Additional Absorption Band Research Articles

Photoinduced Electron Transfer between Sulfur-Containing Carboxylic Acids and the 4-Carboxybenzophenone Triplet State in Aqueous Solution

The mechanism of photoinduced electron transfer was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by six sulfur-containing acids, with varying numbers of COO[sup [minus]] groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3-2.1) x 10[sup 9]M[sup [minus]1] s[sup [minus]1] depending on the charge of the acid molecule. The observation of ketyl radical anions and intermolecular (S S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanism of quenching. An additional absorption band at approximately 410 nm in the transient absorption spectra for some of the acids was assigned to intramolecularly (S O)-bonded species (for acids for which a five-member ring structure was sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of CO[sub 2] formation and CB disappearance from the steady-state measurements were determined. The values of these quantum yields clearly indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is the main photochemical pathway. 29 refs., 4 figs., 3 tabs.

Spectroscopic investigations of sharp Oh/Od Ir-absorption bands in nearly stoichiometric (VTE) LiNbO3

Abstract A narrow stretching absorption band of OH/OD, polarized completely perpendicular to the ferroelectric z-axis, (at RT for OH at 3465.9 cm−1, FWHM = 3.25 cm−1 and for OD at 2561.9 cm−1, FWHM = 2.05 cm−1) is observed in the bulk of nearly stoichiometric LiNbO3 (prepared by the VTE technique [1]). These bands are about an order of magnitude more narrow than the corresponding bands in the bulk of congruent LiNbO3 at RT. The position, halfwidth and integrated absorption of the fundamental as well as the next higher vibrational transition for OH and OD are measured as a function of temperature (RT - LHeT). Electrical and mechanical anharmonicities are discussed on the basis of a Morse-type potential for a diatomic oscillator. Additional OH and OD absorption bands [2] for OH at 4417 cm−1 and for OD at 3271 cm−1 in the bulk of nearly stoichiometric LiNbO3, are shown to belong to a libration-vibration combination band. The OH libration was also observed directly at 960 cm−1 in Raman scattering and the OD l...

Scintillation mechanisms in cerium fluoride

Ultraviolet photoelectron spectroscopy, optical transmission, fluorescence excitation spectroscopy and time-resolved fluorescence spectroscopy are used to investigate the scintillation mechanisms of cerium fluoride (CeF 3) and of lanthanum fluoride doped with cerium in concentrations between 0.01 and 50 mol% cerium. In LaF 3:Ce, the absorption of either optical or ionizing radiation directly or indirectly results in excitation of the Ce 3+ 4f electron to the lowest 5d level followed by 5d→4f fluorescence at 284–300 nm. Whereas for optical excitation the fluorescence has a 20 ns decay time, for ionizing radiation there is an additional faster (2–10 ns) initial decay component. As the cerium concentration increases, another band appears that partially absorbs the 284–300 nm emission and re-radiates it in a broad band peaking at 340 nm and having a longer (∼30 ns) decay time. In the limit of 100% CeF 3, radiation trapping is very pronounced. The additional absorption and emission bands present at large Ce concentrations are attributed to Ce 3+ ions in perturbed sites. The relative efficiency for excitation of unperturbed and perturbed cerium sites via the Ce 3+ 5d and 6s bands, the F - 2p valence band, and the Ce 3+ or La 3+ 5p core levels are determined from fluorescence excitation spectra.

Jahn-Teller effect in V4+ doped Gd3Ga5O12 garnet

We consider the static and dynamic E*∈ and T*∈ Jahn-Teller effects in order to analyze the absorption and emission spectra of octahedrally coordinated V 4+ and absorption of tetrahedrally coordinated V 4+ . Using two-dimensional configuration coordinate diagrams of octahedrally and tetrahedrally coordinated V 4+ we are able to explain the structure of the emission and absorption spectra and the fluorescence kinetics measurements of V 4+ doped GGG. Our calculations support the assumption that V 4+ in tetrahedral sites is responsible for the additional absorption band in the red region

