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Physical review letters 118(1), 013002 (2017). doi:10.1103/PhysRevLett.118.013002, Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He+(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. The measured circular dichroism is unexpectedly found to vary strongly as a function of the NIR intensity. The experimental data are well described by theoretical modeling and possible mechanisms are discussed., Published by APS, College Park, Md.

SO2 poisoning of methane oxidation over alumina-supported, Pd@CexZr1−xO2 nanoparticle catalysts was systematically studied by means of advanced PhotoElectron Spectroscopy (PES) methods. The Pd@CexZr1−xO2 units were synthesized and deposited on two modified-alumina supports, i.e. high surface area modified alumina and a model alumina prepared by Atomic Layer Deposition (ALD) of alumina on Indium Tin Oxide (ITO)/quartz slides. The model support was designed to be suitable for PES analysis and was stable to high temperature treatments (850 °C). Characterization of the high-surface-area (HSA) catalysts by X-Ray Diffraction (XRD), N2 physisorption, CO chemisorption and Transmission Electron Microscopy (TEM) indicated formation of CeO2–ZrO2 (CZ) mixed-oxide crystallites that stabilize the Pd active phase against sintering. Correlation of methane-oxidation rates with PES results demonstrated two distinct mechanisms for deactivation by SO2. Below 450 °C, the presence of SO2 in the feed led to partial reduction of the active PdO phase and to the formation of sulfates on the Pd. Above 500 °C, poisoning by SO2 was less severe due to spillover of the sulfates onto the oxide promoter. Pd@ZrO2 catalysts showed the best resistance to SO2 poisoning, outperforming analogous Pd@CZ mixed-oxide catalysts, because there was less sulfate formation and the sulfates that did form could be removed during regeneration. The authors acknowledge CERIC-ERIC for open access to instrumentation and user support. This paper was also made possible by a NPRP Grant #6-290-1-059 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. Scopus

The atomic composition, local structure and coordination of flash evaporated Ge2S3 films and their changes induced by laser illumination and thermal annealing in vacuum were investigated by means of high resolution synchrotron radiation and X-ray photoelectron spectroscopy. The top surface and subsurface layers of the films are found to be enriched by Ge in comparison with the composition of target glass and contain oxygen and a significant contribution of physisorbed carbon. The influence of laser and thermal treatments of flash evaporated Ge–S films in vacuum on their composition is analyzed. The local atomic coordination obtained by curve fitting of Ge 3d, S 2p, O 1s and C 1s core level spectra is discussed in detail.

We have investigated the role of coadsorbed atomic oxygen during the decomposition of acetic acid on Pt(111) by means of temperature-programmed desorption (TPD) and synchrotron radiation photoelectron spectroscopy (SRPES). Reaction mechanisms have been established through identification of desorbing products and surface species formed during decomposition of acetic acid, both on Pt(111) and oxygen pre-exposed p(2 × 2)–O/Pt(111). Acetate and molecularly adsorbed acetic acid are formed on both samples during the adsorption of acetic acid at 150 K. On p(2 × 2)–O/Pt(111), however, surface acetyl is identified as the principal species. The major decomposition channel for acetate and acetic acid involves formation of ketene and acetaldehyde at 222 K, and this reaction is not affected by coadsorbed oxygen. In the following reactions, partial decomposition of acetaldehyde yielded ethylene, ethylidene, ethylidyne, and small amounts of CO and methoxy on both samples. Above 222 K, decomposition of acetate on Pt(111)...

Platin ist das am vielseitigsten eingesetzte Element in der Katalyse. Allerdings begrenzt der hohe Preis des Edelmetalls die Verwendung in vielen Bereichen, z. B. in Katalysatormaterialien fur Brennstoffzellen. Trotzdem nutzen konventionelle Katalysatoren oftmals nur einen Bruchteil ihres Pt-Gehaltes, namlich diejenigen Atome, die sich auf der Oberflache des Katalysators befinden. Eine effizientere Edelmetallnutzung setzt somit eine hohere, bevorzugt atomare Dispersion der Pt-Atome auf der Oberflache voraus. Tatsachlich ist es moglich, solche atomar dispergierten Pt-Spezies mit sehr hoher Stabilitat auf einer Katalysatoroberflache herzustellen. Mithilfe von DFT-Rechnungen identifizieren wir ein spezifisches Strukturmerkmal solcher Systeme, eine sogenannte Cerdioxid-“Nanotasche”, die Pt2+-Ionen so stabilisiert, dass diese sowohl thermischem Sintern als auch einer Diffusion in tiefere Katalysatorebenen widerstehen. Die theoretisch vorhergesagte auserordentliche Stabilitat wurde experimentell an Modellkatalysatoren verifiziert. In realen Pt-CeO2-Nanomaterialien, die tatsachlich eine sehr hohe katalytische Aktivitat in Brennstoffzellen zeigen, identifizieren wir diese Bindungsstellen ebenfalls. Die so erhaltenen neuen Materialien konnen dazu beitragen, den Bedarf an Edelmetallen fur Katalysatormaterialien erheblich zu reduzieren.

