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.     

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H₂ have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β₂, β₁, and β₀ were detected after adsorption at 250 K. The population of β₂ and β₁ states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O⁺ and CO⁺ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H⁺ signal was recorded following H₂ adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H⁺ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H₂ TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β₂ state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H⁺ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from 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.     

ESD on metal oxides and oxygen adsorption layers: Evidence for different mechanisms in electron stimulated desorption

Electron stimulated desorption (ESD) of several metal oxides (W, Mo, Ta, Fe, Ni, and Cu oxide) and oxygen adsorption layers on Ni and Fe has been investigated. ESD of O⁺ from oxides can be easily understood in terms of the Auger decay model for ESD proposed recently by Knotek and Feibelman (KF). The measured O⁺ desorption from oxygen adsorption layers on Fe and Ni however, can hardly be explained in the frame of the KF model. There is strong evidence that at least two different ESD mechanisms are operative for ionically and covalently bound oxygen, respectively.     

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.     

Adsorption and desorption of oxygen and carbon monoxide on the Si(111) surface studied by ion scattering spectroscopy

The adsorption of O₂ and CO on the Si(111) surface was studied by low-energy helium ion scattering. The adsorption consists of a fast adsorption stage followed by a much slower Sorption process. In the final uptake region CO has a faster rate of increase than O. There is no evidence of He⁺ scattering from C atoms. This fact excludes the CO molecule having its axis parallel to the surface. A comparison of the intensities of the substrate (Si) signals, for the same recorded oxygen content on the surface, shows that carbon monoxide shadows the Si atoms more than oxygen does. An increase in the oxygen signal was observed even after exposures in the range of 10¹⁴–10¹⁵ molecules cm^?2. No substantial diffusion of CO into the bulk can be deduced from these results. Desorption of oxygen by He⁺ ions was observed by following the adsorbate and substrate signals as a function of time. The sputtering cross-section has a maximum for an impact angle of 25° relative to the surface.     

Oxygen adsorption on a Pd(111) surface

The interaction of oxygen with Pd(111) has been studied by means of AES, ELS, thermal desorption (TD), electron stimulated desorption (ESD) and work function measurements. It was found that a very small part ( ～ 2–3%) of the available adsorption sites are contributing to the O⁺ electron stimulated yield, the population of the latter being accompanied by enormously large work function changes (up to ～ 0.9 eV). A mechanism of adsorption and depopulation of these sites involving oxygen bulk and surface diffusion has been proposed.     

Adsorption of hydrogen and carbon monoxide on platinum

TDS spectra and ESD ion energy distributions have been measured for coadsorption of H₂ and CO on recrystallized platinum. Sample exposure to a 90% H₂?10% CO gas mixture results in appearance of structurein both TDS spectra of H₂ and CO. Coadsorption also results in an O⁺ ion energy distribution which is much narrower compared to the distribution resulting from pure CO adsorption. These results are interpreted as evidence for the formation of an HCO surface complex.     

Theory of multiple-pass two-photon Doppler-free spectroscopy

Electron stimulated desorption (ESD) of CO on Ru (001) leads to emission of CO⁺ and O⁺ ions from the same adsorption states. Following earlier work on this system, a correlated study of its ESD behaviour together with LEED, Δϕ, and TPD measurments has been carried out. The filling of the two states found earlier can be seen in ESD also. The dividing line between the high and low energy states is different in room temperature adsorption on the one hand, and desorption or equilibrium measurements at elevated temperatures on the other, which is explained by incomplete attainment of equilibrium in the layer under the first condition. The behaviour of the ESD ion currents with coverage showstthat part of the molecules occupying high energy sites in the ordered layer of intermediate coverage are shifted to less favourable sites at higher coverage, so that the full layer consists of a mixture of states due to the occupation of different sites in the compressed layer, possibly accompanied by lateral interactions. ESD also suggests that the high energy sites are the on-top sites rather than the threefold sites.     