Optical absorption spectroscopy of synthetic tourmalines

Tourmaline solid solutions containing Fe, Fe+ Ti, Cr, Ni, Cu, Co, Mn chromophoric centers have been grown hydrothermally at 650° C and 1,5 kbar on natural seeding plates close to the elbaite composition. The newly grown tourmalines were characterized by chemical analyses and optical absorption spectroscopy in the range 26316-5000 cm-1 at 297 K and in the range 26316-9090 cm-1 at 77 K. Most characteristic of Fe2+, Fe3+-bearing specimens is the presence of intensive σpolarized absorption bands caused by exchange-coupled Fe2+-Fe3+ pairs in Y- and Z-sites of the tourmaline structure. An additional intensive absorption band 12500 cm-1 (σ-polarisation) appears in some specimens but is not yet found in spectra of natural tourmalines. The colour and spectroscopic properties of the Fe3+, Mn3+ and Cu2+ containing tourmalines are significantly affected by the presence of even the smallest Li-contents. The results suggest that Fe2+, Cu2+, Co2+, Ni2+-ions occupy, predominantly, Y-sites of the tourmaline structure, whereas the Cr3+-ions seem to enter the smaller Z-octachedra.

Infrared Spectroscopy of Adsorbed Carbon Monoxide on Platinum/Nonacidic Zeolite Catalysts

Infrared spectroscopy of adsorbed carbon monoxide was investigated on a series of platinum (or palladium) zeolite catalysts plus platinum on silica. The platinum catalysts all showed a strong absorbance band around 2070 ± 20 cm−1 due to linear-bonded CO and a weaker band around 1850 ± 20 cm−1 due to bridge-bonded CO. Four platinum catalysts, Pt/K+ LTL, Pt/K+ MFI, Pt/K+ MTW, and Pt/K+ SiO2, had an additional absorbance near 1970 cm−1. The additional band is not observed for Pt/silica, Pt/La+3 LTL, or platinum supported on H+ LTL or H+ MFI, i.e., neutral and acidic supports. The additional band on the alkalized catalysts is speculated to result from an electrostatic attraction of linear-bonded CO with an alkali cation when Pt is on a nonacidic support. The electrostatic attraction increases the CO adsorption strength and lowers the stretching frequency. The additional linear CO absorption band was also observed on Pd/K+ LTL. The additional band, therefore, is not specific to platinum. Although the presence of this additional infrared band suggests a "new type of metal (catalytic) site," no relation was found between the catalytic activity or selectivity for n-hexane dehydrocyclization and the additional CO absorbance band.

Cluster formation in expanded fluid mercury and dilute amalgams near the liquid-gas critical point

The optical absorption spectrum and the thermopower have been measured for expanded fluid Hg and dilute amalgams containing 0.2 at.% Bi and 0.2 at.% Cd at temperatures and pressures up to 1600°C and 2100 bar. The optical gap closes around 8 g/cm 3. Above 4 g/cm 3 an additional absorption band with negative slope appears in the near-infrared region and grows rapidly with increasing density. The shape of this near-infrared band is appreciably changed by the addition of a multivalent element such as Bi, while it remains unchanged when the divalent Cd is added. The thermopower for Hg remains negative and attains an extremely large value in the vicinity of the critical point. It is suggested that the results may be interpreted by assuming the existence of dielectric clusters in the critical region.

Alpha-keto acids are novel siderophores in the genera Proteus, Providencia, and Morganella and are produced by amino acid deaminases.

Growth promotion and iron transport studies revealed that certain alpha-keto acids generated by amino acid deaminases, by enterobacteria of the Proteus-Providencia-Morganella group (of the tribe Proteeae), show significant siderophore activity. Their iron-binding properties were confirmed by the chrome azurol S assay and UV spectra. These compounds form ligand-to-metal charge transfer bands in the range of 400 to 500 nm. Additional absorption bands of the enolized ligands at 500 to 700 nm are responsible for color formation. Siderophore activity was most pronounced with alpha-keto acids possessing an aromatic or heteroaromatic side chain, like phenylpyruvic acid and indolylpyruvic acid, resulting from deamination of phenylalanine and tryptophan, respectively. In addition, alpha-keto acids possessing longer nonpolar side chains, like alpha-ketoisocaproic acid or alpha-ketoisovaleric acid and even alpha-ketoadipic acid, also showed siderophore activity which was absent or negligible with smaller alpha-keto acids or those possessing polar functional groups, like pyruvic acid, alpha-ketobutyric acid, or alpha-ketoglutaric acid. The fact that deaminase-negative enterobacteria, like Escherichia coli and Salmonella spp., could not utilize alpha-keto acids supports the view that specific iron-carboxylate transport systems have evolved in members of the tribe Proteeae and are designed to recognize ferric complexes of both alpha-hydroxy acids and alpha-keto acids, of which the latter can easily be generated by L-amino acid deaminases in an amino acid-rich medium. Exogenous siderophores, like ferric hydroxamates (ferrichromes) and ferric polycarboxylates (rhizoferrin and citrate), were also utilized by members of the tribe Proteeae.