The interaction of gold nanoparticles (AuNPs) with cysteine and its derivatives is the basis of a number of bionanotechnologies, and for these, the most important process is aggregation (or antiaggregation), which enables an array of colorimetric detection methods. When AuNPs were functionalized with cysteine, its dimer cystine, or the cysteine-derived tripeptide, glutathione, three different mechanisms of aggregation were observed. Both cysteine and glutathione induced aggregation of AuNPs without further pH modification: the first by interparticle zwitterionic interaction and the second by interparticle hydrogen bonding. Cystine, however, did not induce aggregation, although it dissociated into two cysteinate moieties upon adsorption on the AuNPs, which appear to be chemically identical to cysteinate produced from cysteine adsorption. We show that the difference is due to the lower coverage of cysteinate from cystine and differences in charge states of the adsorbates. On modifying the pH to 1.5, the surface species become cationic (neutral COOH and protonated NH3 +), and aggregation of cystine/AuNPs occurs immediately by interparticle hydrogen bonding. Thus, cysteine may induce aggregation by neutral hydrogen bonding or zwitterionic interaction between nanoparticles, but the mechanism depends sensitively on a number of parameters.

Single-photon laser enabled Auger decay (spLEAD) is an electronic de-excitation process which was recently predicted and observed in Ne. We have investigated it using bichromatic phase-locked free electron laser radiation and extensive angle-resolved photoelectron measurements, supported by a detailed theoretical model. We first used separately the fundamental wavelength resonant with the Ne+ 2s–2p transition, 46.17 nm, and its second harmonic, 23.08 nm, then their phase-locked bichromatic combination. In the latter case the phase difference between the two wavelengths was scanned, and interference effects were observed, confirming that the spLEAD process was occurring. The detailed theoretical model we developed qualitatively predicts all observations: branching ratios between the final Auger states, their amplitudes of oscillation as a function of phase, the phase lag between the oscillations of different final states, and partial cancellation of the oscillations under certain conditions.

The adsorption of 5-Fluorouracil (5FU) on Au(111) and Cu(111) surfaces as a function of molecular coverage and temperature has been studied, using x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The nature of 5-Fluorouracil bonding with the two substrates is remarkably different. The Cu substrate forms a chemisorbed complex with 5-FU while the Au substrate shows only physisorption. NEXAFS data at the C, N and O K-edge show a strong angular dependence, indicating that 5-FU lies nearly parallel on the inert Au(111) surface, and at a steep angle on the Cu(111) surface. 5-FU is a biomolecule used for cancer treatment and the results are relevant for those using metal surfaces to prepare 5-FU for applications such as drug delivery.The adsorption of 5-Fluorouracil (5FU) on Au(111) and Cu(111) surfaces as a function of molecular coverage and temperature has been studied, using x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The nature of 5-Fluorouracil bonding with the two substrates is remarkably different. The Cu substrate forms a chemisorbed complex with 5-FU while the Au substrate shows only physisorption. NEXAFS data at the C, N and O K-edge show a strong angular dependence, indicating that 5-FU lies nearly parallel on the inert Au(111) surface, and at a steep angle on the Cu(111) surface. 5-FU is a biomolecule used for cancer treatment and the results are relevant for those using metal surfaces to prepare 5-FU for applications such as drug delivery.

Towards a molecular understanding of the mechanism behind catalytic reforming of bioderived hydrocarbon oxygenates, we explore the C-C bond scission of C2 model compounds (acetic acid, ethanol, ethylene glycol) on ceria model catalysts of different complexity, with and without platinum. Synchrotron photoelectron spectroscopy reveals that the reaction pathway depends very specifically on both the reactant molecule and the catalyst surface. Whereas C-C bond scission on Pt sites and on oxygen vacancies involves intermittent surface carbon species, the reaction occurs without any carbon formation and deposition for ethylene glycol on CeO2(111).

Adsorption of histidine on cerium oxide model surfaces was investigated by synchrotron radiation photoemission, resonant photoemission, and near edge X-ray absorption fine structure spectroscopies. Histidine was evaporated in a vacuum onto ordered stoichiometric CeO2(111) and partially reduced CeO1.9 thin films grown on Cu(111). Histidine binds to CeO2 in anionic form via the carboxylate group and all three nitrogen atoms, with the imidazole ring parallel to the surface. The amino nitrogen atom of the imidazole ring (IM) is deprotonated, and both IM nitrogen atoms form strong bonds via pi-orbitals, while the alpha-amino nitrogen interacts with the oxide via its hydrogen atoms. In the case of CeO1.9, the deprotonation of the amino nitrogen of the imidazole ring is less pronounced and N K-edge spectra do not show a clear orientation of the ring with respect to the surface. A minor reduction of the cerium surface on adsorption of histidine was observed and explained by charge exchange as a result of hybridization of the pi-orbitals of the IM ring with the f and d orbitals of ceria. Knowledge of histidine adsorption on the cerium oxide surface can be used for design of mediator-less biosensors where the histidine-containing proteins can be strongly bound to the oxide surface via the imidazole side chain of this residue.

