Infrared absorption studies on some derivatives of xanthic, dithiocarbamic and trithiocarbonic acids

The infrared absorption spectra of some of the derivatives of xanthic Image dithiocarbamic Image and trithiocarbonic Image acids are studied in the sodium chloride optics region and the bands assigned to group frequencies. The position of C---O---C and C=S bands in the derivatives of xanthic acid has been discussed from theoretical and experimental evidences and it is suggested that the two strong bands around 1200 and 1030 cm−1 are due to the Image group. The bands around 980 and 1050 cm−1 in the derivatives of dithiocarbamic and trithiocarbonic acids respectively have been assigned to C=S group frequencies. These bands shift to lower frequency in the corresponding ionic compounds while the bands around 1030 and 1200 cm−1 in the ionic compounds of xanthic acid shift to higher and lower frequencies respectively.

Investigations of oxide superconductors by x-ray absorption, photoemission and cognate spectroscopies

X-ray absorption spectra, X-ray photoelectron spectra and Auger spectra of cuprate superconductors are discussed. The studies establish the absence of Cu3+ for all practical purposes, but point out the importance of oxygen holes. X-ray photoelectron spectra of BaBi0.25Pb0.75O3 and related compounds are also examined.

Transition metal oxide perovskites by photoelectron and x-ray absorption spectroscopy

X-ray and ultraviolet photoelectron spectroscopy as well as x-ray absorption spectroscopy have been employed to investigate transition metal oxide perovskites of the general formula ABOs (A=La or rare-earth ion, B=trivalent transition metalion). Systematics in the core levels and in the valence bands in the series of LaBOa compounds have been discussed. Lanthanum chemical shifts in the x-ray absorption spectra in this series show interesting trends. Photoelectron spectra of the solid solutions, LaNil_x Coxes, LaNix_x FexO8 and LaFel_x Coxes show that the rigid band model is applicable to these systems. It is shown that x-ray photoelectron spectroscopy can be employed to identify multiple oxidation states of transition metal ions in oxide perovskites.

Electronic Structure of and the Metal-Insulator Transition in La1-xSrxCoO3-δ: A Soft-X-Ray Absorption Study

We report the soft-X-ray absorption spectra at the oxygen K-edge of La1-xSrxCoO3-δ (x = 0.0, 0.1, 0.2, 0.3 and 0.4) series with experimentally determined δ values. We show that the doping of holes by replacing La3+ with Sr2+ induces states within the band gap of the insulating undoped compound for small x and these doped states have a very substantial oxygen 2p character. This indicates that the insulating compounds belong to the charge transfer insulator regime. With increasing Sr content, the doped states broaden into a band overlapping the top of the primarily oxygen p-derived band, leading to an insulator-metal transition at x ≥ 0.2.

Recognition of Polycyclic Aromatic Hydrocarbons and Their Derivatives by the 1,3-Dinaphthalimide Conjugate of Calix4]arene: Emission, Absorption, Crystal Structures, and Computational Studies

Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants as well as well-known carcinogens. Therefore, it is important to develop an effective receptor for the detection and quantification of such molecules in solution. In view of this, a 1,3-dinaphthalimide derivative of calix4]arene (L) has been synthesized and characterized, and the structure has been established by single crystal XRD. In the crystal lattice, intermolecular arm-to-arm pi center dot center dot center dot pi overlap dominates and thus L becomes a promising receptor for providing interactions with the aromatic species in solution, which can be monitored by following the changes that occur in its fluorescence and absorption spectra. On the basis of the solution studies carried out with about 17 derivatives of the aromatic guest molecular systems, it may be concluded that the changes that occur in the fluorescence intensity seem to be proportional to the number of aromatic rings present and thus proportional to the extent of pi center dot center dot center dot pi interaction present between the naphthalimide moieties and the aromatic portion of the guest molecule. Though the nonaromatic portion of the guest species affects the fluorescence quenching, the trend is still based on the number of rings present in these. Four guest aldehydes are bound to L with K-ass of 2000-6000 M-1 and their minimum detection limit is in the range of 8-35 mu M. The crystal structure of a naphthaldehyde complex, L.2b, exhibits intermolecular arm-to-arm as well as arm-to-naphthaldehyde pi center dot center dot center dot pi interactions. Molecular dynamics studies of L carried out in the presence of aromatic aldehydes under vacuum as well as in acetonitrile resulted in exhibiting interactions observed in the solid state and hence the changes observed in the fluorescence and absorption spectra are attributable for such interactions. Complex formation has also been delineated through ESI MS studies. Thus L is a promising receptor that can recognize PAHs by providing spectral changes proportional to the aromatic conjugation of the guest and the extent of aromatic pi center dot center dot center dot pi interactions present between L and the guest.

