Leed,auger, and electron energy loss studies of Ni epitaxially grown on Cu(100)

Nickel was epitaxially deposited onto a clean, flat Cu(100) surface. Low energy electron diffraction I(E) curves were recorded for 0.6, 1.1, and 2.7 monolayer (ML) Ni coverage. Multilayer relaxation was considered in theoretical calculations, which were compared with experiment by means of the R|ΔE| factor. The estimated relaxations of the first and second interlayer spacings are estimated to be − 2% and + 1.5% for clean Cu(100), − 2% and − 1.5% for 1 ML Ni coverage, relative to the bulk Cu interlayer spacing of 1.81 Å, and −1% and 0% for 3 ML Ni coverage, relative to the bulk Ni spacing of 1.76 Å. Decreasing the surface Debye temperature of the Ni layer to 268 K from the bulk value of 440 K improves the agreement between theory and experiment. The optimum inner potential values are − 9 and − 10 eV for clean Cu(100) and Ni on Cu(100), respectively. Auger electron spectroscopy was used to determine the thickness of the Ni films, and LEED indicates layer-by-layer growth until about 4 layers, when the LEED spots begin to spread, indicating island formation. Electron energy loss spectra were obtained with primary electron energies of 150 and 300 eV. The 3p core ionization transition was clearly observed after 0.5 ML Ni coverage. Peaks at 3.8 and 7.5 eV for clean Cu are ascribed to interband transitions, and shift to higher energy with Ni coverage. Peaks at 10 and 16 eV for clean Cu (ascribed to an interband transition and a surface plasmon, respectively) disappear with Ni coverage. Bulk plasmon peaks at 19 and 27 eV remain unshifted with Ni coverage. The effect of 0.9 and 1.3 ML Ni coverage of Cu(100) on the chemisorption of Co and oxygen was also studied. The behavior of the surface towards oxygen chemisorption was similar to that of the pure Ni surface. For a large exposure of oxygen (50 L and more) the EEL and Auger spectra are very similar to those observed for NiO. In the case of CO, for submonolayer Ni coverage, the surface shows a more Cu-like behavior, while for larger Ni coverage (a monolayer and more) there is a great similarity with the behavior of the pure Ni(100) surface.     

CO adsorption on alkali-metal covered Ni(100)

The effect of preadsorbed alkali metal atoms Na, K and Cs on CO adsorption on Ni(100) has been studied using Auger spectroscopy and thermal desorption. It was found that the presence of alkali metals causes an appearance of several more tightly bound states in the CO thermal desorption spectra. The observed difference in carbon and oxygen Auger peak line shape on a bare and alkali modified Ni(100) is indicative that the presence of alkali adatoms induces CO decomposition on the Ni(100) surface. The fraction of dissociated CO increases with the amount of alkali adatoms present. At the same overlayer coverage the dissociation probability increases in the sequence Na, K, Cs. A comparison of the strength of the promoting effect on CO dissociation with the changes in the surface electron density in the presence of alkali adatoms has shown that at low overlayer coverages the electronic factor plays a major role in explaining the action of the surface modificators.     

Electron energy loss and Auger study of epitaxially grown Cu on Ni(100)

Auger and electron energy loss spectra have been measured on films of Cu epitaxially grown on Ni(100). The films were prepared under UHV conditions using a quartz crystal for monitoring the deposition rate. LEED measurements were taken to determine the orientation of the films. The presence of a monolayer of Cu on Ni(100) is enough to suppress the 3p-3d transition on the surface of the sample. The electron energy loss spectra were studied as a function of the primary electron energy (50 to 300 eV). The experimental results were qualitatively analyzed using recent theoretical calculations of Cooper and co-workers. The effect of a small Cu coverage on Ni(100) on the chemisorption of CO and O₂ was also studied. A strong suppression of CO chemisorption at room temperature was observed. In the case of O₂, large exposures are necessary in order to observe a significant amount of oxygen on the surface. The absence of any appreciable chemisorption on the surface of the metal is attributed to the lack of empty d-surface states.     

Oxygen adsorption on Ge(111) surface: I. Atomic clean surface

Oxygen adsorption on a clean Ge(111) surface has been studied in the temperature range 300–560 K by means of Auger electron spectroscopy (AES), thermal desorption (TD), work function (WF) measurements, and electron energy loss spectroscopy (ELS). The adsorption and WF kinetics at 300 K exhibit a shape different from those observed at higher adsorption temperatures. At 300 K oxygen only removes the empty dangling bond surface state, whereas at higher temperature new loss transitions involving chemically shifted Ge 3d core levels appear. The findings imply that at 300 K only a chemisorption oxygen state exists on the Ge(111) surface whereas the formation of an oxide phase requires higher temperatures. The shapes of the TD curves show that the desorption of GeO follows $\text{1}\text{2}$ order desorption kinetics.     

