Quantum size effects of CO reactivity on metallic quantum dots

We study the reactivity of a metallic quantum dot when exposed to a gas phase CO molecule. First, we perform a Newns-Anderson model calculation in which the valence electrons of the quantum dot are confined by a finite potential well and the molecule is characterized by its lowest unoccupied molecular orbital in the gas phase. A pronounced quantum size effect regarding the charge transfer between the quantum dot and molecule is observed. We then perform a first-principles calculation for a selected size interval. The quantum dot is described within the jellium model and the molecule by pseudopotentials. Our results show that the charge transfer between the quantum dot and the molecule depends critically on the size of the quantum dot, and that this dependence is intimately connected with the electronic structure. The key factor for charge transfer is the presence of states with the symmetry of the chemically active molecular orbital at the Fermi level.     

Importance of longitudinal contributions to backward inelastic electron scattering

Coulomb contributions of multipolarity 2 are calculated for backward (θ = 180°) inelastic electron scattering by our improved DWBA code. General features are studied such as nuclear charge (Z) dependence, energy transfer dependence, momentum transfer dependence as well as the angle dependence near θ = 180°, with the help of a simple model for the transition charge density. We compare these features with PWBA and other simple approximations. We show, as a specific example, that the C2 contribution is comparable to the M1 and E2 multipole form factors for inelastic scattering to the rotational excited states of ¹⁸¹Ta.     

Strong local electron correlations in high-temperature superconductors: Mean-field description of the metal-insulator transition

A realistic model of a Cu?O-plane, as the important structural element of high-temperature superconductors, is discussed with emphasis on strong local electron correlations. The Coulomb repulsion of electrons on oxygen sites as well as the one on copper sites is assumed very large, so that a systematic approximation scheme can be based on canonical transformations for elimination of both of these interactions. The resulting multiplet-J-model encorporates correlated hopping as leading processes and interactions between local spins as next leading ones. Here we concentrate on the leading terms and study transitions between a metallic phase and a charge transfer insulator phase via bosonization and mean field theory. The full selfconsistent one particle spectra are derived for the metal and the doped charge transfer insulator. Near the phase transition an analytic solution is given. Numerical calculations are based on realistic tight binding bands and include the influence of van Hove singularities. Physical implications, some formal aspects of mean-field theory and the connection to other approaches a are discussed.     

Size dependent electron momentum density distribution in ZnS

In this paper, we present size dependent electron momentum density distribution in ZnS. ZnS nanoparticles of size 3.8 nm and 2.4 nm are synthesized using the chemical route and characterized by X-ray diffraction (XRD). The Compton profile measurements are performed on both the nano-sized as well as bulk ZnS samples employing 59.54 keV gamma-rays from ²⁴¹Am source. The results reveal that the valence electron density in momentum space becomes narrower with reduction of particle size. To evaluate the charge transfer on compound formation, the ionic model based calculations for a number of configurations of Zn^+xS^−x (0.0≤x≤2) are also performed utilizing free atom Compton profiles. These results suggest different amounts of charge transfer in these materials varying from 1.2 to 2.0 electron from Zn to S atom.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Spin-glass phase in the ensemble of amphoteric impurities in a semiconductor

The transition to the spin-glass phase at T = 0 has been found for a two-dimensional ensemble of amphoteric impurities with charge transfer. The characteristics of the charge clustering found in specimens at low temperatures have been investigated. The finite size scaling with the critical exponents v = 0.95 ± 0.05, η = 0.75 ± 0.05 has been found for the spin-glass susceptibility _χSG(L,T) and Binder parameter g(L,T).     

Effects of charging and doping on orbital hybridizations and distributions in TiO2 clusters

Charging and doping are two important strategies used in TiO₂ quantum dots for photocatalysis and photovoltaics. Using small clusters as the prototypes for quantum dots, we have carried out density functional calculations to study the size-specific effects of charging and doping on geometry, electronic structure, frontier orbital distribution, and orbital hybridization. We find that in neutral (TiO₂)_n clusters the charge transfer from Ti to O is almost size independent, while for the anionic (TiO₂)_n clusters the corresponding charge transfer is reduced but it increases with size. When one O atom is substituted with N, the charge transfer is also reduced due to the smaller electron affinity of N. As the cluster size increases, the populations of 3d and 4s orbitals of Ti decrease with size, while the populations of the 4p orbital increase, suggesting size dependence of spd hybridizations. The present study clearly shows that charging and doping are effective ways for tailoring the energy gap, orbital distributions, and hybridizations.     

