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The interaction of NO with the O-rich RuO 2 (1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with 18 O. During exposure to NO an O···N–O surface group (NO 2 -cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO 2 -cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.

We report results of density functional theory calculations of the interaction of NO with the stoichiometric RuO2(110) surface that provide insights into the experimentally observed lack of reactivity for the system. We find that NO adsorbs on top of the undercoordinated Ru (Ru-cus) with an upright axis, and the adsorption energy (with zero-point contribution) changes from 1.61 eV for 0.5 ML to 1.49 eV for 1 ML coverage. Once all Ru-cus sites are occupied, NO adsorbs on O-bridge sites with adsorption energy of 0.66 eV, forming an asymmetric O···N−O surface complex. We also find a high dissociation barrier of 3.22 eV for NO on Ru-cus. Although the activation energy for oxidation of NO is calculated to be about 1.2 eV, the location of the final state makes the formation of NO2 only transient with a large probability of reverting back to NO. Additionally, the total energy difference for the reaction NO + NO → N2O + O on RuO2(110) is found to be about 1.35 eV. Comparison of results with those for a similar ov...

The GaAs(1 1 2)A surface was prepared and InAs QDs were grown on it by molecular beam epitaxy and investigated using in situ scanning tunneling microscopy and reflection high-energy electron diffraction. We confirm and better specify an earlier result that the bare surface is not flat, but is facetted into {2 5 11}A, {1 1 0}, and (1 1 1)A facets forming shallow holes. The InAs wetting layer is not flat, but undulated and disordered, reflecting in part the structure of the bare surface. InAs QDs are formed with high number density typical for coherent QDs, i.e., without any lattice defect at the interface. However, the size distribution is quite broad what is considered to be typical for the A faces of GaAs substrates. From comparison with the literature we conclude on growth of coherent QDs. The shape of the InAs QDs is complementary to that of the holes at the bare surface. The QDs are rather flat entities.

InAs was deposited onto nominal and vicinal (1.0°-off-oriented) GaAs(2 5 11)A surfaces by means of molecular beam epitaxy and studied by scanning tunneling microscopy and photoluminescence measurements. Both surfaces show step bunches along the [311¯] direction which form fairly large (0 1 1) nano-facets. Large, inhomogeneously distributed InAs islands are formed on these nano-facets. The InAs islands exhibit a wide size distribution and vanishing photoluminescence intensity, both being characteristic for incoherent islands. The shape of the incoherent InAs islands is composed mainly of (1 1 1)A, (0 1 1), (0 0 1), and (3 1 7)A surfaces. During growth the latter undergoes a transition into the steeper (1 0 1) facet. The shape of the incoherent islands exhibits no symmetry in accordance with the missing of any symmetry at the GaAs(2 5 11)A bulk-truncated substrate surface. On the flat terraces of the nominal GaAs(2 5 11)A surface a second kind of QDs develops which are of the same shape but of a sharp size distribution. The photoluminescence intensity of the latter is quenched presumably by the coexistent incoherent InAs islands.

Thin layers of InAs were deposited onto GaAs(0 0 1) substrates using molecular-beam epitaxy. The transition from the two-dimensional wetting layer to three-dimensional quantum dots (QDs) of strained InAs was studied by in situ scanning tunneling microscopy with atomic resolution. Closely before the transition, the wetting layer exhibits a flat morphology with mostly straight and parallel steps. The transition occurs during a coverage increase by less than 0.2 ML only. After the transition the wetting layer shows step meandering and holes. Besides the continuously deposited InAs material from the molecular beams, mass transport from the wetting layer and even out of the substrate is concluded to contribute to QD formation. The location of the QDs with respect to the step edges is discussed within a model.

The adsorption of methane and ethane on the stoichiometric and O-rich RuO2(110) surfaces, respectively, was studied by thermal desorption and high-resolution electron energy loss spectroscopy. The results support weak adsorption (physisorption) of these molecules on the surface, regardless of the presence of undercoordinated Ru and oxygen surface sites. The vibrational spectra recorded at about 90 K are compared with spectra calculated for gaseous CH4 and C2H6. The nearly complete agreement of frequencies and relative intensities is consistent with physisorption. No evidence of adsorption induced molecule activation is found.

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 � C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 · 4) structure. For a disorientation of 0.7� , InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 · 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 � C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying. � 2005 Elsevier B.V. All rights reserved.

