Adsorption of ethylene on clean and oxygen precovered Cu(110) surfaces

UV photoemission spectroscopy (UPS) with He I and He II radiation is used to study the interaction of C₂H₄ with clean and oxygen precovered Cu(110) surfaces at 90 K. On the clean surface only-bonding of the C₂H₄ molecules is observed whereas preadsorbed oxygen causes a second molecular orbital to be involved in the chemisorption. This result is consistent with the differing behaviour of the work function change during thermal desorption of C₂H₄.     

Electron loss spectroscopy of clean and oxygen covered GaAs(110) surfaces

Electron energy loss spectra of clean and oxygen covered GaAs(110) surfaces have been measured with a four grid retarding field analyser. Loss spectra of clean cleaved p- and n-type surfaces are slightly different and different states of adsorption for the oxygen on the two surfaces are found. The loss peaks which are common in the spectra obtained from clean surfaces of both types of material have been interpreted in terms of bulk and surface excitations. The data associated with the bulk excitations are in good agreement with previous optical and electron transmission data while loss peaks at 11.5 and 18.5 eV are interpreted as the surface plasma loss and a surface state transition respectively. For n-type material extra loss peaks were observed. In the case of oxygen adsorption on these surfaces new loss peaks were found at 13.5, 17.2 and 28.1 eV in both spectra and are assumed to be characteristic of the oxygen. Further, for n-type material an extra peak occurs at 8.2 eV.     

Atomic chemisorption of chlorine on Ag(110) studied by high-resolution electron energy loss spectroscopy

We have studied atomic chemisorption at room temperature of chlorine on Ag(110) using high-resolution electron energy loss spectroscopy (HREELS), supplemented by XPS and LEED. The ClAg vibration energy (around 25 meV) and the line-width of this loss peak show well resolved variations with both chlorine coverage and substrate temperature T. The observed shift with T is related to the anharmonicity of the potential. Based on the Morse potential we derive an anharmonicity parameter x_a = 6.2 × 10⁻² for the (2 × 1)Cl-overlayer. This indicates that the anharmonicity is enhanced by about a factor of two as compared to the bulk. In contrast, we find x_a < 0.2 × 10⁻² for c(4 × 2)Cl. By comparison to other data we conclude that the (2 × 1)Cl-phase is a simple overlayer, with no significant reconstruction of the topmost substrate layer.     

Work function measurements with a high resolution electron energy loss spectrometer: Application to the interaction of oxygen with Ag(110)

The monochromatized electron beam of a high resolution electron energy loss (HREEL) spectrometer is used for accurate (±5 meV) measurement of the work function changes during exposure of a Ag(110) single crystal surface to oxygen. Absolute calibration of the results is made by comparison with Kelvin probe data. The procedure allows the precise determination of the electron impact energy, which is an important parameter for quantitative HREELS analysis. Furthermore, in the case of oxygen adsorbed on Ag(110), the occurrence of several LEED (n×1) superstructures enables a calibration of the HREELS data with respect to surface coverage.     

Electron energy loss spectra of a Pd(110) clean surface

Electron energy loss spectra of a Pd(110) clean surface have been measured at primary energies of 40–100 eV. The observed peaks are at the loss energies of ∼ 3, 4.3, 7.5, 11.5, 16, 21.3, 26.5 and 33.8 eV. The 7.5, 26.5, and 33.8 eV peaks are attributed mainly to the bulk plasmon excitations associated with 5s electrons, coupled 5s and a limited number of 4d electrons, and total (4d+5s) electrons, respectively. The rest of the peaks are ascribed mainly to one-electron excitations.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

Hydrogen chemisorption on Ni(110) by high-resolution electron energy loss spectroscopy

