A. Böttcher

Exoelectron emission during oxidation of Cs films

During oxidation of thin Cs films, a nonadiabatic surface reaction manifests itself in the emission of electrons. This effect was investigated in detail by combining measurements of the current and of energy distributions of these exoelectrons with studies on the electronic properties of the surface by means of ultraviolet photoelectron spectroscopy and metastable deexcitation spectroscopy. Exoelectron emission occurs via Auger deexcitation of the empty state derived from the O2 affinity level. This process is confined to the stage Cs2O2→CsO2 in which resonance ionization of the affinity level of the impinging O2 molecule upon crossing the Fermi level EF is efficiently suppressed due to the absence of metallic states near EF. A kinetic model based on the successive steps involved in the oxidation of Cs is developed which describes qualitatively well all the experimental findings.

Characterization of oxygen phases created during oxidation of Ru(0001)

Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction (LEED), and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru(0001). Emission of RuOx (x<4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (Tp<850 K) a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular RuxOy phase takes place within the temperature region of 900–1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly red...

Spectral and spatial anisotropy of the oxide growth onRu(0001)

Scanning photoelectron spectromicroscopy has been used to study the onset and the initial stages of oxidation of Ru(0001) at three oxidation temperatures, 625, 675, and 775 K, and oxygen exposures of about 105 Langmuir. The lateral heterogeneity developed during oxide nucleation and growth and the local composition of the coexisting phases have been determined using as fingerprints the O 1s and Ru 3d spectra, thus combining chemical mapping with spectroscopy from selected features from the maps. The onset of oxide formation is characterized by the appearance of randomly distributed small islands (⩾0.5 μm) identified as germinal patches exhibiting some spectral features of bulk RuO2. The following anisotropic growth of the RuO2 phase and in particular the shape of the oxide islands shows a strong dependence on the oxidation temperature. The spectroscopic information obtained for the areas surrounding the oxide islands reveals an intermediate oxygen state characterized by distinct O 1s and Ru 3d features di...

Preparation and characterization of thin CsAu films

Abstract The formation of CsAu films by successive evaporation of gold and caesium atoms onto a clean Ru(001) substrate surface was followed by UV photoelectron spectroscopy and (subsequent) thermal desorption spectroscopy. No compound but only a chemisorbed layer (and eventually bulk caesium) is formed if caesium is deposited onto a gold layer, if the quantity of the latter itself does not exceed the monolayer capacity. With thicker gold layers, on the contrary, semiconducting CsAu (with a band gap of 2.6 eV and the Fermi level close to the conduction band edge) is formed by interdiffusion at T > 250 K, but the outer surface consists always of a layer of chemisorbed caesium. This even holds for a “sandwich” system for which a thick gold film was evaporated onto a CsAu layer. The surface region of CsAu is highly reactive towards oxygen.

Dynamics of the interaction of O2 with Li surfaces

Abstract Oxidation of thin Li films to Li 2 O is found to be associated with the emission of exoelectrons and even ejection of O − ions. Cluster calculations demonstrate that electron transfer from the substrate causes formation of O 2− 2 species which readily dissociate without noticeable activation barrier. The O − particles formed near the surface deexcite into O 2− ions, partly non-adiabatically causing exoelectron emission, and a very small fraction may escape into the vacuum.

Exoelectron emission during the oxidation of Na films

Oxidation of Na films is accompanied by a low yield of electron emission which is, however, confined to the later stages of reaction in which transformation of peroxide (Na2O2) into Superoxide (NaO2) species at the surface takes place. By probing the electronic properties of the outermost layer by means of metastable deexcitation spectroscopy (MDS) and by recording the energy distribution of the emitted exoelectrons the mechanism of this process was found to be analogous to that established previously for the oxidation of Cs films and as proposed earlier theoretically. It involves decay of a hole state derived from the affinity level of the impinging O2 molecule in front of the surface via an Auger transition which, on the other hand, may efficiently be quenched by resonance ionisation from metallic electrons near the Fermi level. Experiments with Na submonolayers adsorbed on a Ru(0001) substrate reveal that for coverages < 0.6 ML the latter effect dominates so strongly that exoelectron emission is no longer observed.

Oxide-free oxygen incorporation into Ru(0001)

A smooth Ru(0001) surface prepared under ultra-high vacuum conditions has been loaded with oxygen under high-pressure (p approximately 1 bar) and low-temperature (T < 600 K) conditions. Oxygen phases created in this way have been investigated by means of thermal desorption spectroscopy, low-energy electron diffraction, and ultraviolet photoelectron spectroscopy. The exposure procedures applied lead to oxygen incorporation into the subsurface region without creation of RuO2 domains. For oxygen exposures ranging from 10(11) to 10(14) L oxygen contents up to about 4 monolayer equivalent could be achieved. The oxygen incorporation is thermally activated. The CO oxidation reaction conducted at mild temperatures (T < 500 K) at a sample loaded with subsurface oxygen reaches CO --> CO2 conversion probabilities of 10(-3).

Ejection of O− ions by interaction of O2 with Ru(0001) covered with submonolayers of Cs

Exposure of a Ru(0001) surface covered by submonolayer quantities of Cs to O2 leads to the emission (with very low probabilities) of O− ions. The yield of O− reaches a maximum at Cs coverages around 0.3–0.4 monolayers (1 ML is equivalent to an absolute coverage θCs = 0.33), and in this range is much higher for low (∼260 K) than for high sample temperatures. These effects are traced back to the varying degree of “metallization” of the Cs overlayers as substantiated by metastable deexcitation spectroscopy (MDS) experiments.

Mesoscopic-scale pattern formation induced by oxidation of Ru(0001)

Abstract Photoemission electron microscopy (PEEM) has been utilized for monitoring the modifications of a Ru(0001) surface induced by surface oxidation. The PEEM images of the initial oxidation stages reveal bright RuxOy regions surrounded by dark areas formed by the saturated oxygen chemisorption layer. For a narrow particular temperature interval the oxidation starts with the appearance of light nuclei or grains (

Electronic properties of Cs+CO coadsorbed on the Ru(0001) surface

The variation of the Cs 6s and the Cs 5p emission in He* and Ne* metastable deexcitation spectroscopy (MDS) as a function of the CO exposure indicates a demetallization of the Ru(0001)–(2×2)-Cs and the Ru(0001)–(√3×√3)R30°-Cs surfaces upon CO coadsorption. This observation corroborates a (substrate-mediated) charge transfer from the Cs atom to the 2π* orbital of CO. With the Ru(0001)–(2×2)-Cs system even at CO saturation, MD spectra show emission associated with the Cs 6s state, indicating that the Cs atoms are not completely ionized. Exposing the (√3×√3)R30°-Cs-pre-covered Ru(0001) to CO, surplus Cs of the first layer is displaced into a second layer. In this way, CO molecules are able to be accommodated into the first layer. Desorbing this second layer Cs by heating the sample to 600 K produces a (2×2) structure with one Cs and CO in the unit cell as evidenced by MDS and low energy electron diffraction.