Oxygen adsorption on Ge(111) surface: II. Alkali metal-covered surfaces

Oxygen adsorption on an alkali metal (a.m.)-covered Ge(111) surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (ELS), thermal desorption (TD), and work function measurements (WF). It was found that the presence of a.m. results in enhancement of the oxygen adsorption rate. The initial values of the sticking coefficient, S₀, are exponential functions of the work function changes caused by the a.m. adsorption. It was shown that no germanium oxide phases are formed on an alkali-covered Ge surface at 300 K. The oxidation rate at high temperatures is limited by the rearrangement processes taking place in the surface GeO layer. The results obtained show that the alkali metal perturbs the GeO bond to a certain extent but no alkali oxide formation was observed at a.m. covertages under investigation.     

Electron ejection by helium metastable atoms incident on the clean and chalcogen covered Ni(100) surface

An intense, essentially photon free, helium metastable beam has been used to cause electron ejection from the clean, oxygen and sulphur covered Ni(100) surface, in a system equiped with an AES facility to monitor surface cleanness. The ejected electron energy distribution obtained for the clean Ni(100) surface is narrow and peaked at ～2 eV, unlike the distribution obtained from INS studies, and consequently indicates different de-excitation mechanisms for incident ions and excited atoms. The ejected electron distribution from the adsorbate covered surface is also narrow, but peaked at ～1 eV with structure which is essentially independent of the nature of the adsorbate. The yield of ejected electrons is found to increase linearly with coverage of both oxygen and sulphur, in contrast to the results obtained from INS. These data indicate that Auger neutralization does not occur at the surface; the possibility of Auger de-excitation is considered.     

Coadsorption of oxygen and sodium on Ru(001)

The interaction of oxygen with sodium predosed Ru(001) is studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficient of oxygen is found to increase from 0.45 for bare Ru(001) to 1 for Ru(001) with a 0.35 monolayer sodium coverage. The adsorption capacity of the sodium predosed Ru(001) surface towards oxygen is enhanced from θ_O = 0.5 for clean Ru(001) to θ_O = 1.4 for Ru(001) with a 0.7 monolayer sodium coverage. The work function, electron loss changes and thermal desorption data give evidence that as long as θ_Na is less than 0.25, the oxygen chemisorption phase is characterized mainly by oxygen-Ru bonds and by the absence of strong sodium-oxygen interactions. At high sodium coverages (θ_Na > 0.35), the experimental data indicate the formation of a Na-O compound in the second adsorption layer at high oxygen exposures. When Ru(100) is predosed with sodium (θ_Na ? 0.25), this leads to complete suppression of oxygen penetration into the bulk during heating, the latter process being observed for the oxygen-Ru(001) system.     

Initial oxidation of the Mg(0001) surface,an electron spectroscopy study

The initial oxidation of Mg(0001) has been studied using AES (Auger electron spectroscopy), LEED (low energy electron diffraction), and EELS (electron energy loss spectroscopy). The oxidation proceeds through different stages; first oxygen atoms are incorporated to chemisorption sites below the top layer magnesium. This chemisorption phase is followed by the formation of an oxide layer. The oxide layer covers the Mg surface after an oxygen exposure of ～ 10 L O₂. After this exposure the bulk-like MgO formation slowly increases the oxide thickness. The oxide layer formed for exposures up to ≤ 10 L O₂ gives rise to a diffuse LEED pattern of the same symmetry as the original “clean” LEED pattern; the possibility of an epitaxial oxide formation at this stage is discussed.     

A LEED-AES study of the oxidation of cr(110) and Cr(100)

The initial stages of the oxidation of (110) and (100) chromium surfaces have been studied using low energy electron diffraction and Auger electron spectroscopy. The low energy Auger electron peaks were tentatively explained in terms of different chemical states. Thus, the clean chromium surface, the surface covered by chemisorbed oxygen and the chromium oxide surface could be associated with the occurrence of different peaks. The intermediate oxygen chemisorption structures observed at oxidation, have been characterized with respect to symmetry and accurate unit cell dimensions. Lattice parameters were found to range from those of the substrate chromium metal to those of chromium sesquioxide. On the (110) face, the lattice parameter change was observed to be largest in the [11̄0] direction. The observations are in fair agreement with current concepts of misfitting crystalline surface layers.     