The selective oxidation of ammonia to either N2 or NO on RuO2(110) single-crystal surfaces was investigated by a combination of vibrational spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and steady-state rate measurements under continuous flow conditions. The stoichiometric RuO2(110) surface exposes coordinatively unsaturated (cus) Ru atoms onto which adsorption of NH3 (NH3-cus) or dissociative adsorption of oxygen (O-cus) may occur. In the absence of O-cus, ammonia desorbs completely thermally without any reaction. However, interaction between NH3-cus and O-cus starts already at 90 K by hydrogen abstraction and hydrogenation to OH-cus, leading eventually to N-cus and H2O. The N-cus species recombine either with each other to N2 or with neighboring O-cus leading to strongly held NO-cus which desorbs around 500 K. The latter reaction is favored by higher concentrations of O-cus. Under steady-state flow condition with constant NH3 partial pressure and varying O2 pressure, the rate for N2 formation takes off first, passes through a maximum and then decreases again, whereas that for NO production exhibits an S-shape and rises continuously. In this way at 530 K almost 100% selectivity for NO formation (with fairly high reaction probability for NH3) is reached.

InAs quantum dots (QDs) were grown by molecular beam epitaxy on GaAs ( 1 ¯ 1 ¯ 4 ¯ ) B surfaces and studied by in situ scanning tunneling microscopy (STM). Atomically resolved STM images, obtained for the QDs of different size, reveal that the QDs exhibit different shapes at different stages of evolution. First, small islands develop composed mainly of flat { 1 ¯ 3 ¯ 5 ¯ } B and ( 1 ¯ 1 ¯ 2 ¯ ) B facets. Next, steeper { 2 5 ¯ 11 ¯ } A and { 2 ¯ 5 11 ¯ } A facets form at the { 1 ¯ 3 ¯ 5 ¯ } B and ( 1 ¯ 1 ¯ 2 ¯ ) B facets. Then a remarkable shape transition occurs with the aspect ratio increasing from 0.10 to 0.20. The QDs grow mainly in height now by stacking { 2 5 ¯ 11 ¯ } A layers so that new, steep { 0 1 ¯ 1 ¯ } and ( 1 ¯ 1 ¯ 1 ¯ ) B facets appear at the side and the top { 2 5 ¯ 11 ¯ } A facets shrink and finally vanish. The mature QD is terminated by five steep facets besides the flat base. Further growth results in an elongation of the mature QD along the symmetry direction.

The GaAs(1¯1¯2¯)B surface was prepared by molecular beam epitaxy and investigated by in situ scanning tunneling microscopy with respect to structure and morphology. A remarkable surface corrugation with (01¯1¯) and (1¯01¯) facets was observed at Ga-rich condition. The period of the corrugation is about 12 nm. The Ga-rich structure changed into an As-rich structure, when the sample temperature was decreased to below about 520 °C under As2 flux. The As-rich surface is also not flat but undulated on a sub-μm scale. On an atomic scale it is rather rough being composed of fragments of 1D chains along [11¯0]. There is a twofold periodicity along [11¯0], but little periodicity along [111¯].

InAs quantum dots (QDs) were prepared by molecular beam epitaxy on GaAs(1¯1¯2¯)B substrates. Shape and size distribution of the QDs were investigated using in situ scanning tunneling microscopy as function of preparation temperature between 435 and 550°C. The wetting layer is not flat but undulated in submicrometer scale in a similar way as the bare substrate. The atomic structure of the wetting layer is the same as found for the flat base of InAs QDs grown on GaAs(1¯1¯3¯)B substrates. The shape of the QDs is given by {110}, (1¯1¯1¯)B, and {1¯4¯3¯}B bounding facets and a round vicinal (001¯) region. Unexpectedly, the number density increases and the size distribution sharpens, when the growth temperature is increased from 435to470°C, which is attributed to lattice defects incorporated into the QDs during growth at 435°C.

Ethylene adsorbed on the stoichiometric RuO 2 (110) surface at 85 K is investigated applying high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy in combination with isotope labeling experiments. Under the applied conditions (ultra-high vacuum, no continuous reactant supply) ethylene does not react with stoichiometric RuO2(110) and is desorbed molecularly at 320 K. Warming to 260 K leads to a rearrangement of C 2 H 4 from n- to σ-bonded.

The adsorption and reaction of NH3 was studied on the Ru(1120) surface using high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. Following NH3 exposure at 85 K, ...

