X-Ray Photoelectron Spectra and Electronic Structure of Solid Solutions Based on Cubic Boron Nitride

The electronic energy structure of substitution solid solutions based on boron nitride B _1-x NR _x and BN _1-x R_x (R = C, O) (x=0.25) in a diamond-like modification of ZnS type has been investigated by the local coherent potential method in terms of multiple-scattering theory. The total and partial densities of states were calculated for each element in a solid solution. The crystalline potential was calculated using an MT approximation. The lattice parameter was chosen based on X-ray diffraction data for c-BN: 0.3615 nm. The electronic energy structures of the solid solutions and binary c-BN are compared in the framework of a single approximation. The calculated partial densities of states are compared with the experimental X-ray emission and photoelectron spectra of boron, nitrogen, and oxygen in these compounds. The calculated partial charges of electrons at the top of the valence band show that charge transfer from boron to nitrogen takes place in the solid solutions. An analysis of the electronic structures of the solid solutions of boron nitride indicates that the quasicore resonances inherent in binary c-BN are delocalized and that chemical bonding in the solid solutions of boron nitride is weakened.     

X-ray spectra and electronic structure of boron nitride in different crystallographic modifications

The electronic energy structure of boron nitride with ZnS (c-BN) and wurtzite (w-BN) type crystal lattices is calculated by the local coherent potential (LCPA) method in a multiple scattering approximation. The local partial 2p states of boron with c-BN and w-BN are compared with the boron K emission spectra in the corresponding compounds. Fine structure is first obtained in the region of the top of the valence band. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1083–1087, November–December, 1998.     

X-ray spectra and electronic structure of several ternary chalcogenides and their solid solutions

The electronic energy structure of substitution solid solutions CuGa(S_xSe_1−x)₂ is studied within wide limits of sulfur concentration x in the onion sublattice. The SK absorption spectrum is calculated for CuGaS₂ in a high-order multiple scattering approximation using the FEFF7 program. For all concentrations x, partial densities of states are calculated in a full multiple scattering approximation by the local coherent potential method. The calculation schemes for the filled and vacant states are employed, which differ in a choice of the crystalline potential. The effect of a vacancy on the SK level on the density of the free Sp states is considered. The theoretical K absorption spectra and densities of states of CuGaS₂ are compared with the experimental X-ray and X-ray photoelectron spectra. The calculated curves are in good agreement with the experiment. It is established that the densities of the S and Se p states change smoothly with varying concentration of anions. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1076–1082, November–December, 1998.     

Joint X-ray spectroscopic and quantum-chemical study of the electronic structure of pentafluorophenylalkyl ethers

The high resolution X-ray emission O-Kα spectra of pentafluorophenylalkyl ethers C₆F₅OR (R=Et, Prⁱ, and Bu^t) exhibit differences related to a change in the electronic structure of the compounds as R is varied. The search for stable conformers was performed by the semiempirical PM3 method. The most probable structures of C₆F₅OR were determined by the comparison of the experimental and theoretical X-ray spectra plotted for each conformer usingab initio calculations in the 6–31 G basis set. Substituent R in pentafluorophenylalkyl ethers is situated outside of the ring plane. The fluorination of the benzene ring changes the energy level of the lone electron pair of oxygen relative to the levels of orbitals of the ring and substituent R and leads to an increase in the efficiency of interactions in the σ-system. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2443–2450, December, 1998.     

X-Ray Spectra and Electronic Structure of 3C SiC and BN Based Solid Solutions

The local coherent potential approximation is used in the framework of multiple-scattering theory to calculate the electronic energy structure of solid solutions of silicon carbide Si_1-xCR_x and boron nitride BN_1-xR_x and B_1-xNR_x (x = 0-0.75, R = C, Al, Ti) in a diamond-like modification. The total and partial densities of states are calculated for each atom in the solid solutions. The crystal potential is evaluated in an MT approximation. The lattice parameter is determined by Vegard's rule. The electronic energy structures of the solid solutions are compared with each other and with binary analogs in the framework of one approximation. The calculated partial densities of states are compared with the experimental X-ray spectra of silicon in the compounds. The calculation of the partial charges of atoms at the top of the valence band showed that the charge transfer (0.35 e) from boron to nitrogen in binary 3C BN changes sign in B_0.75NC_0.25. In the latter system, nitrogen donates 0.19 e to boron, and carbon acts as a donor for the electronic configurations of boron and nitrogen. An electronic structure analysis of the solid solutions indicates that the quasicore resonance states inherent in the binary compounds are delocalized, probably because of the weakening of chemical binding in the solid solutions.     