High-resolution electron energy loss spectra of hydrogen-covered Ni(110) surfaces both at 100 and 300 K are presented. The adsorbed sites of hydrogen atoms are discussed.High-resolution electron energy loss spectra of hydrogen covered Ni(110) surfaces have been studied. Tentative models for the adsorbed sites of hydrogen atoms are as follows: (1) For the (2 × 1)-H surface, hydrogen is adsorbed in the three-coordinated sites of the rudimentary (111) face of the unreconstructed Ni(110) substrate. (2) For the low-temperature (1 × 2)-H surface, hydrogen is adsorbed in the three-coordinated sites and, probably, in the two-fold hollow sites of the distorted Ni(110) substrate. (3) For the room-temperature (1 × 2)-H surface, hydrogen is disorderedly adsorbed in the three-coordinated, two-fold hollow and short-bridge sites and, possibly, in the octahedral sites of the distorted Ni(110) substrate. Some of the unresolved problems in the above assignments are summarized: (1) Strictly, the three-coordinated sites above are somewhat different from those discussed in the molecular-beam diffraction study [5]. (2) For the low-temperature (1 × 2)-H surface, the loss associated with hydrogen in the two-fold hollow sites is apparently not observed. (3) Intensity changes of the three losses for the room-temperature (1 × 2)-H surface with increasing hydrogen pressure (Fig. 2) are not well understood.     

Electron energy loss spectroscopy studies of nitrogen adsorption on W(100)

Vibrational excitations of nitrogen on W(100) are investigated over the 100–300 K temperature range using elastic and inelastic electron scattering. New vibrational modes of nitrogen are identified that require different mode assignments from previous work. Experimental evidence for a molecular precursor to the atomic β₂ phase of adsorbed nitrogen is presented. Coverage dependent studies of vibrational modes suggests conversion between two different molecular surface phases and between atomic and molecular phases. A new ordered nitrogen phase characterized by a (4 × 1) LEED pattern is observed. The new phase appears to consist of orthogonal domains of p(4 × 1) symmetry that contain atomic nitrogen at the four fold sites (the β₂ atomic phase) with additional bridge-bonded nitrogen atoms in the unit cell.     

Theoretical studies of chemisorbed oxygen on Ag(110)

The interaction of an oxygen atom with a 26-atom cluster model of the (110) face of Ag has been investigated with ab initio self-consistent-field and configuration-interaction theory. The SCF results for the bridge site (C_2v) predict $\text{r = 0.3}\text{A}\text{?}$ and ω_e = 327 cm^?1, in good agreement with the available experimental evidence. The calculated binding energy (D_e = 9 kcal/mole) is roughly an order of magnitude too small. The inclusion of electron correlation increases r_e and ω_e only slightly, but should have a dramatic effect on D_e. The ground state corresponds to a “surface oxide” state. The theoretical projected density-of-states curves exhibit “bonding” and “anti-bonding” O(2p) peaks, separated by ~ 6 eV, in good agreement with recent angle-resolved photoemission data.     

Electron energy loss spectroscopy of sodium covered Ge(111) surfaces

The ELS spectra of sodium covered Ge(111) surfaces are studied. It was found that sodium coverage upto 1 ML leads to a extinction of the ELS features of the clean Ge surface. With increasing sodium coverage a metal induced peak at 3.4 eV and a peak at 31.6 eV due to a sodium 2p-core level transitions appear. ELS spectra of a heat-treated surface covered with sodium imply a formation of a surface Na-Ge “compound” as a result of Na-Ge interdiffusion.     

Electron energy loss spectroscopy of the decomposition of formic acid on Ru(001)

Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO₂ and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H₂,H₂O, CO and CO₂ as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.     

The adsorption of molecular oxygen species on Ag(110)

The adsorption of oxygen on the Ag(110) surface was examined at temperatures down to 123 K. In addition to the dissociatively adsorbed state which desorbed at 590 K, a second oxygen state desorbed at 190 K following adsorption at 150 K and below. This high temperature state appeared to form prior to the development of the low temperature state. The ratio of coverages of the two states was a strong function of both exposure and adsorption temperature. Isotopic exchange experiments indicated that the low temperature state was molecularly adsorbed. The desorption of the molecularly adsorbed oxygen exhibited complex kinetics due to interaction with adsorbed oxygen atoms.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.     