Ethylene (C2H4) adsorbed on the stoichiometric and oxygen-rich RuO2(110) surfaces, exposing coordinatively unsaturated Ru-cus and O-cus atoms, is investigated by applying high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy in combination with isotope labeling experiments. On the stoichiometric RuO2(110) surface C2H4 adsorbs and desorbs molecularly. In contrast, on the oxygen-rich RuO2(110) surface ethylene adsorbs molecularly at 85 K and is completely oxidized through interaction with O-cus and O-bridge upon annealing to 500 K. The first couple of reactions are observed at 200 K taking place on Ru-cus: A change from π- to σ-bonding, formation of −CO and −C−O groups, and dehydrogenation giving rise to H2O adsorbed at Ru-cus. Maximum reaction rate is reached for C2H4 chemisorbed at Ru-cus with O-cus neighbors on each side. A model for the first couple of reactions is sketched. For the final combustion, C2H4 reacts both with O-cus and O-bridge. Ethylene oxide is not detecte...

GaAs(114)A surfaces were prepared using molecular beam epitaxy followed by annealing in As2 pressure as well as in ultrahigh vacuum. Based on low-energy electron diffraction, in situ scanning tunneling microscopy measurements, and ab initio first-principles electronic-structure calculations the surface reconstructions are determined. Under As-rich conditions GaAs(114)A develops a β2(2×1) and/or β2c(2×2) reconstruction which is analogous to the GaAs(001)β2(2×4) one. Although a low surface free energy (46 meV/A2 for As-rich conditions) is found for GaAs(114)Aβ2(2×1), it is split up into mesoscopic (113) and (115) facets. Also typical steps between 150 A wide terraces are observed along [110]. The stoichiometric GaAs(114)A surface is mesoscopically very flat and develops a α2(2×1) reconstruction of a low surface free energy of 53 meV/A2. The surface free energies for the α(2×1) and a newly introduced ω(2×1) variant are larger.

Morphology and structure of the GaAs( 3 1 5 )B surface were investigated under Ga- and As-rich conditions. On atomic scale the Ga-rich surface is not flat, but exhibits a very anisotropic surface morphology. Narrow stripes of 1 × 1 structure extend along [1 2 1 ]. The steps between the stripes often bunch together thus creating ( 1 0 1 ) facets. The As-rich surface is also not flat, but facets into vicinal ( 5 2 11 )B surfaces with steps along the [1 3 1 ]proj. and [ 3 2 1]proj.. ( 1 0 1 ) facets form on the sidewall of the steps along the [1 3 1 ]proj.. In adsorbing a monoatomic layer of InAs the GaAs( 3 1 5 )B surface becomes flat, and exhibits a c(2 × 2) reconstruction.

The O-bridge atoms on a stoichiometric RuO2(1 1 0) surface were removed by reaction with CO. The resulting reduced surface was then further exposed to CO. By means of thermal desorption spectroscopy and high-resolution electron energy-loss spectroscopy three adsorbed CO states were identified on bridge sites and assigned to double-bonded, single-bonded, and single-bonded species in the vicinity of O-bridge residues, respectively.

The interaction of NO with the stoichiometric RuO2(110) surface, exposing coordinatively unsaturated Ru-cus and O-bridge atoms, was studied by thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS). At low exposure (≤0.5 L) and 85 K, NO is adsorbed on-top of Ru-cus with its molecular axis perpendicular to the surface for the first two-thirds of the monolayer and slightly bent when the final third is adsorbed. After saturation of this state, NO reacts with O-bridge in forming an NO2-type surface species which decomposes again at 250 K. At higher exposure, a small amount of N2O-cus is formed, presumably via an (NO)2 dimer intermediate. In parallel, a low-temperature pathway for N2 formation is observed, which leaves the surface instantaneously between 130 and 190 K.

The adsorption of CO on RuO2(110) single-crystalline thin films grown on Ru(0001) was studied with scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS), and high-resolution electron energy-loss spectroscopy. The stoichiometric RuO2(110) surface exhibits alternating rows of O-bridge and coordinatively unsaturated Ru atoms (Ru-cus). We identify two different CO adsorption states: CO-bridge and CO-cus. CO-bridge is known to adsorb at the O-bridge positions after reacting-off O-bridge. CO-cus is linearly bonded to Ru-cus in an on-top position and becomes stabilized only after complete filling of the CO-bridge sites. The CO-cus state exhibits two desorption maxima, at 200 and 320 K, corresponding to a surface coverage of approximately 1.0 (α state) and 0.5 (β state), respectively. At 300 K and without CO gas in the background, CO-cus remains only temporarily present at the surface, and reaches coverages of 0.5. A coverage of 1.0 can be achieved by exposure to CO at 300 K to prepare CO-bri...

