Time-dependent wavepacket calculations of atom scattering from surfaces with impurities

Exact quantum-mechanical calculations are reported on atom scattering from a crystalline surface with isolated impurities. The calculations are for He scattering from a one-dimensional model of a Cu surface with adsorbed Ar atoms. The difficulties of carrying out calculations on scattering from extended but non-periodic structures are overcome by using a time-dependent wavepacket approach. A recently developed method for solving the time-dependent Schrödinger equation is employed. Scattering intensities are given for several energies and incidence angles. Detailed insight is obtained on impurity effects on surface scattering. The main features are: (1) Broad intensity tails are superimposed on each diffraction spike. The width of the tails decreases with increasing diffraction order; (2) Shallow rainbow peaks arise, due to impurity induced local corrugation; (3) Weak intensity maxima arise due to interference between surface and impurity scattering. The intensities are somewhat sensitive to the position of the impurity within the surface unit cell. Physical interpretation of the effects is provided from exact calculations, and from a simple sudden approximation for the scattering intensities. It is argued that He scattering can be used to determine impurity locations on surfaces.     

Size exclusion chromatography with Corona charged aerosol detector for the analysis of polyethylene glycol polymer

A technique of using size exclusion chromatography (SEC) with the Corona charged aerosol detector (CAD) was developed and evaluated in comparison with refractive index (RI) and evaporative light scattering detection (ELSD) for fast screening of polyethylene glycol (PEG), a polymer used in preparing pegylated pharmaceutical compounds. These detection techniques were used in the analysis of multiple lots of PEG reagents. CAD was found to provide more accurate impurity and polydispersity profiles of PEG reagents that better differentiate their quality, while RI was not suitable for this application due to its low sensitivity and ELSD led to underestimation of the impurity and polydispersity. The accuracy of polydispersity determination by SEC-CAD was validated against a commercial reference standard of known polydispersity. The SEC-CAD technique and the observed differences between the three detectors can also be applied to polymer analysis in general.     

Statistical mechanics in rotationally inelastic scattering of molecules from surface within the dynamical Lie algebraic method

The dynamical Lie algebraic (DLA) method of Alhassid and Levine [Phys. Rev. A 18 (1978) 89] is applied to statistical mechanics in rotationally inelastic scattering of molecules from surfaces. Specifically, the method is generalized to include the motion of surface atoms, i.e., phonons. For given Hamiltonian and initial state, the set of constraints required to obtain the solution of the motion equations is determined by an algebraic procedure. It is furthermore found possible to derive the motion equations for the mean values of the constraints. Application of the method to the scattering of NO molecules from a Pt(1 1 1) surface is made. The mean values of the final energies of NO molecules scattered from the surface obtained using the DLA method are in good agreement with experimental results in qualitative trends. The DLA method thus appears to have a wide range of validity for describing the statistical mechanics of the gas-surface scattering.     

Thermal expansion and impurity effects on lattice thermal conductivity of solid argon

Thermal expansion and impurity effects on the lattice thermal conductivity of solid argon have been investigated with equilibrium molecular dynamics simulation. Thermal conductivity is simulated over the temperature range of 20-80 K. Thermal expansion effects, which strongly reduce thermal conductivity, are incorporated into the simulations using experimentally measured lattice constants of solid argon at different temperatures. It is found that the experimentally measured deviations from a T(-1) high-temperature dependence in thermal conductivity can be quantitatively attributed to thermal expansion effects. Phonon scattering on defects also contributes to the deviations. Comparison of simulation results on argon lattices with vacancy and impurity defects to those predicted from the theoretical models of Klemens and Ashegi et al. demonstrates that phonon scattering on impurities due to lattice strain is stronger than that due to differences in mass between the defect and the surrounding matrix. In addition, the results indicate the utility of molecular dynamics simulation for determining parameters in theoretical impurity scattering models under a wide range of conditions. It is also confirmed from the simulation results that thermal conductivity is not sensitive to the impurity concentration at high temperatures.     

