Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.     

Desorption methods as probes of kinetics and bonding at surfaces

This paper is divided into two parts. Firstly, a review of desorption methods is presented, with emphasis on the use of temperature programmed desorption (TPD) and electron stimulated desorption (ESD) for understanding the bonding of adsorbed species to surfaces. Secondly, recent studies of the angular distribution of ESD ions from adsorbed layers on W(011) are discussed. The ESD of O⁺ ions from oxygen adsorbed on a stepped W(011) surface is shown to be sensitive to the presence of atom steps.     

Author index volumes 111–120

Adsorption of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment μ = (0.18 ± 0.02) × 10^?30C m and polarizability α = (6.3 ± 1.3) × 10^?40J V^?2m². Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2 × 2), p(2 × 2) and (1 × 1) structures. Bismuth is thought to behave in a similar way, but, unlike lead, forms a second monolayer which replicates the first. Electron spectroscopy shows that sub-monolayer bismuth removes the surface state (Swanson) peak and at monolayer coverage a new peak emerges and shifts with increasing coverage. Using Gadzuk's theory, this peak is tentatively attributed to the ²P level in bismuth adatoms which form a p(2 × 2) structure in the first and second monolayers. Its shift with coverage is ascribed to an increase in the local surface field. There remains the difficulty of reconciling the proposed occupation of the ²P level with the observed small positive charge on the bismuth adatom.     

Hydrogen adsorption on Pd(133) surface

An approach based on measurements of the total energy distribution (TED) of field emitted electrons is used in order to examine properties of the Pd(133) from the aspect of hydrogen adsorption. The most favourable sites offered to a hydrogen atom to be adsorbed are indicated and an attempt to ascribe the peaks of the enhancement factor R in the TED spectrum to the specific adsorption sites is made.      

The adsorption of CO and oxygen on Cu layers on a W(110) surface

The adsorption of CO, and to a lesser extent that of oxygen on Cu layers deposited on a W(110) surface has been investigated by thermal desorption. Auger, and XPS measurements. For CO the amount adsorbed decreases monotonically with Cu thickness from 1–5 layers. For O there is a slight increase for 1 layer, followed by a steep decrease up to 4 Cu layers where the amount adsorbed levels off. CO adsorption shifts the core levels of Cu (observed for 1 layer of Cu) to higher binding energy by 0.4 eV; the O 1s level of CO is also shifted to higher binding energy by 1.5 eV, relative to CO/W(110) suggesting that electron transfer from CO occurs but is passed on to the underlying W. For O adsorption there is very little shift in the Cu core levels or in the O 1s level, relative to O/W(110). Thermal desorption of CO at saturation coverage from Cu/W(110) shows desorption peaks at 195, 227 and 266 K, as well as small peaks associated with CO desorption from clean W, namely a peak at 363 K and β-desorption peaks at 1080 and 1180 K. As CO coverage is decreased the 195 and 227 K peaks disappear successively; the W-like peaks remain unchanged in intensity. It is argued that the latter may be due to adsorption on bare W at domain boundaries of the Cu overlayer, while the 190–266 K peaks are associated with adsorption on Cu, but probably involve reconstruction of the Cu layer. For n = 2–8 a single but composite peak is seen, shifting from 180 to 150 K as Cu thickness increases as well as a minor peak at 278 K, which virtually vanishes on annealing the Cu deposit at 850 K. The effect of tungsten electronic structure on the behavior of adsorbates on the Cu overlayers, as well as similar effects in other snadwich systems are discussed.     

Application of synchrotron radiation to investigation of the mechanism of increase in the yield of alkali metal ions in electron-stimulated desorption

Core-level photoelectron spectroscopy with synchrotron radiation (hv = 140 eV) has been applied to study the variation in the Si⁺ charge state in silicon films deposited on the W(100) surface after thermal annealing of the substrate. The purpose of this study is to check the mechanism responsible for the sharp increase in the yield of Na⁺ ions in electron-stimulated desorption from a sodium layer adsorbed on the Si/W(100) surface after high-temperature annealing. The evolution of the W 4f _7/2 and Si 2p photoelectron spectra and the valence band photoemission spectra is investigated for two silicon coverages (1 and 3 ML) on the W(100) surface in the temperature range 300<T<2200 K. It is shown that annealing of 1 ML Si on the W(100) surface results in the formation of a W-Si covalent bond, which can weaken the Si-Na bond and lead to an increase in the equilibrium distance X ₀ and, hence, to an increase in the yield of Na⁺ ions in electron-stimulated desorption. The variation in the photoelectron spectra under annealing of 3 ML Si differs from that observed after annealing of 1 ML Si in the direction of charge transfer, thus correlating with the opposite effect of annealing of 3 ML Si/W on the Na⁺ yield in electron-stimulated desorption.     

