Free-electron Metal Research Articles

Influence of quasicrystal bulk structure on oxidized AlPdMn surface properties probed by electronic and vibrational spectroscopies

High-resolution electron energy-loss spectroscopy and x-ray photoemission are used to probe the fivefold AlPdMn oxidized surface. The vibrational spectrum reveals a strong increase of the Al-O stretching mode wave number while the oxygen Auger transition and photoelectron spectra prove the high ionicity of the thin alumina layer. These observations are markedly different from those made with oxidized aluminum and can be related to the low reactivity of the quasicrystal surface. The influence of the quasicrystal bulk structure on the oxidized surface vibrational mode and ionicity can be well understood by image forces theory and the static dielectric constant behavior between the pure free-electron metal and the quasicrystalline alloy.

Effect of an Adsorbed Atom on Resonant Charge Exchange in Atom Scattering on Metal Surfaces

A simple model describing resonant charge transfer between the free-electron metal surface and an atom colliding with it at some distance from another adsorbed atom is presented. The negative ionization probability of a scattered atom is studied within the time-dependent. Anderson-Newns model and the time-evolution operator approach. With appropriate models for the considered system, the ionization probability is shown to oscillate with the distance between the adsorbed atom and the collision point of the scattered atom with the metal surface. These oscillations are results of the indirect interaction between the adsorbed and the scattered atoms due to the coupling of both atoms with the metal energy band.

Lifetime broadening in Compton scattering

Recent high-resolution Compton scattering experiments revealed a smearing of the Fermi break in nearly free electron metals which contradicts Luttinger's theorem of a sharp Fermi surface even for the interacting electron gas. Sternemann et al. [Phys. Rev. B 62, R7687 (2000)] explained this smearing by introducing a spectral function for the recoil electron with a finite lifetime due to final-state interactions. In contrast to the laborious GW approximation (GWA) used by these authors we calculate inelastic mean-free paths both for electron-hole and plasmon excitations which are of striking simplicity. The resulting lifetimes agree very well with those of GWA. In addition, we show that core excitation significantly shortens the lifetimes. The estimate of the influence of final-state interactions is extended to an analysis of (e,2e) experiments.

Wave packet propagation study of the electron transfer in back-scattering of H − ions from alkali adsorbates on an Al surface

The charge transfer between an H − ion and a free-electron metal surface with a single alkali adsorbate (Li and Cs) is studied with the wave packet propagation approach in the back-scattering geometry. Both the static problem for a fixed projectile–surface distance and the problem of charge transfer during a collision are considered. The three body (projectile–adsorbate–surface) aspect of the charge transfer process in this case results in an avoided crossing between the projectile and adsorbate-localized quasi-stationary states. We analyze and discuss the local effect of the adsorbate on the resonant charge transfer and the applicability of the rate equation approach.

Sub-threshold plasmon excitation in free-electron metals by helium ions

In this work, we re-examine sub-threshold plasmon excitation by ions measuring the energy distributions of electrons emitted from Al and Mg under the impact of 0.16–4 keV single charged Helium ions. Electron energy distributions for the case of He + projectiles contain well distinguished structures that confirm the excitation of multipole surface plasmons, due to the potential energy released by the neutralization of incoming ions. It is shown that electron emission from multipole plasmon decay is more intense than from Auger neutralization (AN). Our experiments indicate that scattering and cascade in the solid of electrons excited by plasmon decay and by AN may give an important contribution to electron emission.

Image states on a free-electron metal surface covered by an atomically thin insulator layer

Excited electronic states on a free-electron metal surface are investigated in the case when an atomically thin Ar layer is adsorbed on the surface. It is shown that the dielectric character of the Ar layer allows the existence of image states as well-defined resonances in this system. The image states are confined in vacuum by the Ar layer and decay by electron transmission through the Ar layer. A 3D microscopic description of the Ar layer associated to a wave-packet propagation approach is used to determine the characteristics, energy and lifetime, of the image states in a model Ar/free-electron metal system. The possibility of the observation of these states in a time-resolved experiment is discussed.

FDTD method for computing the off-plane band structure in a two-dimensional photonic crystal consisting of nearly free-electron metals

An FDTD numerical method for computing the off-plane band structure of a two-dimensional photonic crystal consisting of nearly free-electron metals is presented. The method requires only a two-dimensional discretization mesh for a given off-plane wave number k z although the off-plane propagation is a three-dimensional problem. The off-plane band structures of a square lattice of metallic rods with the high-frequency metallic model in the air are studied, and a complete band gap for some nonzero off-plane wave number k z is founded.