Fragmentation of sputtered vanadium and niobium cluster ions,kinetic release and dissociation energy

The generation and unimolecular fragmentation of V _n ⁺ and Nb _n ⁺ clusters formed in sputtering vanadium and niobium surfaces by Xe⁺ ions has been studied. The method of measuring the kinetic energy of fragment ions (kinetic energy release distribution) has been used to determine the dissociation energy. Kinetic energy spectra have been measured in the field-free zone (corresponding to a time window of 10⁻⁵–10⁻⁴ sec after emission) of an ion microanalyzer with double focusing in reverse geometry. The results of spectra measurement were treated using the Rice-Ramsperge-Kassel theory of unimolecular reactions and the “evaporative ensemble”, which allowed us to calculate the dissociation energies of homonuclear V _n ¹ (n= 5–11) and Nb _n ¹ (n = 3–8) clusters.     

An approximate method of self-consistent electronic structure calculations of a crystal surface. The Ir(111) surface

The Slater approximation for atomic electron energies is employed to calculate the electronic structure of the Ir(111) surface. The results are used to estimate the shift of the 4f_7/2 level on the surface relative to its position in the volume. The electron state densities in the neighborhood of the center of the surface Brillouin zone (SBZ) are compared with photoelectronic spectra for emission normal to the Ir(111) surface. On this basis, the accuracy of this approximation is estimated. Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 1, pp. 17–24, January–February, 1994. Translated by O. Kharlamova     

Isotope effect of hydrogen and lithium hydride molecules. Application of the dynamic extended molecular orbital method and energy component analysis

In order to explore the isotope effect including the nuclear–electronic coupling and nuclear quantum effects under the one-particle approximation, we apply the dynamic extended molecular orbital (DEMO) method and energy component analysis to the hydrogen and lithium hydride isotope molecules. Since the DEMO method determines both electronic and nuclear wave functions simultaneously by variationally optimizing all parameters embedded in the basis sets, the virial theorem is completely satisfied and guarantees the relation of the kinetic and potential energies. We confirm the isotope effect on internuclear distances, nuclear and electronic wave functions, dipole moment, the polarizability, and each energy component. In the case of isotopic species of the hydrogen molecule, the total energy decreases from the H₂ to the T₂ molecule due to the stabilization of the nuclear–electronic potential component, as well as the nuclear kinetic one. In the case of the lithium hydride molecule, the energy lowering by replacing ⁶Li with ⁷Li is calculated to be greater than that by replacing H with D. This is mainly caused by the small destabilization of electron–electron and nuclear–nuclear repulsion in ⁷LiH compared to ⁶LiH, while the change in the repulsive components from ⁶LiH to ⁶LiD increases. Received: 24 March 1999 / Accepted: 5 August 1999 / Published online: 15 December 1999     

Semiempirical calculations of the electronic spectra of organometallic compounds: Estimation of solvent and counterion effects

The electronic spectra of the [Ru(NH₃)₅pz]²⁺, [Ru(NH₃)₅pz]³⁺, and [Ru(CN)₅pz]³⁻ complexes are calculated by the CI INDO method. The effect of solvation on the spectra is considered in a point charge approximation and in terms of the parametric model of a charged sphere enveloping the metal fragment. Interrelation of these approaches is discussed. Data on the molecular electrostatic potentials created by the complexes are presented to substantiate the models. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 603–618, July–August, 1996. Translated by I. Izvekova     

Importance of shattering fragmentation in the surface-induced dissociation of protonated octaglycine

A QM + MM direct chemical dynamics simulation was performed to study collisions of protonated octaglycine, gly₈-H⁺, with the diamond {111} surface at an initial collision energy E _i of 100 eV and incident angle θ _i of 0° and 45°. The semiempirical model AM1 was used for the gly₈-H⁺ intramolecular potential, so that its fragmentation could be studied. Shattering dominates gly₈-H⁺ fragmentation at θ _i = 0°, with 78% of the ions dissociating in this way. At θ _i = 45° shattering is much less important. For θ _i = 0° there are 304 different pathways, many related by their backbone cleavage patterns. For the θ _i = 0° fragmentations, 59% resulted from both a-x and b-y cleavages, while for θ _i = 45° 70% of the fragmentations occurred with only a-x cleavage. For θ _i = 0°, the average percentage energy transfers to the internal degrees of freedom of the ion and the surface, and the energy remaining in ion translation are 45%, 26%, and 29%. For 45° these percentages are 26%, 12%, and 62%. The percentage energy-transfer to ΔE _int for θ _i = 0° is larger than that reported in previous experiments for collisions of des-Arg¹-bradykinin with a diamond surface at the same θ _i . This difference is discussed in terms of differences between the model diamond surface used in the simulations and the diamond surface prepared for the experiments.     

