Spatial and electronic structure of the Ni3P surface

To understand the catalytic effect in the Ni-Ni₃P for the growth of carbon nanostructures, the structural and electronic properties of Ni₃P surface are calculated from first-principles calculations. The calculated surface energies for the (0 0 1)-Ni₄P₄-terminated surface, the (0 0 1)-Ni₈-terminated surface, and the (1 1 0)-Ni₈-terminated surface show that the (0 0 1)-Ni₄P₄-terminated surface is energetically more stable within the allowed range of the chemical potential of P. Through the analysis of the partial density of states of Ni and P atoms in surface and bulk states, respectively, it is further found that due to the bond contractions of the surface layer, the core-level shifts of P atoms in the (0 0 1)-Ni₄P₄-terminated surface make P atoms in the Ni₃P particles act as a catalyst. Finally, the obtained results of the work function show that the (0 0 1)-Ni₄P₄-terminated surface has the largest work function when compared with the other two studied surfaces.     

Interaction of electronic structure and nuclear dynamics on the S1 reaction surface for the ring opening of cyclohexadiene

For the ultrafast photoinduced ring opening of cyclohexadiene the S₁ state plays a central role, providing the possibility to rapidly decay to the ground state. In this paper we follow the path of a wavepacket propagating in the reactive coordinate space of the S₁ surface. We present a detailed analysis of the corresponding electronic and nuclear motions. With the help of a projection method and a normal-mode analysis, the vibrational modes driving the reaction might be detected experimentally. Received: 10 November 1999 / Published online: 13 July 2000     

Critical size and surface effect of the hydrogen interaction of palladium clusters

Metal hydrides are used for electrochemical or gaseous storage of hydrogen because considerable amounts of hydrogen are reversibly absorbed and desorbed at interstitial sites. Palladium is often used as a model system. Nanophase material is of interest because properties related to the hydrogen absorption are size dependent. In this study, clusters from the size of 55 to 1415 atoms are investigated and compared with bulk Pd. It turns out that not only the amount of hydrogen per palladium that can be intercalated changes but also kinetics and chemical potentials are dependent on the cluster size. The clusters used for this study were chemically synthesised and stabilised by a ligand shell. Received 9 October 1998 and Received in final form 10 May 1999     

Adsorption of alkanes and their coadsorption with hydrogen on evaporated palladium thin films

Adsorption of methane, ethane and propane and their coadsorption with hydrogen on evaporated palladium thin films at 195 K and 298 K was investigated by means of surface potential change measurements, observation of gas phase composition and volumetric measurements. Adsorption at 195 K occurs without any significant decomposition of hydrocarbon admolecules, while at 298 K approximately 10% of the total deposit of ethane and propane can be decomposed. Propane admolecules strongly promote the incorporation of coadsorbed hydrogen adatoms into the bulk of palladium film at 195 K.     

Interaction of hydrogen with palladium surface: FIM and FEM studies

In the present paper, we focus on the geometrical and electronic changes in palladium surface structure which appeared during its interaction with hydrogen in the presence of an external electric field. The interaction process was examined by using the field ion microscopy (FIM) as well as the field emission microscopy (FEM) techniques. In order to study the geometrical changes in substrate surface structure, the distance distribution function (DDF) was constructed on the basis of FIM patterns of both a clean and hydrogen-covered palladium surface. The electronic changes were examined by the measurement of the total energy distribution (TED) of electrons emitted from the palladium tip surface. The most pronounce examples of such changes are an expansion of the equilibrium interatomic distance in palladium surface and a shift of the Fermi level of the metal. These changes may be explained among others by palladium hydrides formation. This process is the most efficient if the field strength exceeds 23 V/nm.     

Molecular dynamics simulations of hydrogen adsorption/desorption by palladium decorated single-walled carbon nanotube bundle

Utilising molecular dynamics simulations, the hydrogen molecules adsorption isotherms of the (8,?0) palladium decorated single-walled carbon nanotube (SWNT) were obtained. The hydrogen adsorption was studied on the external, interstial and internal surfaces of the SWNT bundle at several temperatures ranging from 77 to 400?K. The results were compared with the bare single-walled carbon nanotube bundle under the same conditions. The decorated carbon nanotube bundle hydrogen adsorption was significantly higher than that of the bare one. The hydrogen desorption and readsorption were studied using temperature as the readsorption/desorption variable. The rate constants were calculated for the hydrogen desorption at different temperatures. The calculated decorated SWNT bundle hydrogen desorption activation energy was higher than that for the bare SWNT bundle. The calculated activation energies for the hydrogen desorption in both nanotube bundles specified the temperature dependency of hydrogen desorption.     

Lattice and electronic contributions to the elastic constants of palladium

We measured second order elastic constants of Pd from 295 to 850 °K and under hydrostatic pressure up to $\text{7000 kg}\text{cm}{}^2$. A study of the temperature behavior and the volume dependence of these constants allows to separate the electronic contributions from those of the lattice. Grüneisen constants are calculated. With our model a certain relation between elastic shear constants and the magnetic susceptibility is successfully applied and the deformation potential is determined.     

Spectral densities and the surface electronic structure of tungsten

The recursion method is used to calculate spectral densities at surfaces for the particular purpose of further analysing the nature of the central peak in the d-band density of states at the tungsten (100) surface.     