Auger parameter studies of aluminium–transition metal alloys

AbstractWithin a general study of transition metal (TM) alloys, the variation of the Auger parameters of Fe–10 at.% Al and Fe–50 at.% Al relative to the pure metal was determined. Initial‐ and final‐state Auger parameters were calculated and related to charge transfer calculations. Iron exhibits almost pure metal behaviour, particularly in Fe10Al, but Al core holes are more poorly screened than in Al metal. Charge transfer inferred from the final‐state Auger parameters suggests that Al carries a positive charge consistent with the electronegativity differences of Fe and Al. However, there is no evidence of Fe carrying a corresponding negative charge: furthermore, positive Al charge is inconsistent with the Al initial‐state parameter. This suggests, rather, that the Alsp–Fesd bonds in the alloys are less polarizable than Al bonds in the nearly free electron metal, a concept supported by the reduction in Al 2p plasmon satellite intensity on alloying and the increased plasmon energy associated with the enhanced valence electron density. Copyright © 2002 John Wiley & Sons, Ltd.

Particle-in-a-box momentum densities compared with electron momentum spectroscopy measurements

We investigate the momentum space representation of the solutions of a particle in a three-dimensional box. By comparing the calculated momentum densities of a particle in a box with the measured distribution of highly symmetric molecules we want to elucidate the nature of the measured orbitals. Finally the measured momentum distribution of a free electron metal is compared with the results of this model.

Tetra point wetting of liquid K–KCl mixtures: Spectroscopic characterization of mesoscopic wetting and prewetting films

The wetting and prewetting transitions at the metal-rich K–KCl melt–sapphire interface have been investigated by spectroscopic ellipsometry in combination with normal incidence reflectivity in the spectral range 0.8⩽ℏω⩽2.2 eV at temperatures up to 730 °C. Along the coexistence curve a salt-rich liquid wetting film is observed which is identified by the spectral features of the liquid state F-center. Unusually thick wetting films are found ranging from 30 nm near 540 °C to 300 nm approaching the monotectic temperature of 751 °C. Their composition has been determined from the absorption coefficients of the F-center band and it corresponds to about 90 mole % salt. At conditions off coexistence and near the prewetting line, similar mesoscopically thick wetting films exist. Crossing the prewetting line towards metal-rich solutions, the optical properties at the interface agree with those of the nearly free electron metal. The high thickness of the prewetting films is qualitatively explainable by charging and double layer formation at the interface. The occurrence of liquid F-center-like states up to 200 K below the monotectic temperature gives evidence of a strong undercooling of the wetting films with respect to the bulk phase. These characteristics of the wetting transition in a metal–molten salt solution can be described by the tetra point wetting scenario for binary fluid mixtures.

Lifetime of excited electronic states at surfaces: CO −(2π *) resonance on Cu(1 1 1) and Cu(1 0 0) surfaces

A model study of the (2π *) resonance in the CO/Cu(1 1 1) and CO/Cu(1 0 0) systems is presented. Although the CO −(2π *) resonance is located inside the surface projected band gap in both systems, it is found to be very short-lived. These results are at variance with the apparently similar alkali/Cu systems where a strong reduction of the resonance decay by one-electron transfer to the substrate as compared to a free-electron metal surface is observed and attributed to the presence of the surface projected band gap. The differences between the two systems are analysed, allowing a discussion of the situations where a projected band gap induced stabilisation can be expected. The present results are compared with recent experimental results obtained by time resolved 2-photon photo emission.

Suppression of projectile-electron excitations in collisions with a free-electron gas of metals

Excitation and ionization of hydrogenic projectiles in collisions with metal free-electron gas are analyzed in the high but nonrelativistic energy regime. Transition matrix elements are calculated in the first Born approximation, and the free-electron gas response is described by using the Mermin-Lindhard dielectric function. Projectile excitation and loss probabilities per unit length are found to be smaller than those corresponding to the collisions with a wave packet of single electrons of equivalent density. This behavior is explained in terms of the collective effect (shielding) of the free-electron gas and depends on the energy transferred to the atomic electron and on the impact velocity. By comparing results obtained using a binary collisional formalism and those using the dielectric formalism, we estimate the plasmon excitation contribution to the total probabilities.

Application of Mathieu potential to photoemission from metals

The Mathieu potential is used to define the crystal potential from which the initial state wavefunction for the surface state is derived. The wavefunction is used for photoemission calculations in the case of free electron metals like Al and Be.