The reaction of CO2 to carbonate CO3δ− is studied on the O-enriched RuO2(110) surface using thermal desorption spectroscopy and high-resolution electron energy-loss spectroscopy. It is known that the epitaxially grown RuO2(110) surface exposes coordinatively unsaturated sites, so-called Ru-cus and O-bridge, and can be O-enriched by dissociative adsorption of O2 giving rise to weakly bound O-cus atoms on top of Ru-cus. CO2 adsorption at 85 K and annealing up to 250 K, results in a stepwise increased carbonate CO3δ− formation which takes place only on Ru-cus sites. Based on isotope substitution experiments the carbonate-related losses are identified among them the symmetric stretching mode at 150.8 meV and the asymmetric one at 174.9 meV. Through interaction of CO2δ− with O-cus, both chemisorbed on neighboring Ru-cus sites, a bidentate transient state and finally a monodentate carbonate CO3δ− is formed. The molecular plane of monodentate CO3δ− is oriented perpendicular to the surface with a tilted RuO–CO2 a...

NH and NH2 reaction intermediates were prepared on the Ru(1 1 2 0) surface by thermal dehydrogenation of NH3 and studied using high-resolution electron energy-loss spectroscopy. NH2 and NH were isolated to a high degree, which allows to unambiguously identify their vibrational modes. From isotope experiments with 15ND3, frustrated translational and rotational (bending, rocking) modes were differentiated. NH2 exhibits a scissoring mode at 189.7 meV, a rocking mode at 165.4 meV and translational modes at 62.2 and 44.3 meV. NH2 is tilted relative to the surface normal within an adsorbate complex of Cs symmetry. For NH two rather low-lying bending modes at 83.2 and 88.2 meV were identified due to NH species presumably adsorbed in the two different threefold-hollow sites within the Ru(1 1 2 0)(1×1) unit cell, also in a tilted geometry. A long-standing wrong assignment of the NH bending mode could be corrected.

We present a brief overview of surface structures of high-index GaAs surfaces, putting emphasis on recent progress in our own laboratory. By adapting a commercial scanning tunneling microscope (STM) to our molecular beam epitaxy and ultra high vacuum analysis chamber system, we have been able to atomically resolve the GaAs(\(\bar{1}\)\(\bar{1}\)\(\bar{3}\))B (8 ×1), (114)Aα2(2×1), (137), (3 7 15), and (2 5 11) surface structures. In cooperation with P. Kratzer and M. Scheffler from the Theory Department of the Fritz-Haber Institute we determined the structure of some of these surfaces by comparing total-energy calculations and STM image simulations with the atomically resolved STM images. We present the results for the {112}, {113}, and {114} surfaces. Then we describe what led us to proceed into the inner parts of the stereographic triangle and to discover the hitherto unknown stable GaAs (2 5 11) surface.

GaAs samples off-oriented from (2 5 11) by 1degrees and 5.2degrees were prepared by molecular-beam epitaxy and characterised in situ by scanning tunneling microscopy. Terraces of the GaAs(2 5 11) surface are separated by steps along three characteristic directions [3 1 (1) over bar], [4 5 (3) over bar], and [2 (3) over bar 1] which are equivalent to the intersections of the (0 1 1) (I I 3)A. and (I I I)A planes with the (2 5 11) plane, respectively. On the basis of atomically resolved step images, step models were developed and analyzed with help of the electron counting rule (ECR). Step bunches along [3 1 (1) over bar] form (0 1 1) facets. According to the derived models the steps along [3 1 (1) over bar] fulfill the ECR. whereas the steps along [4 5 (3) over bar] violate it. Steps along [2 (3) over bar 1] violate the ECR as well. but may fulfill it over mesoscopic areas.

Growth nuclei and surface defects are studied with scanning tunneling microscopy on the GaAs( 1 1 3 )B(8×1) surface prepared by molecular beam epitaxy. Besides steps, three types of distortions of the (8×1) reconstructed surface are found. First, there are growth nuclei formed by small cluster of Ga and As atoms. Second, there are zig–zigzag or zigzag–zag irregularities in the zigzag chains of the (8×1) reconstruction. Third, complete zigzag chains are added or removed. Based on the analysis of these surface defects, the epitaxial growth on the GaAs( 1 1 3 )B(8×1) surface is discussed.

