Scattering of hot electrons by adatoms at metal surfaces

The scattering of electrons in image-potential states by Cu adatoms on Cu(001) surfaces has been investigated by means of time- and angle-resolved two-photon photoemission. Several interband and intraband-scattering mechanisms have been identified and their contributions to the total decay of the states determined quantitatively. The adsorbates mainly cause quasielastic scattering processes. Inelastic processes in contrast are due to interactions with electrons in the substrate and are not significantly increased by Cu adatoms. Quasielastic scattering into bulk bands contributes significantly to the depopulation of surface states.     

Dynamics of image-potential states on stepped Cu(001) surfaces

The dynamical properties of image-potential states on stepped Cu(117) and Cu(118) surfaces were studied by time- and angle-resolved two-photon photoelectron spectroscopy. The interaction with the step-induced potential leads to quasielastic anisotropic scattering between image-potential-state bands. In particular, resonant interband scattering from image-potential states with quantum numbers n2 to the n=1 band and quasielastic intraband scattering within the n=1 band show high efficiency. In spite of the higher step density of Cu(117), resonant scattering is about four times larger on Cu(118). This distinction is attributed to the different step distributions of the two surfaces and to the concomitant correlation of the step arrangement on short- and long-range scales, which has been studied by scanning tunneling microscopy. Details of the surface band structure lead to different boundary conditions and may weakly affect scattering probabilities. PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht     

Inelastic scattering of He atoms from a Cu(110) surface

A He beam has been used to measure inelastic scattering from a Cu(110) surface. The scattering was a result of predominantly single phonon events and both energy gain and loss processes were observed. This was in contrast to Cu(001) where only energy loss was observed. For Cu(110), aligned in the [001] azimuth, partial dispersion relations have been measured which meant that phonons with ΔQ values up to the zone boundary value have not been detected under the present experimental conditions. The elastic incoherent component varied with azimuthal angle for Cu(110).     

Scattering of image-potential-state electrons by steps on Cu(001)

Scanning tunneling spectroscopy (STS) reveals a distinct asymmetry in the scattering properties of an isolated step for the n=1 image-potential state on Cu(001). The elastic scattering probability for an electron traveling downstairs is determined from the strength of density oscillations in front of a step edge and is found to be approximately two times higher than for the opposite upstairs direction. A one-dimensional scattering model is extended to the case of asymmetric transmission and reflection coefficients. The calculations using the asymmetry measured by STS explain the dispersion and the decay rate of the n=1 band on Cu(119) measured by two-photon photoemission. In particular, the asymmetry of the decay rate can be described quantitatively with a minimum of adjustable parameters. While the results can also be transferred successfully to the Cu(1115) surface, the limit of applicability is reached for Cu(117) with a step separation of 3.5 nearest-neighbor distances. PACS 73.20.At; 79.60.Bm; 68.37.Ef; 72.10.Fk     

A theoretical study of dingle temperatures and scattering rates in Cu(Al) and Cu(Ni) within the cluster formalism

The cluster formalism developed by Lodder is used to calculate electron scattering rates and Dingle temperatures in dilute alloys of Cu(Al) and Cu(Ni). The results are compared to quantities obtained experimentally using magnetic induced surface state resonances. The experimental results for Cu(Al) are reproduced nicely. For Cu(Ni) a discrepancy between theory and experiment is found, which remains unsolved.     