Theoretical calculation of vibrations of adsorbed species

The chemisorption of oxygen on Lithium, Aluminum, Nickel and Copper surfaces has been investigated using the ab initio Hartree-Fock cluster model. These substrates have the possibility for different bonding in that Li is a simple s metal, Al an s, p and Ni(Cu) an s, p, d metal. In all cases, we have calculated binding energy curves as a function of the oxygen-metal distance. Using these curves, we have derived oxygen-metal normal vibrational frequencies, and the equilibrium bond distance. We have compared the calculated vibrational energy with electron energy loss spectroscopic (EELS) data for Al and find a satisfactory agreement. We discuss O adsorbed on Ni(100) for which coverage dependent loss peaks have been reported but no generally acceptable interpretation exists to date.     

The interaction of oxygen with Bi(0001): Kinetic,electronic, and structural features

The interaction of oxygen with clean Bi(0001) was studied for adsorption between 118 and 296 K using LEED, Auger, electron energy loss, and work function measurements. Oxygen adsorption kinetics show an activated process with a dissociative sticking probability (<10^?4) which smoothly decreases over two orders of magnitude up to saturation coverage. Changes in surface electronic properties indicate that an oxide-like bond is formed from the onset of adsorption. There is no evidence for a stable chemisorbed phase. LEED shows simultaneous growth of epitaxial BiO(0001) and a $\text{3}$ overlayer. At 296 K the adsorption terminates after about three equivalent monolayers of BiO(0001). Periodic trends extended from the transition metal series suggest that local and atomic characteristic rather than long-range electronic properties determine the low reactivity of this surface toward O₂.     

Changes in Auger spectra of Mg and Fe due to oxidation

Auger electron spectra of clean Mg and Fe surfaces have been investigated under UHV conditions. The main Auger peaks in the low energy Auger spectra of these elements are identified as due to L_2,3VV and M_2,3VV transitions for Mg and Fe respectively. Changes in the low energy spectra of these clean surfaces of Mg and Fe due to chemisorption of residual oxygen in the UHV system, were also studied. The results indicate that for each oxidised surface new larger Auger peaks appear at energies lower than the original main peaks in the clean spectra. The changes in the spectra are believed to be due to the energy shifts of inner energy levels and valence bands involved in the Auger transitions as an oxide is formed.     

Study of adsorption phenomena on aluminium by interatomic Auger transition spectroscopy: I. Initial oxidation of Al

The initial oxidation process of aluminium is studied by using the combined molecular beam evaporation and Auger electron spectroscopy under ultra-high vacuum. At the initial stage of oxygen exposure to the clean aluminium at room temperature, the chemisorption of the oxygen atoms on the surface of aluminium is turned out to be dominant from the behaviour of the energy shifted Auger signal of aluminium. Then, the formation of the alumina (Al₂O₃) type bonding is followed, which is concluded from the interatomic Auger signals.     

ELS study on initial oxidation of Ni(100) surfaces

Electron energy loss spectra of clean and oxygen-covered Ni(100) surfaces were observed with concomitant measurements of LEED, work function change, and Auger peak height ratio O(KL_{2, 3}L_{2, 3})/Ni(L_{2, 3}VV). The observed electronic transitions are interpreted on the basis of primary election energy dependence, and of comparison with the loss spectrum for a UHV-cleaved NiO(100) surface and optical data of Ni. The observed loss peaks at 9.1, 14, and 19 eV in the clean surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electrons, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Three oxygen-derived peaks at 6.0, 8.0, and 10.3 eV in the low oxygen exposure region (?4 L) are ascribed to the O 2p(e) → Ni 3d, O 2p(a₁) → Ni 3d, and O 2p → Ni 4s transitions, respectively. In the high oxygen exposure region (?50 L), the spectra become quite similar to that of the UHV-cleaved NiO(100) surface. The oxidation process consistent with LEED, Auger peak height ratio and work function change measurements is discussed.     

Energy loss spectroscopy of sulphide films on Mo (100)

Energy loss spectroscopy has revealed features on clean Mo(100) due to bulk and surface plasmons, surface state emission, core-conduction band transitions, and multiple losses. On random adsorption of H₂S at room temperature new peaks appear which are associated with chemisorption bonds. Spectra from epitaxially grown MoS₂ on Mo(100) are in broad agreement with those from bulk MoS₂. The results complement other studies of sulphide film growth using low energy electron diffraction and Auger spectroscopy.     