Adsorption and reaction of formic acid on stoichiometric CeO2(111), partially reduced CeO2–x, and Pt/CeO2(111) films prepared on Cu(111) were studied by means of synchrotron radiation photoelectron spectroscopy (SRPES), resonant photoemission spectroscopy (RPES), infrared reflection absorption spectroscopy (IRAS), and temperature-programmed desorption (TPD). On all studied samples, the principal species formed during formic acid adsorption below 160 K were formate and molecular formic acid. In the presence of Pt particles, formate species were predominantly localized on Pt at 100 K, and on ceria at or above 300 K. Below 400 K, molecular formic acid decomposes to formate with partial release of CO2, CO, hydrogen, and water. Analysis of the TPD fragmentation suggests additional evolution of methane. Above 400 K, desorption of CO2, CO, hydrogen, and water is observed. This process is controlled by the stoichiometry of ceria and the presence of Pt particles. In particular, desorption of CO2 is suppressed on C...

The fabrication and control of coordination compounds or architectures at well-defined interfaces is a thriving research domain with promise for various research areas, including single-site catalysis, molecular magnetism, light-harvesting, and molecular rotors and machines. To date, such systems have been realized either by grafting or depositing prefabricated metal-organic complexes or by protocols combining molecular linkers and single metal atoms at the interface. Here we report a different pathway employing metal-organic chemical vapor deposition, as exemplified by the reaction of meso-tetraphenylporphyrin derivatives on atomistically clean Ag(111) with a metal carbonyl precursor (Ru3(CO)12) under vacuum conditions. Scanning tunneling microscopy and X-ray spectroscopy reveal the formation of a meso-tetraphenylporphyrin cyclodehydrogenation product that readily undergoes metalation after exposure to the Ru-carbonyl precursor vapor and thermal treatment. The self-terminating porphyrin metalation protocol proceeds without additional surface-bound byproducts, yielding a single and thermally robust layer of Ru metalloporphyrins. The introduced fabrication scheme presents a new approach toward the realization of complex metal-organic interfaces incorporating metal centers in unique coordination environments.

Practical applications of ionic liquids (ILs) often involve IL/oxide interfaces, but little is known regarding their interfacial chemistry. The unusual physicochemical properties of ILs, including their exceptionally low vapor pressure, provide access to such interfaces using a surface science approach in ultrahigh vacuum (UHV). We have applied synchrotron radiation photoelectron spectroscopy (SR-PES) to the study of a thin film of the ionic liquid [C6C1Im][Tf2N] prepared in situ in UHV on ordered stoichiometric CeO2(111) and partially reduced CeO2-x. On the partially reduced surface, we mostly observe decomposition of the anion. On the stoichiometric CeO2(111) surface, however, a layer of surface-anchored organic products with high thermal stability is formed upon reaction of the cation. The suggested acid-base reaction pathway may provide well-defined functionalized IL/solid interfaces on basic oxides.

A combined experimental and theoretical approach has been used to disentangle the fundamental mechanisms of the fragmentation of the three isomers of nitroimidazole induced by vacuum ultra-violet (VUV) radiation, namely, 4-, 5-, and 2-nitroimidazole. The results of mass spectrometry as well as photoelectron-photoion coincidence spectroscopy display striking differences in the radiation-induced decomposition of the different nitroimidazole radical cations. Based on density functional theory (DFT) calculations, a model is proposed which fully explains such differences, and reveals the subtle fragmentation mechanisms leading to the release of neutral species like NO, CO, and HCN. Such species have a profound impact in biological media and may play a fundamental role in radiosensitising mechanisms during radiotherapy.

The surface functionalization of TiO2-based materials with alkylsilanes is attractive in several cutting-edge applications, such as photovoltaics, sensors, and nanocarriers for the controlled release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES) is able to self-assemble to form monolayers on TiO2 surfaces, but its adsorption geometry and solar-induced photodegradation pathways are not well understood. We here employ advanced experimental (XPS, NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools to investigate the preferential interaction mode of APTES on anatase TiO2. We demonstrate that monomeric APTES chemisorption should proceed through covalent Si–O–Ti bonds. Although dimerization of the silane through Si–O–Si bonds is possible, further polymerization on the surface is scarcely probable. Terminal amino groups are expected to be partially involved in strong charge-assisted hydrogen bonds with surface hydroxyl groups of TiO2, resulting in a reduced propensity to react with other species. Solar-induced mineralization proceeds through preferential cleavage of the alkyl groups, leading to the rapid loss of the terminal NH2 moieties, whereas the Si-bearing head of APTES undergoes slower oxidation and remains bound to the surface. The suitability of employing the silane as a linker with other chemical species is discussed in the context of controlled degradation of APTES monolayers for drug release and surface patterning.

