Investigation of the electronic structure of the BiSBr and BiSeBr clusters by density functional method

The energy levels of valence bands (VB) of the BiSBr and BiSeBr crystals have been calculated for investigation of the photoelectron emission spectra of BiSBr, BiSeBr and BiSI crystals. The molecular model of this crystal has been used for the calculation of VB by the Density Functional Theory (DFT) method. The molecular cluster, consisting of 20 molecules of BiSBr, BiSeBr, has been used for calculations of averaged total density of states, including atom vibrations. The spectra of the averaged total density of states from VB of BiSBr and BiSeBr clusters have been compared with the experimental photoelectron emission spectra from VB of BiSI crystals. The results clarify that the atomic vibrations in A⁵B⁶C⁷ type crystals with chain structure create a smoother appearance of the averaged total density of state spectrum and the experimental X-ray photoemission spectra (XPS).     

Electronic structure of BiSeI clusters

The valence band (VB) energy levels of BiSeI crystals have been calculated and compared to the X-ray photoelectron spectra of SbSI crystals. A molecular cluster consisting of 20 molecules of BiSeI has been used to calculate the total VB density of states by the density functional theory (DFT) method. The spectrum of total VB density of states of the (BiSeI)₂₀ cluster weighted by atomic photoemission cross-sections has been compared with the experimental X-ray photoelectron VB spectrum of SbSI type crystals. The cluster model calculation has showed that the core level energy splitting depends on the difference of ionic charges of the same atoms at the edges of the (BiSeI)₂₀ cluster.     

Time-dependent wave packet theoretical study of femtosecond photoelectron spectra and coupling between the A<Superscript>2</Superscript>Σ<Superscript>+</Superscript> and B<Superscript>2</Superscript>Π states of the NO molecule in a strong laser field

In this work, the femtosecond time-resolved photoelectron spectra and the coupling between the A²Σ⁺ and B²Π states of the NO molecule in a strong laser field have been investigated by the time-dependent wave packet method. We demonstrate that the weak coupling between the A²Σ⁺ and B²Π states of NO plays a key role on the peak centered at 0.37 eV of the photoelectron spectra in the 2+1’ channel.     

Theoretical investigation of the electronic structure of a ferroelectric SbSI cluster at a phase transition

This paper presents the theoretical investigation of energy levels of valence bands (VB) and core levels (CL) of the ferroelectric SbSl single crystals in antiferroelectric and ferroelectric phases. Since the best approximation for the deep VB levels is a calculation by the Hartree-Fock method, the molecular model of a SbSI crystal was used for calculations. This model of the crystal was also used for calculations of the total density of states. It was found that the VB and CL of this ferroelectric semiconductor are sensitive to the small lattice distortion at the phase transition, and that an average of the total density of states, when all atoms participate in oscillations of all normal modes, are more similar to the experimental X-ray photoelectron spectra (XPS). The experimental splitting of CL obtained by XPS was compared with the theoretically calculated one by two different methods. The cluster model calculations showed that the splitting of the CL in SbSI might be caused by photoelectron emission from the atoms, which have different valence state, at the surface.     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.     

Quantitative conversion spectroscopy of the ultrasoft isomeric transition of uranium-235 and the electronic structure of uranium oxides

Combined measurements of conversion electron spectra and the decay constant (76.5 eV, (1/2)⁺→(7/2)⁻) of the E3-isomeric transition of the uranium-235 nucleus have been performed with collection of the isomer atoms on an indium surface. The conversion spectra are interpreted as corresponding to a mixture of two different oxides A and B of uranium, one of which (A) is similar to UO₂, and the other (B) consists of a uranium-oxygen cluster based on the linear uranyl group O-U-O. From a set of mixed experimental spectra conversion spectra have been found corresponding to the chemical states A and B of the isomer atoms, and the variation of the absolute intensities of the conversion lines has been quantitatively investigated for them by varying the chemical composition of the isomer atoms and the ratio between the intensities of various conversion lines of the B spectrum. Experimental ratios between the intensities of the conversion lines are compared with the expected ratios in accordance with the distribution of the 6p electron density in the uranyl group. It is concluded that the experimental data agree with the calculation and that abrupt violations of proportionality of the partial probabilities of conversion of the electron density near the nucleus are absent. In accordance with the hypothesis of proportionality of the partial probabilities of conversion, an experimental estimate is given of the degree of localization of the deep-lying uranium 6p _1/2 shell during formation of the chemical bond in the uranyl group: around 70% of the 6p _1/2 electron density remains in the quasi-atomic uranium shell and around 30% is transferred to hybrid molecular orbitals. Zh. éksp. Teor. Fiz. 112, 574–595 (August 1997)     

Determination of temperature in a plasma jet emanating from a plasma torch with sectioned inter-electrode insert from the molecular emission spectrum of nitrogen

Results of application of a method for measuring the distribution of temperature in a nitrogen plasma jet emanating from a dc plasma torch with sectioned inter-electrode insert from the relative intensities of the molecular emission bands of nitrogen in the N₂ ⁺(B²Σ_u ⁺ − X²Σ_g ⁺) first negative and N₂(C³Π_u ⁺ − B³Π_g ⁺) second positive systems are reported. The emission spectra were registered using a small-size spectrometer with medium-range spectral resolution enabling a contour analysis of ro-vibrational bands in molecular emission spectra. The obtained distribution of temperature was compared with the distribution that was determined from the emission lines due to copper atoms and with the mean-mass plasma temperature of the air plasma jet.     

