Vibrational analysis of cesium on Ru(0001)

We report a high-resolution electron-energy-loss spectroscopy study of the coverage dependence of the Cs-Ru stretch vibration at 300 K. Surprisingly, the frequency of Cs-Ru stretch vibration shifts upward by about 30% with Cs coverage for 0Cs Cs>0.19 is associated with the metallization of the alkali-metal layer including a structural change

The Adsorption of Atomic Nitrogen on Ru(0001): Geometry and Energetics

The local adsorption geometries of the (2x2)-N and the (sqrt(3)x sqrt(3))R30^o -N phases on the Ru(0001) surface are determined by analyzing low-energy electron diffraction (LEED) intensity data. For both phases, nitrogen occupies the threefold hcp site. The nitrogen sinks deeply into the top Ru layer resulting in a N-Ru interlayer distance of 1.05 AA and 1.10 AA in the (2x2) and the (sqrt(3)x sqrt(3))R30^o unit cell, respectively. This result is attributed to a strong N binding to the Ru surface (Ru--N bond length = 1.93 AA) in both phases as also evidenced by ab-initio calculations which revealed binding energies of 5.82 eV and 5.59 eV, respectively.Comment: 17 pages, 5 figures. Submitted to Chem. Phys. Lett. (October 10, 1996

Atomically resolved structure of InAs quantum dots

InAs was grown by molecular-beam epitaxy onto GaAs(001) until quantum dots (QDs) formed. At this point, the growth was interrupted and the uncovered QDs were investigated in situ by scanning tunneling microscopy (STM). Atomically resolved STILI images of the QDs revealed that four dominating bounding facets occur, whose Miller indices were identified to be {137}. The assignment of the facet orientation was based on experiments on planar high Miller index GaAs surfaces. In addition, the latter experiments indicated that (137) facets are thermodynamically stable only up to a certain size. This conclusion is assumed to explain the sharp size distribution of InAs QDs

Experimental evidence for a stable GaAs surface near (113)

GaAs surfaces vicinal to (113) with a continuous range of misorientation angles up to 11.5° in all azimuthal directions were created by grinding a spherical depression into (113) oriented samples. Thin homoepitaxial layers were grown onto these samples by molecular beam epitaxy (MBE), and the surfaces were in situ studied by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface quality in the depression was verified by reproducing LEED patterns of the (113) and (114) surfaces. A stable GaAs surface was found that is oriented from (113) by 9°±2° towards [11̅0̅]. STM and LEED images of this surface are presented

Adatom-induced donor states during the early stages of Schottky-barrier formation: Ga, In, and Pb on Si(113)

We performed angle-resolved ultraviolet and soft-x-ray photoelectron spectroscopy for the early stages of Schottky-barrier formation of Ga, In, and Pb on Si(113) at room temperature. In the coverage region below 0.1 monolayer a band-bending behavior, typical for donor states, is found. The energies of the adatom-induced donor states in the band gap depend on the adatoms. The Schottky barrier reaches its final value at a coverage of about one monolayer. The values are 0.35 eV above the valence-band maximum for In and Ga and 0.425 eV for Pb. Measurements with Xe interlayers were made to verify that these interfaces are not reactive

Vibrational analysis of the (Cs+CO)‐(2×2) compound layer on Ru(0001)

On a Ru(0001) surface Cs and CO form a very well ordered (Cs+CO)‐(2×2) compound layer whose structure was analyzed recently (Cs on‐top, CO in threefold sites). Here we present a vibrational analysis of the same system using high‐resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low‐energy electron diffraction (LEED). The bonding of CO to Ru is both local and nonlocal. Two (C–O) stretch frequencies are observed depending on whether there are one or two CO molecules in the 2×2 cell. They change in energy between 155 and 204 meV depending on CO coverage θCO. Setting θmaxCO=1.0, the evolution of the C–O stretch intensities indicates that up to θCO=0.22 the 1‐CO‐(2×2) phase is formed exclusively implying some mobility of the Cs layer. For θCO≳0.22 the 2‐CO‐(2×2) phase grows additionally until at θCO=1.0 only the 2‐CO‐(2×2) phase is found. Two Ru–CO stretch modes are observed for the first time and are assigned to adsorption in the hcp and fcc hollow sites within the 2×2 unit cell. They are very weak in intensity which is attributed to the threefold‐hollow site and some screening in the 2D compound. With CO adsorption a change of the electronic structure of the Cs adlayer is observed; the adlayer loses metallicity and the Cs–Ru stretch becomes visible. Strong changes of the Cs–Ru stretch energies are observed with CO coverage