The thermal dehydrogenation of NH3 was studied on Ru(11(2) over bar0) using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Following NH3 exposure at 85 K, the expected three dehydrogenation steps could be separated, allowing to isolate successively NH2, NH and N on the surface. The R u(11(2) over bar 0) surface is highly reactive as indicated by the complete dehydrogenation of NH3 for exposures up to 0.2 L. During annealing to 300 K both the channels for desorption and dehydrogenation are opened. Following adsorption at 85 K, the obtained nitrogen coverage of 0.11 ML is low, while for dissociative adsorption of NH3 at 300 K, a maximum coverage of 0.38 ML is achieved, and the stability of NH2 is increased.

The interaction of CO2 with the bare RuO2(110) surface, exposing unsaturated Ru and oxygen atoms, was studied using high-resolution electron energy-loss spectroscopy (HREELS) and thermal desorption...

The steady-state rate rCO2 of CO2 formation from CO + O2 on an epitaxially grown RuO2(110) single-crystal surface was recorded as a function of the partial pressures in the 10-7−10-6 mbar range. The RuO2(110) surface exposes singly coordinatively unsaturated “Ru-cus” sites and 2-fold coordinatively unsaturated “Ru-bridge” sites. Normally, the Ru-bridge sites are saturated by oxygen (O-bridge) which can be replaced by CO. The stage of the surface was controlled by vibrational spectroscopy (HREELS), and analysis of the kinetic data was based on previous information about the structural and adsorptive properties of this surface. Measurements of the rate as a function of temperature up to ≈350 K in a 1:1 mixture of CO and O2, each with 10-7 mbar partial pressure, revealed most remarkable agreement with data reported by Zang and Kisch13 with small supported RuO2 particles prepared from aqueous solution for 1 bar total pressure. Under these conditions of temperature and ratio of partial pressures the reaction w...

The dissociation of CO on the Ru(1 1 2 0) surface was investigated by thermal desorption spectroscopy and high resolution electron energy loss spectroscopy (HREELS). Up to about 20% of a monolayer CO dissociates on the Ru(1 1 2 0) surface at room temperature. Oxygen (68 meV) and carbon (42 meV) are identified as dissociation products by HREELS. At temperatures below 240 K the precursor of dissociation can be stabilized. In this state, CO is tilted and presumably bonded at a kind of a fourfold hollow site. During annealing, CO2 formation is observed from CO reacting with oxygen from the dissociation.

RuO2(1 1 0) single crystal surfaces were prepared by exposing a Ru(0 0 0 1) surface to high doses of O2 at 700 K. This surface exhibits coordinatively unsaturated (cus) Ru atoms (Ru-cus) onto which CO may adsorb at 85 K. Actually, high resolution electron energy loss spectroscopy reveals an energy of 262 meV (2115 cm−1) for the C–O stretch vibration of adsorbed CO indicating relatively weak binding in terminal configuration on Ru-cus. Thermal desorption spectroscopy shows partial desorption for temperatures above 250 K and, in parallel, formation of CO2 through reaction with neighboring surface oxygen atoms called O-bridge. Thereby, underlying Ru atoms become accessible onto which CO adsorbs more strongly giving rise to CO stretch-mode energies of 235 (1895) and 245 (1975) meV (cm−1). Hence, above 250 K, Ru-cus is not able to stabilize CO but serves as transient source for CO (and presumably other adsorbates), thus enabling the remarkable catalytic activity of this surface.

The molecular adsorption of CO was studied on the Ru (1 1 2 0) surface using low energy electron diffraction, high resolution electron energy loss spectroscopy and thermal desorption spectroscopy at room temperature. Two ordered CO phases were observed, a p(1×2) phase formed at θCO=0.25 and a (1×2)p2mg phase formed at θCO=0.5 (saturation). In both phases, the CO molecules occupy the on-top sites. Two structural models are suggested.

HREELS and TDS results are presented for hydrogen adsorption on Ru(1121) at 90 K. Three distinct adsorption states were separated both from TDS and HREELS. These a, β and y states are characterized by desorption temperatures of 115, 175-280, 345 K. The according vibrational energies are (42, 95 meV), (64, 115, 160 meV) and (62, 92, 145 meV). By comparison with literature data and taking into account the site symmetries, these states are assigned to fourfold and threefold symmetric sites, respectively.

The atomic structure of GaAs(2 5 11), a hitherto unknown stable surface, has been determined by in situ scanning tunneling microscopy and first-principles electronic structure calculations. This orientation is located within the stereographic triangle, i.e., far away from all low-index surfaces. A low-energy ( 1x1) reconstruction containing arsenic dimers forms on the surface. The analysis of the surface structure shows that, for semiconductor surfaces, the gain in stability due to minimization of the number of dangling bonds is more important than the gain from rendering a semiconducting ground state.