Fermi surface and band mapping of the cerium/palladium surface alloy

X-ray and UV excitation angle-resolved photoemission spectroscopy of ultra-thin films of cerium deposited on Pd(1 1 1) single-crystal surface has been carried out. Photoelectron diffraction pattern showed that deposition of 1 ML of Ce led to a formation of Ce-Pd substitutional alloy. Valence band spectra measured with high angular resolution permitted to plot valence band maps and Fermi surface scans and showed formation of surface alloy exhibiting d- and f-electron orbital hybridization. A shift of Pd 4d-derived states to higher binding energy in the Ce-Pd systems was observed.     

Population dynamics: Poisson approximation and its relation to the Langevin process

We discuss how to simulate a stochastic evolution process in terms of difference equations with Poisson distributions of independent events when the problem is naturally described by discrete variables. For large populations the Poisson approximation becomes a discrete integration of the Langevin approximation [T. G. Kurtz, J. Appl. Prob. 7, 49 (1970); 8, 344 (1971)]. We analyze when the latter gives a reasonable representation of the original evolution for finite size systems. A simple example of an epidemic process is used to organize the discussion and to perform statistical tests that underline the goodness of the proposed method.     

The valence electronic structure of SeO2 studied by ups and its relation to that of other group VIB dioxides

HeI and HeII spectra of SeO₂ are presented and are analysed with reference to published work on the valence-shell structure of other group VIB dioxides. Orbital trends are discussed and the importance of processes involving two electrons in the valence-shell photoelectron spectra of all members of this group is noted. Where relevant the results of detailed studies of dioxides other than SeO₂ are presented to help clarify the analysis.     

Image quantization and its relation to network structure processes

The relation between image quantization and network structure is described. HalLtoning methods are interpreted as special network types and a learning algorithm for halftoning is presented.     

Nb2H 的电子结构和相互作用

应用基于密度泛函理论的赝势-平面波方法研究了Nb₂H的电子结构和H原子占据点之间的关系.计算结果表明：由4个Nb近邻构成的四面体中心点(T点)为H的稳定俘获点，而由6个Nb近邻构成的八面体中心点(O点)则为相互作用势的极大值点，是不稳定点.相邻T点间存在低能量通道，具有鞍点(S点)结构.在T点及近邻低能通道上,H的1s能级展宽较弱，Nb的4d, 5s 带部分向下延展，与H带杂化后形成孤立带.当H由T向     

A general model for hydrogen trapping at the inclusion-matrix interface and its relation to crack initiation

Abstract

The role of non-metallic inclusions in hydrogen-induced failure of structural materials has been a controversial topic for many years. In this paper, hydrogen trapping and its relation to the crack initiation at the inclusion-matrix interfaces are studied by considering the interfacial structure and the interaction between the dissolved hydrogen atoms and the elastic strains produced by lattice matching and misfit dislocations. A model is proposed to analyse the change of interfacial structure with inclusion size and its relation to hydrogen trapping. Hydrogen accumulation at the interfaces is quantitatively analysed. The obtained results are in good agreement with the experimental observations. The multiple factors, such as interfacial structure, chemical composition, elastic properties of matrix and inclusions, crystallographic relationship between inclusions and matrix, inclusion morphology and size, simultaneously control hydrogen trapping. In addition, the mechanism of hydrogen-induced crack initiation at the interface is investigated. A criterion is proposed to determine critical conditions for crack initiation. For the first time, the inherent relationship between hydrogen trapping and hydrogen-induced cracking at the interface is clarified. This work paves a way for an in-depth understanding of the effects of inclusions on hydrogen-induced degradation of mechanical properties.     

An order-N electronic structure theory with generalized eigenvalue equations and its application to a ten-million-atom system

A linear algebraic theory called the 'multiple Arnoldi method' is presented and realizes large-scale (order-N) electronic structure calculations with generalized eigenvalue equations. A set of linear equations, in the form of (zS - H)x = b, are solved simultaneously with multiple Krylov subspaces. The method is implemented in a simulation package ELSES (www.elses.jp) with tight-binding-form Hamiltonians. A finite-temperature molecular dynamics simulation is carried out for metallic and insulating materials. A calculation with 10(7) atoms was realized by a workstation. The parallel efficiency is shown up to 1024 CPU cores.     

The diamond surface: atomic and electronic structure

Experimental studies of the diamond surface with primary emphasis on the (111) surface are presented. Aspects of the diamond surface which are addressed include (1) the electronic structure, (2) the atomic structure, and (3) the effect of termination of the lattice by foreign atoms. Limited studies of graphite are discussed for comparison with the diamond results. Experimental results from valence band and core level photoemission spectroscopy (PES), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) are used to study and characterize both the clean and hydrogenated surface. In addition, the interaction of hydrogen with the diamond surface is examined using results from vibrational high resolution low energy electron loss spectroscopy and photon stimulated ion desorption (PSID) yield at photon energies just above the carbon k-edge. Both EELS and PSID verify that the mechanically polished 1 × 1 surface is hydrogen terminated and also that the reconstructed 2×2/2×1 surface is hydrogen free. We apply this basic knowledge of the clean diamond surface and of diamond-hydrogen systematics to understanding of the fundamental growth characteristics of diamond films.     

Effect of hydrogen dimers on the electronic structure of graphene

We investigate the effect of hydrogen dimers on the electronic structure of graphene. Using Greenʼs function and the T-matrix approach, we calculate the local density of states of graphene with single hydrogen dimer, as well as the quasiparticle spectral function of graphene with a finite concentration of randomly distributed hydrogen dimers. Our results show that the effect of dimer adsorption is dramatically different from that of monomer adsorption previously studied, and strongly depends on the configuration of the dimer. The features of the plotted spectral function of graphene are relevant to the band gap opening and the metal–insulator transition.