The electronic structures of coumarin and three of its derivatives (7-amino-4-methylcoumarin, 7-amino-4-(trifluoro)methylcoumarin, and 4-hydroxycoumarin) have been studied by theoretical calculations, and compared with experimental valence and core photoelectron spectra to benchmark the predicted spectra. The outer valence band spectra of the first three compounds showed good agreement with theoretical calculations for a single isomer, whereas the spectrum of 4-hydroxycoumarin indicated the presence of more than one tautomer, consistent with published results. Calculations of core level spectra of carbon, nitrogen, oxygen, and fluorine of the first three compounds are also in satisfactory agreement with our measurements. The carbon and oxygen 1s spectra of 4-hydroxycoumarin allow us to identify and quantify the populations of the principle tautomers present. The 4-hydroxy enol form is the most stable isomer at 348 K, followed by the diketo form, with 1.3 kJ·mol(-1) lower energy.

The reactivity of atomically dispersed Pt(2+) species on the surface of nanostructured CeO2 films and the mechanism of H2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt(2+) sites are found to be inactive towards H2 dissociation due to high activation energy required for H-H bond scission. Trace amounts of metallic Pt are necessary to initiate H2 dissociation on Pt-CeO2 films. H2 dissociation triggers the reduction of Ce(4+) cations which, in turn, is coupled with the reduction of Pt(2+) species. The mechanism of Pt(2+) reduction involves reverse oxygen spillover and formation of oxygen vacancies on Pt-CeO2 films. Our calculations suggest the existence of a threshold concentration of oxygen vacancies associated with the onset of Pt(2+) reduction.

In high-intensity laser light, matter can be ionized by direct multiphoton absorption even at photon energies below the ionization threshold. However on tuning the laser to the lowest resonant transition, the system becomes multiply excited, and more efficient, indirect ionization pathways become operative. These mechanisms are known as interatomic Coulombic decay (ICD), where one of the species de-excites to its ground state, transferring its energy to ionize another excited species. Here we show that on tuning to a higher resonant transition, a previously unknown type of interatomic Coulombic decay, intra-Rydberg ICD occurs. In it, de-excitation of an atom to a close-lying Rydberg state leads to electron emission from another neighbouring Rydberg atom. Moreover, systems multiply excited to higher Rydberg states will decay by a cascade of such processes, producing even more ions. The intra-Rydberg ICD and cascades are expected to be ubiquitous in weakly-bound systems exposed to high-intensity resonant radiation., 原子の集団が数珠つなぎに電子を放出する! : 極紫外自由電子レーザーで誘起される新現象解明. 京都大学プレスリリース. 2016-12-12.

Researchers demonstrate correlation of two colours (63.0 and 31.5 nm wavelengths) in a free-electron laser and control photoelectron angular distribution by adjusting phase with 3 attosecond resolution. Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization1, pumping an atomic laser2 and four-wave mixing spectroscopy3. However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence4,5,6,7. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent8,9, and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5 nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution10,11 by adjusting the phase, with a temporal resolution of 3 as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity.

A comparative study of DTBT (1,3-di(thiophen-2-yl)benzo[c]thiophene), Br-DTBT-Br (1,3-bis(4,5-dibromothiophen-2-yl)benzo[c]thiophene), and a model 3T (2,2?:5?,2-terthiophene) oligothiophenes has been performed by investigating the interactions occurring between these compounds and both polycrystalline and single-crystal Ag surfaces. As a main result we report that the interaction of 3T with the Ag surfaces is followed by a fast, low-temperature polymerization. On the contrary, DTBT-like compounds, less reactive at low temperature, undergo thermally activated dissociation processes which hinder the possibility of a surface-induced polymerization of such compounds as a suitable synthetic strategy. Our results are supported by Raman and surface-enhanced Raman spectroscopy (SERS) measurements of the molecules in contact with a polycrystalline Ag substrate, showing the fingerprint of polythiophene chains in the case of 3T and a characteristic S-S stretching signal compatible with ring-opening processes in the case of DTBT-like compounds, not detected for 3T. Moreover, high-resolution X-ray photoelectron spectroscopy (XPS) measurements, probing carbon 1s and sulfur 2p core levels of the same molecules deposited onto a Ag(110) single crystal, confirm the suggestion of a ring-opening process by a C-S bond cleavage induced by both the interaction with silver and thermal treatments. Near-edge X-ray absorption fine structure (NEXAFS) carbon K-edge measurements complete the information, showing a common tendency of the molecules to lay flat on the Ag surface. Finally, density functional theory (DFT) calculations have been carried out to investigate the interaction and molecular conformation/orientation at the 3T and DTBT/Ag(110) interfaces. Theoretical findings support the results of measurements and suggest that a fine interplay between structural and electronic modifications of the benzo-fused rings of DTBT-like compounds in contact with the Ag surface is responsible for the peculiar reactivity of such molecules.