The stoichiometric RuO2(110) surface is terminated by bridge-coordinated oxygen atoms (Oβ) and by coordinatively unsaturated Ru (Rucus) atoms. Exposure to gaseous O2 leads to the formation of two additional surface species: a molecularly chemisorbed state (Oδ) bridging two neighboring Rucus atoms and weakly held O atoms (Oγ) in terminal position above the Rucus atoms. Characterization of the energetics and kinetics as well as structural, vibrational, and electronic properties is achieved by combined application of experimental (low-energy electron diffraction, high-resolution electron loss spectroscopy, thermal desorption spectroscopy) and theoretical (density functional theory) methods. The interplay between the different oxygen species accounts for the high sticking coefficient for dissociative adsorption as well as for the continuous restoration of the surface structure in the course of catalytic oxidation reactions.

H 2 O adsorption on Pt(1 1 1) was studied by high-resolution electron energy loss spectroscopy over a wide exposure range at 85 K. We conclude that the known loss of the ice multilayer at about 30 meV is due to T ⊥4 , the Pt–OH 2 translational mode perpendicular to the surface. This new assignment gives rise to a very consistent understanding of the vibrational spectra for thicker ice layers, for the bilayer, and for sub-monolayer coverages. For the bilayer two perpendicular modes are observed at 33 meV for the top H 2 O molecule and at 16.5 meV for the bottom H 2 O bonding to the surface Pt atom. For a new low-coverage phase only one T ⊥ is found at 15 meV. Following a recent scanning tunneling microscopy result of Morgenstern et al. [1] , the latter is assigned to H 2 O monomers adsorbed in chains at the Pt(1 1 1) step edges. The newly observed monomer translation mode T ⊥1 at 15 meV can be used as an indicator for small amounts of H 2 O co-adsorbed within O or OH layers.

Atomically resolved scanning tunneling microscopy and low-energy electron diffraction were used to determine the surface structure of the $\mathrm{GaAs}(1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}3\ifmmode\bar\else\textasciimacron\fi{})B$ surface prepared by molecular beam epitaxy. An $(8\ifmmode\times\else\texttimes\fi{}1)$ reconstruction was found, which forms by exchanging Ga and As atoms analogously to the $\mathrm{GaAs}(113)A(8\ifmmode\times\else\texttimes\fi{}1)$ reconstruction proposed by Wassermeier et al. [Phys. Rev. B 51, 14 721 (1995)]. The characteristic components of the A and B $(8\ifmmode\times\else\texttimes\fi{}1)$ surface reconstructions are dimers forming zigzag chains along $[3\ifmmode\bar\else\textasciimacron\fi{}3\ifmmode\bar\else\textasciimacron\fi{}2]$ in two atomic levels. While on the A surface the dimers are built of As atoms, on the B surface Ga atoms form the dimers. The morphology of the $\mathrm{GaAs}(1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}3\ifmmode\bar\else\textasciimacron\fi{})B(8\ifmmode\times\else\texttimes\fi{}1)$ surface is rather smooth and does not show the typical roughness known for the $\mathrm{GaAs}(113)A(8\ifmmode\times\else\texttimes\fi{}1)$ surface.

In this contribution, an overview of the research work in our group on the GaAs(112), (113) and (114) surfaces is presented. Samples were prepared by molecular beam epitaxy (MBE) and analyzed in situ by low-energy electron diffraction (LEED), core level spectroscopy and scanning tunneling microscopy (STM). The GaAs(112)A surface is unstable and decomposes into five facets of the orientations {110}, (111) and {124}. Real space images reveal that the facets form depressions whose horizontal cross-section is an irregular pentagon. For the GaAs(113)A surface, our results support the (8×1) reconstruction proposed by Wassermeier et al. On the GaAs(114)A surface a c(2×2) reconstruction was found. A structure model based on the GaAs(001)-(2×4)α reconstruction is in agreement with all our results.

The GaAs(001), (110), (111)A, (111)B, (112)A, (112)B, (113) A, (113)B, (114)A and (114)B surfaces were prepared by molecular-beam epitaxy (MBE) and analyzed in situ by photoelectron spectroscopy of the Ga and As 3d core levels using synchrotron radiation. An overview of the results is given. For each surface also a brief summary of its structure is added. It is shown how surface core-level shifts (SCLS) act as a tool supporting the structure evaluation of these surfaces. The (001) and (110) surfaces are touched only briefly for comparison. The (111) surfaces are discussed more deeply. The high-index surfaces can be characterized as follows: (113)A exhibits a low-energy surface and a new type of reconstruction which consists of As-dimer zig-zag chains; (112)A and (112)B are heavily faceted and (114)A and (114)B are quite well understood in analogy to the (001) surface.

