The adsorption of no on Ni(111); an esdiad/leed study

The ESDIAD method (electron stimulated desorption ion angular distributions) has been combined with LEED (low energy electron diffraction) in a study of the adsorption of NO on Ni(111). For adsorption at 80 K, NO appears to be bonded with its molecular axis perpendicular to the Ni(111) surface at all coverages. Heating the 80 K layer leads to a striking structural change which we interpret as the formation of inclined or bent NO in the range 120 ? T ? 250 K. Upon adsorption at 150 K, only the bent form of NO is present at low coverages; at higher coverages at 150 K, the perpendicular form appears, in agreement with recent electron energy loss spectroscopy (EELS) data of Lehwald, Yates, and Ibach. When NO is coadsorbed with p(2 × 2) oxygen, the perpendicular form of NO dominates at all coverages and temperatures studied. Dissociated NO adsorbed at steps and defect sites on Ni(111) produces a welldefined hexagonal ESDIAD pattern.     

Evidence,by metastable quenching spectroscopy,that CO adsorbs on K/Ni(111) near potassium and lies flat or tilts when heated

By using metastable quenching spectroscopy (MQS) we show that if a K/Ni(111) surface with low K coverage (θ_K = 0.1 and 0.12) is exposed to low doses of CO (0.3 L), the CO molecules are adsorbed near the potassium atoms. Heating from 118 to 298 K results in spectral changes that indicate that the CO molecules closest to the potassium atoms tilt strongly or lie flat on the surface.     

The geometry of CO on Ru(001): Evidence for bending vibrations in adsorbed molecules

As a test of the utility of the ESDIAD method (Electron Stimulated Desorption Ion Angular Distributions) in studies of the geometry of adsorbed molecules, the chemisorption of CO on Ru(001) has been examined. Data previously reported using UPS (ultraviolet photoemission spectroscopy) and EELS (electron energy loss spectroscopy) have indicated that CO is terminally bonded to the Ru surface through the C atom, with the CO axis perpendicular to the surface. The ESDIAD results for CO confirm this orientation; for all CO coverages in the temperature range 90 K to ~ 350 K, the angular distributions of O⁺ and CO⁺ ESD ions are centered about the surface normal. The widths of the ion beams are temperature dependent; for both O⁺ and CO⁺, the half widths at half maximum, α, of the ion cones are ~16° at 300 K, and ~12° at 90 K. This temperature dependence, coupled with a simple model calculation, indicates that the dominant factors contributing to the width of the ESD ion beams are the CO surface bending vibrations, i.e., initial state effects. Thus, the data suggest that both the directions and widths of ESDIAD beams are determined largely by the structure and dynamics of the initial adsorbed state.     

Nickel carbonyl adsorption on the Ni(111) surface

Overlayers formed by the adsorption of Ni(CO)₄ in CO on the Ni(111) surface at 100 K were characterized using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At temperatures below 135 K, molecular nickel carbonyl adsorbs on the CO saturated Ni(111) surface as suggested by several observations. Vibrational transitions characteristic of molecular Ni(CO)₄ are dominant. The energy dependence of both the elastic and inelastic electron scattering cross sections are dramatically altered by Ni(CO)₄ adsorption. All of the mass spectrometer ionization fragments typical of molecular Ni(CO)₄ are observed in the narrow thermal desorption peak at 150 K. The inelastic scattering cross sections for both adsorbed nickel carbonyl and adsorbed CO on the Ni(111) surface suggest that a nonresonant dipole scattering mechanism is dominant.     

Adsorption of H2O on Al(111)

The adsorption of H₂O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H₂O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H₂O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH_ads and hydrogen, which leaves the surface as H₂. Following adsorption of H₂O onto oxygen-precovered Al(111), additional OH_ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H₂ desorbs at temperatures considerably higher than that seen for H₂O on clean Al(111). The general behavior of H₂O adsorption on clean and oxygen-precovered Al(111) (θ_O ? monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H₂O to form OH _adsis noted on the oxygen-dosed surface around room temperature.     