Transition charge densities of low-lying collective states in even Zn isotopes

The inelastic electron scattering cross sections for the quadrupole transitions to the 2₁⁺ and 2₂⁺ states in the even Zn isotopes ⁶⁴Zn, ⁶⁶Zn and ⁶⁸Zn and for the hexadecapole transition to the 4⁺₁ state in ⁶⁴Zn have been measured in a momentum transfer range up to q = 2.2 fm^?1. In the frame-work of the vibrational model these states are considered as one- and two-quadrupole-phonon states. The measurements are characterized by high statistical accuracy and by an overall resolution of δE/E₀ = 10^?3 which permitted separation of almost all members of the two-phonon triplet. The measured cross sections are analyzed with phenomenological models as well as with a Fourier-Bessel expansion of the transition charge density. The latter analysis yields realistic error bands for the transition charge densities and model-independent values for the reduced transition probabilities and transition radii.     

Charge transition densities for the excitation and nucleon decay of the giant dipole resonance in16O

Predictions on charge transition densities in¹⁶O for the excitation of the giant dipole resonance are given within a continuum self-consistent RPA-SK3 theory. The nuclear states are allowed to decay with proton or neutron ejection. The discussion is focused on the analysis of the radial behaviour of transition densities in terms of their microscopic structure. The rôle of RPA ground state correlations is clarified. The surface properties of resonant nuclear states extracted from photonuclear and (e, e′ x) reactions at low momentum transfer are confronted with the full radial dependence of the resonant structure shown in charge transition densities.     

Nonlocal Coulomb correlations in metals close to a charge order insulator transition

The charge ordering transition induced by the nearest-neighbor Coulomb repulsion V in the 1/4-filled extended Hubbard model is investigated using cellular dynamical mean-field theory. We find a transition to a strongly renormalized charge ordered Fermi liquid at V(CO) and a metal-to-insulator transition at V(MI)>V(CO). Short range antiferromagnetism occurs concomitantly with the CO transition. Approaching the charge ordered insulator, V approximately 相似文献   

151Eu Mössbauer Spectroscopy in La0.38Eu0.29Ca0.33MnO3

Electrical conductivity with and without magnetic field, d.c. magnetization and ¹⁵¹Eu Mössbauer studies were carried out in La_0.38Eu_0.29Ca_0.33MnO₃ perovskite manganite system. An insulating ground state is found throughout the temperature range with charge ordered (CO) state emerging at T _CO ～ 140 K, where as an external magnetic field of 6 T induces metal-insulator transition at ～120 K. D.C. magnetization measurements show the antiferromagnetic (AFM) transition occurring at T _N ≈ 48 K. The temperature dependent ¹⁵¹Eu Mössbauer measurements showed that the substituted Eu replaces La³⁺ in the 3+ charge state and a small magnetic moment gets induced at the Eu nucleus at low temperatures. The anomalous variation of the f- factor with temperature occurring around T _N and T _CO corroborates the occurrence of antiferromagnetic (AFM) and charge ordering (CO) transition, respectively.     

CO vibration as a probe of ligand dissociation and transfer in myoglobin

We report femtosecond visible pump, midinfrared probe, spectrally integrated experiments resolving the dynamics of CO in myoglobin upon photodissociation. Our results show a progressive change in absorption strength of the CO vibrational transition during its transfer from the heme to the docking site, whereas the vibrational frequency change is faster than our time resolution. A phenomenological model gives good qualitative agreement with our data for a time constant of 400 fs for the change in oscillator strength. Density-functional calculations demonstrate that indeed vibrational frequency and absorption strength are not linearly coupled and that the absorption strength varies in a slower manner due to charge transfer from the heme iron to CO.     

Volume of formation and electronic configuration in dilute alloys

A method is presented to relate the volume of formation to the electronic configuration of impurities in metals. The electronic configuration determines the effective size of the impurity through a modified ThomasFermi-Dirac model combined with elasticity theory. The size of the impurity determines the macroscopic volume change in the crystal, and the electronic configuration can be varied to fit the experimental volume of formation. Alloys of mainly transition metals are studied. The general conclusion of the work is that the charge transfer deduced from the changes in electronic configuration agrees with the expectations of the electronegativity scales, except for a few exceptions. For most of the alloys studied the changes consist of reoccupation of d and non-d levels at each site, maintaining the local neutrality. Only in a few cases were ionic configurations found, and the degree of ionicity was very small.     

Influence of Bi3+ doping on electronic transport properties of La0.5−xBixCa0.5MnO3 manganites

Electronic transport properties of La_0.5?xBi_xCa_0.5MnO₃ (x=0, 1/16, 1/8, 1/4, 3/8 and 1/2) compounds have been studied systematically to investigate their charge ordering (CO) behaviors. The results show that the CO temperature increases with the substitution of Bi³⁺ ion for La³⁺ ion, suggesting that the charge ordering is enhanced. This is attributed to the special role of the 6s² lone pair of Bi³⁺. It is found that for all the samples the adiabatic small polaronic conduction mechanism is responsible for the transport behavior above CO transition, whereas Mott's variable range hopping mechanism dominates below the CO transition. In addition, the electronic transport behavior of La_0.5?xBi_xCa_0.5MnO₃ compounds is high sensitive to an external magnetic field, which could raise fresh opportunities for application in magnetic sensors.     