A compact ultrahigh vacuum (UHV) system has been built to study growth and properties of III/V semiconductor surfaces and nanostructures. The system allows one to grow III/V semiconductor surfaces by molecular beam epitaxy (MBE) and analyze their surface by a variety of surface analysis techniques. The geometric structure is examined by scanning tunneling microscopy (STM), low-energy electron diffraction and reflection high-energy electron diffraction. The electronic properties of the surfaces are studied by angular resolved photoemission either in the laboratory using a helium discharge lamp or at the Berlin Synchrotron Radiation Facility BESSY. In order to meet the space restriction at BESSY the system dimensions are kept very small. A detailed description of the apparatus and the sample handling system is given. For the UHV-STM (Park Scientific Instruments, VP2) a new, versatile tip handling mechanism has been developed. It allows the transfer of tips out of the chamber and furthermore, the in situ tip cleaning by electron annealing. In addition, another more reliable in situ tip-preparation technique operating the STM in the field emission regime is described. The ability of the system is shown by an atomically resolved STM image of the c(4×4) reconstructed GaAs(001) surface.

The GaAs(112)A and (112)B surfaces have been prepared by molecular-beam epitaxy (MBE) and analyzed in situ by low-energy electron diffraction (LEED) and surface core-level spectroscopy of the Ga 3d and As 3d core levels using synchrotron radiation. The morphology of these surfaces has been studied in situ by scanning tunneling microscopy (STM) and ex situ by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Neither of the surfaces is stable, but both decompose into facets under standard MBE preparation conditions. The (112)A surface is covered by regular depressions (inverted pyramids) with a pentagonal base and with side walls formed of low-energy (111), {110}, and as yet unobserved {124} facets. The (112)B surface exhibits depressions with (111), {011}, and (113) facets on a rectangular base. These results are in agreement with ab-initio calculations of surface energies based on density functional theory. The theoretical findings show that, by forming roof-like structures employing {110} or {111} and {113} surfaces, the surface free energy can be lowered below the values found for a large number of reasonable GaAs(112)A and (112)B model surfaces.

The GaAs(112)A and B surfaces were prepared by molecular beam epitaxy (MBE) and characterized in situ by scanning tunneling microscopy (STM). Both surfaces are unstable. On the (112)A surface five facets with the orientations (111), {110} and {124} appear, while on the ( 1 1 2 )B surface four facets with the orientations ( 1 1 1 )B, { 1 1 0 } and ( 1 1 3 )B were observed. In both cases, the facets form depressions covering the entire surface. In addition, total-energy calculations employing density-functional theory were carried out for these surfaces. For the (112)A surface a roof-like structure of {111} and {113} planes has the lowest calculated energy. According to the calculations, the ( 1 1 2 )B surface forms under As-rich conditions a roof-like structure of { 1 1 1 }B and { 1 1 3 }B planes, and it decomposes under Ga-rich conditions into { 1 1 0} planes. The experimental and theoretical findings for the ( 1 1 2 )B surface are in good agreement. The occurrence of {124}-facets on the (112)A surface was not taken into consideration in the calculations and needs to be investigated further.

Results of a combined experimental and theoretical study are presented. GaAs{113} and {112} surfaces have been prepared by molecular beam epitaxy and analyzed in situ by low-energy electron diffraction and ex situ by atomic-force microscopy. The experimentally found structures are in full agreement with surface energies calculated by density-functional theory. The {112} surfaces are unstable under faceting into low-index planes, whereas the {113} surfaces appear as singular surfaces. Particularly, for GaAs(113) the surface energy is comparable with the values for the low-index surfaces. The impacts of these results on the interface between InAs quantum dots and the embedding GaAs matrix are discussed.

The GaAs (113)A and ( 1 1 3 )B surfaces were prepared by molecular beam epitaxy (MBE) and ion bombardment and annealing (IBA). The surfaces were investigated in situ by means of low-energy electron diffraction and surface core-level (SCL) spectroscopy. Both orientations show 1×1 reconstructions after preparation with IBA. For the (113)A face a stable, 8×1 reconstructed surface was prepared by MBE. SCL shifts (SCLSs) are observed for As 3d (+0.53 eV) and Ga 3d (−0.46, +0.36 eV) which support a recently proposed model by Wassermeier et al. [Phys. Rev. B 51 (1995) 14721]. The (113)A(8×1) surface exhibits surface resonances at −1.0 and −3.8 eV below the valence band maximum. The dispersion of these resonances is only weak and does not follow the symmetry of the reconstructed surface. The ( 1 1 3 )B surface shows { 1 1 1 }B and {110} facets after preparation with MBE. This is confirmed by SCLSs for As 3d (−0.63, +0.57 eV).