Step structure on GaAs(113)A studied by scanning tunneling microscopy

The GaAs(113)A surface was prepared by molecular-beam epitaxy and in situ characterized by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The occurrence of an (8×1) reconstruction as proposed by Wassermeier et al. [Phys. Rev. B 51, 14 721 (1995)] was confirmed. Overview STM images reveal a striking anisotropy in the step structure of this surface. While steps along [332¯] (the 1× direction of the reconstruction) are straight for up to 2000 Å, steps along [11¯0] are extremely rough. In this direction, kinks occur typically after less than 100 Å. The ratio of the respective lateral step densities is 8±4. This anisotropy is explained by applying the electron counting rule (ECR) to one-dimensional islands. While islands along [332¯] fulfil the ECR, it is violated by islands along [11¯0]. Thus, if structures formed additionally perpendicular to step edges along [332¯], they would be energetically unfavorable. Hence, growth occurs mainly by propagation along [332¯]. It is suggested that the determining structural element of GaAs(113)A−(8×1) is the zigzag chain of As dimers

Gravity wave flux modulation by planetary waves in a circulation model

Mit Hilfe eines Zirkulationsmodells der mittleren Atmosphäre wird die Ausbreitung der Quasi-Zwei-Tage-Welle simuliert. Das Modell verfügt über eine aktuelle Schwerewellenparametrisierung und ermöglicht daher die detaillierte Beschreibung der Wechselwirkung planetarer Wellen mit Schwerewellen. Bei Anwesenheit der Quasi-Zwei-Tage-Welle wird der Schwerewellenfluss mit der Periode von zwei Tagen und der räumlichen Struktur der Quasi- Zwei-Tage-Welle moduliert. Modellergebnisse zeigen, dass sich die Quasi-Zwei-Tage-Welle nicht gut in die untere Thermosphäre ausbreitet. Phasenvergleiche zwischen Quasi-Zwei-Tage-Welle und Divergenz des Eliassen-Palm-Flusses der Schwerewellen zeigen, dass dies eine Folge sekundärer Anregung der Quasi-Zwei-Tage-Welle durch brechende Schwerewellen ist, die außer Phase mit der Originalwelle erfolgt

Experimental evidence for chiral melting of the Ge(113) and Si(113) 3×1 surface phases

Results of a spot-profile-analysis low-energy-electron-diffraction study of the 3×1 order-disorder phase transition of the Ge(113) and Si(113) surfaces are reported. For Ge(113) agreement with predictions for chiral melting with isotropic scaling is found. For Si(113) we compare our findings to those of other LEED and x-ray-scattering studies

Vibrations, coverage, and lateral order of atomic nitrogen and formation of NH₃ on Ru(10̅̅10)

The dissociative chemisorption of nitrogen on the Ru(10̅10) surface has been studied using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). To prepare a surface covered by atomic nitrogen we have used ionization-gauge assisted adsorption. A saturation coverage of θN=0.6 is achieved of which about 30% is in the subsurface region. At saturation coverage a (-1/2 1/1) pattern is observed. Then v ǁ(Ru–N) mode at 41 meV and the v_l_(Ru–N) mode at 60 meV are identified. Upon exposing the nitrogen covered surface to hydrogen at 300 K we have observed the formation of NH3 which is characterized by its symmetric bending mode δs(NH3) at 149 meV. At 400 K, NH3 could not be detected. The reaction intermediate NH is stable up to 450 K and has been identified by its vibrational losses ν(Ru–NH) at 86 meV, and ν(N–H) at 408 meV. The TD spectra of mass 14 show three desorption states of nitrogen, Nα at 740 K (from subsurface N), Nβ shifting from 690 to 640 K with increasing coverage, and Nϒ at 550 K. The activation energy for desorption via the Nβ state is 120±10 kJ/mol. The TD spectra of mass two showed three desorption states at 450, 550, and 650 K due to the decomposition of NHx