Photoinduced desorption of potassium atoms from a two dimensional overlayer on graphite

CO chemisorption on Bi-modified Ni(100) surfaces, along with the structure and growth of vapor-deposited Bi adlayers on Ni(100), was characterized by Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), energy loss spectroscopy (ELS), UV photoelectron spectroscopy (UPS), work function measurements, and high-resolution electron energy loss spectroscopy (HREELS). Bi growth on Ni(100) at 500 K proceeds via a layer-plus-island (Stranski−Krastanov) growth mode and the gradual formation of a c(2 × 2) structure near monolayer coverage. Desorption of Bi from the first monolayer on Ni(100) occurs with an activation energy Ed = 290−240 kJ mol-1. Bi desorption from Bi multilayers has Ed = 200 kJ mol-1. Adsorbed Bi changed the work function of the Ni(100) surface only slightly, indicating an initial dipole moment of only −0.5 D and thus relatively little charge transfer between Bi and Ni compared to other modifier adlayers. CO chemisorption was used to probe the reactivity of Ni(100) surfaces modified by preadsorbed Bi adlayers, denoted as Bi/Ni(100). Only a small decrease (4 kJ mol-1) occurs for the CO adsorption energy as determined by CO TPD. Site-blocking effects dominate over electronic (ligand) effects on the surface chemistry of CO on Bi/Ni(100). A comparison of these results to those on Bi/Pt(111), where Bi has been used as a model inert site-blocking agent, indicates that Bi modifies the electronic structure of Ni(100) even less than on Pt(111). Therefore, Bi adatoms may allow useful probing of adsorption and reaction ensemble requirements on Ni surfaces that contain modifiers as adatoms.

Ziel dieser Arbeit war es, die Reaktivität von V2O3(0001) zu untersuchen. In dieser Arbeit wird sich zunächst mit dem epitaktischen Wachstum von V2O3-Filmen auf Au(111)und W(110) befaßt. Stöchiometrie und Geometrie der dünnen Filme wurden mit Röntgenphotoelektronenspektroscopie (XPS), Röntgenabsorptionsspektroskopie (NEXAFS) und Beugung niederenergetischer Elektronen (LEED) charakterisiert. Wir haben gezeigt, dass die Oberfläche zwei Terminierungen aufweist, die sich durch die An- bzw. Abwesenheit von zusätzlichen Sauerstoffatomen auf der Oberfläche unterscheiden. Diese Sauerstoffatome bilden Vanadylgruppen mit den Oberflächenvanadiumatomen, deren Streckschwingung mit Infrarotabsorptionsspektroskopie (IRAS) detektiert werden kann. Die elektronische Struktur des V2O3(0001) dünnen Filmes wurde mittels UV-Photoelektronenspektroskopie (UPS), XPS und NEXAFS untersucht. Wir haben bewiesen, dass die Bildung von Vanadylgruppen an der Oberfläche einen Metall-Isolator Übergang hervorruft. Für jede Oberflächenterminierung wurde ein elektronenenergieverlustspektrum (HREELS) gezeigt und mit einem Spektrum des isomorphischen Cr2O3 verglichen. Wasseradsorptionsexperimente zeigen, dass Wasser sowohl molekular als auch dissoziativ auf beiden Oberflächen adsorbiert. Die Dissoziationswahrscheinlichkeit hängt von der Terminierung und von der Bedeckung ab. Sie ist am höchsten bei großer Bedeckung auf der -V=O Oberfläche. CO2 Adsorption wurde mit UPS, XPS, HREELS und IRAS untersucht. CO2 physisorbiert auf der -V=O Oberfläche. Den IRA Spektren entnehmen wir, dass CO2 auf der -V Oberfläche als gewinkelte Spezies adsorbiert. Heizen dieser Spezies auf 200 K führt zu Karbonatbildung. Die Adsorption von CO verhält sich ähnlich wie die von CO2: nur kleine Menge adsorbieren auf der -V=O Oberfläche, während die -V Oberfläche viel reaktiver zu sein scheint. Winkelaufgelöste UPS Messungen deuten auf eine flache CO Adsorptionsgeometrie auf der -V=O Oberfläche hin. NEXAF- und IRA-Spektren zeigen dagegen, dass bereits bei 90 K sich CO2 auf der -V Oberfläche bildet.