Coadsorption of oxygen and carbon monoxide on tungsten: Desorption spectra,electron stimulated desorption and field emission microscopy

CO/W desorption spectra are characterized by an α state and multiple β states; using electron stimulated desorption (ESD) the α state was shown to comprise two sub-states, α₁ and α₂. In this paper the consecutive interactions of O₂ and CO on W are investigated using ESD, flash desorption and field emission microscopy (FEM).Desorption spectra show that the α-CO state is displaced by O₂, in two stages. The ESD probe provides an identification of the first stage with the removal of the α₁-CO state, and energy analysis of ESD ions reveals a large energy shift (～ ? 1.5 eV) during O₂ coadsorption which can be attributed to an incresae in the α₁-CO WC bond length of ～ 0.15 Å. During this O₂-induced displacement, the two β peaks converge into a single peak at the β₁ position; this is ascribed to adatom interactions in the mixed O and C adlayer. Isotope exchange experiments with ²⁸CO and ³⁶O₂ reveal (i) no exchange in the α-CO states, and (ii) complete exchange in the β-CO states, which is consistent with dissociative adsorption in the latter. The amount of coadsorbed O₂ is estimated from these results, and from FEM data: a full monolayer of O adatoms can be coadsorbed on CO-saturated W, but CO pre-adsorption inhibits the formation of W oxides. The β₁-O₂ (ESD active) state also forms on the CO-covered surface: this state is identical in population, ESD cross section and ion energy distribution to β₁-O₂ on clean W, and retains its identity in the mixed layer (it does not undergo isotopic exchange). CO₂ desorption spectra from the mixed layer were also characterised, complete isotopic scrambling being observed.Pre-exposure of tungsten to O₂ inhibits CO adsorption: a monolayer of O₂ is sufficient to prevent CO adsorption, and at low O₂ coverages, every O₂ molecule preadsorbed prevents one CO molecule from adsorbing. Isotopic exchange is again complete in the β states, and a lateral interaction model for desorption kinetics, based on dissociative adsorption in the β-CO state, quantitatively describes the CO desorption spectra.     

Kinetic theory description of electron stimulated desorption

A first principles kinetic theory is developed, applicable to angular dependent emission in Electron Stimulated Desorption. Expressions for the ionic and neutral atom ESD cross sections are formulated and applied in a model calculation of O⁺ emission from W(111). Strong focussing of the outgoing ions was found, with the use of a model ion-solid potential in which the substrate was free of excitation. Off-axis spot groups were simulated. The peak ion energies obtained with this model are, however, small compared to experimental energies for the high coverage case. The need to introduce substrate excitations to describe this case is discussed.     

Electron stimulated desorption ion energy distribution (ESDIED) and surface structure: O on W(100)

The yield and energy distributions of O⁺ ions ejected by electron stimulation into selected angular regions is studied as a function of oxygen exposure and annealing temperature of the adsorbate on W(100). Yield and energy distributions are related to coverage and structure via AES, LEED and ISS which indicates that ESDIED measurements supply useful information on adsorbate bonding.     

Electron stimulated desorption of carbon monoxide from a (111) niobium surface

Electron stimulated desorption of CO from the (111) face of a Nb single crystal produced both CO⁺ and O⁺ ions after adsorption at 150°K on a clean surface. When the surface was heated to above 250 °K only O⁺ ions were observed, and this current disappeared as the temperature was increased to 700 °K. Readsorption (at 150 °K) was inhibited following the 700 °K heating. These data indicate the formation on heating of a tightly bound surface phase with very low ionic desorption cross section. Threshold energies for CO⁺ and O⁺ ion production were 10.0 ± 0.5 eV and 19.0 ± 0.5 eV, respectively. The cross section for electron stimulated depopulation of the O⁺ producing phase was (4 ± 1) × 10^?18 cm² for 100 eV electrons.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

Study of lithium adsorption on the TiO2 surface by electron-stimulated desorption

This paper reports on a study of electron-stimulated desorption (ESD) of O⁺ and Li⁺ ions from titanium dioxide as a function of the preheating temperature T and of the concentration of lithium adsorbed at 300 K, which was carried out with a static magnetic mass spectrometer combined with a retarding-field energy analyzer. For T>1500 K, the TiO₂ surface undergoes irreversible rearrangement. At temperatures from 300 to 900 K and at lithium coverages Θ<1, the ESD cross sections of the O⁺ and Li⁺ ions vary in a reversible manner with temperature, while for lithium coverages Θ>1, the changes in the Li⁺ and O⁺ ESD cross sections become irreversible. For θ<1, the appearance threshold of the Li⁺ and O⁺ ions is 25 eV, whereas for θ>1, the ESD threshold of Li⁺ ions shifts to 37 eV.     