Resonance Raman Spectroscopic Evidence for an Anionic Flavin Semiquinone in Bovine Liver Monoamine Oxidase

The flavoprotein monoamine oxidase B (MAO B) from bovine liver, as isolated, has an unusual additional absorption band at 412 nm, which is similar to the absorption of its anionic flavin semiquinone form, (Fl∸), and other typical (Fl∸) flavoproteins. Denatu ration of the enzyme results in the elimination of this anomalous absorption. The resonance Raman (RR) spectrum of MAO B as isolated is virtually identical to that of its dithionitereduced (Fl∸) form. Both spectra show features similar to those of the RR spectrum of the (Fl∸) form of Aspergillus niger glucose oxidase (GO) in the region between 300 and 1700 cm−1 with 406.7 nm excitation. These features are readily distinguishable from those of oxidized flavin, neutral fiavin semiquinone, and hemoprotein, strongly suggesting the presence of an (Fl∸) form in MAO B as isolated, even with preparations isolated in the absence of light. There are significant differences between the RR spectra of the (Fl∸) form of MAO B and those of GO or the published RR spectra of the (Fl∸) form of Damino acid oxidase with excess substrate analog. At least some of these differences can be attributed to the different binding of flavin in the three enzymes. No EPR signals due to (Fl∸) are observed in MAO B as isolated. The dithionite-reduced (Fl∸) form exhibits 50% less EPR signal than that expected from the absorption spectrum, which suggests a possible coupling of the (Fl∸) fiavin with a paramagnetic center of unknown identity in the protein. The implications of these observations on MAO B with the current view of its catalytic mechanism are discussed.

Early steps of formation of silver/polyacrylate complexed oligomer clusters. A pulse radiolysis study

Pulse radiolysis has been carried out on aqueous solutions of silver ions complexed with polycrylate (PA). Over the time scale 10 −8−10 s, only a few kinetic steps have been observed which have been identified through spectroscopic and kinetic analysis. The reaction rate of e - aq with Ag + is lowered in the presence of PA (3.6×10 9lmol −1s −1 when [PA]/[Ag +]=100). The first products (Ag 0 λ max≈360 nm, Ag + 2 λ max=310 nm) are similar to those obtained in the absence of PA, except for an additional absorption band (350–450 nm) which indicates that Ag + 2 is itself complexed. Then through dimerization (2k=3×10 9lmol −1s −1) and reaction with the alcoholic radical (in the presence of 2-PrOH, 1.5×10 9lmol −1s −1), Ag + 2 leads to an “Ag 2” species peaking at 300, 320 and 360 nm, noticeably different from the reported Ag 2+ 4 species. Two further steps are observed which are assigned to a ligand reorganization of “Ag 2” in the millisecond range (10 3 s −1), followed by a coalescence to “Ag 4” in the range of seconds (10 5lmol −1s −1). This last product is thought to be the stable Ag a cluster known to be formed by γ-radiolysis. A kinetic simulation accounts for the whole mechanism which is much lower than in the absence of PA.

Near‐infrared probe for in situ characterization of nafion thin films

AbstractA near‐infrared (NIR)‐absorbing cyanine dye was used as a probe for in situ characterization of Nafion thin films. This NIR dye was very sensitive to changes in the hydrophobicity of the environment and proved to be very suitable for probing the Nafion coatings, which involve a two‐Phase structure of hydrophobic and hydrophilic zones. Sorption phenomena of water and aqueous alkali salt solutions by Nafion were investigated. For our study systems of thin‐coating films prepared from 5% wt alcoholic solution of Nafion 117, the absorption spectrum was dominated by higher‐order aggregates when in dry form. In pure water, the absorption maximum of the monomer dye appears at 773 nm and can be used as an indication of the swelling process of the film. The water uptake reaches saturation in only a few seconds. After the swelling process, an additional absorption band with a maximum at 555 nm appears primarily at the expense of the NIR monomer dye absorption band. The peak transition process, which is a very slow process and is a strong function of the water content inside the matrix, can be used as an indication of the establishment of equilibrium between the two‐phase structure due to the water uptake. Since the absorption maximum of the study system is around 800 nm, semiconductor lasers can be used. This technique shows high potential in other applications where polymers are used as support material, e.g., in situ thin‐film thickness measurements. In the study, the feasibility of this approach was illustrated.

