Changes In Optical Spectra Research Articles

Charge ordering and disordering transitions inPr1−xCaxMnO3 (x=0.4)as investigated by optical spectroscopy

The magnetic-field dependence of optical spectra and their anisotropy have been investigated for a single crystal of ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3} \phantom{\rule{0ex}{0ex}}(x=0.4)$ at 30 K. The charge-ordered (CO) state is transformed into a ferromagnetic metallic state by a magnetic field of 6.5 T, which is manifested in a huge change of optical spectra over a wide photon-energy region (0.05 eV--3 eV). The observed change in magnitude and anisotropy of the optical spectra with the external magnetic field has been elucidated in terms of the spin- and orbital-ordering structures in the CO state.

Spectral changes of CR-39 induced by irradiation and heat treatment

The structure of CR-39 was investigated under the effect of either gamma radiation or thermal treatment, by following the induced changes in its spectrum in the visible and ultraviolet region. Samples were irradiated with gamma doses of up to 100 kGy in air while heat treatment was carried out by holding the samples at temperatures of up to 180 °C in an air oven for 1 h. The spectra of the samples before and after gamma exposure and thermal treatment were analysed by the direct-allowed transitions suggested by Mott and Davis. The calculated optical energy gap ( E opt) values and the induced changes in absorption spectra of the irradiated samples are interpreted in terms of a degradation process at these doses. On the other hand, the temperature coefficient of the gap is evaluated as 1 × 10 −3 eV K −1 from the obtained E opt values for the heat-treated samples. The induced changes in optical spectra suggest crosslinking, by a free radical mechanism, at lower temperatures and bond rupture at oxygen-containing sites of the main chain at higher temperatures. An abrupt change in the UV spectrum of the sample heated at 180 °C is attributed to a phase transition in the amorphous structure of the polymer. In both cases of treatment, a colour centre band is induced at about 340 and 364 nm for the irradiated and heat-treated samples, respectively.

Studies of rotational level Λ-doubling by rf-optical double resonance spectroscopy: application to NaK D1Π

We report here the application of optical-radio frequency double resonance spectroscopy for individual rotational levels of the NaK D 1 Π state. Lambda doubling constant q values for five ν′, J′ levels are obtained. These data are combined with measurements of dc e- f Stark-mixing-induced changes in optical spectra, and the electric dipole moment d p in the D 1 Π state is determined.

Low-fluence excimer laser irradiation-induced defect formation in indium-tin oxide films

DC sputtered indium-tin oxide (ITO) films of 500 nm thickness are irradiated with single pulses of fluences between 190 and 510 mJ/cm2 from a KrF excimer laser. The irradiation induced changes in optical spectra are consistently described through the behaviour of four (Gaussian-like) absorption bands at 0.7, 1.0, 1.6 and 2.6 eV. Being absent in the original films and emerging at fluences exceeding 300 mJ/cm2, the 2.6 eV contribution is most characteristic to excimer laser processing. X-ray photoelectron spectroscopic analysis suggests that the irradiation-induced changes should be associated with oxygen displacement within the atomic network rather than surface reduction via oxygen removal. Thermal model calculations reveal that the principal effect of single pulse processing in this fluence domain is deep melting of the films. Defects created during molten phase resolidification are assumed to be responsible for the irradiation-induced changes in the short range chemical structure of the films.

Optical and EPR characterization of Desulfovibrio vulgaris (Hildenborough) sulfite reductase and ligand adducts