Here, we report the observation of electron transfer mediated decay (ETMD) involving Mg clusters embedded in helium nanodroplets which is initiated by the ionization of helium followed by removal of two electrons from the Mg clusters of which one is transferred to the He environment neutralizing it while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. The photoelectron spectrum reveals a low energy ETMD peak. For Mg clusters larger than 5 atoms we observe stable doubly-ionized clusters. We argue that ETMD provides a new pathway to the formation of doubly-ionized cold species., 5 pages submitted to Phys. Rev. Lett

The influence of near bandgap laser irradiation on the structure of the As50Se50 thin film has been investigated by synchrotron radiation photoelectron spectroscopy. The As 3d and Se 3d photoemission peaks of the irradiated sample show significant differences in shapes and positions in comparison with those obtained for non-irradiated amorphous film. The experimental data processing and quantifications are performed analyzing As 3d and Se 3d core-level components obtained by curve fitting. The relative contribution of the As and Se atoms in different chemical states to the whole As 3d and Se 3d signal, its structural origins as well as their relation to the As50Se50 nanolayers structure before and after laser irradiation is analyzed and discussed in detail.

The influence of air and near bandgap laser irradiation on the surface structure of As40Se60 nanolayers has been investigated by synchrotron radiation photoelectron spectroscopy (SRPES). The As 3d and Se 3d photoemission peaks of the irradiated sample show significant differences in shapes and positions in comparison with those obtained for non-irradiated amorphous films. The experimental data processing and quantification were performed by analyzing As 3d, Se 3d, C 1s and 0 1s core-level components obtained by curve fitting. The relative contribution of the As, Se, C and 0 atoms in different chemical states to the whole As 3d, Se 3d, C is and 0 is signals, its structural origins as well as their relation to the As40Se60 surface and subsurface nanolayers structure before and after laser irradiation is analyzed and discussed in detail. An atomic model of induced structural changes on the surface of As40Se60 nanolayers under ambient conditions and after laser irradiation in air is proposed. (C) 2012 Elsevier B.V. All rights reserved.

X-ray photoelectron spectra of the core and valence levels of the fundamental building blocks of beta-lactam antibiotics have been investigated and compared with theoretical calculations. The spectra of the compounds 2-azetidinone and the 2- and 4-isomers of thiazolidine-carboxylic acid are interpreted in the light of theoretical calculations: The Spectra of the two isomers of thiazolidine-carboxylic acid are rather. similar, as expected, but show clear effects due to isomerization. Both isomers are analogues of proline, which is well-known to populate several low energy conformers in the gas phase. We have investigated the low energy conformers of thiazolidine-4-carboxylic acid theoretically in more detail and find some spectroscopic evidence that multiple conformers may be present. The measured, valence levels are assigned for all three compounds, and the character of the frontier orbitals is identified and analyzed.

We have studied the interaction of water with stoichiometric CeO2(111)/Cu(111), partially reduced CeO2–x/Cu(111), and Pt/CeO2/Cu(111) model catalysts by means of synchrotron–radiation photoelectron spectroscopy (SRPES), resonant photoemission spectroscopy (RPES) at the Ce 4d edge, infrared reflection absorption spectroscopy (IRAS), and density functional (DF) calculations. The principal species formed during adsorption of water at 160 K on CeO2(111) films is chemisorbed molecular water. On the surface of CeO2–x water partially dissociates yielding hydroxyl groups. By use of core-level PES, differentiation between chemisorbed water and hydroxyl groups is complicated by the overlap of the corresponding spectral features. Nevertheless, we determined three characteristic indicators for OH groups on ceria: (i) the presence of 1π and 3σ states in valence band (VB) PES; (ii) an increase of the binding energy (BE) separation between the O 1s spectral components of lattice oxygen and OH/H2O; (iii) an increase of t...

We have performed a synchrotron radiation photoelectron spectroscopy (SR-PES) study on the mechanism of SO2 decomposition on Pt/CeO2(111) model catalysts prepared on Cu(111). Resonant photoemission spectroscopy (RPES) was applied to monitor changes of the cerium oxidation state. Adsorption of SO2 at 150 K on Pt/CeO2(111)/Cu(111) yields surface sulfites (SO32–) on ceria and upright standing molecular SO2 (SO2st) on Pt along with traces of atomic sulfur. Partial decomposition and desorption of the adsorbed species in the form of SO2 occur between 150 and 300 K. In this temperature region, slow accumulation of atomic sulfur is accompanied by reduction of the ceria support. Above 300 K, all surface species completely transform into atomic sulfur; this process is accompanied by a strong reoxidation of ceria. As probed by CO adsorption, the majority of the atomic sulfur species is located on the surface of the Pt particles. Possible reaction mechanisms involving spillover of numerous sulfur-derived species are ...

