Formation mechanism of the O-induced added-row reconstruction on Ag(110): A low-temperature STM study

The formation of the O-induced added-row reconstruction on Ag(110) was studied by scanning tunneling microscopy at 190 K. At this temperature critical surface processes are slow enough to be followed on the atomic scale. Ag atoms, which are required for the formation of the reconstruction, detach from steps in a row-wise fashion, starting from kink sites. The growth rate of the added rows is constant and controlled by the detachment rate of Ag atoms from the steps. The data provide evidence for a reconstruction mechanism in which the release of the metal atoms from the steps and the formation of lateral oxygen metal bonds are independent processes

Existence of a “Hot” Atom Mechanism for the Dissociation of O₂ on Pt(111)

The dissociation of O2 on a Pt(111) surface was studied by variable temperature scanning tunneling microscopy at 150–106 K. The 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 10−6.3cm2s−1, 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

Adsorbate-adsorbate interactions from statistical analysis of STM images: N/Ru(0001)

Atomic nitrogen on Ru(0001) was prepared by dissociative chemisorption of N2 and studied by scanning tunneling microscopy (STM) at 300 K. Nitrogen occupies the hcp threefold hollow site and is imaged as a depression with a diameter of about 5 Å. Interactions between the adsorbed nitrogen atoms were obtained by statistical analysis of STM images, by extraction of the two-dimensional pair distribution function from the arrangement of the N atoms. Since the nearest-neighbor separations could be identified with atomic precision, the pair distribution function g and hence the potential of mean force Veff were obtained as a function of the discrete neighbor sites j up to the tenth nearest neighbor. A comparison with Monte Carlo calculations for balls with a hard-sphere potential provides information about the pair potential Vpair(j): The nearest-neighbor site is strongly repulsive, the second-neighbor site is weakly repulsive, and the third-neighbor site is weakly attractive. These findings rationalize the absence of island formation and of a well-ordered 2×2 phase for the N/Ru(0001) system: At temperatures ≥300 K the attractive interaction on the third-neighbor site is too weak, while at lower temperatures the diffusion barrier of 0.9 eV represents a kinetic obstacle. The fact that the range of the interaction is identical to the diameter of the N-atom features in the STM topographs is taken as evidence that the interaction is caused by substrate-mediated electronic forces

Diffusion and Atomic Hopping of N Atoms on Ru(0001) Studied by Scanning Tunneling Microscopy

The dynamic behavior of N atoms adsorbed on a Ru(0001) surface was studied by scanning tunneling microscopy. N atoms formed by dissociation of NO molecules show an initial sharp concentration profile at atomic steps. Its decay was followed as a function of time, providing a quasicontinuum diffusion constant; the activation energy is 0.94 eV and the prefactor is 2×10−2cm2s−1. The diffusion constant was determined also at equilibrium, from statistical jumps of individual N atoms in a uniform overlayer, and is found to be identical to the Fickian value

Enhanced reactivity of adsorbed oxygen on Pd(111) induced by compression of the oxygen layer

The reaction between O atoms and CO molecules on Pd(111) was investigated by scanning tunneling microscopy (STM). CO was dosed on the (2x2)O-covered surface at temperatures between 100 and 190 K, and the structure changes were monitored by STM. CO adsorption causes compression of the (2x2)O overlayer into islands of the (2x1)O structure, followed by reaction of the O atoms to give CO2. The (2x2)O overlayer does not react with CO at temperatures up to 180 K, whereas the (2x1)O phase reacts at temperatures as low as 136 K. The analysis of the reaction kinetics reveals an activation energy for the O+CO reaction of 0.4 eV and a reaction order of 1 with respect to the O coverage

Coadsorption phases of CO and oxygen on Pd(111) studied by scanning tunneling microscopy

The adsorption of CO on an oxygen precovered Pd(111) surface was investigated between 60 and 300 K. Applied methods were variable temperature scanning tunneling microscopy (STM) and video STM to analyze the coadsorption structures. The STM data are compared with simulated STM images for the various surface phases in order to identify the appropiate structural model for each case. Low-energy electron diffraction and reaction isotherms by means of mass spectrometry were used to correlate the phases with the reaction yielding CO2. The video-STM data recorded during CO adsorption at 300 K on the (2x2)O phase show a fast phase transition into the (√3x√3)R30°O structure, followed by reaction to CO2. The reaction only starts after completion of the phase transition, indicating that the (√3x√3)R30°O structure plays a crucial role for the reaction. At temperatures between 170 and 190 K the phase transition is slow enough to be monitored with STM. The experimental images of both the (2x2)O and the (√3x√3)R30°O structures are well reproduced by the simulations. Further CO adsorption caused a second phase transition into a p(2x1)O structure. The STM simulations strongly support a pure oxygen p(2x1) structure, rather than a mixed O + CO structure, in contrast to previous experimental work. The CO molecules form the same structures between the O islands that are known from the pure Pd(111)/CO system. At lower temperatures, between 110 and 60 K, a so far unknown (2x2) phase was observed. The formation of this structure, and its imaging by the STM, show that it constitutes a mixed p(2x2)O+CO structure, where the oxygen atoms remain unchanged, and the CO molecules occupy hcp sites between the O atoms

