Photoionization cross sections: Interpretation of band intensities in He I and He II photoelectron spectra

Relative photoionization cross sections of molecules consisting of atoms from the first three rows of the Periodic Table are computed by a theoretical method developed previously which involves the plane-wave approximation for the photoelectron and the use of semiempirical LCAO-SCF-MO's of the CNDO- or MINDO- type for the initial orbital. The calculated values are compared with experimental photoelectron band intensities obtained by integrating the band areas in He I and He II photoelectron spectra. The observed relative intensity changes of bands in the spectrum in going from He I to He II excitation are attributed to variations in the one- and/or two-centre contributions to the cross sections from the electron density at the atoms and in the bonds, respectively. The analysis of the relative intensities of bands in the He I and He II spectra thus leads to conclusions about the electron density distribution in the initial orbital and about the assignment of the bands.In a qualitative sense, the experimental He I/He II intensity changes are usually correctly predicted by our theoretical method, but there are often considerable quantitative discrepancies between the measured and calculated values which probably arise from the inherent simplifications of our approach.     

Theory of one-phonon scattering of atoms from metal surfaces: Application to He/Ag(111)

We present explicit expressions for one-phonon scattering of atoms from metal surfaces, within the distorted wave Born approximation by including Van der Waals forces. A comparison is made with the recent high-resolution time of flight spectra for He/Ag(1 1 1). We explain in quantitative terms the position and the intensity of the peaks due to scattering from the Rayleigh wave of silver. Also the bulk phonons contribute to the spectra, but the calculated intensities underestimate the data.     

Surface of a 3He crystal: crossover from quantum to classical behavior

3He crystals start to show facets on their surface only at about 100 mK, well below the roughening transition temperature. To understand the reason for that, we have performed the first quantitative investigation on the growth dynamics of the basic (110) facet at 60-110 mK. The obtained values of the step free energy suggest an extremely weak coupling of the solid-liquid interface to the crystal lattice which we show to be the result of quantum fluctuations of the interface. The renormalization group approach by Nozières and Gallet, modified to incorporate quantum fluctuations, explains well the temperature dependence of the step energy measured in this work and at ultralow temperatures by Tsepelin et al., where the coupling is known to be strong. We have thus shown that, paradoxically, the role of quantum fluctuations is at higher temperatures much larger than at low temperature.     

Calculation of dipole moment functions with density functional theory: application to vibrational band intensities

We have calculated fundamental and overtone XH stretching vibrational band intensities for H₂O, benzene, cyclohexane, 1,3-butadiene, and HCN. The band intensities were calculated with a simple harmonically coupled anharmonic oscillator local mode model and a series expanded dipole moment function. The dipole moment functions were obtained from local, non-local and hybrid density functional theory calculations with basis sets ranging from 6–31G(d) to 6–311++G(3df,3pd). The calculated band intensities have been compared with intensities calculated with conventional ab initio methods and with experimental results. Compared with conventional correlated ab initio methods, a carefully chosen density functional method and basis set seems to give better fundamental and overtone intensities with far less resources used. We have found that the density functional methods appear to be less sensitive to the choice of basis set, with little difference between the results obtained with a non-local or hybrid density functional method.     

An application of theta functions to the band problem

The utility of theta function-modulated plane wave bases — the modulation is essentially by site-centered Gaussians — in the band problem is shown. The Kronig-Penney model is used as a test system.     

Surface transport in premelted films with application to grain-boundary grooving

We present a new model of surface transport in premelted films that is applicable to a wide range of materials close to their melting points. We illustrate its use by applying it to the evolution of a grain-boundary groove in a high vapor pressure material and show that Mullins's classical equation describing transport driven by gradients in surface curvature is reproduced asymptotically. The microscopic contact angle at the groove root is found to be modified over a thin boundary layer, and the apparent contact angle is determined. An explicit transport coefficient is derived that governs the evolution rate of systems controlled by surface transport through premelted films. The transport coefficient is found to depend on temperature and diverges as the bulk melting temperature is approached.     

New universality class for the three-dimensional XY model with correlated impurities: application to 4He in aerogels

Encouraged by experiments on 4He in aerogels, we confine planar spins in the pores of simulated aerogels (diffusion limited cluster-cluster aggregation) in order to study the effect of quenched disorder on the critical behavior of the three-dimensional XY model. Monte Carlo simulations and finite-size scaling are used to determine critical couplings K(c) and exponents. In agreement with experiments, clear evidence of change in the thermal critical exponents nu and alpha is found at nonzero volume fractions of impurities. These changes are explained in terms of hidden long-range correlations within disorder distributions.     