The electronic properties of the low molecular mass sulfite reductase (SiR) from the sulfate-reducing bacterium Desulfovibrio vulgaris (Hildenborough) have been examined in both oxidized and reduced forms. Two well-resolved changes in optical spectra are observed following the addition of the first and second electron equivalents. One-electron reduced SiR shows a general decrease in the optical absorbance, with retention of all features. After addition of the second electron equivalent the broad absorbance at ∼400 nm in the oxidized spectrum was observed to split and more resemble the spectral features of the larger dissimilatory sulfite reductase from the same organism. Electron paramagnetic resonance spectroscopy (EPR) has also demonstrated the stepwise reduction of siroheme and cluster in a manner consistent with the observed changes in the optical spectrum. The signal from the oxidized siroheme was found to disappear prior to the appearance of the signal arising from the reduced cluster; that is, the one-electron reduced enzyme is EPR silent. Oxidized sulfite reductase shows a rhombic low-spin hexacoordinate siroheme ( S = 1 2 , g = 2.45, 2.38, 1.78 ), and the reduced enzyme shows a characteristic ferredoxin-like Fe 4S 4 EPR spectrum ( g = 2.04, 1.93) that differs from spectra obtained with E. coli sulfite reductase (P.A. Janick and L.M. Siegel, Biochemistry 21 (1982) 3538). The AsO 2 − adduct of the fully reduced enzyme displays distinctive optical characteristics. Following oxidation, the AsO 2 − ligand remains bound, and the complex yields an optical spectrum that is again different from the native enzyme.

Effects of Spatial Coherence and Dispersive Diffraction on a Change in Optical Spectrum

This paper describes a change in optical spectrum on propagation in free space for a particular class of spatially partially coherent fields emanating from a polychromatic secondary source. A rectangular opening aperture in an opaque screen, illuminated with an extended incoherent polychromatic primary source, works as the secondary source. The spectral change is explored in detail theoretically and experimentally in association with spatial coherence in the secondary source as well as dispersive diffraction by the secondary source. The peak shift of the spectrum is associated with the coherence area for characterizing the secondary source. The peak shift becomes maximum if the secondary source is spatially coherent, but no shift occurs if the secondary source is spatially incoherent.

Temperature Dependence of Reflectivity in MnSr0.7Ca0.3O3

Abstract We measured the temperature dependence of the optical reflection spectrum of MnSr0.7Ca0.3O3, which was prepared by the solid reaction method. It was found that the compound showed distinct optical spectrum change in the cryogenic temperature.

Optical spectra of Pr2-xCexCuO4- delta crystals: Evolution of in-gap states with electron doping.

The systematic change of optical conductivity spectra with carrier doping and their anisotropy have been investigated for single crystals of Pr 2-x Ce x CuO 4-δ ; the crystals were grown by a traveling solvent floating-zone method. The evolution of in-gap states with x has shown up not only in the growth of midinfrared absorption below the charge-transfer gap but also in higher-lying interband transitions between O 2p and Cu 4s states. Possible roles of the reducing procedure, which is indispensable for producing superconductivity in the electron-doped cuprates, are also discussed in terms of observed changes of the optical spectra

Stability of ruby in solid hydrogen at megabar pressures.

High-pressure spectroscopic investigations carried out with diamond-anvil cells demonstrate that solid hydrogen undergoes a phase transition and changes in electronic properties near 150 GPa. An alternative interpretation has been proposed that ruby, which has been used as a pressure calibrant in such studies, is reduced by hydrogen to form aluminum metal and aluminum hydroxide, and that the resulting composite is responsible for the observed changes in optical spectra at these pressures. Direct measurements are performed and analyzed to test this hypothesis. We demonstrate that the hypothesis is inconsistent with the experimental data and that the spectral changes observed at 150 GPa are not associated with aluminum.

Role of ring torsion angle in polyaniline: Electronic structure and defect states.

The role of phenyl-ring torsion angle in determining the nature of the ground and charged-defect states of polyaniline is explored. The coupling of the transfer integral between nitrogen atom and phenyl-ring constituents of the polyaniline chain to the dihedral angle of the rings competes with the substantial steric repulsion between adjacent rings in determining the conformation of these systems. The ring conformation of the leucoemeraldine-base (LB) form of polyaniline is described by a novel ring-torsion-angle order parameter. The anharmonicity of the interring steric potential leads to a temperature-dependent mean order parameter, and thus to the prediction of thermochromic effects consistent with experiment. Furthermore, changes in optical spectra accompanying derivatization of the rings can be understood by the response of the substituted polymer to modifications of the steric potential. The existence of two degenerate ring-torsion-angle phases in LB implies that both polaronic and solitonic ring-angle-alternation defect states may be relevant in describing the charged states in polyaniline. As these defects involve substantial changes in ring torsion angle, they are expected to possess large kinetic mass, in agreement with photoinduced absorption experiments on polyaniline. The anticipated Peierls ground state of the oxidized pernigraniline-base form of polyaniline can be regarded in part as a ring-torsion-angle dimerized state; consequently, the charge states of this material are also expected to be massive defects in the ring-rotational order. The importance of ring rotations in other ring-containing electronic polymers, such as poly(paraphenylene sulfide), is discussed.

