Yu. Suchorski

ELECTROSTATIC FIELDS ABOVE INDIVIDUAL ATOMS

This review describes recent field ion microscopic measurements of local electrostatic fields in the immediate vicinity of individual surface atoms. The studies were stimulated and supported by self-consistent theoretical calculations on the field distribution close to single atoms of different electronic structure adsorbed on a metal surface in the presence of high external fields. The experimental and theoretical data establish the existence of strongly varying local field distributions above different surface atoms. New details concerning localized field ionization and image formation processes occurring in the field ion microscope are revealed.

Local electric fields at individual atomic surface sites : field ion appearance energy measurements

Abstract A new method of determining local electric fields is reported. Under conditions of highest field ion image contrast, absolute appearance energy measurements were carried out at a weakly corrugated step of a Rh(001) plane and at strongly corrugated step across the atom rows of a Rh(113) facet. Data of local fields, F hkl loc , are derived from appearance energies. A comparison is made with external electric field strengths, F hkl 0 , determined by electron energy spectroscopy and i - V (Fowler-Nordheim) measurements. Significantly different enhancement factors, F hkl loc / F hkl 0 , were found, 1.27 for steps at the (001) plane, and 1.54 for steps across the rows of Rh(113). In both cases the step-site atoms were covered with field-adsorbed Ne. The experimental results are discussed in view of recent self-consistent calculations of local-field distributions in the proximity of protruding surface atoms. The occurence of field ionization of image gas atoms, released from field-adsorbed states at step sites by atom-collision processes, is in accord with the present observations.

Lithium field desorption microscope : a new tool for surface investigations

Abstract A new method of imaging a surface with field-desorbed metal (Li + ) ions is reported. The long-lasting supply of the imaging substance (Li) is achieved by surface diffusion of Li from a multilayer of Li deposited on the shank of the tip. The surfaces of [110]-oriented W and [111]-oriented Rh tips were imaged in continuous, pulsed and mixed continuous-pulsed field modes. Different types of images, formed by ions field-desorbed from di- or triatomic Li layers on the surface, were observed. The mechanisms of image formation in the lithium field desorption microscope (FDM) are discussed on the basis of known properties of field desorption and surface diffusion of Li in comparison with experimental data. The perspectives and possible applications of Li FD for surface investigations are outlined.

Comparative studies on field ionization at surface sites of Rh, Ag and Au—differences in local electric field enhancement

Abstract Field ion imaging conditions were studied for clean Rh, and for Rh epitaxially covered with Ag and Au, respectively. Significant, metal-specific decreases of the image voltages have been observed. This is caused by different enhancements of local fields in the vicinity of protruding Rh, Ag and Au atoms due to their different electronic structures. The interpretation is based on self-consistent calculations of local electrostatic field distributions in the vicinity of single atoms adsorbed on a metal surface. For different facets of a surface, a close correlation between local field distributions and the degree of localization of the field ionization process is establihed from the measured full width at half maximum (FWHM) of field ion energy distributions and is discussed in association with possible field ionization mechanisms.

Alkali metal effect on catalytic CO oxidation on a transition metal surface: a lattice-gas model

Abstract We present a lattice-gas-type model which accounts for short-range correlations between coadsorbates to describe analytically the alkali-modified CO oxidation reaction on a transition metal surface. The effect of the adsorbed alkali near the surface is described in terms of long-range fields which change the binding energies of adsorbed CO and oxygen, and of the coadsorption-modified sticking coefficients. An decrease of the binding energy of CO in chemisorbed state which provides an increase of oxygen coverage on the surface and an alkali-induced delocalization of adsorbed CO accompanied by a lowering of the CO coverage is predicted. As net result the reactive state (oxygen covered surface) is enlarged towards higher pCO pressures in agreement with the experimentally obtained phase diagrams (pCO, 1/T).

Fluctuations during catalytic CO oxidation on different crystal planes of a Pt field emitter

Abstract Fluctuations during catalytic CO oxidation on the (100), (111) and (113) planes of the [100]-oriented Pt field emitter tip are studied by FEM (field electron microscope). Intensity profiles and local time series are obtained by digitizing the image brightness in a rectangular probing window oriented along the [110]-direction. From the local time series the probability density distributions as well as the auto- and cross-correlation functions are computed. The results demonstrate that the fluctuation behavior is quite different on the different facets of a field emitter tip. On the facets studied here, the fluctuations observed in the monostable and in the bistable region of the reaction exhibit substantial differences in the degree of time- and spatial correlation.