We investigated GaAs (1 1 3) surfaces prepared by molecular beam epitaxy (MBE) and metal-organic vapour phase epitaxy (MOVPE) with reflectance anisotropy spectroscopy (RAS) and low energy electron diffraction (LEED) and found two different static (non-growth) reconstructions: For many conditions the well known (8×1) reconstruction is observed, while under more arsenic rich conditions a reconstruction a p(1×1) symmetry shows up. The activation energy for the transition from the p(1×1) to the (8×1) reconstruction was determined to (1.50±0.02) eV. A structure model for the arsenic rich p(1×1) reconstruction is presented in accordance with the above findings.

Some typical examples are presented for the investigation of vibronic properties of adsorbates on metal surfaces using high-resolution electron energy loss spectroscopy under the use of the new-generation Ibach spectrometers. Two examples—NH 3 /Ru( 11 2 1 ) and Cs/Ru(0001)—concern chemisorption and one is for physisorption of H 2 on Ag(111). Finally, ozone production by electron bombardment of a thin O 2 multilayer is described.

Velocity distributions of desorbing product CO 2 in CO oxidation on Pt(133) were analyzed in both the active and inhibited regions at the steady state. The translation temperature is high in the active region where the reaction is in the first order with respect to CO. It decreases sharply with increasing CO pressure when the reaction shifts to the inhibited region where CO retards the reaction. The translational energy of CO 2 increases linearly with the surface temperature.

The photodesorption of oxygen admolecules was studied on a stepped Pt(113)=(s)2(111)×(001) surface with 193 nm irradiation at 110 K. Multidirectional desorptions were found to collimate at ±12–20° and ±45–49° off the surface normal and also along the surface normal in a plane along the trough. The first component is always dominant, and the weak second component only appears at higher oxygen coverages. The normally directed desorption is not significant. The translational energy of desorbing O2 peaks around 15–20° and 50°, confirming the inclined desorptions. It is proposed that these inclined components are due to the desorption induced by the impact of oxygen admolecules with hot oxygen atoms from the photodissociation of adsorbed molecular oxygen, emitted along the trough. A simple cosine distribution was found to fit the thermal desorption from oxygen admolecules and also the recombinative desorption of oxygen adatoms. The 193 nm irradiation also produces additional, less tightly bound oxygen adatoms, which yield a desorption component collimated at 15° from the surface normal in the step-down direction.

Using a recently developed MBE apparatus, the GaAs ( 1 1 1 ) B(2×2) surface was prepared and studied in situ by photoemission and LEED intensity analysis. The measured surface core level shifts of the Ga and As 3d core levels support the atomic geometry of the As-trimer model whose detailed structure was determined by LEED intensity analysis.

L'irradiation d'un film d'oxygene moleculaire par des electrons conduit a la production d'ozone. O 3 a ete mis en evidence par spectroscopie de perte d'energie d'electrons (HREELS) qui permet d'observer les modes d'excitation vibrationnelle des molecules. L'identification de O 3 est confirmee par l'etude de l'effet isotopique. Le seuil energetique de formation est de l'ordre de 3,5 eV. Il correspond a la dissociation de O 2 par attachement d'un electron : 0 2 +e→O( 3 P)+O-( 2 P), suivi de la reaction spontanee de recombinaison: O + O 2 +O 2 →+O 3 +P 2 . A des energies superieures a 5,1eV, l'excitation de O 2 vers des etats de Rydberg et du continuum de Schumann-Runge est la principale source d'oxygene atomique et donc d'ozone. Des mesures en fonction du temps d'irradiation montrent que la quantite d'ozone forme sature.

We discuss scanning electron micrographs and atomic force microscope images of thermally etched GaAs(1 1 3) surfaces. The GaAs(1 1 3)A and GaAs(1 1 3)B surfaces are compared. The polarity of the surface leads to a different morphology for the two surfaces after thermal etching. It is found that the Ga-enriched droplets, which form under As-deficient conditions at higher temperatures, are sitting on characteristic pedestals, which are different for the two faces. The facets occurring after this thermal etching process are identified. They represent thermally favourable surfaces under the arsenic-deficient conditions of the thermal etching process. © 1998 Chapman & Hall