Oxygen and carbon monoxide adsorption on W(111): An investigation with angular resolved ESD,AES and LEED

Oxygen and carbon monoxide adsorption on clean W(111) surfaces have been studied by angular resolved ESD emission (ESDIAD). In addition, the specimen could be characterized in situ with AES and LEED. Adsorption was performed at room temperature. The electron stimulated desorption yielded O⁺ ions from the two investigated adsorption layers. Upon oxygen adsorption followed by subsequent annealing at least eight different ESDIAD patterns have been obtained. However, a convincing interpretation on the basis of the surface geometry can only be presented for three patterns produced without annealing as well as for one pattern at a very high annealing temperature. The difficulties are a consequence of complex structure changes which the surface undergoes in the intermediate annealing temperature range. This may influence the little known neutralisation probability of the desorbing ions. In this special case ESDIAD probably reflects in contrast to LEED a picture of some specific adsorption sites (minority species) and therefore, no clear correlation of the two techniques can be seen. ESDIAD from carbon monoxide shows four different patterns and supports the model of linear bonded CO molecules at room temperature with oxygen in the “standing up” position. At T > 900 K, CO starts to dissociate and results in similar ESDIAD patterns as obtained from O₂ adsorption.     

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.     

The adsorption of H2O on clean and oxygen-dosed silver single crystal surfaces

The adsorption of H₂O on the surface of a single-crystal sphere of silver with exposed (111), (100) and (112) facets has been examined using ESDIAD (electron stimulated desorption ion angular distribution), LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The purpose of the study was (a) to examine the influence of substrate geometry for adsorption of H₂O on a metal surface for which the adsorbate-substrate interaction is weak, and (b) to study the influence of a surface impurity, oxygen, on the surface chemistry and local bonding structure of H₂O on Ag. We have found no evidence for either long-range or short-range local bonding order for adsorbed H₂O at 80 K on any of the surfaces studied. This appears to be a consequence, in part, of the lattice mismatch between the Ag crystal structure and the two-dimensional H₂O ice crystal structure. Adsorbed H₂O reacts with preadsorbed oxygen to form OH species which are bonded with the molecular axis perpendicular to Ag(111) and (100) but “inclined” on (112) surfaces, as identified using ESDIAD. The “inclined” OH species are associated with atomic steps on the (112) surface.     

Oxygen chemisorption on Cr(110): II. Evidence for molecular O2(ads)

Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O₂(ads) vibrational frequency of 1020 cm⁻¹ and a six beam ESDIAD pattern are suggestive of super-oxo O₂(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.     

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: desorption of CO from Pt(111)

The first quantum-resolved angular distribution measurements of electronically-excited neutral molecules which have undergone electron stimulated desorption (ESD) are presented. Two-dimensional imaging of laser resonance-enhanced multiphoton ionization (REMPI) is used to obtain angular distributions of CO^* in the v=0 vibrational level of the metastable a³Π_r state desorbed from CO/Pt(111) by 350 eV electrons. For saturation CO coverages at 90 K, sharp Gaussian distributions peaked about the surface normal (6° ± 0.5° half-width at half maximum) are observed, consistent with previously reported data acquired by ESDIAD (ESD ion angular distributions). The (1 + 1) photon REMPI scheme for state-specific CO^* detection involves the b³Σ(v = 0) ← a³Π(v = 0) transition at 283 nm, followed by photoionization at the same wavelength. In this paper, the overall experimental technique for REMPI imaging of products from electron stimulated processes is discussed. Thus specific CO^* data as a function of coverage and temperature is presented for comparison with the ESDIAD results.     

Ion angular distributions in electron stimulated desorption: Oxygen and CO on W(111)

The ion angular distributions resulting from electron stimulated desorption (ESD) of oxygen and carbon monoxide chemisorbed on a tungsten (111) crystal have been determined. The O⁺ ions released during ESD of adsorbed oxygen exhibit three-fold symmetric angular distributions in orientational registry with the W(111) substrate. The CO⁺ and O⁺ ions released during ESD of a monolayer of CO are desorbed normal to the (111) surface. Models for both oxygen and CO adsorption are discussed. The data for CO are consistent with adsorption of CO in “standing up” carbonyl structures in the virgin and α-CO binding states.     

Enhancement of the ESDIAD method for imaging bond directionality in chemisorbed species

The ESDIAD method for imaging adsorbate bond directions by photographic observation of positive ion angular distributions during electron stimulated desorption suffers from inherent low contrast due to background effects. The use of a digital acquisition system designed to overcome this difficulty in ESDIAD measurement is presented. Measurements on a Ni(110) single crystal substrate show the presence of a significant background signal due to soft X-ray generation by electron impact. By subtraction of the background signal, a significant enhancement of positive ion signal-to-noise ratio is achieved in ESDIAD, converting the ESDIAD method into a high contrast, high resolution surface measurement technique. Quantitative studies of the soft X-ray background have shown it to be linearly dependent on electron current density and electron energy, with no change in angular shape. These properties permit an accurate background subtraction procedure to be employed to significant;y enhance the capability of the ESDIAD method.     