The adsorption, desorption, and condensation of methanol on a clean Pd(100) surface has been studied in UHV at 77 K using high-resolution electron energy loss spectroscopy, thermal desorption spectroscopy, UV photoemission, work function measurements, and low energy electron diffraction. CH3OH adsorbed readily with a high initial sticking probability which remains nearly constant up to monolayer coverages (∼6×1014/cm2) indicating precursor state adsorption kinetics. The first 20% of this methanol spontaneously decomposes, but can largely recombine to desorb as methanol. The nature of this species and other decomposition products are discussed in part II. The remaining methanol chemisorbs nondissociatively having a bonding energy of 10.8 kcal/mol; no long-range order can be observed. The first physisorbed and later condensed layers are readily distinguishable and have desorption energies of 9.0 and 7.2 kcal/mode, respectively. The adsorption of methanol is accompanied by a continuous work function decrease to a final value of −1.60 eV after the deposition of ∼3 layers. A layer-by-layer growth mechanism is observed with physisorbed methanol showing first order kinetics and the absence of hydrogen bonding. This physisorbed layer still alters the surface dipole layer and shows some signs of additional interactions with the surface not present in the subsequent condensed layers. The subsequent condensed layers exhibit fractional order kinetics, which is interpreted to reflect hydrogen bonded ’’chains’’ of methanol molecules similar to that occurring in the melt of crystalline methanol.

Ethene adsorption on Pt 3Cu(111)

Interaction of water with the (1×1) and (2×1) surfaces of α-Fe 2O 3(012)

Decomposition mechanism of triethylindium (TEI) on a [formula omitted] surface studied by LEED, AES, TPD and HREELS

Adsorption and decomposition of t-butylphosphine (TBP) on a GaP(0 0 1)-(2 × 1) surface studied by LEED, HREELS, and TPD

The interaction and bonding of atoms and molecules on metal surfaces is explored under ultra-high vacuum conditions using a variety of surface science techniques: high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), work function measurements, and second harmonic generation (SHG). 164 refs., 51 figs., 3 tabs.

The adsorption of ethene (C2H4) has been studied on Pd(111) and ordered Sn/Pd(111) surface alloys using temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS), high-resolution electron energy loss spectroscopy (HREELS), and low energy electron diffraction (LEED). Two surface alloys were prepared by thermal treatment of Sn-films, which were vapor deposited on Pd(111) at room temperature. Depending on the preparation conditions, surface alloys giving a p(2×2) or a (√3×√3)R30° LEED pattern were produced. Below 250 K ethene adsorbs on pure Pd(111) in an undissociated – but substantially distorted – form relative to the molecular structure in the gas phase: HREELS suggests an adsorption in the di-σ bonded state. A π-bonded ethene species was, however, found to coexist with this strongly rehybridized form, probably as a result of hydrogen coadsorbed from the residual gas. TPD and annealing experiments followed by UPS and HREELS indicated that most of the adsorbed ethene desorbs reversibly in the temperature range between 150 K and 350 K, while a small amount dehydrogenates. After adsorption at room temperature, ethylidyne (≡CCH3) has been identified as the most important species. Alloying Pd(111) with Sn results in a decreasing ethene-substrate interaction with increasing Sn-content in the topmost layer of the substrate. Only π-bonded ethene was formed on both surface alloys and decomposition reactions were suppressed.

HREELS and TDS studies of NO adsorption and NO + H 2 reaction on Pt(100) surfaces

The interaction of atomic hydrogen with the Ag(111) surface was studied with low-energy electron diffraction (LEED), high-resolution electron-energy-loss spectroscopy (HREELS), thermal desorption spectroscopy, and work function measurement. The adsorption of atomic hydrogen at 100 K induces a reconstruction of the Ag(111) surface, as indicated by an intense (2\ifmmode\times\else\texttimes\fi{}2) (at 0.25--0.5 ML) and a mixture of (2\ifmmode\times\else\texttimes\fi{}2) and (3\ifmmode\times\else\texttimes\fi{}3) (at 0.5--0.6 ML) superstructures in LEED. The H-induced work function change (\ensuremath{\Delta}${\mathrm{\ensuremath{\Phi}}}_{\mathrm{max}}$=+0.32 eV) is uncharacteristically large for the close-packed surface and indicative of the surface structural change. HREELS data show that H occupies a single site of high symmetry for all coverage. The observation of two vibrational modes (one dipole active at 106 meV and the other not dipole active at 87 meV at \ensuremath{\sim}0.5 ML) allows an assignment of bonding at a threefold hollow site. Both modes show measurable shifts in energy with increasing coverage, indicating an appreciable H-H lateral interaction. Atomic hydrogen adsorbed on Ag(111) recombines and desorbs at a temperature \ensuremath{\sim}180 K, appreciably lower than those for Cu and transition metals. The properties of H adsorption on Ag will be compared to Cu and to transition metals.