Electron energy-loss spectroscopy study of oxygen chemisorption and initial oxidation of Fe(110)

The initial stages of the interaction of oxygen with an Fe(110) surface have been studied at 300 K by electron energy-loss spectroscopy with in-situ combined low energy electron diffraction, Auger electron spectroscopy and work-function change measurement. From all the results, four different stages of the oxygen interaction are distinguished: (I) a first dissociative chemisorption up to 3 L, characterized by the c(2×2)-O structure, (II) a second dissociative chemisorption between 3 and 7 L, characterized by the c(3×1)-O structure, (III) incorporation of O adatoms into the selvage between 7 and 30 L, and (IV) oxidation above 30 L leading to the formation of FeO(111), characterized by the diffuse hexagonal diffraction pattern. The sticking probability was found to be initially near unity and fall off rapidly to a minimum value of ≈0.05 at ≈1 L. Particular emphasis was placed upon the investigation of the change in surface electronic properties from those characteristic of them metal to those of the oxide. In stage (I) an energy-loss peak, being attributed to the transition from the 2p orbital of the chemisorbed oxygen, was observed at 6.0 eV, while in stage (II) two additional peaks of the same origin appeared at 7.5 and 9.3 eV due to the formation of the O 2p band. The energy-loss spectrum in the oxide phase was characterized by the peaks at 4.8 and 7.5 eV due to the O²⁻ 2p → Fe²⁺3d charge-transfer transitions and by a peak at 2.4 eV due to the ligand-field d → d transitions of an Fe²⁺ ion in FeO. It is shown that the Fe 3d_yz,zx and 4sp electrons play a major role in the chemisorption bond (O adatoms located in the long-bridge site), and that for the incorporation process the Fe 3d_y² electrons are also involved in bonding by the symmetry breaking. The change in the Fe 3p-excitation spectrum during oxidation was also investigated. The differences between the experimental results on Fe(100) and (110) surfaces are summarized.     

The adsorption of fluor-carbon complexes on GaAs(110) studied by electron energy loss spectroscopy

The room temperature adsorption of CF₃COOH, CH₃COOH and CO on cleaved GaAs(110) surfaces has been studied by vibrational electron energy loss spectroscopy (HRELS), second derivative electron energy spectroscopy (ELS) and electron diffraction (LEED). CO does not adsorb on the GaAs surfaces in measurable quantities. Acetic acid CH₃COOH is dissociatively adsorbed as an acetate bonded to Ga surface atoms with the split-off hydrogen on As surface atoms. The fluorated acid CF₃COOH decomposes via an acetate intermediate CF₃COO into active CF₃ groups which adsorb on Ga surface atoms. The split-off hydrogen sticks to surface As atoms while the generated CO₂ desorbs. The adsorption models are consistent with the LEED c(2×2) superstructure observed after saturated adsorption of both acids.     

Electron energy loss spectroscopy of the vibration modes of water on Ag(100) and Ag(115) surfaces and comparison to Au(100), Au(111) and Au(115)

Motivated by rather similar behavior of the Helmholtz capacitances of stepped Au(11n) and Ag(11n) electrodes we have extended a previous study on the vibration spectrum of water adsorbed at low temperatures on stepped gold surfaces to Ag(100), Ag(115) and Au(111) surfaces. On Ag(100) surfaces, the spectra show the presence of the typical H-bonded network of water molecules. The rather weak intensity, the absence of non-hydrogen bonded hydrogen atoms, the similarity to the infrared spectrum of ice crystallites, and the increase in the angular spread of the elastic peak are indicative of adsorption in form of three-dimensional clusters. This is stark contrast to Au(100) and Au(111) where the spectra match to a model involving stacks of water bilayers. The low coverage spectra on Ag(115) resemble the results on Au(115): A considerable fraction of the H-atoms remains in the non-H-bonded state and spectral features of water adsorbed at step-sites are identified. The first layer of water on Ag(115) surfaces should therefore have a similar structure as recently proposed in a theoretical study concerning water on Au(115). For larger doses, the experimental results on Ag(115) suggests the formation of three-dimensional clusters. This is contrary to Au(115) where the layered structure with a constant fraction of non-hydrogen-bonded H-atoms persists at higher doses.     

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO₂ on Ag(110) and the reaction of SO₂ with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO₂ adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO₂-amine charge transfer complexes and in transition metal complexes. SO₂ exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol^?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO₂ reacted with oxygen adatoms to form SO_3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO₂ resulted in a c(6 × 2) pattern for SO_3(a). The adsorbed SO_3(a) decomposed and disproportionated upon heating to 500 K to yield SO_2(g), SO_4(a) and subsurface oxygen.     

Photon energy dependence of the differential cross section for atomic-field bremsstrahlung

In a coincidence experiment we have measured the absolute fully differential cross section for atomic-field bremsstrahlung over a range of photon energies for electrons of 140 keV incident on gold and aluminum. The results are compared with the available calculations.