Distinctive high frequency spectrum of liquid methyl fluoride

Absorption coefficient and refractive index were measured for methyl fluoride in the frequency range 70-320 GHz and for temperatures ranging from the melting point to the triple point. The present results in combination with previous spectroscopic data give the complete 0-THz dipolar spectrum. The experimental results are well described by the confined rotator model with the exception of the part of the spectrum around 600 GHz where an additional absorption band is observed. The general features of this extra absorption are discussed in the framework of several molecular dynamics mechanisms, and show qualitative agreement with the contribution from induced moments originating in translational motion.

Theoretical analysis of the absorption spectra of diacetylene single crystals with color zones.

Absorption spectra of polymer chains in TS6, FBS, and TS-FBS diacetylene single crystals (TS6 and FBS are symmetrically substituted with R=R': -${\mathrm{CH}}_{2}$-O-${\mathrm{SO}}_{2}$-?-${\mathrm{CH}}_{3}$ and R=R': -${\mathrm{CH}}_{2}$-O-${\mathrm{SO}}_{2}$-?-F, respectively) are described theoretically by means of a Franck-Condon model that considers a chain-length dependence. The electronic transition energies and matrix elements are calculated by means of a linear-combination-of-atomic-orbitals molecular-orbital calculation in the H\"uckel approach. The considered diacetylene crystals can show chromism in the form of stable, sharply separated reddish and yellow color zones. These color zones are most probably caused by edge dislocations in small-angle grain boundaries, which arise during crystal growth and have a Burgers vector in the [010] direction. We are able to describe the absorption spectra for the reddish color zone for FBS, TS-FBS, as well as for TS6, which undergoes a phase transition resulting in a splitting of the absorption lines. These spectra contain the absorption of polymer chains with lengths of about 10--15 monomer units. The absorption spectra of the yellow color zones exhibit for TS-FBS one and for FBS several additional absorption bands. An assignment of these absorption energies to conjugations of distinct lengths is possible. Only lengths are found that are equivalent to even numbers of monomer units. The absorption spectra can be well simulated by the calculations. An evaluation of the parameters shows an influence of defects on the absorption linewidths as inhomogenous broadening. This broadening does not depend on the lengths of the absorbing conjugation domains. Hints to a defect-induced lowering of the activation energy for the polymerization reaction are found in the yellow zones.

Infrared spectra of the gaseous iodides of chromium, iron and nickel

The infrared spectra of the vapours above chromium, iron and nickel diiodide have been studied by high-temperature infrared spectroscopy. The gaseous molecules CrI 2, FeI 2 and NiI 2 were all identified and the interpretation of the spectra is in agreement with a linear structure. Additional strong absorption bands in the spectra of the vapour phase above liquid CrI 2 and FeI 2 were assigned to dimeric (MI 2) 2 molecules. Valence force constants and thermodynamic quantities have been calculated.

Radiation-induced anionic species of maleic anhydride in glassy matrices

In pulse radiolysis and steady state radiolysis experiments with optical absorption spectroscopy the radiation-induced formation and decay of anionic species of maleic anhydride (MA) in rigid 2-methyltetrahydrofuran (MTHF) solutions were investigated at low temperatures. Absorption bands in the u.v. and n.i.r. regions were observed in the pulse radiolysis experiments and in agreement with literature data assigned to monomeric radical anions and radicals and to dimeric radical anions of MA, respectively. Additional absorption bands have been detected in the vis. range but could not be ascribed. The formation and decay of the radical anions and radicals were characterized kinetically.

Optical spectroscopy of field-induced charge in poly(3-hexyl thienylene) metal-insulator-semiconductor structures: Evidence for polarons.