Angular distribution of the desorption of CO2 produced on Pt(111) surface at low temperatures

A new CO₂ formation process was observed in the CO oxidation over Pt(111) surface below 200 K. The desorption flux of the product CO₂, which is formed from the interaction between chemisorbed CO and adsorbed oxygen molecules O₂^2? (a), showed a very sharp angular distribution along the surface normal.     

ESDIAD studies of the structure of TiO2(110)(1 × 1) and (1 × 2) surfaces and interfaces in conjunction with LEED and STM

The TiO₂(110)-(1 × 1) surface and its reconstruction as a (1 × 2) form have been studied with low energy electron diffraction (LEED), electron stimulated desorption ion angular distribution (ESDIAD) and scanning tunnelling microscopy (STM). Oxygen ion desorption occurs within a lobe perpendicular to the (1 × 1) surface, changing to two off-normal lobes for the (1 × 2) reconstruction. This transformation in the ESDIAD pattern is consistent with the added Ti₂O₃ row model of the (1 × 2) reconstruction proposed by Onishi and Iwasawa. STM studies of the stoichiometric and electron irradiated surfaces reinforce the association of the O⁺ ESD contribution with majority sites at the surface. Adsorption of acetic acid on the (1 × 1) surface produces a (2 × 1) overlayed and induces a reconstruction of the underlying substrate. ESDIAD reveals H⁺ ions emitted off-normally from dissociatively adsorbed acetate, and along the surface normal from surface hydroxyls. Adsorption of acetic acid on the (1 × 2) surface does not modify the LEED pattern, but ESDIAD reveals H⁺ desorption with a weaker off-normal contribution consistent with the Ti₂O₃ model of the reconstruction.     

A TDS and HREELS study of CO adsorbed on a potassium promoted Fe(111) surface

The adsorption behavior of CO on a potassium promoted Fe(111) surface was investigated in the range from zero to several monolayers of preadsorbed potassium. TD spectra show that the presence of potassium decreases the amount of CO which is desorbed in the α (molecular) desorption state and increases the desorption temperature of this state. In addition, it gives rise to second, β (recombination) desorption state which is correlated to K desorption. The total CO uptake is comparable to that for the clean surface for precoverages of up to one monolayer, beyond this, however, it increases and at three potassium monolayers it is about twice the clean surface value. At K precoverages above 0.5 monolayer the initial sticking coefficient for CO is greatly reduced so that CO exposures of up to several thousand Langmuirs are required in order to saturate the surface. The three stretch frequencies which are observed in HREELS for CO adsorbed on clean Fe(111) are all affected by the presence of potassium. At potassium precoverages between zero and 0.5 monolayers these frequencies shift both in energy and relative intensity; however, between 0.5 and 1 preadsorbed potassium monolayers the spectra are greatly modified and now show only two losses in the CO stretch region. The lower-frequency one of these gives evidence for a close interaction of CO with the coadsorbed potassium.     

CO on Pt(111): TDS and ESD study

Electron stimulated desorption of ionic species from CO adsorbed on Pt(111) has been studied and comparison made with EELS results. The “on-top” site which, according to EELS data, fills first is observed to yield O⁺ ion. The bridge adsorption site appears to release CO⁺ during electron bombardment. Coadsorption of H₂ and CO was also examined and compared with the polycrystalline platinum case. Only very weak coadsorption effects are seen on the Pt(111) surface, as evidenced by presence of a weak low energy component associated with the O⁺ ESD energy distribution.     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.     

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.     

Thermal decomposition of CO on a stepped Ni surface

The adsorption-desorption behaviour of CO on the stepped Ni(S) [5(111) × (11̄0)] and the smooth Ni(111) plane are compared by using LEED, thermal flash desorption and AES. Above room temperature flash desorption from the Ni(111) face yields a single α peak characteristic of molecularly adsorbed CO whereas from the stepped surface in addition to the a peak α second desorption peak (β2) appears around 550°C which is assigned to associative CO desorption. If carbon and oxygen are separately chemisorbed on Ni(111) associative desorption of CO leads to a desorption peak around 350° C. It is concluded that steps lower the activation energy for CO decomposition but increase the activation energy for associative desorption.