Electronic and optical properties of Cu, CuO and Cu2O studied by electron spectroscopy

The electronic and optical properties of Cu, CuO and Cu(2)O were studied by x-ray photoelectron spectroscopy (XPS) and reflection electron energy-loss spectroscopy (REELS). We report detailed Cu 2p, Cu LVV, O 1s and O KLL spectra which are in good agreement with previous results. REELS spectra, recorded for primary energies in the range from 150 to 2000 eV, were corrected for multiple inelastically scattered electrons to determine the effective inelastic scattering cross section. The dielectric functions and optical properties were determined by comparing the experimental inelastic electron scattering cross section with a simulated cross section calculated within the semi-classical dielectric response model in which the only input is Im(-1/ε) by using the QUEELS-ε(k,ω)-REELS software package. By Kramers-Kronig transformation of the determined Im(-1/ε), the real and imaginary parts (ε(1) and ε(2)) of the dielectric function, and the refractive index n and extinction coefficient k were determined for Cu, CuO, and Cu(2)O in the 0-100 eV energy range. Observed differences between Cu, CuO and Cu(2)O are mainly due to modifications of the 3d and O 2p electron configurations.     

X-ray photoelectron diffraction investigation of the (2 × 2) overlayers of Cs and K on Cu(111)

X-ray photoelectron diffraction measurements have been made for the Cu(111)(2 × 2)-Cs and Cu(111)(2 × 2)-K systems to explore the possibility of using substrate emission XPD for the elucidation of adsorbate-substrate registry in these cases of strongly scattering adsorbate atoms and anticipated atop adsorption sites. Although scattering effects within the substrate, for sub-surface emitters, clearly complicate the interpretation of the data, simple symmetry arguments do give substantial support to the identification of atop adsorption sites in both systems.     

Role of elastic scattering in electron dynamics at ordered alkali overlayers on Cu(111)

Scanning tunneling spectroscopy of p(2 x 2) Cs and Na ordered overlayers on Cu(111) reveals similar line widths of quasi-two-dimensional quantum well states despite largely different binding energies. Detailed calculations show that 50% of the line widths are due to electron-phonon scattering while inelastic electron-electron scattering is negligible. The mechanism of enhanced elastic scattering due to Brillouin zone backfolding contributes the remaining width.     

Scattering of electrons on the Cu(001) surface by Co adatoms

Excited electrons at surfaces can be scattered by adsorbate atoms or defects, which changes the energy or momentum. Such scattering processes can be studied by energy, time and angle-resolved two-photon photoelectron spectroscopy. In this article the influence of statistically distributed Co adatoms on a Cu(001) surface on the dynamics of electrons in image-potential states is investigated. Different scattering mechanisms, such as interband, intraband, and bulk scattering are identified and analyzed quantitatively. Cobalt adatoms cause mainly quasielastic scattering of electrons in image-potential states. Inelastic processes are due to interactions with electrons in the substrate and are not significantly increased by Co adatoms. The results are compared to previous experimental and theoretical work on Cu adatoms. PACS 73.20.At; 68.49.Jk; 79.60.Ht     

Metastable (2S) helium atom scattering from NiO(100) and Cu(100) surfaces

The scattering of metastable 2³S He atoms (He*) from cleaved NiO(100) as well as from clean and CO-covered Cu(100) surfaces has been studied. For these varied surfaces, which were characterized in situ by ground state He atom scattering, only broad He* angular distributions without any diffraction peaks were observed. For metastable He atoms scattered from the clean Cu(100) surface a total survival probability of 1×10⁻⁶ was determined. For NiO(100) and the CO-covered Cu(100) surface values of about 1×10⁻⁵ were obtained. Time-of-flight spectra of the surviving He* atoms revealed a substantial energetic broadening which increases with the substrate temperature. This behaviour indicates a large well depth for the He*–surface interaction potential and is discussed in terms of an enhanced multiphonon excitation and/or trapping probability upon the scattering.     

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.     

Crystal effects in the neutralization of He+ ions in the low energy ion scattering regime

Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.     

Oxygen superstructures throughout the phase diagram of (Y,Ca)Ba2Cu3O6+x

The doping dependence of short-range lattice superstructures in (Y,Ca)Ba2Cu3O6+x has been studied with high-energy x-ray scattering. We observe diffuse features with a well defined periodicity which depend on the oxygen concentration but not on the charge carrier concentration. In addition, we find that diffuse scattering is absent in underdoped YBa2Cu4O8, which does not sustain oxygen defects. Our combined data highlight that the diffuse scattering arises from short-range oxygen ordering and associated lattice distortions. Signatures of stripe ordering or fluctuations are not seen and therefore must be much weaker.     

