Angle-resolved photoemission and LEED from rare-earth metals

Angle-resolved photoemission and LEED studies of single-crystal surfaces of the rare-earth metals are still relatively few in number. The reasons for this are outlined and comparisons are made between the procedures used to obtain clean and well-ordered surfaces suitable for study. Valence-band photoemission spectra are compared with surface electronic structure and photocurrent calculations in order to identify surface and bulk features. Core-level spectroscopy indicates that the 5p levels of the lanthanides show intensity variations with emission angle that are not explicable in terms of a simple core-level shift. Although the 4p levels of Y have some band character, detailed comparison of the 4p photoemission lineshape with those produced by photocurrent calculations enables the surface core-level shift to be determined. Quantitative LEED studies of close-packed (0001) surfaces show essentially a bulk-terminated structure for Sc, a structural reconstruction of the divalent surface of Sm and a magnetic reconstruction of ferromagnetic Gd. The more open (11 0) and (10 0) surfaces of all the rare-earth metals studied undergo extensive reconstructions into a close-packed structure almost identical to that of a (0001) surface. Angle-resolved photoemission and LEED have yet to distinguish between these structures, indicating a reconstruction depth of at least five atomic layers.     

Angle-resolved transmission measurements through anisotropic two-dimensional plasmonic crystals

We report on high-accuracy angle-resolved optical transmission measurements through anisotropic 2D plasmonic crystals made of gold films with large-area rectangular arrays of nanoscale square holes, deposited on GaAs substrates. The measurements reveal the dispersion relations of air-gold and gold-GaAs surface plasmon polaritons. The crystal anisotropy induces a separation between plasmonic modes propagating in different directions. Their symmetry and dispersion properties are discussed.     

An electron spectrometer for LEED fine structure measurements

LEED fine structure analysis requires intensity versus energy data for very low energy electrons (typically 5–25 eV) at various angles of incidence and azimuth. An electron spectrometer capable of fulfilling these requirements was built and used to obtain data for Cu(111). This spectrometer differs from others used for these type of measurements in that it used a retarding field analyzer to collect the elastically scattered beams instead of a deflecting analyzer. This considerably simplifies the apparatus required and the experimental procedure as the collector can be used at a fixed angle to collect data from a number of different angles of incidence. The experimental design is discussed and sample results, showing that three peaks were resolved, are presented.     

Fast LEED intensity measurements with a video camera and a video tape recorder

The measurement of LEED intensities with a TV camera-computer system, which has been published in an earlier paper, is reported to be improved with respect to the speed of data collection by a factor of about 10². Other properties such as sensitivity, reliability and handling facility have also been improved. The new measuring mode, which applies a video tape recorder, allows the simultaneous measurement of all appearing diffraction spots down to a beam current of 10⁻¹¹ A with a rate of 50 energy points/s. This pushes the total measuring time for LEED spectra to the order of seconds and makes possible to monitor non-stable surfaces. As an example intensity measurements for different adsorption stages of hydrogen on W(100) are presented.     

Angle-resolved photoelectron measurements on the first two orbitals of acetylene using synchrotron radiation

The angle-resolved photoelectron spectrum of acetylene has been measured using synchrotron radiation over a range of photon energies from 12 to 28 eV.     

Angle-resolved photoemission study of a vacancy-induced state in substoichiometric VC

A vacancy induced state has been identified at about 1.8eV binding energy on the (100) face of VC_0.80 using angle-resolved photoelectron spectroscopy. The recorded spectra are compared with calculated spectra for VC_1.0 and the results show that the origin of the observed state cannot be accounted for by the energy band structure of VC_1.0. Photoemission spectra recorded after exposing the clean surface to O₂ and CO are also presented and used in a discussion of the sensitivity of the vacancy induced state to surface contamination.     

Adsorption of krypton on the basal plane of graphite: LEED and Auger measurements

The adsorption of krypton on (0001) graphite has been studied by LEED and Auger. Stepwise isotherms are observed and thermodynamic quantities such as the latent heat of two dimensional adsorption and the binding energy of a krypton atom are determined. The mean free path of electrons in krypton is measured. The LEED pattern of the krypton layer shows a √3 × √3 superstructure. Some vibrational properties are examined by LEED and possible implications of the experimental findings are discussed. The potentials and limitations of the techniques used in the present work are critically examined with respect to other techniques.     