The photoinduced ion chemistry of acetonitrile is investigated by using tuneable VUV synchrotron radiation in an octupolar ion guide with mass spectrometric detection. The appearance energies and yields of primary photoions and secondary ionic products deriving from self-reactions at thermal collision energies are measured. Comparison of the appearance energies of the different product ions provides insights into the reaction mechanisms. Experimental data are interpreted with the help of theoretical calculations (energies and structures at the G3 level of theory) for the various isomers of the most relevant ionic products. We find that the ion C2H2N+ is formed at the threshold with a cyclic structure, but only its linear isomer (lying 0.47 eV higher in energy) can react with acetonitrile to form CH 2 CN CH 2 + plus HCN via a cyclic transition state.

The immobilization of 5-halouracils on Au(111) has been studied by soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Multilayer and monolayer films of 5-chlorouracil were deposited on the Au(111) surface by evaporation in vacuum while films of 5-bromouracil were absorbed in two modes: from the gas phase under ultrahigh vacuum conditions and from solution. The photoemission spectra of C, N and O 1 s, Br 3d and Cl 2p as well as the absorption spectra at the N and O K-edges were measured for monolayers of 5-halouracil films and the nature of the bonding with the Au(111) surface has been determined. From the angular dependence of the NEXAFS spectra at the O and K-edges, we conclude that these 5-halouracils are lying nearly parallel to the Au(111) surface. Distinct chemical states and surface adsorption geometry of the 5-BrU molecules for monolayer coverage prepared in two different ways have been found. (C) 2011 Elsevier B.V. All rights reserved.

Interaction of water with fully oxidized and partially reduced CeO2(1 1 1) thin film model catalyst grown on a Cu(1 1 1) surface was investigated by photoelectron spectroscopy (PES), scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). On the stoichiometric surface water adsorbs molecularly at low temperatures (

We have employed resonant photoemission spectroscopy to investigate the phenomenon of hydrogen spillover on well-defined model Pt/ceria catalysts. On Pt/CeO2(1 1 1)/Cu(1 1 1), hydrogen spillover and reverse spillover give rise to reversible changes of the oxidation state of the surface cerium ions. These changes are monitored with highest sensitivity via resonant enhancement of Ce3+- and Ce4+-related features in the valence band photoemission spectra. The temperature regions are precisely determined in which specific processes such hydrogen spillover, Ce3+ transport, and reverse spillover are activated.

The catalytic activity of cerium dioxide can be modified by deposition of alkaline earth oxide layers or nanoparticles or by substitutional doping of metal cations at the Ce site in ceria. In order to understand the effect of Mg oxide deposition and doping, a combination of experiment and first principles simulations is a powerful tool. In this paper, we examine the interaction of Mg with the ceria (111) surface using both angle resolved X-ray (ARXPS) and resonant (RPES) photoelectron spectroscopy measurements and density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). With DFT + U, we also examine the interaction of Mg with the ceria (110) surface. The experiments show that upon deposition of Mg, Ce ions are reduced to Ce(3+), while Mg is oxidised. When Mg is incorporated into ceria, no reduced Ce(3+) ions are found and oxygen vacancies are present. The DFT + U simulations show that each Mg that is introduced leads to formation of two reduced Ce(3+) ions. When Mg is incorporated at a Ce site in the (111) surface, one oxygen vacancy is formed for each Mg to compensate the different valencies, so that all Ce ions are oxidised. The behaviour of Mg upon interaction with the (110) surface is the same as with the (111) surface. The combined results provide a basis for deeper insights into the catalytic behaviour of ceria-based mixed oxide catalysts.

The adsorption of ethylene on the bridge adatom–restatom position of the Si(111)–7 × 7 surface was studied with scanning tunneling microscopy, photoelectron spectroscopy, and theoretical calculations. The electronic structure and the corresponding vibrational states were calculated for a single molecule adsorbed in different binding sites located within the 7 × 7 unit cell. We found that there is no significant difference in the electronic structure between the absorption sites that were considered: center and corner adatom–restatom. Moreover, the different electron occupation of the restatoms and adatoms has a strong effect on the electronic structure of the adsorbed molecule near the Fermi level, and this leads to the reduction of the molecular symmetry to Cs or even to C1. In the case of the Si 2p core level, beside the previously reported continuous quenching of the restatom state, we found clear evidence of the existence of a peak corresponding to the Si–C bond separated by 0.44 eV from the bulk comp...

We have performed a systematic synchrotron radiation photoelectron spectroscopy (SR-PES) study of the interaction of SO2 with stoichiometric CeO2(111) and reduced CeO2–x films supported on Cu(111). Resonant photoelectron spectroscopy (RPES) was utilized to monitor changes in the cerium oxidation states. Adsorption of SO2 at 150 and 300 K results mostly in formation of sulfite (SO32–) species on both reduced and stoichiometric ceria. Other species such as sulfate (SO42–), SO2 chemisorbed at cationic sites, anionic SO2–, atomic sulfur, surface sulfides, and bulk oxy-sulfides were detected. The origin of these species is discussed in relation to the changes in the oxidation state of ceria cations at the surface. The thermal evolution of the species formed upon adsorption at 150 and 300 K was studied systematically on both the stoichiometric and the reduced films. It was found that the sulfite partially desorbs to release SO2 and partially decomposes. In general, the reduced CeO2–x surface was found to be muc...