Interaction of oxygen with Al(111) at elevated temperatures

The interaction of oxygen with Al(111) was investigated by STM at temperatures between 350 and 530 K, by annealing an oxygen precovered surface and by adsorption of oxygen on the hot surface. For exposures up to 10 L and temperatures up to 470 K a considerable part of the oxygen exists still in the chemisorbed state, another part transforms into Al oxide. In contrast to 300 K chemisorbed Oad atoms are mobile at elevated temperatures, and compact, hexagonal (1×1)Oad islands develop by an ordinary nucleation and growth scheme. This evidences attractive interactions between the oxygen atoms on (1×1) sites. From the lateral distribution of Oad islands a diffusion barrier of 1.0–1.1 eV is derived. The imaging of the islands of the (1×1) phase by STM depends on their size, which is understood by a different imaging of the Oad/Al adsorbate complexes at the island borders. Defects in the islands and bright features at the edges are interpreted as nuclei of aluminum oxide. Additional features which appear as topographic holes may be attributed to nonconducting Al oxide grains

Interactions between alkali metals and oxygen on a reconstructed surface: An STM study of oxygen adsorption on the alkali-metal-covered Cu(110) surface

Room-temperature adsorption of oxygen on potassium- and cesium-precovered Cu(110) surfaces was studied by scanning tunneling microscopy. Depending on the alkali-metal precoverage, two different scenarios exist for the structural evolution of the surfaces. For alkali-metal coverages θalk≤0.13 ML [θalk=0.13 corresponds to the (1×3) missing-row reconstructed Cu(110) surface], oxygen adsorption leads first to a transient contraction of the missing rows into islands of a (1×2) structure. After longer exposures it causes the local removal of the alkali-metal-induced reconstruction, and the (2×1) Cu-O ‘‘added-row’’ structure with θO=0.5 is formed. In this structure the alkali-metal atoms are incorporated in the Cu-O chains. For higher alkali-metal precoverages, in the range of the (1×2) reconstruction (θalk≊0.2), more than one-half a monolayer of oxygen can be incorporated into the (1×2) phase with only a minor structural effect before, at higher oxygen coverages, complex oxygen–alkali-metal–Cu structures with oxygen coverages well above 0.5 ML are formed. The saturation oxygen coverage is drastically enhanced beyond θO=0.5, the quasisaturation value of the clean surface. Based on mass-transport arguments the substrate is reconstructed for all ratios of oxygen and alkali metal investigated here. Hence, adsorbate-substrate interactions are essential for these structures; they are not dominated by interactions between alkali metals and oxygen, i.e., by adsorbate-adsorbate interactions

Direct observation of mobility and interactions of oxygen molecules chemisorbed on the Ag(110) surface

The energetics of thermal motions and interactions of oxygen molecules chemisorbed on a Ag(110) surface were investigated by scanning tunneling microscopy at 60–100 K. Surface mobility is anisotropic, preferably in the [1̅10] direction with an activation energy of 0.22±0.05 eV and a preexponential factor of 1×1013±3 s−1. Along the [1̅10] direction a repulsive interaction between nearest neighbors of about 0.02 eV and an attraction of 0.04±0.01 eV between next nearest neighbors were derived. Along [001] appreciable repulsion exists between nearest neighbors, while a ''diagonal'' arrangement of molecules is associated with an attraction of 0.02±0.01 eV. The data are indicative for the operation of indirect, substrate-mediated molecule-molecule interactions

Dual-Path Mechanism for Catalytic Oxidation of Hydrogen on Platinum Surfaces

The catalytic formation of water from adsorbed hydrogen and oxygen atoms on Pt(111) was studied with scanning tunneling microscopy and high resolution electron energy loss spectroscopy. The known complexity of this reaction is explained by the strongly temperature dependent lifetime of the product H2O molecules on the surface. Below the desorption temperature water reacts with unreacted O adatoms to OHad, leading to an autocatalytic process; at higher temperatures sequential addition of H adatoms to Oad with normal kinetics takes place