Emissive Properties and Intramolecular Charge Transfer of Pyrazoloquinoline Derivatives

The fluorescence properties of several pyrazoloquinoline derivatives were performed by stationary as well as by time-resolved spectrosopy. Non-donor-substituted compounds show a high quantum yield; transition dipole moments of absorption and fluorescence as well as experimental and calculated lifetimes are in excellent agreement. The donor-substituted compound DMA-DPPQ exhibits a charge transfer fluorescence in polar solvents. Additionally, dual fluorescence appears in polar protic solvents. The nature of the charge transfer state is discussed with respect to the the TICT model.     

用Xα-DV方法研究CO,NO在Ⅷ族过渡金属表面的化学吸附(Ⅰ)——CO吸附在Rh(111)面的电子结构

本文用自洽Hartree-Fock-Slater分子丛方法计算了CO在Rh(111)面上(θ≤1/3)的电子结构。计算了分子丛的总能量、基态能级随吸附高度的变化。从总能量曲线确定的最佳键长为1.85?与实验值1.95±0.1?符合得较好。相应的吸附能为0.98eV比实验值1.3eV略小。在以上最佳键长处计算了总态密度,考虑终态和弛豫效应后与UPS实验结果符合更好。通过CO分子接近表面时各分子轨道能量本征值的变化,讨论了各轨道的成键、反键特征。通过Mulliken总数分析和用CO分子波函数展开总波函数的系数分析着重讨论了CO分子被过渡金属Rh吸附前后的电荷转移。这种电荷转移导致被吸附CO分子的活化。 关键词：     

气体分子在类石墨烯材料负载单个金属原子表面的吸附特性

采用密度泛函理论方法研究了不同种类和数量的气体小分子在类石墨烯材料(graphenylene)衬底支撑的金属原子(M=Co, Mo和Pd, gra-M)表面的吸附特性,系统地分析了吸附不同数量的NO和CO分子的稳定构型,吸附能,电荷转移量以及引起的体系电子结构和磁性变化.研究结果表明:1) NO、CO气体小分子的稳定吸附位在金属原子顶位,吸附物与衬底间的电荷转移量表明负载不同的金属原子能够有效地调制类石墨烯材料的气敏特性;2)单个和两个气体分子吸附能够引起gra-M体系的自旋电荷密度分布发生变化,进而使得气体吸附体系表现出不同磁矩大小.     

Neutral-to-ionic transition in tetrathiafulvalene-p-chloranil as investigated by optical reflection spectra

Polarized reflection spectra and their temperature-dependence as well are investigated on tetrathiafulvalene-p-chloranil (TTFCA) single crystals over the temperature range from 2 K to room temperature. Abrupt changes are observed at T_c = 84.4 ± 0.5 K in the charge transfer and intramolecular excitation bands. These experimental results strongly indicate that the dominant part of the neutral-to-ionic transition in this crystal takes place at a sharply defined critical temperature, in contrary to the recent observation by Torrance et al. [Phys. Rev. Lett.47, 1747 (1981)] that the transition occurs in a temperature range of about 30 K in width. A simple model is presented to account for the observed shift of the TTF⁺ intramolecular transition bands at T_c.     

第一性原理研究CO在Cu(110)表面的吸附

本文采用基于密度泛函理论的第一性原理方法, 并同时考虑范德华力的作用, 计算并分析了CO在Cu(110)表面的吸附情况. 结果表明: 1) CO在两个表面Cu原子的短桥位位置吸附最强, 吸附能为1.28 eV. 第二稳定吸附位置为表面Cu原子的顶位, 吸附能为1.23 eV. CO在其他两个位置, 表面两个Cu的长桥位和表面四个Cu的中心位的吸附要弱一些, 约为0.86 eV 和 0.83 eV. 2) 在Cu表面吸附的CO的C-O键长有部分拉长, 这与较强的吸附能和电荷转移相应. 3) 电荷分析表明所有吸附的CO整体上从衬底上面获得部分电荷, 约为0.2 个电荷.     

第一性原理研究CO在Cu(110)表面的吸附

本文采用基于密度泛函理论的第一性原理方法,并同时考虑范德华力的作用,计算并分析了CO在Cu(110)表面的吸附情况.结果表明:1)CO在两个表面Cu原子的短桥位位置吸附最强,吸附能为1.28 e V.第二稳定吸附位置为表面Cu原子的顶位,吸附能为1.23 e V.CO在其他两个位置,表面两个Cu的长桥位和表面四个Cu的中心位的吸附要弱一些,约为0.86 e V和0.83 e V.2)在Cu表面吸附的CO的C-O键长有部分拉长,这与较强的吸附能和电荷转移相应.3)电荷分析表明所有吸附的CO整体上从衬底上面获得部分电荷,约为0.2个电荷.