Origin of the insulating state in NaCuO2

We study the electronic structure of NaCuO2 by analysing experimental core level photoemission and X-ray absorption spectra using a cluster as well as an Anderson impurity Hamiltonian including the band structure of the oxygen sublattice. We show that the X-ray absorption results unambiguously establish a negative value of the charge transfer energy, A. Further, mean-field calculations for the edge-shared one-dimensional CuO2 lattice of NaCuO2 within the multiband Hubbard Hamiltonian show that the origin of the insulating nature lies in the band structure rather than in the correlation effects. LMTO-ASA band structure calculations suggest that NaCuO2 is an insulator with a gap of around 1 eV.

Theoretical analysis of x-ray-absorption near-edge fine structure at the O and metal K edges of LaFeO3 and LaCoO3

We present experimental x-ray-absorption spectra at the oxygen and 3d transition-metal K edges of LaFeO3 and LaCoO3. We interpret the experimental results in terms of detailed theoretical calculations based on multiple-scattering theory. Along with providing an understanding of the origin of various experimental features, we investigate the effects of structural distortions and the core-hole potential in determining the experimental spectral shape. The results indicate that the core-hole potential as well as many-body effects within the valence electrons do not have any strong effect on the spectra suggesting that the spectral features can be directly interpreted in terms of the electronic structure of such compounds.

Temperature dependence of the energy gap and free carrier absorption in bulk InAs0.05Sb0.95 single crystals

Temperature dependence of the energy gap and free carrier absorption in a high-quality InAs0.05Sb0.95 single crystal was studied between 90 K and 430 K through the absorption spectra. At this alloy concentration, the room-temperature energy gap was measured to be 0.15 eV. Varshni- and the Bose–Einstein-type fit parameters were obtained from the measured temperature dependence of the energy gap, and the latter gave the zero-temperature gap to be 0.214 eV. It was found that although Weider’s empirical formula for the dependence of the energy gap on temperature and the alloy concentration agrees with the value of the gap at room temperature, it is inaccurate in describing its temperature dependence. From the free carrier absorption measurements, the phonon limited cross section of 7.35×10−16 cm2 at 15 μm was deduced at room temperature.

Triplet excited electronic state switching induced by hydrogen bonding: A transient absorption spectroscopy and time-dependent DFT study

The solvent plays a decisive role in the photochemistry and photophysics of aromatic ketones. Xanthone (XT) is one such aromatic ketone and its triplet-triplet (T-T) absorption spectra show intriguing solvatochromic behavior. Also, the reactivity of XT towards H-atom abstraction shows an unprecedented decrease in protic solvents relative to aprotic solvents. Therefore, a comprehensive solvatochromic analysis of the triplet-triplet absorption spectra of XT was carried out in conjunction with time dependent density functional theory using the ad hoc explicit solvent model approach. A detailed solvatochromic analysis of the T-T absorption bands of XT suggests that the hydrogen bonding interactions are different in the corresponding triplet excited states. Furthermore, the contributions of non-specific and hydrogen bonding interactions towards differential solvation of the triplet states in protic solvents were found to be of equal magnitude. The frontier molecular orbital and electron density difference analysis of the T-1 and T-2 states of XT indicates that the charge redistribution in these states leads to intermolecular hydrogen bond strengthening and weakening, respectively, relative to the S-0 state. This is further supported by the vertical excitation energy calculations of the XT-methanol supra-molecular complex. The intermolecular hydrogen bonding potential energy curves obtained for this complex in the S-0, T-1, and T-2 states support the model. In summary, we propose that the different hydrogen bonding mechanisms exhibited by the two lowest triplet excited states of XT result in a decreasing role of the n pi* triplet state, and are thus responsible for its reduced reactivity towards H-atom abstraction in protic solvents. (C) 2016 AIP Publishing LLC.

I. Spectroscopic evidence for slow vibrational relaxation in excited electronic states of diatomics in rare gas solids. II. Static crystal field effects in the electronic spectra of isotopically mixed benzene crystals

Part I

Studies of vibrational relaxation in excited electronic states of simple diatomic molecules trapped in solid rare-gas matrices at low temperatures are reported. The relaxation is investigated by monitoring the emission intensity from vibrational levels of the excited electronic state to vibrational levels of the ground electronic state. The emission was in all cases excited by bombardment of the doped rare-gas solid with X-rays.