A combined experimental and theoretical study of resonance emission spectra of SO2( B2)

Experimental and theoretical resonance emission spectra are obtained for a number of vibrational states of SO₂( ¹B₂). The experimental emission spectra are dominated by (n₁ν₁, n₂ν₂, 0) bands, but some weak activity in the anti-symmetric stretch (ν₃) is observed. The calculated emission spectra based on an empirical near-equilibrium potential energy surface agree reasonably well with experiment for two lowest states investigated here, but fail to reproduce higher ones. Resonance Raman spectra are also calculated and agree well with an earlier experiment.     

t-Si64: A Novel Silicon Allotrope

Utilizing first principle calculations, a novel Si₆₄ silicon allotrope in the I4₁/amd space group with tetragonal symmetry (denoted as t-Si₆₄ below) is proposed in this work. In addition, also its structural, anisotropic mechanical, and electronic properties along with its minimum thermal conductivity κ_min were predicted. The mechanical and thermodynamic stability of t-Si₆₄ were evaluated by means of elastic constants and phonon spectra. The electronic band structure indicates that t-Si₆₄ is an indirect band gap semiconductor with a band gap: 0.67 eV (primitive cell) compared to a direct band gap of 0.70 eV with respect to a conventional cell. The minimum thermal conductivity of t-Si₆₄ (0.74 W cm⁻¹ K⁻¹) is much smaller than that of diamond silicon (1.13 W cm⁻¹ K⁻¹). Therefore, Si−Ge alloys in the I4₁/amd space group are potential thermoelectric materials.     

Low temperature luminescence spectra of the d10s2 complexes Cs2MX6 (M = Se,Te and X = Cl,Br). The Jahn—Teller effect in the Γ−4(3T1u) excited state

The emission spectra of the title compounds in microcrystalline form have been measured at 10 K. The extensive vibrational progression in the e_g mode is indicative of a tetragonal Jahn—Teller distortion in the Γ^?₄(³T_1u) excited state. The vibronic coupling of a threefold electronic state with a doubly degenerate e_g mode (T—e coupling), linear in the nuclear coordinates, has been reinvestigated considering spin—orbit coupling up to second order perturbation on energy levels which result from an a¹_1gt¹_1u electron configuration. For an estimation of Jahn—Teller coupling constants, the intensity distributions in the progressions were compared with the theoretical line shape functions which were obtained from a model which also permits the determination of potential energy minima and vibrational fundamentals of the excited state. The unusually large increase in the e_g vibrational frequency compared to the ground state is due to Jahn-Teller forces which distort the potential surface, yielding steeper excited state energy curves.     

Photoelectrochemical properties of TiO2 films obtained by electrical explosion

The electrical explosion of wires was used to prepare titanium dioxide nanopowders alloyed with silver nanoparticles. The photoelectrochemical properties and electronic structure of these materials were studied. The quantum yield for the photoelectrochemical current η and the flat band potential E _fb for TiO₂/Ag films were found to be proportional to the content of the Ag⁰ phase on the electrode surface.     

Electronic Energy Structure of TiCu and Ti2Cu

The electronic energy structure (EES) of the valence band in tetragonal TiCu and Ti₂Cu was studied experimentally and theoretically. The experimental study of valence band EES was carried out by X-ray photoelectron spectroscopy (XPS). The calculations were performed in terms of the cluster version of multiple scattering theory in a self-consistent field approximation. The results are compared with X-ray emission spectroscopy data available in the literature. The density of state curves agree well with spectroscopic data. The major contribution to XPS is from the copper d-states. The specifics of chemical bonding in the compounds leading to the observed changes in the shape of the valence band X-ray photoelectron spectra are discussed.     