Resonances in the cesium atom yield in electron-stimulated desorption from tungsten coated with a germanium monolayer

The yield and energy distributions of Cs atoms emerging from cesium layers, which are adsorbed on tungsten coated with a thin germanium film (1-to 2-monolayers thick), have been measured as a function of the incident electron energy, the amount of adsorbed cesium, and the substrate temperature. The measurements were performed by the time-of-flight technique with a surface ionization detector. At low cesium coverages (Θ < 0.1), the Cs atom appearance threshold at a substrate temperature T = 160 K is ～24 eV, which correlates with the Cs 5s-level ionization energy. As the electron energy is increased, the yield passes through a broad plateau and reaches saturation. The signal intensity in the plateau region decreases gradually with increasing cesium coverage and tends to zero for Θ > 0.14. For Θ ≥ 0.15, the cesium atom appearance threshold shifts to ～30 eV, which corresponds to the Ge 3d-level ionization energy and the plateau is replaced by a resonance peak at ～38 eV, which can be identified with the ionization energy of the W 5p _3/2 level. This peak is observed only for Θ < 0.3 and T = 160 K. For Θ ≥ 0.3, there appears a resonance peak at ～50 eV, and for Θ ≥ 0.5, another resonance peak appears at ～80 eV. These peak positions correlate with the ionization energies of the W 5p _1/2 and W 5s levels, and their intensity is maximum at Θ = 1. The Cs atom energy distributions for Θ < 0.15 consist of a bell-shaped peak with a maximum at ～0.55 eV, and those for Θ ≥ 0.15 contain two nearly resolved maxima, a broad one peaking at ～0.5 eV and a narrow one at ～0.35 eV. The above results argue for the existence of three channels of Cs atom desorption. One channel involves reverse motion of the Cs²⁺ ion; another channel, neutralization of the adsorbed Cs⁺ ion following the Auger decay of a vacancy in the Ge atom; and the third channel involves desorption of a CsGe molecule as it is repelled from a W core exciton.     

Interactions of BaO with mixed-metal substrates: W---Ir

Surface studies of BaO on W---Ir alloy substrates of different composition show work functions lower for the alloys than on either pure W or pure Ir. In particular, the lowest work function ( 1.8 eV) appears near the 50% W, 50% Ir concentration. Peak height characterizations and energy shifts of interatomic Auger lines indicate an adsorbate-substrate interaction is responsible for the function lowering. The interaction consists of the O of the adsorbed Ba⁺O⁻ molecule receiving electrons from the W and the Ba from the same molecule contributing electrons to the Ir. The resulting increase in surface dipole (as observed from interatomic O 2p and Ba 6s peak heights) is in very good agreement with the actual work function lowering. If the substrate consists of significant “patches” of W and Ir, then the Ba and O from the same molecule cannot interact with the different substrate components. When this happens, neither is the adsorbate-substrate interaction seen in the interatomic Auger spectra nor is any work function-lowering observed. The minimum in work function near 50% W, 50% Ir is the point where the dipole lowering (i.e., charge transfer) is most efficient because at thatconcentration there is one Ir atom available for each Ba atom and one W atom available for each O atom on the surface.     

Hydrogen adsorption on a clean palladium ribbon; Thermal desorption and work function measurements

The adsorption of hydrogen on a palladium ribbon has been studied by thermal desorption and work function measurements. It has been established that several heating cycles of the sample covered with hydrogen, up to 700 K lead to the repeated appearance of hydrogen thermal desorption peaks. Analogous experiments of adsorption and repeated heating cycles up to 700 K have shown work function changes decrease to zero as a result of heating and an increase again almost up to the initial value following cooling, in a much shorter time than that required for adsorption. The experimental results show that only a small part of the adsorbed hydrogen is desorbed in the temperature range of the thermal desorption peak. The major part of adsorbed hydrogen penetrates below the surface which leads to a nonequilibium increase of the bulk concentration.     

Direct reaction of adsorbed molecular oxygen with hydrazine on a clean Pt(111) surface

The oxidation of hydrazine on the clean Pt(111) surface has been investigated by temperature-programmed reaction spectroscopy (TPRS) in the temperature range 130–800 K. Direct reaction of molecular oxygen is observed on the Pt(111) surface for the first time, as indicated by the desorption of nitrogen beginning at 130 K with a maximum rate at 145 K, below the molecular oxygen dissociation temperature. Direct reaction of hydrazine with adsorbed molecular oxygen results in the formation of water and nitrogen. With excess hydrazine, all surface oxygen is reacted, forming water. When only adsorbed atomic oxygen is present, the low-temperature nitrogen yield decreases by a factor of 3 and the peak nitrogen desorption temperature increases to 170 K. No high-temperature (450–650 K) nitrogen desorption characteristic of nitrogen atom recombination is seen, indicating that during oxidation the nitrogen-nitrogen bond in hydrazine remains intact, as observed previously for hydrazine decomposition on the Pt(111) surface and hydrazine oxidation on rhodium. Two water desorption peaks are observed, characteristic of desorption-limited (175 K) and reaction-limited (200 K) water evolution from the Pt(111) surface. For low coverages of hydrazine, only the reaction-limited water desorption is observed, previously attributed to water formed from adsorbed hydroxyl groups. When excess hydrazine is adsorbed, the usual hydrazine decomposition products, H₂, N₂ and NH₃, are also observed. No nitrogen oxide species (NO, NO₂ and N₂O) were observed in these experiments, even when excess oxygen was available on the surface.     