Application of the Lie algebraic approach to diffractionally and rotationally inelastic molecule–surface scattering

The Lie algebraic approach of Alhassid and Levine [Phys. Rev. A 18 , 89 (1978)] is applied to the molecule–surface scattering. Specially, the diffractionally and rotationally inelastic scattering of a diatomic molecule from a solid surface is dealt with. Within the framework of the close-coupling method, we construct a Hamiltonian for the scattering system and use it to generate a dynamical algebra h₆. By solving equations of motion for the group parameters, the scattering wave functions near the surface are obtained. Computed transition probabilities of diffractively and rotationally inelastic scattering of H₂ from LiF(001) surface with the use of Lie algebraic method are seen to agree well with the coupled-channel calculations. The Lie algebraic method thus appears to have a wide range of validity for describing the dynamics of gas–surface scattering. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 981–989, 1997     

Surface segregation of silicon impurities in organic materials

The present revolution in novel organic materials is driven by the synthesis of new materials exhibiting specific functional properties. Traces of silicon compounds are often present in these materials and, although the bulk concentrations of these impurities may be low, segregation can seriously modify the surface composition. Surfaces and interfaces play an important role in many applications, and the intrinsic properties of the materials are thus often obscured by the presence of segregated impurities. By studying silicon impurity segregation in poly‐dialkoxy phenylenevinylene (PPV), polycarbonate and dendrimer macromolecules, we demonstrate how low‐energy ion scattering may be used to determine the surface impurity fraction and to observe which groups at the surface are shielded by the segregated species. We demonstrate that the performance of PPV‐ based light‐emitting diodes is significantly reduced for submonolayer coverages of siloxanes. We find that the kinetics of the segregation process depend strongly on the materials and the sample preparation conditions. We find that the presence of solvents is needed to enable segregation at room temperature. Heating does enable siloxane impurity segregation in polycarbonate in the solid phase, whereas for polydimethylsiloxane in PPV films we find that segregation in the solid phase does not occur up to 200 °C. The siloxane molecules are found to segregate to preferential sites at the surface, shielding the polar groups. Finally, we demonstrate that purification of the surface is often possible through simple procedures that provide an easy way to study the intrinsic properties of the materials. Copyright © 2004 John Wiley & Sons, Ltd.     

Angle-resolved x-ray photoelectron spectroscopy

In this review, various aspects of angle-resolved x-ray photoelectron spectroscopy (ARXPS) as applied to solid state- and surface chemical- studies are discussed. Special requirements for $\text{instrumentation}$ are first considered. The use of $\text{grazing-emission}$ angles to enhance surface sensitivity and study surface concentration profiles of various types is then discussed. Various effects that may limit the accuracy of such measurements such as surface roughness, electron refraction, and elastic scattering are considered. Several examples of surface-specific electronic structure changes as studied by grazing-emission ARXPS (e.g., valence-band narrowing and core-level shifts) are also reviewed. The use of $\text{grazing-incidence}$ geometries for surface enhancement is also briefly considered. Single-crystal studies providing additional types of information via ARXPS are next discussed. For core-level emission from single-crystal substrates or adsorbed overlayers, $\text{x-ray photoelectron diffraction (XPD)}$ is found to produce considerable fine structure in polar- or azimuthal- scans of intensity. Such XPD effects can be very directly related to the atomic geometry near a surface, for example, through simple intramolecular or intermolecular scattering processes. A straightforward single scattering or kinematical theory also appears to describe such effects rather well, thus far permitting several structures to be solved by analyses of azimuthal intensity scans. Likely future developments and possible limitations of such XPD structure studies are also discussed. Finally, $\text{valence-band ARXPS}$ is considered, and it is shown that pronounced direct-transition effects can be observed provided that the specimen Debye-Waller factor is not too small. A simple free-electron final-state model is found to predict these direct-transition effects very well, and future studies at low temperatures and with higher angular resolution seem promising.     