Angle-resolved mapping of the fermi velocity in a quasi-two-dimensional organic conductor

We demonstrate a new method for determining the Fermi velocity in quasi-two-dimensional (Q2D) conductors. Application of a magnetic field parallel to the conducting layers results in periodic open orbit quasiparticle trajectories along the Q2D Fermi surface. Averaging of this motion over the Fermi surface leads to a resonance in the interlayer microwave conductivity. The resonance frequency is simply related to the extremal value of the Fermi velocity perpendicular to the applied field. Thus, angle dependent microwave studies enable a complete mapping of the in-plane Fermi velocity. We illustrate the applicability of this method for the highly 2D organic conductor kappa-(BEDT-TTF)2I3.     

Is there a future for LEED intensities?

The application of LEED intensities within surface crystallography is illuminated with respect to its today's standard, frontier and hopes. It is shown that structure determination is possible with high accuracy in case of simple surfaces. For more complex surfaces with about a handful of atoms in the surface unit-cell structural parameters are still reliably determined, however, with tremendously increasing computational amount of work. Some approximative approaches which could show a way out of the misery are described. However, importance is attached to other and newer developments of LEED as well. So the possibility to extract structural informations from diffuse intensities in case of disordered adsorption is demonstrated as well as the use of intensities to probe the critical phenomena and kinetics of surface phase transitions. Examples are given which make the future of LEED intensities appear quite promising.     

Simple sound transmission loss measurements using a modified impedance tube technique

A method of comparing the sound transmission characteristics of various materials, and combinations of materials, is presented, using a modified impedance tube technique. The procedure proved to be relatively quick and inexpensive in comparison with standard reverberation suite tests, and is therefore particularly useful for the qualitative ranking of multiple samples.The limitations of the technique are discussed in some detail, and particular emphasis is given to the problems of small sample size and method of mounting in the apparatus.     

LEED,AES and work function measurements on clean and cesium covered polar faces of GaP

After argon bombardment and annealing both the (111) and (1̄1̄1̄) faces of GaP show a (1 × 1) LEED pattern. The stabilization of the polar termination is probably obtained by charging of surface states. Measurements of the work function, the Auger spectrum and the LEED pattern during cesium deposition at room temperature suggest disordered cesium adsorption limited to a monolayer.     

Wave dynamics in a modified quintic complex Ginzburg-Landau system

We consider physical systems described by the modified quintic complex Ginzburg-Landau equation and its derivative forms and examine numerically the dynamics of its shock type wave solution. Discussions on the behaviours of this shock wave are introduced and it is shown how the ratios of diverse velocities of this wave could be exploited to explain and collect information concerning the spatial patterns formation in the system.     

The potential profile in LEED theory

The effect of variations in the inner potential at a crystal surface on low-energy electron diffraction (LEED) is investigated. It is shown in particular that surface structural determination using LEED is sensitive to the local value of the inner potential if energies roughly below 100 eV are used. It is however not sensitive to scattering effects at potential steps, except if intensities near beam emergences are considered, in which case LEED is shown to be a suitable tool for examining these potential steps.     

7 × 7 Si(111)Cu interfaces: Combined LEED,AES and EELS measurements

CuSi(111) interfaces have been examined as a function of substrate temperature and deposition time using different surface sensitive techniques. At room temperature a layer by layer growth was found. Strong intermixing of Cu and Si takes place and beyond 10 monolayers an ordered α Si phase in Cu was found. At high temperature, the growth follows the layer plus island or Stranski-Krastanov growth mode. Three-dimensional crystallites, probably consisting of Cu₃Si, grow epitaxially on an ordered intermediate layer.     

变形耦合发电机系统中的混沌控制

分别利用反馈和非反馈方法研究了变形耦合发电机系统的混沌控制问题，并基于Lyapunov直接法和Routh-Hurwitz判据讨论了受控变形耦合发电机系统的混沌轨道达到不稳定平衡点或极限环时的条件，同时给出了理论上的证明.数值模拟进一步验证了这两种方法的有效性. 关键词： 变形耦合发电机系统 反馈法 非反馈法 混沌控制     

Angle-resolved desorption and removal of surface nitrogen in deNOx

This paper reports on recent progress on angle-resolved desorption leading to structure-sensitive desorption dynamics. The sensitivity is exemplified in NO and N₂O reduction on Pd and Rh surfaces. The energy partitioning in the repulsive desorption of hyper-thermal products into their rotational and translational modes is an indispensable concept to examine the structure of a reaction site from desorbing molecules because it connects the structure of a transition state with each energy of desorbed products. The extent of the energy partitioning will be derived from the desorption-angle dependences of both the rotational and translational energies at each vibrational state. Such energy analysis has never been completed for any thermal reactive desorption. A new type of measurement is thus proposed. Additionally, we discuss the inadequate use of the detailed balance principle in desorption dynamics, which has prevented desorption dynamics from being sensitive to surface structures.