The diatomics studied and the band systems seen are: N₂, Vegard-Kaplan and Second Positive systems; O₂, Herzberg system; OH and OD, A ²Σ⁺ - X²II_i system. The latter has been investigated only in solid Ne, where both emission and absorption spectra were recorded; observed fine structure has been partly interpreted in terms of slightly perturbed rotational motion in the solid. For N₂, OH, and OD emission occurred from v' > 0, establishing a vibrational relaxation time in the excited electronic state of the order, of longer than, the electronic radiative lifetime. The relative emission intensity and decay times for different v' progressions in the Vegard-Kaplan system are found to depend on the rare-gas host and the N₂ concentration, but are independent of temperature in the range 1.7°K to 30°K.

Part II

Static crystal field effects on the absorption, fluorescence, and phosphorescence spectra of isotopically mixed benzene crystals were investigated. Evidence is presented which demonstrate that in the crystal the ground, lowest excited singlet, and lowest triplet states of the guest deviate from hexagonal symmetry. The deviation appears largest in the lowest triplet state and may be due to an intrinsic instability of the ³B_1u state. High resolution absorption and phospho- rescence spectra are reported and analyzed in terms of site-splitting of degenerate vibrations and orientational effects. The guest phosphorescence lifetime for various benzene isotopes in C₆D₆ and sym-C₆H₃D₃ hosts is presented and discussed.

I. Electronic states of heavily-doped molecular crystals--naphthalene. I. Theoretical. II. Experimental. II. Lowest singlet and triplet excited states of pyrazine

PART I

The energy spectrum of heavily-doped molecular crystals was treated in the Green’s function formulation. The mixed crystal Green’s function was obtained by averaging over all possible impurity distributions. The resulting Green’s function, which takes the form of an infinite perturbation expansion, was further approximated by a closed form suitable for numerical calculations. The density-of-states functions and optical spectra for binary mixtures of normal naphthalene and deuterated naphthalene were calculated using the pure crystal density-of-state functions. The results showed that when the trap depth is large, two separate energy bands persist, but when the trap depth is small only a single band exists. Furthermore, in the former case it was found that the intensities of the outer Davydov bands are enhanced whereas the inner bands are weakened. Comparisons with previous theoretical calculations and experimental results are also made.

PART II

The energy states and optical spectra of heavily-doped mixed crystals are investigated. Studies are made for the following binary systems: (1) naphthalene-h₈ and d₈, (2) naphthalene--h₈ and αd₄, and (3) naphthalene--h₈ and βd₁, corresponding to strong, medium and weak perturbations. In addition to ordinary absorption spectra at 4˚K, band-to-band transitions at both 4˚K and 77˚K are also analyzed with emphasis on their relations to cooperative excitation and overall density-of-states functions for mixed crystals. It is found that the theoretical calculations presented in a previous paper agree generally with experiments except for cluster states observed in system (1) at lower guest concentrations. These features are discussed semi-quantitatively. As to the intermolecular interaction parameters, it is found that experimental results compare favorably with calculations based on experimental density-of-states functions but not with those based on octopole interactions or charge-transfer interactions. Previous experimental results of Sheka and the theoretical model of Broude and Rashba are also compared with present investigations.

PART III

The phosphorescence, fluorescence and absorption spectra of pyrazine-h₄ and d₄ have been obtained at 4˚K in a benzene matrix. For comparison, those of the isotopically mixed crystal pyrazine-h₄ in d₄ were also taken. All these spectra show extremely sharp and well-resolved lines and reveal detailed vibronic structure.

The analysis of the weak fluorescence spectrum resolves the long-disputed question of whether one or two transitions are involved in the near-ultraviolet absorption of pyrazine. The “mirror-image relationship” between absorption and emission shows that the lowest singlet state is an allowed transition, properly designated as ¹B_3u ← ¹A_1g. The forbidden component ¹B_2g, predicted by both “exciton” and MO theories to be below the allowed component, must lie higher. Its exact location still remains uncertain.

The phosphorescence spectrum when compared with the excitation phosphorescence spectra, indicates that the lowest triplet state is also symmetry allowed, showing a strong 0-0 band and a “mirror-image relationship” between absorption and emission. In accordance with previous work, the triplet state is designated as ³B_3u.