Influence of impurities on the x-ray photoelectron spectroscopy and Raman spectra of single-wall carbon nanotubes

The effect of impurities on the properties of single-wall carbon nanotubes (SWNTs) was investigated with multiple analytical techniques. Charge transfer is believed to occur between the impurities and the SWNTs as observed by combining the Raman scattering and x-ray photoelectron measurements. The impurity condition (type and level) was found to strongly affect the electronic and vibrational properties of the SWNT. The metal catalysts in the impurity usually behave as electron donors, which can downshift the graphitic (G) band as well as the radial breathing mode frequencies. The low temperature air oxidation of as-prepared SWNT material usually upshifts the radial breathing mode Raman peaks to higher frequencies.     

New treatment of the theory for small-angle x-ray scattering with applications to polystyrene crazes

The phenomenon of surface scattering of electromagnetic waves by single or multiple layers of films is reviewed and a special treatment for the total reflection of x rays is developed. This theory is applied to the analysis of the surface scattering observed in small-angle x-ray scattering (SAXS) studies of two-phase matter in polymers having lamella stacks or a flat interfacial boundary structure. Important features of this vector theory are the ability to calculate the surface scattering invariant, the absolute scattering intensity, and the surface roughness, which gives rise to dispersion of specular reflection from perfectly smooth surfaces. By considering the interfacial surface roughness of polystyrene crazes, the surface scattering spectrum is calculated theoretically and compared with some experimental results. Also the theory is presented in such a way as to compare surface scattering with volume scattering; i.e., both two- and three-dimensional scattering events can be simultaneously treated. This provides a new basis for quantitative analysis of crazes in polystyrene.     

The Preparation of Surface Chemically Pure Sodiumn-Dodecyl Sulfate by Foam Fractionation

A novel procedure for the purification to surface chemical standard of sodiumn-dodecyl sulfate and its retrieval as a solid is presented. As a result of their nature and relative surface activity with respect to the product, very small amounts of impurity can significantly alter the result of surface tension experiments. In addition to the preparation, the surface chemical behavior of the product is analyzed and compared with previously published data. Additional suggested criteria for purity are also applied, and their validity is discussed.     

Structural Change in Dextran: Mechanism of Insolubilization by Adsorption on the Air-Liquid Interface

To clarify the insolubilization mechanism of water-soluble dextran, the association of dextran in water was studied by dynamic light scattering measurements and a surface chemical approach. Dynamic light scattering measurements indicated that insolubilization of dextran is accompanied by a structural change in dextran. Surface tension data for dextran molecules revealed a structural change in dextran molecules at the air-liquid interface. These results suggest that insolubilization of dextran molecules occurred through an adsorption process at the air-liquid interface. Insolubilization of dextran molecules can be reduced by inhibition of this structural change in dextran molecules. The presence of boron as an impurity was found to trigger precipitation based on inductively coupled plasma mass spectrometry measurements and precipitation tests. Copyright 1999 Academic Press.     

微量稀土对工业纯铝中杂质相的变质行为

采用SE,EDAX,TEM他稀土对工业纯铝中富Fe(Si)杂质相的变质作用及机制,结果表明：富铈混合稀土（RE）是一种有效变质剂,可使铝中杂质相由粗大长针（条）状或骨骼变细小的团球状或短棒状,且分布均均匀,提高了材料的力学性能（尤其是塑性）,其变质机制主要是因稀土在固／液界面前前沿的富集,导致了稀土进入杂质相形成（AlFeSiRE)的复杂化合物,或吸附在杂质相表面阻碍其长大,但过量稀土易导致第二相数量增多,降低材料塑性,其加入量应小于 0．07％（质量分数）。     

Adsorption effects on resonant charge transfer in atom-surface scattering

The probability of positive ionization of an atom impinging on a solid surface is calculated by numerical solution of the equations of motion of the time-dependent molecular orbitals of the system. The model is applied to the scattering of Na atoms, in the energy range 50–1000 eV, from a W(110) surface. The presence of Na adatoms is considered and found to hinder the ionization process. This reduction in the charge-transfer probability arises because of the lowering of the surface work function brought about by increasing adatom concentration.     