Lateral interactions in the thermal desorption of H2 from clean Cu/Ni(110) alloy surfaces

The adsorption of hydrogen on a clean Cu10%/Ni90% (110) alloy single crystal was studied using flash desorption spectroscopy (FDS), Auger electron spectroscopy (AES), and work function measurements. Surface compositions were varied from 100% Ni to 35% Ni. The hydrogen chemisorption on a-surface of 100% nickel revealed strong attractive interactions between the hydrogen atoms in accordance with previous work on Ni(100). Three desorption states (β₁, β₂ and α) appeared in the desorption spectra. The highest temperature (α) state was occupied only after the initial population of the β₂-state. As the amount of copper was increased in the nickel substrate, desorption from the higher energy binding α-state was reduced, indicating a decrease in the attractive interactions among hydrogen atoms. The hydrogen coverage at saturation was not affected by the addition of copper to the nickel substrate until the copper concentration was greater than 25% at which a sharp reduction in saturation coverage occurred. This phenomenon was apparently due to the adsorption of hydrogen on Ni atoms followed by occupation of NiNi and CuNi bridged adsorption sites, while occupation of CuCu sites was restricted due to an energy barrier to migration.     

Adsorption of H2O on clean and on boron-contaminated Rh surfaces

The adsorption and dissociation of H₂O on Rh(111) and Rh foil surfaces have been studied in UHV using Auger electron, electron energy loss (in the electronic range) and thermal desorption spectroscopy. H₂O adsorbs weakly on clean Rh samples at 110 K. The adsorption is accompanied by the appearance of a broad loss feature at 14–14.5 eV. At higher exposures new losses appeared at 8.6 and 10.5 eV. The desorption of H₂O took place in two stages, with T_p = 183 K (β, chemisorption) and 158 K (α, multilayer formation). There was no indication of dissociation of H₂O on a clean Rh(111) surface. Similar results were obtained for a clean Rh foil. However, when small amounts of boron segregated on the surface of Rh, they exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociation of H₂O. This was exhibited by the formation of H₂, by the buildup of surface oxygen, by the appearance of an intense new loss at 9.4 eV, identified as B-O surface species, and by the development of “boron-oxide”-like Auger fine structure.     

Mechanisms for oxygen adsorption on the Si(110) surface studied by Auger electron spectroscopy

Oxygen adsorption on the Si(110) surface has been studied by Auger electron spectroscopy. For a clean annealed surface chemisorption occurs, with an initial sticking probability of ～6 × 10^?3. In this case the oxygen o_kll signal saturates and no formation of SiO₂ can be detected from an analysis of the Si L_2,3VV lineshape. With electron impact on the surface during oxygen exposure much larger quantities are adsorbed with the formation of an SiO₂ surface layer. This increased reactivity towards oxygen is due to either a direct effect of the electron beam or to a combined action of the beam with residual CO during oxygen inlet, which creates reactive carbon centers on the surface. Thus in the presence of an electron beam on the surface separate exosures to CO showed adsorption of C and O. For this surface subsequent exposure in the absence of the electron beam resulted in additional oxygen adsorption and formation of SiO₂. No adsorption of CO could be detected without electron impact. The changes in surface chemistry with adsorption are detectable from the Si L_2,3VV Auger spectrum. Assignments can be made of two main features in the spectra, relating to surface and bulk contributions to the density of states in the valence band.     

Deposition of iron on MgO(100): Reaction of CO and H2

Ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (EELS) and surface extended energy loss fine structure (SEELFS) were used to study the deposition of Fe on MgO(100) and to identify the surface compounds formed after reaction of CO/H₂ (1:1). The clean MgO(100) surface was characterized using the above techniques and the effect of argon ion bombardment damage to the surface was investigated. With the deposition of iron, metallic characteristics appear in the photoemission spectrum; the electron energy loss peaks of the MgO(100) substrate diminish in intensity with no significant shifts in loss energies. Fine structure analysis of the oxygen K-edge of the MgO(100) surface with less than 2 monolayers (ML) of iron suggests that the iron atoms bond with the oxygen at the surface of the MgO(100) lattice. For less than 4 ML of iron, the EEL spectra show that the deposited iron is oxidized after reaction of CO/H₂. Higher iron coverages result in carburization of the surface. Carbon deposition was observed with CO for all Fe coverages. Measurement of the fine structure above the carbon K-edge suggests that the types of carbide formed depend on the iron coverage; one carbide has a short CFe distance of 1.78 Å and the other a distance of 2.06 Å (high metal coverage).