We have studied the electronic properties of single crystal based ceria and magnesia−ceria model catalysts, the CO2 adsorption, and the CO2-induced reoxidation of these systems by synchrotron radiation photoelectron spectroscopy (SR-PES). All model systems were prepared starting from a fully stoichiometric and well-ordered CeO2(111) film grown on Cu(111). Different magnesia−ceria mixed oxide films were prepared by physical vapor deposition (PVD) of magnesium, oxygen treatment, and subsequent annealing. The preparation procedure was varied to obtain samples with different oxidation state, structure, and surface composition. Different carbon-containing species were identified, including surface carbonates formed in the vicinity of Mg2+ and Ce3+/4+ and surface carboxylates. The presence of Mg2+ was observed to strongly enhance carbonate formation but suppress the formation of carboxylates. Changes in the oxidation state of ceria upon CO2 exposure were monitored with highest sensitivity by resonant photoelect...

Interaction of aluminium with cerium oxide was studied by photoelectron spectroscopy of Al/CeO 2 (1 1 1) and CeO 2 /Al(1 1 1) model systems. It was found in both cases that metallic aluminium was immediately oxidized, CeO 2 was partially reduced and a mixed oxide with cerium present as Ce 3+ was formed. The compound is probably cerium aluminate CeAlO 3 mixed with Al 2 O 3 or Ce 2 O 3 . In both cases the intermixing was limited by the diffusion of aluminium into ceria. The excess of deposited material above this limit formed AlO x and CeO 2 overlayers on the top of the mixed oxide + aluminate/CeO 2 and mixed oxide + aluminate/Al films, respectively.

The adsorption of guanine on Cu(110) has been studied by soft X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The valence band spectra, carbon and nitrogen 1s XPS and N K-edge absorption spectra were measured for a thin film, sub-monolayer and saturation coverage. We show that hydrogen bonding causes a significant core level shift in the thin film. At saturation coverage, guanine is tilted at 70° with respect to the surface, and bonds through the N1 nitrogen atom. On reducing the coverage, the orientation changes and the tilt angle of the molecular plane is reduced. For both coverages, guanine is singly deprotonated at the N1 site, and annealing to a higher temperature (500 K) is probably accompanied by further deprotonation of the amino groups.

Thin epitaxial alumina films were grown on Cu(111), Cu–9 at.%Al(111), Ni(111) and NiAl(110) single crystals. The alumina films grew in such a manner that hexagonal or pseudo-hexagonal oxygen lattices were parallel to the surface of the substrates. Photoelectron spectra were obtained either with synchrotron or Al K-alpha radiation. We measured Al 2p spectra and determined the atomic species that terminated the interface between the alumina films and the substrates. The influence of Al in the substrates on the species that terminated the interface has been discussed based on thermodynamics. From valence band spectra, p-type Schottky barrier height (energy difference between the Fermi level of the metallic substrates and the valence band maximum of the alumina films, band offset) was determined. Differences in interface terminating species resulted in variations in p-type Schottky barrier height, or band alignment.

Thin epitaxial alumina layers were grown on the Cu(111) surface using simultaneous aluminum deposition and oxygen exposure. Different substrate temperatures during the deposition resulted in layers with different thicknesses, growth rates, crystallinity and epitaxy. Low energy electron diffraction measurements confirmed the epitaxial growth for substrate temperatures above 870 K. The Al 2p doublet was studied by means of photoelectron spectroscopy in order to determine the alumina termination at the metal-oxide interface. A strong dependence on the preparation temperature was found and both aluminum and oxygen terminated interfaces were created.

The activation of CO2 on ceria-based catalysts opens chemical routes to the use of CO2 as an oxidizing agent. Toward a better understanding of the underlying surface reactions, we have performed a model study on well-defined CeO2−x thin films on Cu(1 1 1) under ultra-high-vacuum conditions. Resonant photoelectron spectroscopy is used as a highly sensitive tool to monitor changes in the oxidation state of cerium as a function of CO2 exposure and thermal treatment. It is shown that partial reoxidation of CeO2−x by CO2 occurs with high reaction probabilities (i) even at temperatures as low as room temperature, (ii) on pure CeO2−x in the complete absence of any noble metal co-catalysts, and (iii) in the absence of surface hydroxyl groups or water. In contrast to previous studies on ceria powders, the findings clearly demonstrate the high activity of pure hydroxyl- and metal-free ceria surfaces for CO2 activation, even at low reaction temperatures.