Observation of Ion Electrosorption in Metal–Organic Framework Micropores with In Operando Small‐Angle Neutron Scattering

A molecular‐level understanding of transport and adsorption mechanisms of electrolyte ions in nanoporous electrodes under applied potentials is essential to control the performance of double‐layer capacitors. Here, in operando small‐angle neutron scattering (SANS) is used to directly detect ion movements into the nanopores of a conductive metal–organic framework (MOF) electrode under operating conditions. Neutron‐scattering data reveals that most of the void space within the MOF is accessible to the solvent. Upon the addition of the electrolyte sodium triflate (NaOTf), the ions are adsorbed on the outer surface of the protrusions to form a 30 Å layer instead of entering the ionophobic pores in the absence of an applied charging potential. The changes in scattering intensity when potentials are applied suggests the ion rearrangement in the micropores following different mechanisms depending on the electrode polarization. These observations shed insights on ion electrosorption in electrode materials.     

Scattering of a potassium atom beam from potassium promoted catalyst surfaces via electronically excited clusters

Surface scattering of potassium atom beams is observed from surfaces of a potassium promoted catalyst, which is known to emit Rydberg K* species and clusters K _n ^* . The surfaces studied are cut flat from pellets of an industrial catalyst, the promoted iron oxide catalyst for styrene production. The scattering is studied in the temperature range 500–1000 K in an UHV apparatus with a K atom beam at 45° towards the normal, with surface ionization and ion detection over an angular range of ?90° to +90° with respect to the surface normal. Bilobular scattering patterns are observed, which are mainly back-scattering at low temperatures, below 750 K. A large signal due to ions emitted in the backwards direction is also found with a voltage on the sample. This back-scattering indicates that the scatterers are heavy clusters outside the surface. The ion formation in the backwards direction is proposed to be due to collisions with electronically excited clusters K _n ^* of the type recently observed by field ionization detection (Kotarba et al. 1994). The bilobular scattering transforms into asymmetric patterns with a larger forward (specular) lobe at higher temperatures, above 800 K. Only a small fraction of the beam molecules is scattered off the surface. The scattering is well described by inelastic surface scattering theory. This shows that the actual scattering surface is rather flat, which is proposed to be due to an antibonding Rydberg type interaction, of long range (hundreds of Å), between the impinging excited K atom and the surface. The temperature dependence of the neutral scattering gives a barrier of 0.96 eV, close to what is generally found for Rydberg species emission from such surfaces. At larger K surface densities, the contributions to the peaks from the beam flux is shown to agree with this picture involving collisions with excited clusters outside the surface.     

Observation of Ion Electrosorption in Metal–Organic Framework Micropores with In Operando Small-Angle Neutron Scattering

A molecular-level understanding of transport and adsorption mechanisms of electrolyte ions in nanoporous electrodes under applied potentials is essential to control the performance of double-layer capacitors. Here, in operando small-angle neutron scattering (SANS) is used to directly detect ion movements into the nanopores of a conductive metal–organic framework (MOF) electrode under operating conditions. Neutron-scattering data reveals that most of the void space within the MOF is accessible to the solvent. Upon the addition of the electrolyte sodium triflate (NaOTf), the ions are adsorbed on the outer surface of the protrusions to form a 30 Å layer instead of entering the ionophobic pores in the absence of an applied charging potential. The changes in scattering intensity when potentials are applied suggests the ion rearrangement in the micropores following different mechanisms depending on the electrode polarization. These observations shed insights on ion electrosorption in electrode materials.     

