Surface Structure Dynamics Research Articles

Helium atom scattering experiments and molecular dynamics simulations of the structure and lattice dynamics of 15-layer acetylene films on KCl(001)

The surface structure and lattice dynamics of thick films of 15 to 40 layers of acetylene adsorbed on a KCl(001) single crystal surface have been investigated at about 40 K by combining helium atom scattering (HAS) experiments with classical molecular dynamics simulations. As rarely occurs for physisorbates on ionic crystals layer-by-layer growth is observed by HAS up to at least 15 layers producing films with a (√2 × √2)R45° geometry. The simulations confirm the stability and structure of these layers up to about 100 K. The time-of-flight spectra reveal along the [110] direction at least four phonon modes, which are assigned by simulations based on the calculated power spectra of the velocity and angular autocorrelation functions.

Effects of Solvent Viscosity on Protein Dynamics: Infrared Vibrational Echo Experiments and Theory

The influence of solvent viscosity on the surface and internal structural dynamics of the protein myoglobin is studied using ultrafast infrared vibrational echo measurements of the pure dephasing of the A1 CO stretching mode of myoglobin−CO (Mb−CO). The dephasing reflects protein structural fluctuations as sensed by the CO ligand bound at the protein's active site. Measurements made as a function of solvent viscosity at 295 K show that the pure dephasing has a marked dependence on viscosity. In addition, the pure dephasing of Mb−CO in the solvents trehalose and 50:50 ethylene glycol:water are compared as a function of temperature T (10−295 K). The pure dephasing data in the two solvents have identical T1.3 temperature dependences at low temperatures, where both solvents are glassy solids. At higher temperatures, the Mb−CO pure dephasing has a much steeper temperature dependence in ethylene glycol:water, which is a liquid, than in trehalose, which is a glass at all temperatures studied. The steep temperatu...

An in situ grazing incidence X-ray absorption study of ultra thin Ru xSe y cluster-like electrocatalyst layers

A novel metal chalcogenide cluster material (Ru xSe y) was investigated in situ using X-ray absorption spectroscopy at grazing angles (GIXAS). The electrocatalyst in form of thin films was deposited onto conducting glass substrates (SnO 2:F) from colloidal solution. The detected signal was the result of X-ray reflection from the substrate. The in situ measurements at different atmospheres in the electrochemical system (Ar or N 2, and O 2) as a function of the applied electrode potential provided evidence for surface structure dynamics in electrocatalysis. Although the detected signals were at the limit of detection due to the small quantity of deposited catalyst material, we conclude that the co-ordination of oxygen to the catalytic centres enhances the local disorder. The change in the distance amounts to circa 12% at the vicinity of the ruthenium. This change decreases when changing to N 2 or Ar (non electrocatalytic conditions), therefore, giving strong evidence of reversibility.

Coherent soft x-ray scattering from InP islands on a semiconductor substrate

Coherent soft x-ray scattering experiments from a semiconductor sample consisting of InP islands on a smooth semiconductor substrate are described. The soft x-ray scattering was performed with 266 eV photons produced by an undulator source. Using a position sensitive detector, we are able to detect diffusely scattered x rays in the vicinity of the specular reflection, with an in-plane momentum transfer of up to 6 μm−1. Using Huygens–Fresnel theory and atomic force microscope images of the surface structure, we simulated the scattering assuming a finite lateral coherence length for the incident radiation. The lateral coherence length of the incident beam was found to be 20 μm from a fit to the observed diffraction pattern from a pinhole. The effect of changes in the surface morphology on the speckle pattern was simulated to explore the potential of coherent soft x-ray scattering for the study of surface structure dynamics.

Surface Ship Shock Modeling and Simulation: Two‐Dimensional Analysis

The modeling and simulation of the response of a surface ship system to underwater explosion requires an understanding of many different subject areas. These include the process of underwater explosion events, shock wave propagation, explosion gas bubble behavior and bubble‐pulse loading, bulk and local cavitation, free surface effect, fluid‐structure interaction, and structural dynamics. This paper investigates the effects of fluid‐structure interaction and cavitation on the response of a surface ship using USA‐NASTRAN‐CFA code. First, the one‐dimensional Bleich‐Sandler model is used to validate the approach, and second, the underwater shock response of a two‐dimensional mid‐section model of a surface ship is predicted with a surrounding fluid model using a constitutive equation of a bilinear fluid which does not allow transmission of negative pressures.