Theoretical study on S<Subscript>1</Subscript>(<Superscript>1</Superscript>B<Subscript>3u</Subscript>) state electronic structure and absorption spectrum of pyrazine

Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S₀(¹ A _g)) and the first (S₁(¹ B _3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S₁(¹ B _3u) state of pyrazine. The assignment of main vibronic transitions is made for S₁(¹ B _3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν_6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S₁(¹ B _3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S₁(¹ B _3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν_10a. The calculated S₁(¹ B _3u) absorption spectrum is in reasonable agreement with the experimental spectra. Supported by Taiwan National Science Council (Grant Nos. NSC 96-2113-M-009-021 and NSC 96-2811-M-009-023)     

Electronic structure and spectral characteristics of Bi-Sr-Ca-Cu-O films

The electronic structure of high-temperature superconducting films of Bi-Sr-Ca-Cu-O is determined using the linearized augmented plane wave method. Total and local partial densities of states are calculated for each film using the results of band structure calculations. Theoretical OK_α and CuL_α X-ray emission spectra are found. The dependence of the electronic structure and spectra on the number of CuO₂ layers in the unit cell is discussed. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1067–1075, November–December, 1998.     

A CSF-based multi-reference coupled pair approximation III. An application to F2, As2 and As+2

Using the newly developed multi-reference coupled pair approximation program code, the adiabatic potential curves of the ground states of F ₂, As ₂ and As ₂ ⁺ were calculated. Computed spectroscopic constants of these molecules were found to be in good agreement with experimental values. The resulting binding energy of As ₂ (3.86 eV) was compared with the experimental value of 3.99 eV [15] and the best multi-reference configuration interaction value (3.58 eV) reported previously by the present authors. The calculated first adiabatic ionization potential of As ₂ (9.67 eV) was found to be in good agreement with the experimental result. Received: 5 July 1997 / Accepted: 27 August 1997     

Should a franck-condon or a curve-crossing picture be applied to ion-target collisional activation? A study of keV CO<Stack><Subscript>2</Subscript><Superscript>+·</Superscript></Stack>/He collisions by emission spectroscopy

Collision-induced photon emissions (CIE) were observed for keV CO₂^+·/He collisions from 190 to 1020 nm. The emissions were assigned to the Δν=0 band of the CO₂^+· B ²Σ_u⁺ → X ²Π_g electronic transition and the Δν = +3, +2, +1, 0, −1, −2, −3 vibrational transition progression in the CO₂^+· A ²Π_u → X ²Π_g electronic transition. The other peaks arise from the emissions of excited O^· fragment atoms and the target gas. The relative intensities of the CO₂^+· and O^· emissions are independent of the ion translational energy above 3 keV, supporting the curve-crossing mechanism for collisional excitation. Investigation of the relative intensities within the A ²Π_u → X ²Π_g emission of CO₂^+· indicates that the vibrational distribution is well described by the Franck-Condon principle at high collision energy, a consequence of short collision time but not necessarily an indication of vertical transitions. Below 3 keV ion translational energy, vibrational excitation in the A ²Π_u electronic state was observed. The observation is consistent with the explanation that the reaction occurs at small impact parameters, in which short-range, repulsive interactions between the projectile and the target result in direct translational-vibrational excitation.     

CaWO_4∶xEu~(3+),ySm~(3+),zLi~+红色荧光粉的制备及光学性能

采用微波固相法制备了CaWO_4∶xEu~(3+),ySm~(3+),zLi~+红色荧光粉。测量样品的XRD图、激发谱、发射谱及发光衰减曲线,研究并分析了Eu~(3+)、Sm~(3+)、Li~+的掺杂浓度,对样品微结构、光致发光特性、能量传递及能级寿命的影响。结果表明,Eu~(3+)、Sm~(3+)、Li~+掺杂并未引起合成粉体改变晶相,仍为CaWO_4单一四方晶系结构。Eu~(3+)、Sm~(3+)共掺样品中,Sm~(3+)掺杂为3%时,Sm~(3+)对Eu~(3+)的能量传递最有效。Li~+掺杂起到了助熔剂和敏化剂的作用,使样品发光更强。在394 nm激发下,与CaWO_4∶3%Eu~(3+)样品比较,3%Eu~(3+)、3%Sm~(3+)共掺CaWO_4及3%Eu~(3+)、3%Sm~(3+)、1%Li~+共掺CaWO_4样品的发光分别增强2倍及2.4倍。同一激发波长下,单掺Eu~(3+)样品寿命最短,Sm~(3+)、Eu~(3+)共掺样品随Sm~(3+)浓度增加,寿命先减小后增加,且掺杂了Li~+的样品比不掺Li~+的样品~5D_0能级寿命有所增加。