Change in the interfacial reaction of Hf-silicate film as a function of thickness and stoichiometry

Medium energy ion scattering and high-resolution transmission electron microscopy are used to investigate the depth of the interfacial reaction of Hf-silicate film. The interfacial reaction is critically affected by the film thickness and the mole fraction of HfO(2) in silicate film. The interfacial compressive strain generated at the surface of the Si substrate is dependent on the film thickness during the postannealing process in film with a thickness of approximately 4 nm. Finally, the phase separation phenomenon demonstrates critically different behaviors at different film thicknesses and stoichiometries because the diffusion of Si from interface to surface is dependent on these factors. Moreover, the oxidation by oxygen impurity in the inert ambient causes SiO(2) top formation.     

A colloid "digesting" route to novel, thermally stable high surface area ZrO2 and Pd/ZrO2 catalytic materials

Zirconia having high thermal stability and high surface area (up to 160 m(2)/g at 700 degrees C) has been prepared by a colloidal "digesting" process. This material having demonstrated high surface areas at elevated temperatures was then applied as a catalyst support. A Pd colloid with diameter of approximately 12 nm has been successfully deposited on the high surface area zirconia material. All systems have been well characterized by TEM, X-ray diffraction, N2 adsorption isotherms, FTIR, elemental analysis and dynamic light scattering techniques. The colloidal Pd particles have been found homogeneously well dispersed in the hydrous zirconia matrix without aggregation. The Pd/ZrO2 catalysts have been screened for cyclohexene and 1-hexene hydrogenation activity and it was found that the catalyst is extremely active.     

Exploiting time-independent Hamiltonian structure as controls for manipulating quantum dynamics

The opportunities offered by utilizing time-independent Hamiltonian structure as controls are explored for manipulating quantum dynamics. Two scenarios are investigated using different manifestations of Hamiltonian structure to illustrate the generality of the concept. In scenario I, optimally shaped electrostatic potentials are generated to flexibly control electron scattering in a two-dimensional subsurface plane of a semiconductor. A simulation is performed showing the utility of optimally setting the individual voltages applied to a multi-pixel surface gate array in order to produce a spatially inhomogeneous potential within the subsurface scattering plane. The coherent constructive and destructive electron wave interferences are manipulated by optimally adjusting the potential shapes to alter the scattering patterns. In scenario II, molecular vibrational wave packets are controlled by means of optimally selecting the Hamiltonian structure in cooperation with an applied field. As an illustration of the concept, a collection (i.e., a level set) of dipole functions is identified where each member serves with the same applied electric field to produce the desired final transition probability. The level set algorithm additionally found Hamiltonian structure controls exhibiting desirable physical properties. The prospects of utilizing the applied field and Hamiltonian structure simultaneously as controls is also explored. The control scenarios I and II indicate the gains offered by algorithmically guided molecular or material discovery for manipulating quantum dynamics phenomenon.     

Time-dependent wave packet propagation using quantum hydrodynamics

A new approach for propagating time-dependent quantum wave packets is presented based on the direct numerical solution of the quantum hydrodynamic equations of motion associated with the de Broglie–Bohm formulation of quantum mechanics. A generalized iterative finite difference method (IFDM) is used to solve the resulting set of non-linear coupled equations. The IFDM is 2nd-order accurate in both space and time and exhibits exponential convergence with respect to the iteration count. The stability and computational efficiency of the IFDM is significantly improved by using a “smart” Eulerian grid which has the same computational advantages as a Lagrangian or Arbitrary Lagrangian Eulerian (ALE) grid. The IFDM is generalized to treat higher-dimensional problems and anharmonic potentials. The method is applied to a one-dimensional Gaussian wave packet scattering from an Eckart barrier, a one-dimensional Morse oscillator, and a two-dimensional (2D) model collinear reaction using an anharmonic potential energy surface. The 2D scattering results represent the first successful application of an accurate direct numerical solution of the quantum hydrodynamic equations to an anharmonic potential energy surface.