Ab initiomolecular-dynamics simulation of liquidGaxAs1−xalloys

We report the results of ab initio molecular dynamics simulations of liquid Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600 K, just above the melting point of GaAs. The liquid is predicted to be metallic at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity maximum near x = 0.5, consistent with the Faber-Ziman expression. The electronic density of states is finite at the Fermi energy for all concentrations; there is, however, a significant pseudogap especially in the As-rich samples. The Ga-rich density of states more closely resembles that of a free-electron metal. The partial structure factors show only a weak indication of chemical short-range order. There is also some residue of the covalent bonding found in the solid, which shows up in the bond-angle distribution functions of the liquid state. Finally, the atomic diffusion coefficients at 1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in Ga_{0.2}As_{0.8}.

Coupling of carbon nanotubes to metallic contacts

The modeling of carbon nanotube-metal contacts is important from both basic and applied view points. For many applications, it is important to design contacts such that the transmission is dictated by intrinsic properties of the nanotube rather than by details of the contact. In this paper, we calculate the electron transmission probability from a nanotube to a free electron metal, which is side-contacted. If the metal-nanotube interface is sufficiently ordered, we find that k-vector conservation plays an important role in determining the coupling, with the physics depending on the area of contact, tube diameter and chirality. The main results of this paper are: (i) conductance scales with contact length, a phenomena that has been observed in experiments and (ii) in the case of uniform coupling between metal and nanotube, the threshold value of the metal Fermi wave vector (below which coupling is insignificant) depends on chirality. Disorder and small phase coherence length relax the need for k-vector conservation, thereby making the coupling stronger.

Investigation of the monochromatic emissivity of liquid copper

The monochromatic normal emissivity of copper at a wavelength of 0.656 mem is experimentally investigated in the temperature range from 1300 to 1700 K. The experimental data are obtained for samples of different purity, which are indicative of the influence of the chemical composition of metal on its emissivity in the molten state. A change in the emissivity is found during the transition of metal from the solid phase into the liquid one. It is shown that the model including the effect of the relaxation time of free electrons of metal produces the best agreement with the experiment in the liquid phase.

First principles calculation of ion induced kinetic electron emission from nearly free-electron metals below the plasmon threshold

The excitation of electrons by decay of plasmons generated directly during the interaction of the impinging particle with the system of conduction electrons of the metal results in an important contribution to the electron emission at higher impact energies. However, also below the so-called plasmon threshold (nearly 40 keV for proton impact on Al), when a direct excitation of plasmons is impossible, several experimental investigations show a remarkable contribution of kinetic excitation of plasmons. Starting from first principles, a theoretical description of the plasmon excitation during the electron capture process as well as by fast secondary electrons generated by binary ion–electron collisions is given. Explicit calculations were performed for proton impact on Al.

Role of the 2D surface state continuum and projected band gap in charge transfer in front of a Cu(111) surface

Electron capture by Li+ and H projectiles in grazing scattering from Cu(111) and Cu(110) surfaces is studied experimentally and theoretically. Whereas data for Cu(110) can be described by established theoretical methods treating resonant charge transfer with a free-electron metal, data for Cu(111) show pronounced deviations from this approach. We interpret our observations by the effect of the projected L-band gap of the Cu(111) surface. In particular, the quantum states of reduced dimension (2D surface state continuum) play a dominant role in electron transfer.

Femtosecond laser-induced heating of electron gas in aluminium

The distribution function of free electrons in metal is calculated for irradiation of aluminium with an ultra- short laser pulse of moderate intensity. We consider inverse Bremsstrahlung absorption, electron-electron and electron- phonon interaction. Our theoretical model is based on Boltz- mann equations and describes each considered process by a corresponding collision integral. The results show the exci- tation and relaxation of the free electron gas. Energy transfer to the phonon gas is calculated. Our model predicts linear absorption for intensities up to damage threshold. The cal- culated absorbed energy compares very well with known ab- sorption characteristics. PACS: 61.80 ba; 79.20 Ap

Vibrational excitation in low-energy electron scattering by H 2 molecules physisorbed on a metal surface

The electron impact vibrational excitation of H 2 molecules physisorbed on a free-electron metal surface is studied theoretically at collision energies of a few eV. The role of the short-lived low-lying 2Σ u resonance is investigated. The electron scattering by a free H 2 molecule is described by the R-matrix method and the corresponding results are used to model the electron scattering by the physisorbed molecule with the coupled angular mode (CAM) method. The strength of the vibrational excitation and, in particular, the overtone vibrational excitation ratio are found to be smaller for the physisorbed molecule than for the free molecule. However, the energy dependence of the vibrational excitation process is found to be weakly influenced by the physisorption, as observed experimentally by Demuth et al. [Phys. Rev. Lett. 47 (1981) 1166].