Surface coverage measurements by sims for CO adsorption on a number of metals and for CO and H2S CO-adsorption on Ni(110), (100) and (111)

A detailed study of CO adsorption on Ni(100) utilizing static SIMS and a comparison of the data with surface coverage data from the literature shows that there is a linear relationship between CO coverage and the peak intensity ratios (MCO⁺/M⁺ and M₂CO⁺/M⁺₂) of the CO-containing secondary ions, in the region of coverage below which the adlayer becomes compressed. Adsorption isobares were obtained using the intensity ratios and from these, adsorption isosteres were derjved to give heats of adsorption as a function of coverage. These data are in very close agreement with the literature. Confirmatory data were obtained for this relationship for CO adsorption on polycrystalline Ni, Pd, Pt and Cu and Cu(100). The application of this technique of surface coverage measurements to a study of the extent to which H₂S coadsorption reduces the coverage of adsorbed CO on Ni(110), (100) and (111) shows that these faces are poisoned in the order (100) > (111) > (110). Surface coverage measurements on the non-closepacked (110) face are affected by the apparent insensitivity of SIMS to adsorbates located in the “channels”.     

Theory of penning ionization of adsorbed Co by metastable helium (21S, 23S) beams

Theoretical analysis of Surface Penning Ionization (SPI) is reported and applied to the process He¹ + CO → He + CO⁺ + e^? for CO molecules adsorbed on a Pd(111) surface. Potential curves, ionization rates, and angular distributions of the emitted electrons are calculated. The results exhibit the extreme surface sensitivity of SPI and indicate its usefulness in studying the electronic and geometrical properties of adsorbates.     

Static sims studies of adsorbate structure: II. CO adsorption on Pd(111); adsorbate-adsorbate interactions on Ru(001), Ni(111) and Pd(111)

In a study of CO adsorption on Pd(111) it is shown that the secondary ion mass spectrum contains information on both adsorbate site geometry and adsorbate coverage. The fractional yields of PdCO⁺, Pd₂CO⁺ and Pd₃CO⁺, as a function of CO coverage are correlated with the changing site geometries suggested by reflection IR data. A relationship between secondary ion emission and the adsorbate-adsorbate interactions revealed by IR and EELS is also demonstrated for CO adsorption on Ru(001), Ni(111) and Pd(111).     

ESD and IID study of CO on nickel

Electron and ion induced desorption has been used to study release of ionic species from CO on nickel. O⁺, CO⁺, and O^? are found to be released by electron impact. Thermal desorption spectra indicate the presence of two binding states. The O⁺ species is observed to be released from state 1, the state of higher binding energy, while state 2, a more loosely bound state is the source of CO⁺. O^? species appear to be released from both states. The cluster ions NiCO⁺ and Ni₂CO⁺ released by argon ion impact were observed to originate from state 2.     

电子束引起的残余含氧气体在镍(001)面上的吸附

当一束具有一定能量和强度的电子束轰击超高真空系统中残余的水汽、一氧化碳和二氧化碳时,将导致这些气体分子通过如下反应:H₂O→O_ad+H₂,CO₂→O_ad+CO,CO→O_ad+C_ad分解并共吸于镍表面。碳和氧的原子各自占据镍(001)面部份四重吸附位置,形成结构为p(2×2)或c(2×2)的许多独立的吸附畴,电子束轰击促进畴的成核、长大、连结和有序化。当氧和碳的原子占据了镍(001)面约一半的四重吸附位后,上述吸附反应将与导致氧和碳的脱附反应:C^*+O_ad→CO,O^*+C_ad→CO平衡,氧化镍与碳化镍开始成核。由于残余含氧气体中氧的含量超过碳,氧化镍成核占优势,使碳的吸附被排斥,已吸附的碳被排挤,形成电子束斑内氧高碳低、束斑外碳高氧低的“互补”分布。电子束轰击过程中碳的俄歇峰形的变化反映着碳原子与基底原子的不同结合状态。电子束的解离效应在吸附的初始阶段起重要作用,而其热效应对氧化镍的长大起重要作用。 关键词：