The adsorption of CO on the (100) plane of tungsten; thermal and work function measurements

Thermal desorption and work function measurements indicate that a largely molecular layer, with some dissociation, is formed at 80–100 K, with an increase in work function of 0.55 eV. The coverage in this layer is 11.5 × 10¹⁴ molecules/cm², or CO/W = 1.15. On heating, equal amounts of a β precursor, possibly dissociated, and a molecular α species are formed at ≈300 K, with abundances of 5 × 10¹⁴ molecules/cm² each. The α desorption is complete at 360 K. The β precursor evolves on heating without desorption in the range 400–700 K as indicated by work function decreases, to β-CO, which is almost certainly dissociated. This change occurs at lower temperatures for low coverages. Thermal desorption shows 3 peaks, which have been traditionally labelled β_{1, β2}, and β₃ at 930, 1070, and 1375 K. Of these only β₃ corresponds to a well defined state. Readsorption after heating to 950 or 1150 K results in a doubly peaked spectrum at 1070 and 1375 K. The β₁ and β₂ peaks obey complex desorption kinetics, probably corresponding to desorption and rearrangement. The coverage of β₃ is 2.5 × 10¹⁴ molecules/cm², suggesting that the c(2 × 2) LEED pattern corresponds to occupany of every other unit cell by a C or an O atom. For coverages ? 1.5 × 10¹⁴ molecules/cm² β₃ desorption obeys second order kinetics with an activation energy of 83 ± 3 kcal/mole. For β₃ the work function decreases from the clean W value by 0.1 eV, suggesting adsorption of C and O in the center of the W unit mesh, below the surface layer of W atoms. Readsorption on β and β precursor layers leads to formation of electropositive α-CO, with a multiply peaked thermal desorption spectrum, indicating the existence of different binding sites. Adsorption-heatingreadsorption, -heating-readsorption sequences indicate that additional changes in the α desorption spectrum occur, suggesting reconstruction in the β layer.     

Adsorption studies of trichloroethylene (TCE) on MgO(100)/Mo(100)

Employing ultraviolet photoelectron spectroscopy (UPS, He I), the more surface sensitive metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) measurements of the adsorption properties of the pollutant trichloroethylene (TCE) on thin MgO(100) films, grown on a Mo(100) single crystal, have been investigated. From TPD spectra of different coverages it is concluded that TCE interacts only weakly with MgO, which is attributed to physisorption. For increasing coverages a change from one peak to two peaks in the TPD spectra, one at higher, the second at lower temperatures with respect to the single peak is detected. Additionally, the observation of a local minimum for the work function (WF) for both MIES and UPS spectra is presented. Such a local minimum has been reported previously for the adsorption of metals with outer s valence electrons on transition metal substrates and adsorption of metals with outer s valence electrons on metal oxide films. Herein, we present the first WF minimum observed for a system of organic molecules adsorbed on an insulating surface. Two different models are discussed in order to understand the presented results.     

Esdiad from CO on W(100): Evidence for non-vertically adsorbed CO

Mass-discriminating measurements of the angular distributions of electron-stimulated ion desorption (ESDIAD) from CO adsorbed on W(100) and coadsorbed with C and O have been performed. The O⁺ beams indicate normal and off-normal (by 5 to 13°) desorption which is interpreted as due to CO molecules bound in symmetric and in two types of unsymmetric bridges. Preadsorption of C suppresses the vertical state, while oxidation of the surface suppresses the off-normal states.     

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25-4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.     

Interactions between NO and CO on Pt(111)

Temperature programmed desorption (TPD) of coadsorbed NO and CO on Pt(111) shows that no reaction occurs (less than 2%) up to the desorption temperature of NO. At 100 K, adsorption is competitive, but neither gas displaces the other from the surface. Coadsorbed CO causes the NO desorption temperature to be lowered by as much as 100 K, but NO does not affect the CO desorption temperature. TPD spectra for NO depend on which gas is adsorbed first, indicating that equilibrium between species is not established on the surface during desorption. Electron energy loss spectra show that the vibrational spectrum of each gas is only weakly affected by the other. When NO is adsorbed first, CO does not affect the ratio of bridged and terminal NO but lowers the frequencies of the bridged NO by approximately 50 cm^?1 and lowers the intensities of vibrational peaks of both species by a factor of about four. When CO is adsorbed first, the ratio of terminal to bridged NO increases for given coverage of NO, and the frequency of the bridged NO remains at the pure NO value. These results are explained in terms of CO island formation, repulsive interactions between NO and CO, and low adsorbate mobilities.     