The adsorption geometry of the DNA nucleoside thymidine on Au(111) and Cu(110) surfaces has been determined from experimental results of soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The XPS of C, N, and O 1s as well as the absorption spectra at the N and O K-edges were measured for monolayer thymidine films, and the nature of the bonding with the two metal surfaces has been determined. The NEXAFS data at the N and O K-edge show a strong angular dependence of the π*/σ* intensity ratios. We conclude that the thymine moiety is lying nearly parallel to the Au(111) surface, while on the Cu(110) surface, it adsorbs at a steep angle.

Multilayer, monolayer, and submonolayer films of the dipeptide glycyl-glycine on the Cu(110) surface have been studied by a combination of soft X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory calculations. Detailed models of the electronic structure and adsorption geometry at each coverage have been proposed. After adsorption, the molecules bind to the copper surface in two forms, zwitterionic and anionic, and the latter is dominant after annealing. In the monolayer, the anionic form of the dipeptide coordinates to the surface via the oxygen atoms of the carboxylate or peptide group and amino nitrogen atom, whereas in the submonolayer regime, the amino group interacts with the surface through a hydrogen atom.

Both epitaxial and amorphous ultra-thin alumina films were grown on a Cu-9 at.%Al(1 1 1) substrate by selective oxidation of Al in the alloy in ultra high vacuum. The crystallinity of the alumina films was controlled by oxidation temperature. The photoelectron spectra of Al 2p, O 1s and valence band were measured in-situ during oxidation. The influence of the crystallinity on the interface structure between the alumina films and the substrate was discussed by analyzing the Al 2p spectra composed of multiple peaks. The energy difference between the Fermi level of the substrate and the valence band maximum of the alumina films (band offset) was derived from the valence band spectra. The energy band alignment at the interface between each of the two alumina films and the substrate was revealed by combining the binding energy values of the core levels with the band offset values. The influence of the alumina crystallinity on the band alignment was discussed.

The inner shell ionization of pyrimidine and some halogenated pyrimidines has been investigated experimentally by X-ray photoemission spectroscopy (XPS) and theoretically by density functional theory (DFT) methods. The selected targets 5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine allowed the study of the effect of the functionalization of the pyrimidine ring by different halogen atoms bound to the same molecular site, or by the same halogen atom bound to different molecular sites. The theoretical investigation of the inductive and resonance effects in the C(1s) ionization confirms the soundness of the resonance model for a qualitative description of the properties of an aromatic system. Moreover, the combination of the experimental results and the theoretical analysis provides a detailed description of the effects of the halogen atom on the screening of a C(1s) hole in the aromatic pyrimidine ring.

The high-resolution outer and inner valence band (VB) spectra, as well as core level X-ray photoemission (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectra at the carbon, nitrogen, and oxygen K-edges of glycyl-glycine in the gas phase have been measured, and the results interpreted by means of ab initio calculations. By comparison with our previous glycine spectra, the features characteristic of the peptide bond in the carbon, nitrogen, and oxygen XPS and NEXAFS spectra of glycyl-glycine have been identified and assigned. The K-edge NEXAFS spectra at all edges were found to be in reasonable agreement with published electron energy loss spectra, but showed more prominent structure due to the higher resolution. Effects on the nitrogen K-edge NEXAFS spectrum of glycyl-glycine due to intramolecular hydrogen bonding were observed. The N(NH(2)) 1s --pi*(CN) resonance shifts to 0.6 eV lower energy, due to internal hydrogen bonding between the NH(2) and NH groups.

Lead submonolayers on the Si(1 1 1) surface in ordered structures of symmetry ( 3 × 3 ) R 30 ° were studied with photoelectron diffraction and ab initio calculations. At 1/3 monolayer coverage Pb atoms flip between up and down positions. This fluctuating geometry is connected with the charge redistribution in the basic triangular bipyramid formed by an adsorbed Pb atom and its four closest Si neighbors. The dynamic balance is reflected in the switching of bipyramids from semiconductor to metal character as a result of the interaction between an unsaturated Pb atom in the T 4 position and a saturated Si atom beneath. Substitution of a Pb atom in the 1/3 monolayer coverage with a Si atom results in stabilisation of the neighboring Pb atoms in the up position which is characteristic for Pb atoms in the 1/6 monolayer mosaic phase.

The core level photoemission and near edge X-ray photoabsorption spectra of guanine in the gas phase have been measured and the results interpreted with the aid of high level ab initio calculations. Tautomers are clearly identified spectroscopically, and their relative free energies and Boltzmann populations at the temperature of the experiment (600 K) have been calculated and compared with the experimental results and with previous calculations. We obtain good agreement between experiment and the Boltzmann weighted theoretical photoemission spectra, which allows a quantitative determination of the ratio of oxo to hydroxy tautomer populations. For the photoabsorption spectra, good agreement is found for the C 1s and O 1s spectra but only fair agreement for the N 1s edge.