The vibronic structure of the phosphorescence spectrum is very complicated. Previous work on the analysis of this spectrum all concluded that a long progression of v_6a exists. Under the high resolution attainable in our work, the supposed v_6a progression proves to have a composite triplet structure, starting from the second member of the progression. Not only is the v_9a hydrogen-bending mode present as shown by the appearance of the C-D bending mode in the d₄ spectrum, but a band of 1207 cm^-1 in the pyrazine in benzene system and 1231 cm^-1 in the mixed crystal system is also observed. This band is assigned as 2v_6b and of a_1g symmetry. Its anonymously strong intensity in the phosphorescence spectrum is interpreted as due to the Fermi resonance with the 2v_6a and v_9a band.

To help resolve the present controversy over the crystal phosphorescence spectrum of pyrazine, detailed vibrational analyses of the emission spectra were made. The fluorescence spectrum has essentially the same vibronic structure as the phosphorescence spectrum.

Synthesis of nickel(II) complexes of alpha-isoxazolylazo-beta-diketones and their absorption of spin-coating films and thermal properties for HD-DVD-R

Three kinds of new nickel(II) complexes of alpha-isoxazolylazo-beta-diketones with blue-violet light absorption were synthesized. Their structures were postulated based on elemental analysis, MS and FT-IR spectra. Smooth films on K9 glass substrates were prepared using the spin-coating method. The absorption properties and thermal stability of these complexes were discussed. The static optical recording test for high density digital versatile disc-recordable (HD-DVD-R) system was also studied. (c) 2005 Elsevier B.V. All rights reserved.

Optical parameters and absorption of copper (II)-azo complexes thin films as optical recording media

Smooth thin films of three kinds of azo dyes of 2-(5'-tert-butyl-3'-azoxylisoxazole)-1, 3-diketones and their copper (II)-azo complexes were prepared by the spin-coating method. Absorption spectra of the thin films on a glass substrate in the 300-600 nm wavelength region were measured. Optical constants (complex refractive index N=n+ik) and thickness of the thin films prepared on single-crystal silicon substrate in the 300-600 nm wavelength region were investigated on rotating analyzer-polarizer type of scanning ellipsometer, and dielectric constants epsilon(epsilon=epsilon(1)+i epsilon(2)), absorption coefficients alpha as well as reflectance R of thin films were then calculated. In addition, one of the copper (II)-azo complex thin film prepared on glass substrate with an Ag reflective layer was also studied by atomic force microscopy (AFM) and static optical recording. AFM study shows that the copper (II)-azo complex thin film is very smooth and has a root mean square surface roughness of 1.89 nm. Static optical recording shows that the recording marks on the copper (II)-azo complex thin film are very clear and circular, and the size of the minimal recording marks can reach 200 nm. (c) 2004 Elsevier B.V. All rights reserved.

Synthesis, spectroscopic properties and thermal stability of metal(II) tetraazaporphyrin complexes with two strong wavelength absorption

Three kinds of metal(II) tetraazaporphyrin complexes with blue-violet and red light wavelength absorption were synthesized by refluxing tetraazaporphyrin ligand and different metal(II) ions, respectively. Their structures were confirmed by elemental analysis, LDI-TOF-MS, FT-IR and UV-Vis. The solubility of metal(II) tetraazaporphyrin complexes in organic solvents and absorption properties of their chloroform solution and films on K9 glass in the region 250-800 nm were measured. The influence on the difference of absorption maximum from metal(II) tetraazaporphyrin complexes to tetraazaporphyrin ligand by different metal(II) ions was studied. In addition, the thermal stability of the complexes was also evaluated. (c) 2006 Elsevier Ltd. All rights reserved.

Effects of Yb concentration on the fluorescence spectra of Yb-doped YAlO3 single crystals

0.5 at.% Yb:YAlO3(YAP), 5 at.% Yb:YAP and 15 at.% Yb:YAP were grown using the Czochralski method. Their absorption and fluorescence spectra were measured at room temperature and their emission line shape was calculated using the method of reciprocity. It was observed that the fluorescence spectra changed appreciably with the increasing of Yb concentration. For 0.5 at.% Yb:YAP, the line shape of fluorescence is very similar with the calculated emission line shape; with the increasing of Yb doping concentration, the line shape of fluorescence is very different from the calculated emission line shape. These phenomena are caused by the strong self-absorption at 979 and 999 nm for Yb:YAP. (c) 2005 Elsevier B.V. All rights reserved.