Structure of zinc-borate low-melting glasses derived from IR spectroscopy data

The IR spectra of glasses of the ZnO—SrO—B₂O₃ system with constant additions of PbO, Al₂O₃, and Li₂O (20 mol. % in sum) were studied. It is established that on replacement of B₂O₃ by ZnO, the structure of the glasses is characterized by the presence of groupings with the bridge bonds B^III— O—B^III, B^III—O—B^IV, B^IV—O—B^IV and end groups B^III— O⁻; ZnO practically exerts no influence on the coordination transition [BO₃] → [BO₄]. At a high content of ZnO, zinc ions are present in both a six-and a four-coordinated state. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 778–781, November–December, 2005.     

Photoelectron spectra of thin films of the high-T c superconductor YBa2Cu3O7−δ

The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra of the valence band of the compound YBa₂Cu₃O_7−δ for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa₂Cu₃O₆ films is observed to increase relative to YBa₂Cu₃O₇ films. Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997)     

Electronic structure of lithium tetraborate Li2B4O7 crystals. Cluster calculations and x-ray photoelectron spectroscopy

The results of an investigation of the electronic structure of lithium tetraborate crystals using experimental (x-ray photoelectron spectroscopy) and theoretical (quantum-chemical modeling) methods are reported. The experimental spectrum of the valence-band states of the crystal lies 2–15 eV below the Fermi level and is due primarily to boron-oxygen groups (B₄O₉). The quantum-chemical calculations were performed self-consistently, using the standard variant of the scattered-wave method in the model of a cluster embedded in a lattice of point charges. The data obtained on the partial contribution of the model densities to the one-electron spectrum of the [B₄O₉]⁶ cluster make it possible to interpret the fine structure of the experimental spectrum of the valence-band states. Fiz. Tverd. Tela (St. Petersburg) 41, 57–59 (January 1999)     

Luminescence spectra fine structure for uranyl sulfate and oxalate crystals

Quasi-line low-temperature (T = 77 and 4.2 K) luminescence spectra of uranyl sulfates and oxalates have been recorded and analyzed. It has been shown that the spectral structure is determined by the manifestation of the internal and external vibrations of both the uranyl ion and molecular groups (SO₄)^2– and (C₂O₄)^2–bonded to the uranyl ion. The spectral line half-width is of the order of ≈5–7 cm^–1; the line shape, close to Lorentzian. The fluorescence spectra may be complicated by structural defects in the crystals.     

The BΣ state of the SO radical

Spectra of the B³Σ⁻- X³Σ⁻ transition in SO above the first dissociation limit are recorded using degenerate four wave mixing. These spectra are combined with earlier work involving laser induced fluorescence, absorption spectra and Fourier transform emission spectra, to enable a rotational analysis and deperturbation of vibrational levels of the B state up to v′ = 16. Numerous perturbations were noted within the B³Σ⁻ state, and the origin of these is discussed. In a number of cases, these perturbations can be attributed to interactions with specific other electronic states of SO, such as A³Π, C³Π, d¹Π, and A″ ³Σ⁺.     

New analysis of the Comet-Tail (AΠi − XΣ) system in the CO isotopic molecule

The emission spectra of the 0-2, 4-2, and 6-1 bands of the Comet-Tail (A²Π_i − X²Σ⁺) system in the ¹⁴C¹⁶O⁺ isotopic molecule, comprising nearly 600 lines, have been recorded and analyzed for the first time. The spectra have been photographed under high resolution by using conventional spectroscopy, and it was possible to separate and observe most of the lines of all the 12 branches of this transition. The reduction of the individual bands’ spectra has been performed by nonlinear least-squares procedure and by means of effective Hamiltonians of Brown et al. the rovibronic structure parameters have been obtained. The currently investigated bands of the Comet-Tail system and the earlier analyzed bands of the A − X and B − A systems in the ¹⁴C¹⁶O⁺ molecule have been merged together. The results of this global fit made it possible to derive a new set of the equilibrium molecular constants for the A and X states. Then the RKR potential curve parameters for both A and X states and Franck-Condon factors as well as r-centroids for the A − X transition have been calculated for the ¹⁴C¹⁶O⁺ molecule.     

Low-temperature vibrational properties of KGd(WO4)2: (Er, Yb) single crystals studied by Raman spectroscopy

Temperature-dependent polarized Raman spectra of KGd(WO₄)₂: (Er, Yb) single crystals have been analyzed over the 77-292 K temperature range. The A_g and B_g spectra obtained are discussed in terms of factor group analysis. The spectra have been found to reveal the bands related to internal and external vibrations of WO₄²⁻, WOW and WOOW molecular groups. Strong depolarization of the majority of the Raman bands has been observed in the whole temperature range. Some anomalies in the spectral parameters of selected Raman bands below 175 K have been discussed in terms of the local distortion of WO₄²⁻ ions in KGd(WO₄)₂: (Er, Yb) crystals.     