Quantum size effects in a heat-treated CdSe-doped glass

An oxide glass doped with CdSe has been made. The growth kinetics of CdSe microcrystals in the glass matrix has been studied by relating the changes in optical spectra of heat-treated glasses to quantum size effects. Particle size distrobutions have been measured using transmission electron microscope (TEM) and X-ray diffraction (XRD) and their effect on the absorption spectrum has been examined using a theoretical model.

Optical spectroscopic observation of a metastable form of sperm whale myoglobin generated by reconstitution

The optical spectrum of Sperm Whale myoglobin, which has been freshly reconstituted with iron protoporphyrin-IX, is shown to be different from that obtained from the native myoglobin, and from the reconstituted, incubated myoglobin (These last two have equivalent absorption spectra.). The effect is immediately evident as a shift of about +1 nm in the Soret band during incubation of freshly reconstituted metMb. Difference spectroscopy can be used to deconvolute changes in optical spectra occurring during and after Mb reconstitution into two components. The initial phase reflects incorporation of hemin into the protein matrix; this is already known to produce two forms, differing by relative hemin orientation. The rate of the second process follows the known pH dependance of iron protoporphyrin-IX reorientation. Presence of the second process indicates that the absorption spectrum of each of the two hemin-insertion Mb forms is unique, so interconversion between the two forms is monitored. Thus iron protoporphyrin-IX reorientation in proteins may be studied by visible spectroscopy.

Physical properties of AIIBVI semiconductor crystals after plastic deformation at low temperature

The influences of plastic deformation at low temperature (Tdef: 4.2 K; 77 K) on optical absorption, luminescence emission, and photoconductivity in the AIIBVI semiconductors CdS and CdSe are investigated. It is shown that even very small plastic deformation (e ≈ 10−4) leads to spatially inhomogeneously distributed remarkable changes of optical spectra in both the near band edge region and the range of deep centers. The main role of metastable point defect complexes introduced by the plastic flow is pointed out. Crystallographic nature and optical and electrical properties of the defect centers are discussed. The optical transitions in the defect centers are explained by a bound exciton mechanism. Furthermore, a variation of carrier lifetime due to dislocation glide destroying non-radiative centers is concluded. [Russian Text Ignored].

Kinetic study of growth of Cd(S,Se) microcrystals in a glass matrix

Using quantitative X-ray diffraction and transmission electron microscopy the growth kinetics and compositional changes of Cd(S,Se) crystals in a glass matrix have been studied in relation to heat treatment temperature and time. An activation energy of 230 kJ mol-1 for diffusion during coarsening was determined, similar to that found by studying optical spectra changes associated with quantum size effects.

Spectral properties of protonated Schiff base porphyrins and chlorins. INDO-CI calculations and resonance Raman studies

INDO-CI calculations successfully reproduce the striking changes in optical spectra that occur upon protonation of mono- and disubstituted porphyrin, chlorin, and bacteriochlorin Schiff base complexes. They ascribe the changes to Schiff base C=N ..pi..* orbitals which drop in energy upon protonation and mix with and perturb the ..pi..* orbitals of the macrocycle, a result consistent with resonance Raman data. The perturbation is predicted to affect not only transition energies and intensities but also dipole moment directions. The symmetry of the porphyrin and the substitution site of the chlorin are shown to play an important role, especially in governing whether the lowest energy transition will red shift or blue shift. Blue shifts are calculated for protonation of ketimine and enamine isomers of pyrochlorophyll a (PChl). Comparison with reported optical spectra suggests that PChl a Schiff base may undergo isomerization upon protonation. Resonance Raman data on CHO, CHNR, CHNHR/sup +/, and pyrrolidine adducts of chlorin demonstrate the isolation of the peripheral C=O and C=N groups from the macrocycle ..pi.. system intramolecular hydrogen bonding, and selective enhancement of v/sub C=N/ for those species with a split Soret band. V/sub C=N/ is observed with 488.0-nm excitation into the lower-energy Soret and absent for 406.7-nmmore » excitation into the higher-energy Soret, a result predicted by the calculations. 44 references, 10 figures, 2 tables.« less

