2,672 research outputs found
The structure of atomic nitrogen adsorbed on Fe(100)
Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 Ă 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED IâV spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Ă
above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Ă
). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe4N, thus supporting the view of a âsurface nitrideâ and providing a consistent picture of the structural and bonding properties of this surface phase
Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)
The formation of chemisorbed O-phases on Ru(0001) by exposure to O_2 at low
pressures is apparently limited to coverages Theta <= 0.5. Using low-energy
electron diffraction and density functional theory we show that this
restriction is caused by kinetic hindering and that a dense O overlayer (Theta
= 1) can be formed with a (1x1) periodicity. The structural and energetic
properties of this new adsorbate phase are analyzed and discussed in view of
attempts to bridge the so-called "pressure gap" in heterogeneous catalysis. It
is argued that the identified system actuates the unusually high rate of
oxidizing reactions at Ru surfaces under high oxygen pressure conditions.Comment: RevTeX, 6 pages, 3 figures, to appear in Phys. Rev. Let
Laser-induzierte schnelle thermische Desorption von FestkÜrper-Oberflächen
Absorption of a pulse of laser light in the surface region of a solid causes a fast temperature increase, followed, after the end of the laser pulse, by a rapid decrease to the base temperature. Experiments with a Nd laser (power 105â106 W/cm2, half width of the pulse 150 Âľsec) showed that desorption of CO from a Pd surface reaches its maximum rate within the time constant of the system (âź 0.5 msec). This effect can be used for example to investigate the kinetics of adsorption/desorption processes by means of relaxation methods. Š 1972, Walter de Gruyter. All rights reserved
Electrodeposition of Cu onto Reconstructed Pt(100) and Pt(110) Surfaces
The structural properties of reconstructed Pt(100) and (110) surfaces in H2S04 solution and after underpotential deposition (UPD) of Cu were investigated by means of LEED, RHEED, Auger electron spectroscopy and voltammetry. Potential cycling between â0.2 and +0.2 V leaves the surface reconstructions unaffected. While with Pt(110) the 1 X 2 surface periodicity survives Cu UPD as well as subsequent stripping, with Pt(100) the surface reconstruction is lifted by Cu UPD. Structural models for Pt(110) covered by 1 and 2 monolayers (ML) of Cu are presented which are consistent with the various experimental observations. Specific adsorption of anions (SO 24- and Br-) on Cu/Pt(110) causes the formation of ordered overlayers under certain conditions
Fritz Stern (1926â2016)
Well respected in America and beyond, but a household name in Germany, not least because of his published conversations with former Chancellor Helmut Schmidt, Fritz Stern dedicated his life's work to the study of the cultural historyâa blending of intellectual and political historyâof modern Germany and Europe. âWhy and how did the universal human potential for evil become an actuality in Germany?â was the âburning questionâ he sought to answer and the âfragility of freedomâ was, as he put it, âthe simplest and deepest lesson of [his] life.â Apart from studying history, Fritz Stern also made history, when he advised the British Prime Minister (Margaret Thatcher) on German reunification, or the American Ambassador to reunified Germany (Richard Holbrook) shortly after the âfifth Germanyâ that Stern had known came about (unless otherwise noted, the quotes herein are taken from Fritz Stern's autobiography entitled Five Germanys I have Known). Fritz Stern's insights into the workings of both liberalism and illiberalism were based on his profound historical case studies of politics and finance during the âlongâ 19th century that also encompassed little-known ideologues involved in preparing the soil for the rise of Nazism and its anti-Semitism. Well aware that Germany's rise to preeminence on the world stage following the unification of 1871 was rooted in its scientific-technological culture, Fritz Stern created a portrait gallery of the key protagonists of that âage of geniusâ: Paul Ehrlich, Albert Einstein, Fritz Haber, Max Planck, Walter Rathenau, Chaim Weizmann. These portraits were drawn with a personal touch, enabled by Fritz Stern's family lineage
Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering
The in-plane effective mass of quantum well states in thin Pb films on a Bi
reconstructed Si(111) surface is studied by angle-resolved photoemission
spectroscopy. It is found that this effective mass is a factor of three lower
than the unusually high values reported for Pb films grown on a Pb
reconstructed Si(111) surface. Through a quantitative low-energy electron
diffraction analysis the change in effective mass as a function of coverage and
for the different interfaces is linked to a change of around 2% in the in-plane
lattice constant. To corroborate this correlation, density functional theory
calculations were performed on freestanding Pb slabs with different in-plane
lattice constants. These calculations show an anomalous dependence of the
effective mass on the lattice constant including a change of sign for values
close to the lattice constant of Si(111). This unexpected relation is due to a
combination of reduced orbital overlap of the 6p_z states and altered
hybridization between the 6p_z and 6p_xy derived quantum well states.
Furthermore it is shown by core level spectroscopy that the Pb films are
structurally and temporally stable at temperatures below 100 K.Comment: 7 pages, 6 figure
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
Vibrations, coverage, and lateral order of atomic nitrogen and formation of NH<sub>3</sub> on Ru(10Ě Ě 10)
The dissociative chemisorption of nitrogen on the Ru(10Ě
10) 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 Ç(RuâN) mode at 41 meV and the v_l_(RuâN) 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(RuâNH) at 86 meV, and ν(NâH) at 408 meV. The TD spectra of mass 14 show three desorption states of nitrogen, NÎą at 740 K (from subsurface N), Nβ shifting from 690 to 640 K with increasing coverage, and NĎ at 550 K. The activation energy for desorption via the Nβ state is 120Âą10 kJ/mol. The TD spectra of mass two showed three desorption states at 450, 550, and 650 K due to the decomposition of NHx
Existence of a âHotâ Atom Mechanism for the Dissociation of O<sub>2</sub> on Pt(111)
The dissociation of O2 on a Pt(111) surface was studied by variable temperature scanning tunneling microscopy at 150â106 K.âThe two oxygen atoms created by the dissociation appear in pairs, with average distances of two lattice constants. Since thermal random walk sets in only at around 200 K, with a diffusion barrier of 0.43 eV and a preexponential factor of 10â6.3cm2sâ1, the distribution of distances at around 160 K evidences nonthermal processes during the dissociation. It is concluded that transient ballistic motion exists, where the short range traveled is in agreement with recent molecular dynamics studies
Coverage, lateral order, and vibrations of atomic nitrogen on Ru(0001)
The N/Ru(0001) system was studied by thermal desorption spectroscopy (TDS), lowâenergy electron diffraction (LEED), and highâresolution electron energyâloss spectroscopy (HREELS). Atomic nitrogen was prepared by NH3 decomposition at sample temperatures decreasing from 500 to 350 K during NH3 exposure. A maximum N coverage of θN=0.38 could thus be achieved. â, split 2Ă2 and 2Ă2 LEED patterns were observed for decreasing θN. After NH3 decomposition and before annealing the sample to a temperature above 400 K, the surface is composed of adsorbed N, H, and NH species. This composite layer exhibits a split â LEED pattern due to domains of size 4 with heavy walls. This phase decays through dissociation of NH leading to sharp firstâorder type desorption peaks of H2 and N2. From the weak intensity of the ν(RuâNH) stretch mode it is concluded that NH is adsorbed at threefoldâhollow sites. The energy of the ν(RuâN) mode shifts from 70.5 to 75.5 meV when θN is increased from 0.25 to 0.38
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