Temperature dependence of the dual phosphorescence from xanthone in n-hexane matrices

The phosphoresence spectrum and lifetime of xanthone embedded in an n-hexane matrix has been investigated as a function of temperature (1·6-100 K). Vibrational analyses of the spectra reveal that emission occurs from three sites, two of which are dominant. Emission from one site (B) occurs from the ³ nπ* state of a planar xanthone molecule and is characterized by strong totally symmetric carbonyl stretching vibrations and a short lifetime (2·4 ms). Emission from the other site (C) is shown to arise from the ³ππ* state of an out-of-plane distorted xanthone molecule. It displays a vibrational structure rich in modes of a ₁, b ₁, and b ₂ (C _2v notation) symmetry and a long lifetime (115 ms). Both direct spin orbit coupling via configurational mixing of the nπ* and ππ* states due to the non-planarity of the molecule in its ³ππ* state and spin-orbit vibronic interaction involving ³ A ₁(ππ*)-¹ A ₂(nπ*) spin-orbit and ¹ A ₂(nπ*)-¹ B ₂(ππ*) vibronic interaction via out-of-plane b ₁ vibrations are shown to be responsible for the C site emission intensity. Vibronic mixing between the ³ππ* and ³ nπ* states is not important. With increasing temperature, the phosphorescence intensity from the B site (³ nπ*) emitters increases at first, reaches a maximum at ～20 K and then decreases. The C site (³ππ*) intensity simply decreases with rising temperature. At a given temperature, the phosphorescence lifetimes are identical and exponential for all emission bands regardless of site origin. These observations indicate an equilibrium between emitters in the two sites throughout the lattice. A phonon-assisted energy transfer mechanism is proposed to account for these observations.     

Excitons in monoclinic zinc diphosphide. Longitudinal exciton and the mixed mode

Exciton absorption spectra in high-quality β-ZnP₂ single crystals have been investigated at T=1.7 K for various directions of the wave vector and various polarization states of the radiation. It has been unambiguously established that the additional high-energy A series, which in some works has been called a D series and ascribed to ZnP₂ crystals, of so-called “rhombic” symmetry,^1,8,10,11 is an intrinsic exciton of the β-ZnP₂ series. A mixed mode has been detected for the first time, and the energy of the longitudinal exciton has been determined. The selection rules for the exciton transitions have been analyzed by a group-theoretical approach, and the symmetry of the nS states of the single exciton has been established on the basis of the experimental data — Γ ₂ ⁻ (z). Fiz. Tverd. Tela (St. Petersburg) 41, 193–202 (February 1999)     

Chemical bond and density of states of silver and copper halides based on DV-Xα method

We have calculated the electronic states of AgX and CuX (X = Cl, Br, I) by using the DV-Xα method. The density of states (DOS) and the bond overlap populations (BOP) of AgX and CuX are calculated by assuming the several model clusters with the nearest, second, and third-neighbor atoms. The cluster-size variations of DOS and BOP are discussed from the viewpoint of the chemical bond between the neighboring ions. We found that the BOP of AgI and CuX are larger than those of AgCl and AgBr. This result is consistent with the Phillips ionicity for A ^N B^8-N compounds. Furthermore, we have investigated the variation of the BOP between Ag and I ions along the diffusion path of the mobile Ag ions in the model cluster for α-AgI.     

Study of the electro-absorption spectrum of a nickel acceptor exciton in a ZnO:Ni crystal based on a calculation of vibrations associated with a Ni+1 impurity

Localized vibrations in ZnO crystals due to the substitution impurity Ni⁺¹ are modeled. The calculations were performed in the shell model using a recursive method for vibrations with A ₁ and E type symmetry. Numerical calculations allowed us to analyze the vibronic structure in the electro-absorption spectra for nickel acceptor excitons in ZnO:Ni. Fiz. Tverd. Tela (St. Petersburg) 41, 986–990 (June 1999)     

First principles study of structural, electronic and vibrational properties of heavily boron-doped diamond

The structural, electronic and vibrational properties of a series of heavily B-doped diamond models have been investigated using the density functional theory within a local density approximation. The doped models C_64 − nB_n (n=1-3) were constructed using supercell techniques. The structural and electronic calculations confirmed that the B dimers are always energetically stable and electrical inactive. The superconducting transition temperature TC is not only decided by the B concentration, but also by the lattice configurations of boron atoms. The vibrational frequencies and eigenmodes were determined using the linear response approach, while Raman intensities were obtained by the second response method. The Raman analysis in terms of atomic vibrations found that the “500 cm⁻¹” and “1230 cm⁻¹” bands are both superposed bands including not only C vibrations but also B-B vibrations and B-C vibrations, respectively. The calculated Raman spectra with isotopic substitutions are in excellent agreement with corresponding experimental results. The reasonable explanation was provided for no obvious Raman shift of main bands from ¹⁰B¹²C to ¹¹B¹²C model.