are well-known traditional materials for ultraviolet optics. Crystal sizes are limited and glass is better for fabricating optics. For lens systems, a variety of glasses with different refractive indices and dispersion are required. The UV resonance wavelengths of FP glasses with a low content of phosphate using a two-term Sellmeier dispersion formula are comparable with those of silica and fluoride single crystals. It is known, that UV radiation induces the generation of several defect centers leading to additional absorption bands. The investigation of the kinetics of defect generation is very important for the prediction of transmission losses in case of long-time irradiation. To predict the kinetics of defect generation, first the separation of absorption bands is necessary. Experiments were carried out using UV-lamps, the KrF excimer laser (ns- and fs-pulses) and the ArF excimer laser (ns-pulses). The healing behavior of radiation-induced absorption bands was investigated using thermal treatment (temperature-dependence) and bleaching experiments (radiation-dependence). The results of these experiments enable the separation of absorption bands and the prediction of the defect generation depending on the used radiation source. The healing of strong defects at room temperature is postulated to be a diffusion-controlled process.

Correlation Between the Impurity Content, the Average Charge State of Chromium Cations and Optical Absorption in Sillenite Crystals Doped by Chromium in Wide Range of Concentrations

Single crystals of sillenites (Bi 12 SiO 20 , Bi 12 TiO 20 ) doped with chromium in a wide range of concentrations from 1 x 10 -3 up to 1.8 x 10 -2 wt. % were grown by the Czochralski and top-seed solution growth (TSSG) techniques. To estimate the content and the average charge state of chromium in the grown crystals the chemical analysis by the modified diphenylcarbazide method was applied. The dependencies of the chromium distribution coefficient, the average charge state of Cr cations, and optical absorption on the Cr concentration were found. Both the shift of the absorption edge toward lower frequencies and the appearance of an additional absorption band in the near IR were observed when the chromium concentration in the crystals was increased. The experimental data suggest that chromium has at least two charge states and occupies probably different positions in the sillenite unit cell.

Ionic strength and pH effect on the Fe(III)-imidazolate bond in the heme pocket of horseradish peroxidase: an EPR and UV–visible combined approach

The effects of chloride, dihydrogenphosphate and ionic strength on the spectroscopic properties of horseradish peroxidase in aqueous solution at pH=3.0 were investigated. A red-shift ( λ=408 nm) of the Soret band was observed in the presence of 40 mM chloride; 500 mM dihydrogenphosphate or chloride brought about a blue shift of the same band ( λ=370 nm). The EPR spectrum of the native enzyme at pH 3.0 was characterized by the presence of two additional absorption bands in the region around g=6, with respect to pH 6.5. Chloride addition resulted in the loss of these features and in a lower rhombicity of the signal. A unique EPR band at g=6.0 was obtained as a result of the interaction between HRP and dihydrogenphosphate, both in the absence and presence of 40 mM Cl −. We suggest that a synergistic effect of low pH, Cl − and ionic strength is responsible for dramatic modifications of the enzyme conformation consistent with the Fe(III)–His170 bond cleavage. Dihydrogenphosphate as well as high chloride concentrations are shown to display an unspecific effect, related to ionic strength. A mechanistic explanation for the acid transition of HRP, previously observed by Smulevich et al. [Biochemistry 36 (1997) 640] and interpreted as a pure pH effect, is proposed.

Influence of above-barrier and edge states on absorption spectra of shallow GaAs/AlGaAs superlattices of finite length in strong electric fields

An investigation is made of the absorption spectra of a GaAs(82 A)/Al0.07Ga0.93As(37 A) superlattice at 10 K in electric fields between 0 and 60 kV/cm. By comparing the experimental absorption spectra with calculations of the energies and oscillator strengths of transitions between states of the Wannier-Stark ladder it is established that in strong electric fields above-barrier states do not form a structureless continuum but a fan of states. Intersection of electric-field-localized electron states with the fan of above-barrier states leads to broadening, splitting, and nonmonotonic changes in the intensity of an intrawell transition band. The additional absorption band observed can be attributed to intrawell transitions between states in the left and right quantum wells of the superlattice.

Xanthophylls of the major photosynthetic light-harvesting complex of plants: identification, conformation and dynamics

The electronic transitions of lutein and neoxanthin in the major light-harvesting complex, LHCIIb, have been identified for the first time. It was found that 0–0, 0–1 and 0–2 transitions of neoxanthin were located around 486, 457 and 430 nm, whilst those for lutein were dependent on the oligomerisation state. For the monomer, the absorption bands of lutein were found at 495, 466 and 437 nm. Trimerisation caused a decrease in lutein absorption and the parallel appearance of an additional absorption band around 510 nm, which was identified by resonance Raman excitation spectra to originate from lutein. Circular dichroism measurements together with analysis of the ν 4 resonance Raman region of xanthophylls suggested that this lutein molecule is distorted in the trimer. This feature is not predicted by the LHCIIb atomic model of Kühlbrandt and co-workers [Kühlbrandt, W., Wang, D.N. and Fugiyoshi, Y. (1994) Nature 367, 614–621] and is an important step in understanding pigment dynamics of the complex. Oligomerisation of trimers led to a specific distortion of the neoxanthin molecule. These observations suggest that the xanthophylls of LHCIIb sense the protein conformation and which may reflect their special role in the assembly and function of the light-harvesting antenna of higher plants.

Transient Absorption Spectroscopy of a Photochromic Dinitrobenzylpyridine

Transient absorption of 2-(2,4-dinitrobenzyl)pyridine (α-DNBP) has been measured by a nanosecond laser photolysis to elucidate the dynamical behavior of a photoinduced intramolecular proton transfer (PIPT). On UV light photolysis, α-DNBP changes from a colorless to a dark blue form. The dark blue form consists of the OH, OH− and NH forms whose absorption bands are located at ∼400, ∼490 and ∼570 nm, respectively. We found an additional absorption band located at 660 nm. From the observed transient behavior of the band, we assign this band to be the lowest absorption band of the OH form. The PIPT reaction pathways and their time constants of α-DNBP also have been determined.