Karl Jacobi

Vibrational and structural properties of OH adsorbed on Pt(111)

OH species adsorbed on Pt(111) were studied in a combined investigation using scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). OH was formed by two different reactions, by reaction of H2O with O, and as an intermediate in the reaction of O with hydrogen to H2O. In both cases, two ordered OH phases were observed, a (√3×√3)R30° and a (3×3) structure, for which models are proposed. Both structures have OH coverages of 2/3, and their formation is driven by hydrogen bond formation between the adparticles; the OH adsorption site is most likely on top. OH molecules at defects in the adlayer, in particular at island edges, are spectroscopically distinguishable and contribute significantly to the vibrational spectra in disordered OH layers. This is important for the water formation reaction, where the OH islands are small. The discrepancies between previous HREELS studies on OH can be explained by the different degree of order under the various formation conditions.

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 STM 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.

Angular resolved UV photoemission from ordered layers of carbon monoxide on a nickel(100) surface

Abstract Angular resolved UPS measurements have been performed on the (√2 × √2)R45° and compressed structures of the adsorption system CO/Ni(100). Due to the operation of dipole selection rules the dependence of normal exit photoemission on angle of photon incidence is found to be a sensitive probe of initial state symmetry. The polar angular dependence of emission from the 4σ orbital follows closely that predicted by Davenport, provided that a correction for refraction and total photoelectron current conservation is applied. Such polar angular dependences show band shifts indicative of dispersion effects, which become more marked at higher CO coverages. An azimuthal angular dependence of emission was not observed despite the presence of ordered adlayers.

Energetics and Vibrational States for Hydrogen on Pt(111)

We present a combination of theoretical calculations and experiments for the low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111) surface. The vibrational band states are calculated based on the full three-dimensional adiabatic potential energy surface obtained from first principles calculations. For coverages less than three quarters of a monolayer, the observed experimental high-resolution electron peaks at 31 and 68meV are in excellent agreement with the theoretical transitions between selected bands. Our results convincingly demonstrate the need to go beyond the local harmonic oscillator picture to understand the dynamics of this system.

Sticking coefficient for dissociative adsorption of N2 on Ru single‐crystal surfaces

The dissociative chemisorption of N2 on Ru(0001), Ru(1010), and Ru(1121) surfaces at 300 K was studied by means of high‐resolution electron energy loss spectroscopy and thermal desorption spectroscopy. The initial sticking coefficient was determined to s0=(1±0.8)×10−12, within the limits of error independent of surface orientation. On Ru(1010) and Ru(1121) small amounts of N can be dissolved into the subsurface region.

Angle resolved photoemission of the p(2x2) oxygen overlayer on Ni(001): Measurements and calculations

Abstract Two oxygen induced peaks are found in the angle resolved UPS using He II (40.8 eV). They are identified by comparing the dependence on the incident angle of light and on the emission direction with calculations. It is shown that anisotropies of the initial states as well as of the final states must be included in calculations in order to get a good agreement.

The oxidation of CO on RuO2(110) at room temperature

RuO2(110) surfaces were prepared by exposing Ru(0001) to 107 L of O2 at 700 K. Postexposure of O2 at 300 K resulted in an additional oxygen species (O-cus) adsorbed on coordinatively unsaturated Ru atoms (Ru-cus). The surface was then exposed to CO at 300 K and studied by thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS). It is demonstrated that CO is oxidized at 300 K through reaction with both the O-cus as well as with surface O-atoms held in bridge positions (O-bridge). Although—at room temperature—CO adsorbs intermediately on the Ru-cus atoms, it is stable only at the Ru atoms underneath the O-bridge after the latter has been reacted off. At room temperature only surface oxygen takes part in the CO oxidation and the oxygen-depleted surface can be restored by O2 exposure, so that under steady-state flow conditions an oxygen-deficient surface will exist whose stoichiometry will be determined by the ratio of partial pressures.

Dissociative chemisorption of nitrogen on Ru(0001)

The dissociative chemisorption of nitrogen on clean and cesiated Ru(0001) surfaces has been studied using high‐resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). N2 (at 300 K) chemisorbs dissociatively with a sticking coefficient of 2×10−6, independent of substrate temperature which was varied between 420 and 700 K. The saturation coverage is found at 0.5 monolayer. The energy of the N–Ru stretching vibration is 71 meV at the bare surface and 69 meV at the cesiated Ru(0001) surface. The activation energy for desorption is about 190 kJ/mol for small coverages. The kinetic data suggest the existence of an activation barrier in the entrance channel of adsorption. Preadsorption of 0.08 monolayer of Cs increases the sticking coefficient only by a factor of 1.3, and the maximum amount of adsorbed N is reduced due to blocking of adsorption sites through Cs.

Change of InAs/GaAs quantum dot shape and composition during capping

Using plan-view and cross-sectional scanning tunneling microscopy, the shape and composition of InAs/GaAs quantum dots are investigated before and after capping by GaAs. During capping, the original pyramidally shaped quantum dots become truncated, resulting in a flat (001) top facet and steeper side facets. The InAs quantum dots are found to be intermixed at their top with GaAs due to material rearrangement. Since the bottom interface of quantum dots and wetting layer is always sharp, this intermixing occurs during capping and not during quantum dot growth. Considering strain energies, a model for the capping is presented.

The adsorption of atomic nitrogen on Ru(0001): geometry and energetics

Abstract The local adsoprtion geometries of the (2 × 2)-N and (✓3 × ✓3)R30°-N phases on the Ru(0001) surface are determined by analyzing low-energy electron diffraction 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 and 1.10 A in the (2 × 2) and the (✓3 × ✓3)R30° unit cell, respectively. This result is attributed to a strong N binding to the Ru surface (Ru-N bond length = 1.93 A ) in both phases as also evidenced by an initio calculations which revealed binding energies of 5.82 and 5.59 eV, respectively.