基质折射率对金属粒子散射特性的影响

基于Mie散射理论,给出了金属粒子的散射、吸收和消光截面以及散射场强度的计算公式,并计算了三种金属(金、银、铜)粒子在不同折射率基质中的光学截面和散射强度.结果表明,在近红外区,这三种金属粒子的散射行为随基质折射率的变化规律相同,折射率越大散射特性越明显.     

The determination of the He-Cu interaction potential by atomic beam scattering

Experimental results on the intensity of helium diffraction peaks for the scattering from Cu(110) and for the selective adsorption from Cu(113) and Cu(115) are reported. The usual hard corrugated wall potential is definitely unable to fit the data. While the corrugated Morse potential, recently proposed by Armand and Manson, gives a very much better agreement. The corrugation is found to be dependent upon incident energy, in good agreement with the prediction of Hamann's calculation     

Neutron scattering study of short-range correlations and ionic diffusion in copper selenide

The paper reports the results of a neutron scattering study of Cu_{2 - d}Se {\hbox{C}}{{\hbox{u}}_{{2 - \delta }}}{\hbox{Se}} superionic compounds. The crystallographic model fitted to the diffraction data shows the occupation of 8c and 32f sites by Cu atoms. Observed diffuse background is related to correlated thermal vibrations of Se and Cu atoms, with Se↔Cu (8c,32f) and Cu (8c)↔Cu (8c) correlations being most important. The quasi-elastic neutron experiments show the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu ions between diffusion hops. Combination of neutron diffraction, diffuse scattering and quasi-elastic scattering experimental data suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.     

Photorefractive light scattering in LiNbO3: Cu crystals

The time dependence of the photorefractive light scattering in lithium niobate single crystals of the congruent composition (LiNbO₃) doped by “photorefractive” Cu cations (0.015 wt %) is studied for a radiation power of 160 mW. The data for the photorefractive scattering are confirmed by changes in Raman scattering spectra.     

Cu(100)表面上H2/D2散射的六维态-态量子动力学研究：位点平均模型的有效性

基于由optPBE-vdW密度泛函计算的数千个数据点拟合得到的精确的神经网络势能面,本文采用含时波包方法对H₂/D₂在刚性Cu(100)表面上的态-态散射进行了六维量子动力学计算. 与以往的理论和实验比较了H₂和D₂在Cu(100)中的振转(非)弹性散射的结果. 特别是通过将六维的(非)弹性散射几率与从十五个位点加权平均四维几率的结果比较,测试了在该体系中位点平均近似模型的有效性. 具体来说,位点平均模型很好地重现了振动弹性散射几率,尽管对于高能下的振动非弹性散射结果没那么好. 结果说明在未来研究重双原子或多原子分子从金属表面的态-态散射动力学时,可以使用位点平均模型来降低全维计算过高的成本.     

Phase and population decay of image-potential states – the influence of defects

Quantum-phase and population decay of image-potential states have been investigated by two-photon photoemission with femtosecond time resolution. The influence of steps and defects on quasielastic and inelastic scattering processes is illustrated for a vicinal Cu(119) surface and diluted adsorbate layers of CO and Cu on Cu(001). Received: 19 April 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Charge and spin structure in of YBa(2)Cu(3)O(6.35)

Neutron scattering has been used to measure the charge and spin structure in the highly underdoped superconductor YBa(2)Cu(3)O(6.35). Incommensurate static charge ordering is found that remains at high temperatures. The magnetic pattern is complex with a resonance and incommensurate structure observed at low temperatures. The results clarify the role of striped phases in YBa(2)Cu(3)O(6+x) superconductors.