The chemisorption and electrochemical activity of 2 ,5 -dihydroxythiophenol (DHT) and 2-(8mercaptooctyl)-1,4-benzenediol (DHOT) on well-defined Pd(111) surfaces were studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energyloss spectroscopy (HREELS), and electrochemistry (EC). Results confirm that DHT is chemisorbed in two discrete orientations such that at low concentrations, DHT is oxidatively bound to the surface through the diphenol and mercapto groups as quinonoid and S moieties, respectively, whereas at high concentrations, the molecule is coordinated oxidatively through the –SH group in a vertical S- η 1

The chemistry of methanol was explored on the vacuum annealed TiO2(110) surface, with and without the presence of coadsorbed water and oxygen, using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED). The vacuum annealed TiO2(110) surface possessed about 8% oxygen vacancy sites, as determined with H2O TPD. Although evidence is presented for CH3OH dissociation to methoxy groups on the vacuum annealed TiO2(110) surface using SSIMS and HREELS, particularly at vacancy sites, the majority of the adlayer was molecularly adsorbed, evolving in TPD at 295 K. Although no evidence of irreversible decomposition was found in the TPD, dissociative CH3OH adsorption at 135 K on the vacuum annealed TiO2(110) surface led to recombinative desorption states at 350 and 480 K corresponding to methoxys adsorbed at non-vacancy and vacancy sites, respectively. Coadsorbed water had little or no influence on the chemistry of CH3OH on the vacuum annealed TiO2(110) surface, however new channels of chemistry were observed when CH3OH was adsorbed on the surface after O2 adsorption at various temperatures. In particular, O2 exposure at 300 K resulted in O adatoms (via dissociation at vacancies) that led to increased levels of CH3O–H bond cleavage. The higher surface coverage of methoxy then resulted in a disproportionation reaction to form CH3OH and H2CO above 600 K. In contrast, low temperature exposure of the vacuum annealed TiO2(110) surface to O2 resulted in low temperature state of O2 (presumably an O2- species) that oxidized CH3OH to H2CO by C–H bond cleavage. These results provide incentive to consider alternative thermal and photochemical oxidation mechanisms that involve the interaction of organics and oxygen at surface defect sites.

In order to understand the mechanism of adsorbate-substrate and adsorbate-adsorbate interactions on catalytically important surfaces, the surface structures and thermal chemistry of molecular overlayers on Rh(111) and Rh(100) single crystal surfaces have been studied under ultra-high vacuum conditions. For C-H and C-C bond activations the bonding and reactivity of unsaturated hydrocarbons, acetylene, ethylene, and benzene, chemisorbed on Rh(100) at 100-800 K were analyzed by using high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). For N-O bond activation, the surface ordering, structure and activity of the (2 /times/ 2)-3NO and C(4 /times/ 2)-CCH/sub 3/ + NO monolayers on Rh(111) have been studied by dynamical LEED analysis and HREELS. Elucidation of adsorbate-adsorbate interactions was made possible by monitoring HREELS and work function changes upon coadsorption on Rh(111) and Rh(100) surfaces. By coadsorbing antiparallel dipole adsorbates, such as CO + hydrocarbons or CO + alkali metals, a new ordered, intermixed structure, can be formed. 292 refs., 60 figs., 17 tabs.

The adsorption and reaction of acetylene on both planar and faceted Ir(210) have been studied utilizing temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy (HREELS). Following adsorption of C2H2 at 300 K or 100 K, TPD data indicate that H2 is the dominant desorption product, and that decomposition of adsorbed C2H2 occurs in a stepwise fashion. Multiple carbon-containing species are formed on Ir(210) upon adsorption of acetylene at high coverage, which are different from those formed at low coverage. Our HREELS results show that the dominant surface hydrocarbon species formed at high coverage are mainly acetylide and ethylidyne while acetylide dominates at low coverage. In contrast to reaction measurements on an Ir organometallic complex that catalyzes cyclization of C2H2 to C6H6, no evidence for the cyclization reaction is found on Ir(210). The results are compared with adsorption and decomposition of C6H6 on Ir(210); as for C2H2, the dominant desorption product is H2, but there are differences in the reaction sequence. In addition, evidence has been found in TPD measurements for structure sensitivity in decomposition of acetylene over the clean faceted Ir(210) surface versus the clean planar Ir(210) surface, which is attributed to nanometer scale structures on the faceted surface. The HREELS data give complementary information to TPD and AES results and provide insights into the reaction mechanisms for acetylene surface chemistry.