Atomic dynamics and structure of the Ge(111)c(2 x 8) surface.

We present results of a first-principles molecular-dynamics study of the surface structure and atomic dynamics of Ge(111)c(2\ifmmode\times\else\texttimes\fi{}8) at T=0 and T=300 K. The T=0 ground-state structure shows asymmetries between the two adatoms and the two restatoms in the surface unit cell, which are seen experimentally in low-energy electron diffraction. These asymmetries, which are found to survive at T\ensuremath{\sim}300 K, lead to zone folding and splitting of the surface phonon modes. Two pairs of prominent surface modes are identified, mostly localized on the adatoms and second-layer atoms just below. The values of the mean square displacements for the various layers at different temperatures moreover indicate an enhanced surface anharmonicity with respect to the bulk.

An Application of Classical Molecular Dynamics Simulation and AB Initio Density-Functional Calculation in Surface Physics

Abstract Classical molecular dynamics simulation and ab initio mixed basis Car-Parrinello methods are discussed and applied to the investigation of the results of a recently performed STM-based experiment involving the adsorption of C60 molecules on the dimerized Si surface. We show that these methods are capable of providing the theoretical basis for this experiment and test the validity of the associated conjectures. A mixed-basis all-electron formalism for the Car-Parrinello method is proposed to obtain the detailed understanding of the electronic states and dynamics of surface structure. A band structure calculation using this formalism is performed for the c(4 × 3) structure of C60 adsorbed on Si (100) surface and is compared with the experimental results.

Helium atom scattering: a gentle and uniquely sensitive probe of surface structure and dynamics

The scattering of molecular beams of 4He atoms from solid surface provides a very sensitive diffraction probe of surface structure and surface dynamics. The experimental method is described and illustrated by some studies of surface structure of metal surface. Because of the high monochromaticity of the helium beams ( Delta nu / nu approximately=1%) surface phonons with energies from 0.5 meV upwards can be resolved and entire surface phonon dispersion curves can be measured. On Cu(111) and Cu(001) an anomalous longitudinal resonance with large intensity is observed. On W(001) and W(110)+H(1*1) sharp anomalies give evidence of electron-phonon coupling which in the former system is related to the reconstruction at 280 K. Finally, two experiments on the dynamics of thin films as a function of layer thickness are described.

APPLICATIONS OF ULTRAFAST LASER SPECTROSCOPY TO THE STUDY OF SEMICONDUCTOR PHYSICS

This article reviews the application of some of the principal methods of picosecond and femtosecond laser spectroscopy to the investigation of the dynamics of carriers, phonons and surface structure in semiconductors. The measurement of the temporal evolution of photoinduced luminescence, absorption, reflection and scattering in semiconductors makes it possible to obtain the lifetimes of photogenerated electrons, holes, excitons and phonons in both the bulk and quantum wells and superlattice structures. The information produced by these studies is necessary for the basic understanding of the underlying physics of semiconductors. In addition, the parameters obtained from these studies are needed for evaluating ultrafast transport, switching, photoconductive response and imaging in semiconductor materials, which will determine their limitations for use in high-speed and high-frequency devices and computers. For measuring time resolved luminescence, the principal techniques used, namely, the streak camera, the optical Kerr gate and the up-conversion gate are thoroughly discussed. Several pump and probe methods are described for the determination of time resolved absorption, reflection and Raman scattering. For absorption measurements where the probe wavelength differs from the pump, the former is generated in nonlinear media by means of stimulated Raman scattering and the supercontinuum for the UV and visible regions and by parametric and difference frequency generation for the near- and mid-IR. Nonlinear optics techniques considered are degenerate and nondegenerate four-wave mixing and transient grattings among which photon echoes yield the momentum relaxation of hot electrons. Coherent anti-Stokes Raman scattering (CARS) and phase conjugate Raman scattering (PC) are described to determine phonon dephasing times and the nonlinear susceptibility, χ3.

He-atom scattering study of the temperature-dependent charge-density-wave surface structure and lattice dynamics of 2H-TaSe2(001).