We have fabricated metal-insulator-semiconductor structures with poly(3-hexyl thienylene) as the active semiconductor. Optical-absorption bands due to injected carriers seen near 0.4, 1.80, and 2.16 eV are assigned to the optical transitions of the singly charged polaron, which is expected to be the thermodynamically favored excitation under these experimental conditions. Additional absorption bands at 0.5 and 1.18 eV are assigned to optical transitions of doubly charged bipolarons. We contrast the strong confinement deduced for the polaron (gap states separated by 1.80 eV) with the weak confinement observed for bipolarons (gap states separated by 0.7 eV).

Optical properties of one- and two-dimensional Hubbard and t-J models.

The optical conductivity for the Hubbard and t-J models is calculated numerically by use of a Lanczos algorithm. One- and two-dimensional systems of up to 16 sites with periodic and open boundary conditions are examined for the t-J model, as well as Hubbard-model systems of up to 9 sites. In the one-dimensional case, the low-frequency part of the conductivity away from half filling is shown to consist almost entirely of a single \ensuremath{\delta} function (Drude peak) even for J of the order of t. This is a direct consequence of the decoupling of charge and spin in one dimension. In two dimensions there is an additional broad absorption band above the Drude peak, consistent with the experimentally observed midinfrared band in the copper oxide superconductors. The importance of vertex corrections is demonstrated by comparing the exact conductivity with the convolution integral of the exact single-particle Green's function.

LRP equipment characterisation at low temperatures and application to polysilicon bipolar emitter processing

A plasma-stimulated LPCVD rapid thermal processor has been developed. This processor is capable of operation in the limited reaction processing (LRP) regime and offers sequential in situ processes such as silicon deposition, plasma oxidation and interface pre-cleaning. Incident power with wavelength greater than 1.1 mu m is absorbed by free carriers and therefore sample doping density has a strong effect on the temperature-time profile obtained. It has been shown experimentally that heavily doped n-type samples reach higher temperature than lightly doped samples and exhibit faster ramp up times. The doping dependence is not so strongly observed for p-type samples. However, in general p-type samples do not attain as high a temperature as n-type samples. This is believed to be due to an additional absorption band for n-type silicon centred around wavelengths of 2.3 mu m. Direct monitoring of the process wafer is considered essential for accurate temperature-time profiles. The system has been used to fabricate n-p-n polycrystalline silicon emitter bipolar transistors. Transistors which received an in situ CF4 plasma pre-clean prior to polysilicon deposition exhibited gains similar to reference long emitter devices. The results were characterised by excellent uniformity across the samples indicating a clean, well controlled interface between the polysilicon and the silicon. This is highly important if LRP systems are to be applied to low-temperature epitaxy and heterojunction production on silicon. Transistors which received an in situ plasma clean, plasma oxidation followed by polysilicon deposition schedule exhibited higher gains. This supports the theory that the thin tunnellable oxide between the polycrystalline and monocrystalline silicon is responsible for the increased gain in the polysilicon emitter devices.

Pulse radiolysis of concentrated sulfuric acid

The pulse radiolysis method is used to study the optical absorption spectra in azeotropic sulfuric acid at 298 and 373 K. The spectra showing maximum absorption at 460 and 275 nm, observed at 298 K in the acid saturated with N 2, are associated with the radicals of HSO 4 and HSO 3, respectively. An additional absorption band of HSO 5 radical having a maximum at 245 nm appears in the acid saturated with O 2 at 298 K and in the acid saturated with N 2 at 373 K. It is suggested that the short-lived (under 1 μs) absorption bands with the maximums at ca 380 and 290nm belong to the transient species H 2SO - 4 and H 2SO + 4, respectively. The obtained results are used to construct the machanism of the early stages of the low- and high-temperature radiolysis of concentrated sulfuric acid.

Observation of collision-induced-dipole absorption bands in strontium–rare-gas mixtures. I. The 5s–4d bands

Reduced absorption coefficients have been measured for the collision-induced-dipole (CID) absorption bands associated with the 5s–4d transition of atomic strontium perturbed by He, Ne, Ar, Kr, Xe, and Sr. The main CID absorption band, assigned to 5sΣ–4dΣ, extends from the position of the atomic 5s–4d transition towards the shorter-wavelength side for every perturber. For Xe and Kr, an additional small absorption band is seen at the foot of the atomic 5s–4d transition. The former 5sΣ–4dΣ band is interpreted to result from collisional mixing of the 5p state of atomic Sr into the molecular 4dΣ state for small internuclear separations, while the latter band is interpreted to result from the collisional quadrupole (strontium)–dipole (Xe or Kr) interaction.