Field ion appearance energy spectroscopy of CO+ originated from Rh(111) and Au(111) surface step sites

Abstract Field ionization of CO at step sites of Rh(111) and Au(111) was studied with field ion microscopy and mass-to-charge resolved field ion energy spectroscopy. Using a newly developed retarding potential technique, absolute values for CO + field ion appearance energies were derived from measurements of the high energy onset of ion retardation curves and from in situ measurements of the work function of the retarding electrode. From analyses of the CO + field ion appearance energies, field ionization of CO on Rh and Au surfaces was found to proceed by field desorption. Applying a thermionic cycle, field-dependent values of the binding energy for CO on Rh(111) and Au(111) were derived. For comparison, CO field ionization was also studied at a passivated Rh surface. The mechanism of field ion imaging of metal surfaces with CO + ions and the field-dependent chemisorption of CO on different metal surfaces is discussed on the basis of experimental results and theoretical calculations.

Field desorption and field evaporation of metals: In memoriam Professor J.H. Block

Abstract In this review we present a detailed study, both experimental and theoretical, of the field desorption and field evaporation of alkali- and transition metals looking in particular at the site specificity and the coverage dependence. A novel experimental approach based on the retarding potential analysis of metal ions emitted in a continuous field desorption mode is used. With this approach, absolute values of the field ion appearance energy have been measured and binding energies have been obtained for atoms extracted from selected surface sites under high field conditions. We discuss results of the mass-to-charge resolved retarding potential analysis of lithium ions, desorbed from W(111), and of rhodium ions evaporated from Rh(100) and Rh(111). Appearance energies of Li+ and Rh2+ were derived from the ion retardation curves, and activation energy data were evaluated from desorption rate measurements. Applying a thermionic cycle, the binding energies of Li adatoms on W(111) as well as of Rh at Rh(100) and Rh(111) step sites are obtained. The cluster embedded in jellium model, based on density functional theory, is used to interpret the experimental data. Local field enhancements, binding and activation energies are calculated for Li field desorption and Rh field evaporation as a function of field strength and surface geometry.

Initial stages of oxide formation on the Zr surface at low oxygen pressure: An in situ FIM and XPS study

An improved methodology of the Zr specimen preparation was developed which allows fabrication of stable Zr nanotips suitable for FIM and AP applications. Initial oxidation of the Zr surface was studied on a Zr nanotip by FIM and on a polycrystalline Zr foil by XPS, both at low oxygen pressure (10−8–10−7 mbar). The XPS data reveal that in a first, fast stage of oxidation, a Zr suboxide interlayer is formed which contains three suboxide components (Zr+1, Zr+2 and Zr+3) and is located between the Zr surface and a stoichiometric ZrO2 overlayer that grows in a second, slow oxidation stage. The sole suboxide layer has been observed for the first time at very early states of the oxidation (oxygen exposure ≤4 L). The Ne+ FIM observations are in accord with a two stage process of Zr oxide formation.

Ordered phases in alkali redistribution during a catalytic surface reaction

Reaction fronts in the O2+H2 reaction on a Rh(110) surface predosed with potassium have been shown to be associated with a redistribution of the potassium from the oxygen freed to the still oxygen covered parts of the surface. As stable final state a stationary pattern results under reaction conditions formed by K+O coadsorption islands of macroscopic size. Here low energy electron microscopy (LEEM) in combination with mirror electron microscopy (MEM), photo electron emission microscopy (PEEM) and small area selected LEED (μ-LEED) were used to identify ordered phases in this process in situ and to resolve fine structures in the reduction fronts. In the O2+H2 reaction without coadsorbed alkali metal a (2×2)p 2 mg and a c(2×6) were identified besides the c(2×8)–O and the (1×1) representing oxygen covered and oxygen freed surface, respectively. With coadsorbed potassium one finds in the front region a (2×2)p 2 mg and further inside the oxygen covered area a dominant (1×2) reconstruction with satellite spots reflecting a (n×2) K+O coadsorption structure with n=8–12. In the stationary pattern a (8×2)−K+O structure forms the core of the coadsorption islands while the boundary region exhibits a (2×2)p 2 mg−K+O overlayer as ordered phase.