Elastic and inelastic He-atom scattering has been used to measure the surface structure and surface dynamics of the layered transition-metal dichalcogenide 2H-${\mathrm{TaSe}}_{2}$(001) crystal. The results cover temperatures from 60 to 140 K. Below ${\mathit{T}}_{0}$=122 K an incommensurate charge-density wave (CDW) is formed, which becomes commensurate below approximately 90 K. The measured intensities of the CDW diffraction peaks continuously increase with decreasing temperature below 122 K. From the diffraction intensities, the temperature-dependent amplitude of the surface potential corrugation is determined. The corrugation amplitude is used as an order parameter and from its temperature dependence, on cooling, a critical exponent of \ensuremath{\beta}=0.33 is extracted. Time-of-flight spectra are used to determine the surface-phonon dispersion curves. Although the spectra are nearly the same at 60 and 140 K, a softening in the Rayleigh mode is observed for intermediate temperatures (around 110 K) at Q=0.53 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$, which is near the middle of the Brillouin zone. The difference between the bulk and the surface dynamics is interpreted through the use of the dispersive linear-chain model.

Design, construction and use of a fast atom scattering spectrometer

Atom-surface scattering can provide information on surface composition, surface structure and surface dynamics without many of the problems associated with charged particle beams. The technique of ion-scattering spectrometry (ISS) is now well established and this paper describes a development of this technique involving the use of an uncharged fast atom beam (FAB) source in place of an ion source. The particular problem associated with the use of neutral particles in this application is in the accurate measurement of particle energy where conventional electrostatic methods cannot be employed. This was overcome by developing a time-of-flight (TOF) system consisting of a nanosecond pulsing system for the source together with detection and data acquisition and processing systems. The design, development and construction of the system is described in detail. The use of the TOF system to characterize the source fully is described, and the results of some initial atom-surface scattering experiments are given.

Application of reflectance difference spectroscopy to molecular-beam epitaxy growth of GaAs and AlAs

We perform an accuracy analysis of several possible reflectance–difference (RD) configurations that are compatible with standard molecular-beam epitaxy (MBE) growth chambers, and describe in detail an optical-bridge system that can determine relative changes in RD signals as small as 5×10−5 under standard growth conditions. Using this system, we determine the RD response of GaAs for changes in surface conditions at different wavelengths and correlate these to simultaneously measured reflection high-energy electron diffraction (RHEED) intensities. Maximum anisotropies are found at 2.0–2.5 and 3.5 eV for Ga on GaAs and Al on AlAs, respectively, providing a way of spectrally distinguishing Ga–Ga and Al–Al dimers for surface-chemical investigations, and suggesting that these photon energies are also optimal for modifying growth by light. At photon energies well removed from these anisotropy maxima, RD signals follow changes in surface structure, as RHEED. Our RD-RHEED correlations provide insight concerning crystal growth by MBE and establish a common experimental link between MBE and non-UHV methods of crystal growth where RHEED cannot be used. Finally, our results illustrate various possibilities of using reflectance difference spectroscopy to investigate surface structure, surface chemistry, and surface dynamics.

Femtosecond-Time-Resolved Surface Structural Dynamics of Optically Excited Silicon

The dynamics of the structural changes that take place on a silicon surface following excitation with an intense optical pulse are observed with 90-fs time resolution. The threefold rotational symmetry of the silicon $〈111〉$ surface becomes rotationally isotropic within a picosecond after excitation consistent with a transition from the crystalline to the liquid molten state.

Fentosecond Time Resolved Surface Structural Dynamics of Optically Excited Silicon

ABSTRACTLaser processing of materials has raised a number of interesting issues relating to phase transitions and the structure of optically excited semiconductors. In this paper we describe the dynamics of structural changes that take place on a silicon surface following excitation with an intense optical pulse. Second harmonic generation from the silicon surface is used as a tool to measure crystalline order with 90 femtosecond resolution. The three-fold rotational symmetry of the silicon <111> surface is observed to become rotationally isotropic within a picosecond after excitation consistent with a transition from the crystalline to the liquid molten state.

Calculation of structure-sensitive surface peak intensity in mev ion scattering

The surface peak intensity from a silicon crystal has been calculated by the Monte Carlo simulation method assuming a variety of surface structure models and dynamics. Effects of thermal vibration, displacements and relaxation of surface atoms, and also the shadowing effect by adsorbed layers and the effect of multiple scattering of the projectile in penetrating surface layers have been evaluated in the He + ion incident energy range from 0.1 to 2.0 MeV. Since many real surfaces of crystals are covered with adsorbed (deposited) layers or otherwise reconstructed when they are clean, it is of practical usefulness to obtain the calculation results about such surfaces in order to apply the MeV ion scattering technique for surface analyses. The results show the sensitivity and limits of this technique.