A. Baraldi

Temperature programmed X-ray photoelectron spectroscopy : a new technique for the study of surface kinetics

Abstract The desorption of CO from the (2 × 1)p2mg layer on Rh(110) was studied by means of a novel method — temperature-programmed X-ray photoelectron spectroscopy (TPXPS). The new information on the variation of the concentration of top and bridge CO during thermal desorption was used to evaluate the difference of the adsorption enthalpies. A model for the structural evolution of the adsorbed layer is suggested, taking into account the CO bonding configurations and the role of the CO CO repulsive interactions.

Nitrogen-induced reconstruction on Rh(110): effect of oxygen on the growth and ordering of Rh-N chains

The formation of (3 × 1) and (2 × 1)-N phases on a Rh(110) surface has been studied by STM and LEED. When the Rh(110) surface is exposed to NH3 at 400–450 K strings running in the [001] direction appear in the STM images which are due to nucleation and growth of -Rh-N- chains. The weak corrugation of ~ 0.2 A along the strings with a periodicity equal to the [001] lattice constant and the large corrugation of ~ 0.7 A and spacing ⩾ two [110] lattice constants across the strings are indicative of a reconstruction involving absence of [001] rows. The nucleation of -Rh-N- chains occurs preferentially at step edges. With increasing coverage most of the strings grow on the lower terrace in an added-row mode remaining attached to the terrace boundaries. The periodicity in the [110] direction changes from random to tripled and doubled lattice constant with (3 × 1) and (2 × 1) islands preferentially formed in the vicinity of the steps. The (3 × 1) and (2 × 1) islands coexist with areas on the terraces with a (n × 1) chain periodicity with n > 3, which accounts for the streaking in LEED patterns even at saturation. The presence of small amounts of oxygen was found to change the growth mode by increasing the nucleation and growth rate on the terraces, promoting the completion of the (3 × 1) and (2 × 1)-N phases. The mechanism of the N-induced reconstruction and the effect of oxygen are discussed considering the thermodynamic and kinetic factors driving the restructuring process.

Comparative study of the adsorption of CO, NO and hydrogen on(1 × 1) and (1 × 2) Rh(110)

The adsorption of CO, NO and hydrogen on the (1 × 1) and (1 × 2) Rh(110) surfaces has been studied by means of LEED, TPD and AES at temperatures above 125 K. On the (1× 2) surface, CO adsorption resulted in c(2 × 4) (for 0.40 < θ < 0.63 ML) and (2 × 2)pg (for θ = 1 ML) LEED patterns while saturation adsorption of NO gave rise to a (1 × 4) pattern. Unlike on the (1 × 1) surface, no H induced LEED patterns were observed on the (1 × 2) surface. The effect of the different structure of the substrate depends strongly on the type of molecule. In particular, H is substantially more strongly adsorbed on the (1 × 2) surface, while NO partly desorbs and partly dissociates on both surfaces, but a larger fraction desorbs molecularly from the (1× 2) surface. CO is only slightly more strongly bound on the (1 × 1) surface. The different adsorption states are described and tentative models are provided.

Structural determination of molecules adsorbed in different sites by means of chemical shift photoelectron diffraction: c(4×2)-CO on Pt(111)

Abstract In this letter we present a chemical shift photoelectron diffraction (CS-PED) study of the c(4×2) phase of CO on Pt(111), in which two molecular species are present. The angular dependence of both chemically shifted C1s core levels was independently determined in a high resolution photoemission experiment. By means of a full multiple scattering analysis, the local structure around each of the two CO species has been quantitatively evaluated. A configuration with half of the CO molecules on top and half in the bridge position gives good agreement with the experimental data, in accord with previous structural studies of this system. Our result highlights the capabilities of CS-PED for the study of complex adsorption systems where the same molecule occupies different adsorption sites.

CO adsorption on unreconstructed and reconstructed Rh(110) surfaces: LEED and XPS studies

Abstract The adsorption of CO on the (1 × 1) unreconstructed and (1 × 2) reconstructed Rh(110) surfaces has been studied by means of XPS and LEED. The O 1s spectra of CO were used for measuring the coverage and determining the binding sites of the adsorbed molecules. It has been found that on both surfaces CO adsorption occurs in two bonding configurations characterized by O 1s binding energies at 530.8 and 531.9 eV. They are assigned to occupation of bridge and on-top sites, respectively. On the (1 × 1) surface initially the on-top sites are occupied yielding an ordered c(2 × 2) layer at a coverage of 0.5 ML. At higher coverage CO changes its bonding configuration and a (2 × 1)p2mg structure is formed near saturation when the coverage approaches 1 ML. It is suggested that the (2 × 1)p2mg structure consists of zig-zag chains of bridging tilted CO. When CO adsorbs on the (1 × 2) surface simultaneous occupation of on-top and bridge sites is observed. Molecules in both bonding configurations contribute to the observed (2 × 2), c(2 × 4) and (2 × 2)p2mg structures formed on the (1 × 2) surface. The intensities of the CO O 1s spectra indicate that the amount of CO adsorbed on the (1 × 1) and (1 × 2) surfaces is the same. Comparable amounts of CO adsorbed on the [110] rows and (111) microfacets of the (1 × 2) surface are suggested in order to explain the observed (2 × 2), c(2 × 4) and (2 × 2)p2mg structures.