Structural and Spectral Studies of Glassy MBBA

Abstract Structure measurements were carried out by neutron diffraction on the glassy nematic liquid crystalline and crystalline phases of MBBA. The change of optical spectra of intermolecular vibrations was also followed by Raman scattering around the phase transition from the fast-cooled state to the crystalline one. Parallel investigations of solid state polymorphism were also carried out using both methods.

ON THE MECHANISMS OF PHOTOLYSIS OF COMPOUNDS I AND II OF HORSERADISH PEROXIDASE AT 77 K*

Abstract—When Compounds I and II of horseradish peroxidase in glycerol/water glasses at 77 K were irradiated with the light of a mercury lamp, some changes in optical spectra as well as the development of strong EPR signals of a free‐radical type were detected. In case of photolysis of Compound I the light of wavelengths around 400 nm was the most effective, while only short wavelength (≥ 280 nm) light affected Compound II. The results of experiments with thawing and freezing the product of photolysis of Compound I (called Intermediate Y) provide further evidence for the suggestion that the ferric state of the heme iron is generated in the course of the photolysis. The quantum yields of the photolysis of Compound I at different wavelengths of irradiating light were obtained and mechanisms of the photolysis of Compounds I and II are proposed.

Cytochrome P-450 of bovine adrenal mitochondria. Ligand binding to two forms resolved by EPR spectroscopy.

The binding of cholest-5-ene-3beta,20alpha-diol (20alpha-hydroxycholesterol), 11-deoxycorticosterone, and aminoglutethimide to cytochrome P-450 in bovine adrenal mitochondria was measured by changes in optical spectra at room temperature and by EPR spectra at 14 K. The two methods provided nearly identical quantitation of these interactions with cytochrome P-450. Two distinct high spin forms of cytochrome P-450 were revealed by EPR spectra. The predominant high spin species (g = 8.2) was decreased by addition of 20alpha-hydroxycholesterol and elevated pH but was increased by addition of cholesterol. The minor high spin species (g = 8.1) was incrreased by addition of deoxycorticosterone but decreased by low concentrations of metyrapone. The two forms were evidently not in equilibrium and have been assigned to distinct forms of cytochrome P-450 involved in, respectively, cholesterol side chain cleavage (P-450scc) and steroid 11beta hydroxylation (P-450(11)beta). The high spin states are derived from complexes of these P-450 cytochromes with endogenous substrates, which are, respectively, cholesterol and deoxycorticoids. A high to low spin transition was observed when these complexes were turned over by initiating hydroxylation with malate. The contributions of cytochromes P-450(11)beta and P-450scc to the low spin spectrum were also resolved by similar means. At least 20% of P-450scc is in the low spin state while about 90% of P-450(11)beta is low spin in isolated beef adrenal mitochondria. Low spin complexes of cytochrome P-450scc with 20alpha-hydroxycholesterol and 3beta-hydroxypregn-5-ene-20-one (pregnenolone) gave distinct EPR spectra. Aminoglutethimide interacted with the total cytochrome P-450 content of the bovine adrenal mitochondria forming low spin complexes. Both optical and EPR data indicated binding to two forms of cytochrome P-450. These results suggest a detailed correlation between the spin state and absorbance changes seen at room temperature, illustrate that EPR allows the distinction of two principal forms of P-450, and suggest that there is no appreciable change in the spin state of either cytochrome between 14 K and 300 K.