Surface effects in nanoparticles: application to maghemite -Fe O

We present a microscopic model for nanoparticles, of the maghemite (-Fe₂O₃) type, and perform classical Monte Carlo simulations of their magnetic properties. On account of M?ssbauer spectroscopy and high-field magnetisation results, we consider a particle as composed of a core and a surface shell of constant thickness. The magnetic state in the particle is described by the anisotropic classical Dirac-Heisenberg model including exchange and dipolar interactions and bulk and surface anisotropy. We consider the case of ellipsoidal (or spherical) particles with free boundaries at the surface. Using a surface shell of constant thickness ( nm) we vary the particle size and study the effect of surface magnetic disorder on the thermal and spatial behaviors of the net magnetisation of the particle. We study the shift in the surface “critical region” for different surface-to-core ratios of the exchange coupling constants. It is also shown that the profile of the local magnetisation exhibits strong temperature dependence, and that surface anisotropy is responsible for the non saturation of the magnetisation at low temperatures. Received 1 September 1999 and Received in final form 3 November 1999     

Surface diffusion of tungsten adatoms induced by low-energy bombardment with He atoms

Using low-temperature field ion microscope techniques, we studied at the atomic level the elementary events of radiation-induced surface diffusion of tungsten adatoms on the ideally pristine surface. The experiments on surface-damage formation and adatom displacements have been performed in situ with a source of neutral helium atoms with an energy of 5?keV. It was demonstrated that the low-energy He atomic bombardment at grazing incidence was able to induce a substantial surface mobility of adatoms. Most of the radiation-induced adatom displacements were oriented along the direction of irradiation. The long impact-induced jumps of adatoms, spanning more than a nearest-neighbor distance, were revealed. Performed molecular dynamics simulations reproduce the general experimental trends and conclusions.     

Conduction and valence band density of states of SnS2: Theory and experiment

The band structure of SnS₂ has been investigated over a wide energy range by pseudopotential band structure calculations and synchrotron radiation photoemission spectroscopy techniques. A good correspondence has been found between energy positions of the theoretical density of states features and structure in the constant initial state (CIS) and energy distribution curves (EDC's) for the conduction and the valence bands respectively. In the energy region between — 8 eV and 15 eV from the top of the valence band we observe four valence band and six conduction band peaks.     

A strangeonium potential model with completeO(v 2/c 2) hyperfine splittings

We model the strangeonium system by numerically solving the Schrödinger equation with a Coulomb plus linear scalar confining potential. All hyperfine terms toO(v ²/c ²) are treated in detail; we show that neglect of spin-independent terms is unjustified. The model is fit to the ?(1019.6) andf′ (1516), and masses for excitedss states are predicted: the strangeonium parameters are contrasted with the values found in charmonium.     

The Nernst-Ettingshausen coefficient in conductors with a narrow conduction band: Analysis and application of its results to HTSC materials

A theoretical analysis is made of the Nernst-Ettingshausen coefficient Q for the case of a narrow conduction band present in the band spectrum of a material. It is shown that the presence of such a band results in a qualitative change in the mechanism responsible for the Nernst-Ettingshausen effect as compared to the classical case of a broad conduction band and that the behavior of this coefficient reveals a number of specific features that are different from the case of the classical theory of transport coefficients in semiconductors and metals. It is demonstrated that the pattern of the Q(T) relation in the case of a narrow band is drastically affected by the asymmetry of the dispersion curve, whereas the other features of the band spectrum and of the properties of the carrier system, including the character of the energy dependence of the relaxation time, are less significant and, in a first approximation, can be disregarded. The calculated Q(T) curves are in qualitative agreement with the experimental relationships obtained for doped HTSCs of the YBa₂Cu₃O_y system. The possibility of using this approach for a complex analysis of the experimental temperature dependences of the four transport coefficients in the normal phase of HTSC materials is demonstrated.     

An orthonormalization method: Theory and application to the j-j coupling of fermions in a single j-shell

An orthonormalization method suitable to deal with an overcomplete and non-orthogonal set of vectors is derived. It is then used to obtain the orthonormal set of states connected with the angular momentum coupling of N fermions in a single j-shell.     

The Broadening of He I lines including ion dynamic corrections,with application to λ4471Å

A theory is developed for the broadening of He I lines with forbidden components which is valid at low densities (where the forbidden line is reasonably well isolated). This theory takes into account the effects of dynamic ion broadening. Application to λ4471Å gives good agreement with experiment and with a recent calculation of Lee; reasons for agreement with Lee are discussed in detail. Comprehensive tables for 4471Å are presented. These have been extended further into the line wings than previous tabulations. Static ion theory is found to be adequate for densities greater than ～ 5 × 10¹⁵ cm^?3. A simple numerical profile for allowed lines, taking into account dynamic ion effects, is also presented.     

He II spectra of La,Ce and Yb: Novel features in the valence band region

He II photoelectron spectra of La, Ce and Yb show features which cannot be explained in terms of single electron excitations. It is proposed that these are due to formation of electron-hole paris. Communication No. 227.     

填充碳纳米管/石墨的复合型电磁波屏蔽膜

介绍一种填充碳纳米管／石墨的复合型电磁波屏蔽膜的组成、制备及其对耐老化性能,实验发现：当碳纳米管／石墨的配比为1／7－1／2、有机聚合物／导电填料的配比为29．6／70．4－32．4／67．6时,该屏蔽具有最佳的电性能、屏蔽性能和加工性能,且在一定条件下具有负的温度系数。用多层结构模型讨论了该屏蔽膜的导电性,并与铜、镍、危险性膜的屏蔽特性进行了比较。