P. Dolle

Thermal desorption of strained monoatomic Ag and Au layers from Ru(001)

Model‐independent or complete analysis of thermal desorption spectra suggests, in agreement with low‐energy electron diffraction, that attractive lateral interactions between Ag atoms on a Ru(001) substrate lead to island formation for Ag coverages above 0.15 ML, with zeroth‐order desorption kinetics and coverage‐independent activation energy of desorption E, and preexponential factor ν. For the system Au/Ru(001), on the other hand, E and ν decrease monotonically with coverage, indicating that Au–Au interactions on Ru(001) are repulsive. For ΘAg<0.15 and ΘAu<1 the systems exhibit a clear compensation effect between E and ν.

Ethylene π species on bare and cesiated Pt(111) surfaces

The adsorption of ethylene on bare and cesiated Pt(111) surfaces has been investigated by means of thermal desorption spectroscopy and ultraviolet photoemission spectroscopy, at temperatures ranging between 37 and 800 K. On the bare Pt(111) surface, a new π‐bonded C2H4 species has been observed at T 52 K, it transforms into di‐σ C2H4 and then into ethylidyne (∼300 K). Conversely, cesium preadsoption gives rise to reversibly adsorbed π‐bonded ethylene, meanwhile it inhibits the usual reaction path (di‐σ species leading to ethylidyne). Furthermore, valence‐level binding energy shifts induced by cesium adatoms have been shown to be consistent with the theoretical picture of the electrostatic potential surrounding adsorbed alkali elements.

A NEW UHV DIFFRACTOMETER FOR SURFACE STRUCTURE AND REAL TIME MOLECULAR BEAM DEPOSITION STUDIES WITH SYNCHROTRON RADIATIONS AT ESRF

Abstract We describe a new surface X-ray diffractometer, which is optimized to combine surface X-ray diffraction (SXRD), absorption spectroscopy (SEXAFS) and grazing incidence small angle scattering (GISAXS). This instrument is particularly well suited for real time studies of material elaborated in-situ with molecular beam deposition (MBD) techniques. The goniometer allows for large in-plane and out-of-plane momentum transfer with high accuracy. Owing to the flipping mechanism of the sample holder, it can perform absorption experiments with polarisation directions normal and parallel to the sample surface, while keeping a grazing incidence. Several MBD sources as well as complementary surface sensitive electron techniques (RHEED and Auger spectroscopy) can be used simultaneously with X-rays. Finally, the samples can easily be inserted in the X-ray chamber via an UHV transport system and a fast entry load-lock module.

Chemical ordering and reconstruction of Pt25Co75(100): an LEED/STM study

The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1–0.4 A above that of Co, whereas LEED shows that Pt atoms are geometrically ∼0.04 A higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (〈C1〉 = 62.6% Pt, 〈C3〉 = 53.5%) and Pt-depleted layers (〈C2〉 = 6.9%, 〈C4〉 = 2.7%). Chemical ordering is present in the third layer and the four-layer surface slab stabilises with a structure and a composition quite similar to that of the L12 PtCo3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis.

The adsorption of oxygen on a cesiated Ni(111) surface: Evidence for the formation of molecularly chemisorbed oxygen species

Abstract Oxygen adsorption is investigated by means of UPS, XPS and work function measurements on a cesiated Ni(111) surface at 100 K (precovered with one cesium monolayer). Upon oxygen exposure of the cesiated surface, the work function decreases, passes through a minimum and then increases as to reach a plateau. It is shown that up to the work function minimum a peroxo-like species (O 2− 2 ) is formed, meanwhile after the work function minimum a superoxo-like species (O − 2 ) is identified too.

The effect of K, Cs and O atoms on ethylene adsorption on the Pt(111) surface

Abstract The adsorption of C2H4 on bare and Cs-, K-, O-precovered Pt(111) surfaces has been investigated, using UPS and TDS. All coadsorbates induce the appearance of more loosely (π-bonded) adsorbed species. In the presence of alkali adatoms, these new species are almost undistorted compared to gaseous, multilayered ethylene and ethylene adsorbed as a monolayer at 37 K. However, some distortion exists when ethylene is coadsorbed with oxygen atoms. The C2H4 valence-level shifts observed in the presence of K (Cs) are consistent with the electrostatic field existing in the vicinity of the alkali metal adatoms.

Propriétés thermodynamiques du film de tétrafluorométhane adsorbé sur graphite

Abstract A set of adsorption isotherms of tetrafluoromethan (CF 4 ) on a homogeneous graphite surface has been determined by volumetric method between 77 and 115 K. The following information can be deduced: the 2D critical point temperature of the different adsorbed monolayers is located above the triple point temperature characterizing the 3D adsorbate (89.5 K). No two-dimensional triple point has been observed, the corresponding temperatures being probably appreciably lower than 77 K. Moreover, the triple points of 2nd, 3rd,… layers are presumably situated in the supersaturation domain; accordingly, the number of the film layers would be limited and would increase with temperature.

Thickness effect on alloying of ultrathin Co films on Pt(111): a real time and in situ UHV study with synchrotron X-ray diffraction

Abstract We study the alloying of ultrathin Co deposits (3 and 10 monolayer) on a Pt(111) substrate. The surface film evolution is followed in real time during annealing by X ray diffraction, which provides the depth profile with atomic resolution. We fully characterize the formation of Pt–Co surface alloys before dissolution; the kinetics is shown to depend on the Co thickness. For the 10 monolayer film, the dominant hcp phase exhibits a strong resistance to interdiffusion while for the 3 monolayer deposit the surface film is progressively enriched in Pt. Finally, whatever the annealing treatment and the film thickness, the stabilized alloy is close to Pt60Co40 bulk-like, after heating around 450°C.

Thermal stability of atomic Ag/Au and Au/Ag interfaces on A Ru(001) substrate

Abstract The thermally activated intermixing in Ag/Au and Au/Ag bilayers supported on a Ru(001) substrate has been studied using the PAX-technique (photoemission of adsorbed xenon). It is found that the Ag/Au interface (on Ru) is thermally more stable than the Au/Ag interface. In the first case Ag atoms do not penetrate into the Au underlayer at 275 K, while in the second case An atoms do penetrate into the Ag underlayer at this temperature. These investigations also represent a very successful model study which may be carried over the other metal/metal combinations.

Microscopic pooperties of two-dimensional silver and gold metal- and alloy-films on Ru(001)

Structural changes in submonolayer codeposits of gold and silver on Ru(001) are studied using the PAX technique. It is shown that the formation of two-dimensional AgAu alloys from Ag islands and co-deposited Au atoms is a two-step process: first the Au atoms diffuse from the Ag silands, only then the Au atoms interalloy from island boundaries.