A comparison of the electronic properties of CO adsorbed on a thick copper film,and on a copper monolayer deposited on Ru(0001), as determined by metastable quenching spectroscopy

We use thermal desorption and metastable quenching spectroscopy to study the properties of CO chemisorbed on Ru(0001); on a Cu monolayer deposited on Ru(0001); and on a Cu film. We find that CO binds more strongly to the Cu monolayer than the Cu film; that the Penning ionization peaks of CO are more prominent on the film; and that addition of CO causes a greater lowering of the work function of the Cu film, than that of the Cu monolayer. We also observe that the 2π¹ emission of CO adsorbed on Ru(0001) has a double peak; that the saturation of Ru(0001) with CO seems to lead to CO tilting; and that the Penning spectrum of CO on Cu has two small features which have not been observed for CO adsorbed on other metals.     

The adsorption of CO,O2, and H2 on Pt(100)-(5×20)

The adsorption/desorption characteristics of CO, O₂, and H₂ on the Pt(100)-(5 × 20) surface were examined using flash desorption spectroscopy. Subsequent to adsorption at 300 K, CO desorbed from the (5×20) surface in three peaks with binding energies of 28, 31.6 and 33 kcal gmol^?1. These states formed differently from those following adsorption on the Pt(100)-(1 × 1) surface, suggesting structural effects on adsorption. Oxygen could be readily adsorbed on the (5×20) surface at temperatures above 500 K and high O₂ fluxes up to coverages of $\text{2}\text{3}$ of a monolayer with a net sticking probability to ssaturation of ? 10^?3. Oxygen adsorption reconstructed the (5 × 20) surface, and several ordered LEED patterns were observed. Upon heating, oxygen desorbed from the surface in two peaks at 676 and 709 K; the lower temperature peak exhibited atrractive lateral interactions evidenced by autocatalytic desorption kinetics. Hydrogen was also found to reconstruct the (5 × 20) surface to the (1 × 1) structure, provided adsorption was performed at 200 K. For all three species, CO, O₂, and H₂, the surface returned to the (5 × 20) structure only after the adsorbates were completely desorbed from the surface.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

Vibrational characterization of carbon monoxide adsorption on sulfur modified Ni(100) surfaces

Carbon monoxide adsorption has been studied on a series of presulfided Ni(100) surfaces using vibrational spectroscopy. The sulfided Ni(100) surfaces were characterized using Auger electron spectroscopy and low energy electron diffraction, binding states were isolated by heating CO-dosed surfaces to prescribed temperatures, corresponding to the desorption temperatures of the CO. Adsorption of CO on Ni(100) with a p(2 × 2) array of sulfur lead to CO stretching frequencies of 1740 and 1930 cm^?1 corresponding to desorption temperatures of 370 and 290 K, respectively. Adsorption of CO into the c(2 × 2)S structure resulted in a CO stretching frequency of 2115 cm^?1 and a desorption peak near 140 K. The binding sites on the p(2 × 2)S structure were interpreted as metal four-fold hollows and bridging sites. The high frequency state was interpreted as weak bonding into the four-fold hollow with back donation into the π¹ orbital on CO restricted by stearic hindrance due to adsorbed sulfur. Both the thermal desorption and vibrational results indicated that local CO-sulfur interactions are dominant on the presulfided Ni(100) surface in the coverage range studied.     

Adsorption of oxygen on W(100): Adsorption kinetics and structure

The adsorption of oxygen on W(100) single crystal surfaces is studied by Auger electron spectroscopy (AES), flash desorption, low-energy electron diffraction (LEED) and retarding field work function measurements with the aim of obtaining a better understanding of the adsorption kinetics and of the structures of the adsorbed layer. The AES results reveal step-wise changes of the sticking coefficients in the coverage range 0 to 1, and activated adsorption at higher coverages. Upon room temperature adsorption a series of complex LEED patterns is observed. In layers adsorbed at 1050 K and cooled to room temperature, the well-known p(2 × 1) structure is the first ordered structure observed. This structure shows a reversible order-disorder transition between 700 K and 1000 K and is characterized by a work function which is lower than that of the clean surface. Heating room temperature adsorbates changes their structure irreversibly. At temperatures below 750 K some new structures are observed. Combining the results obtained in this study with other published work leads to a considerable revision of the previously accepted model of the adsorption of oxygen on W(100).