Atomic nitrogen on steps: A fast x-ray photoelectron spectroscopy study of the NO uptake on Rh(533), Rh(311), and Rh(111)

The interaction of NO with a flat and two stepped Rh(111) surfaces was studied at different temperatures by monitoring in situ the evolution of the O 1s and N 1s spectra during NO exposure using fast high resolution x-ray photoelectron spectroscopy. The O 1s and N 1s intensities and binding energies were used to fingerprint the types of adsorbed species and to monitor the changes in their coverage and adsorption sites. From the O 1s intensity during uptake on Rh(533) it was determined that the NO initial sticking coefficient is temperature independent between 330 and 490 K. The N 1s spectra revealed the consecutive appearance of two atomic nitrogen species. By comparing the N 1s spectra on Rh(533) with those measured on Rh(111) and Rh(311) these species were attributed to adsorption on terrace sites, NT, and step sites, NS. NT is the only species formed in the initial stage of adsorption, whereas the NS species appears later on. This finding indicates that the NO dissociation occurs on terrace adsorption ...

Formation of channels for oxygen migration towards subsurface sites by CO oxidation and growth of the surface oxide phase on Ag(0 0 1)

The mechanism of oxygen incorporation in Ag is still poorly known. As recently demonstrated [Phys. Rev. B 63 (2001) R1404], oxygen adatoms removal by CO oxidation leaves the Ag(0 0 1) surface in a modified state in which oxygen segregation from the bulk and the formation of a surface oxide phase can be induced by further CO exposure. Here we show that the same channels, forming during CO oxidation and linking surface and subsurface sites, allow also for the migration of oxygen adatoms into the subsurface region. When dosing O2 on an Ag(1 0 0) surface, on which oxygen had been previously removed by CO oxidation, we observe indeed the formation of the same surface oxide phase produced by oxygen segregation. A characterisation of this phase by X-rays photoemission spectroscopy and high resolution electron energy loss spectroscopy is given, from which we deduce that it extends several layers deep into the volume. 2002 Elsevier Science B.V. All rights reserved.

Carbon monoxide adsorbed on Ru(101̄0)

Abstract The adsorption of CO on the Ru(1010) surface has been studied by means of XPS, LEED and TPD. The CO O1s spectra, including the satellites, were used to obtain information about the CO bonding configuration and the population of different adsorption sites as a function of the CO coverage and adlayer ordered structures. The on-top bonded CO, which is the only species up to one monolayer, is characterized by an O1s binding energy of 531.7 eV at coverages less than 0.6 ML. The slight shift of the on-top CO O1s binding energy to 531.9 eV at higher coverages has been ascribed to the compression and displacement of CO from the equilibrium on-top position. Beyond 1 ML the adsorption of extra 0.25 ML CO in a higher coordinated site results in a second O1s peak at 530.9 eV. On the basis of the XPS and LEED data a structural model for the arrangement of the CO molecules in the compressed ( 4 −1 1 2 ) saturated layer has been suggested. The coverage dependence of desorption energy, Ed, and pre-exponential factor, v, evaluated by using two methods for fitting the TPD data, have shown almost constant Ed and v values of ∼ 140 kJ mol−1 and 1014 s−1 up to ∼ 0.33 ML, a linear decline to ∼ 80 kJ mol−1 and 109 s−1 at 0.8–1.0 ML and a sharp drop to ∼ 50 kJ mol−1 and 106 s−1 at saturation (∼ 1.25 ML). This coverage dependence has been explained in terms of CO-CO repulsive interactions and reduced CO-substrate coupling in the higher coordinated site.

Core level spectra of amorphous carbon nitride

The nature of bonding in amorphous carbon nitride was studied with core level spectroscopies such as x-ray absorption, photoemission, and Auger electron spectroscopy, using synchrotron radiation as the excitation source. The changes in local structure were systematically studied as a function of sample preparation conditions. The correspondence between nitrogen local environment and spectral features is discussed. It is found that thermal annealing increases the average coordination number and decreases bond multiplicity at nitrogen sites.

Structural effects on water formation from coadsorbed H + O on Rh(100)

The effect of adsorbate coverage, adsorption sequence and temperature on the structure, composition and reactivity of coadsorbed layers, produced by dissociative adsorption of O2 and H2 at 200 K on a Rh(100) surface, has been studied by combined TPD, XPS and LEED measurements. The emphasis is on the impact of the structure and composition of the mixed O + H layers on the synthesis of hydroxyl and water as a result of the O + H surface reaction. The difference in the O 1s binding energies of adsorbed O (529.9 eV) and OH species (530.8 eV) was used as a fingerprint to monitor the formation of the OH species. The H2O TPD spectra show substantial variations of the desorption temperature range and the amount of water evolved with coadsorbate coverage and structure: from 270 to 350 K and from 0 to 0.08 ML, respectively. It has been found that dense O + H adlayers, where the O coverage is in the range 0.25-0.4 ML, favor the formation of stable OH species. The maximum amount of stable hydroxyl OH species (∼ 0.16 ML) can be produced by heating of these dense adlayers to ∼ 260 K. This results in reordering of the adspecies to form a new O + OH − (2 × 6) structure, where hydroxyls react readily to evolve 0.08 ML of water in a sharp desorption peak at ∼ 280 K. The effect of the adlayer density and restructuring on the production of OH and H2O is discussed.