Adsorption of water and methanol on GaAs(110) surfaces studied by ultraviolet photoemission

UV photoemission spectroscopy (UPS) with He 1 radiation (hν = 21.2 eV) has been used to study the interaction of H₂O and CH₃OH with GaAs(110) surfaces prepared by cleavage in ultrahigh vacuum (UHV). For H₂O two molecularly adsorbed phases can be distinguished at 300 K: at low coverage H₂O is chemisorbed by its oxygen lone-pair orbital to the surface whereas for higher exposures a less perturbed species which resembles more a “physisorbed” or condensed H₂O layer is found. At 180 K only the less perturbed species can be identified. Also CH₃OH is chemisorbed molecularly at lower coverage with its oxygen end to the GaAs surface. For higher exposures two additional emission bands are observed which are interpreted as due to the methoxy radical CH₃O resulting from a partial decomposition of CH₃OH.     

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.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Adsorption states and Ga depletion during oxygen adsorption on clean polar GaAs surfaces

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS) and Auger (AES) measurements were used to study oxygen adsorption on sputtered an annealed GaAs(111)Ga, (1̄1̄1̄)As, and (100) surfaces. Two forms of adsorbed oxygen are seen in UPS. One of them is associatively bound and desorbs at 400–550 K mainly as molecular O₂. It is most probably bound to surface As atoms as indicated by the small amounts of AsO which desorb simultaneously. The second form is atomic oxygen bound in an oxidic environment. It desorbs at 720–850 K in the form of Ga₂O. Electron irradiation of the associatively bound oxygen transforms it into the oxidic form. This explains the mechanism of the known stimulating effect of low energy electrons on the oxidation of these surfaces. During oxygen exposure a Ga depletion occurs at the surface which indicates that oxygen adsorption is a more complex phenomenon then is usually assumed. The following model for oxygen adsorption is proposed: oxygen impinges on the surface, removes Ga atoms and thus creates sites which are capable of adsorbing molecular oxygen on As atoms of the second layer and are surrounded by Ga atoms of the first layer. This molecular oxygen is stable and simultaneously forms the precursor state for the dissociation to the oxidic form.     

X- ray photoemission study of the interaction of oxygen and air with clean cobalt surfaces

The interaction of oxygen with polycrystalline cobalt surfaces has been studied at 300 K (1 × 10^?6 to 1 × 10^?5 Torr) using high-resolution (monochromatized) X-ray photoemission. At high exposures (> 100 L nominal) CoO is identified as the product from the nature of the $\text{Co}\text{2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{, 2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{, 3}\text{s}$, and valence band spectra. There is no evidence for measurable amounts of Co₃O₄ or Co₂O₃. Two O 1s features are observed at both high and low (10L) exposures. The dominant O 1s feature at 529.5 ± 0.2 eV corresponds to the oxide and a minor feature at 531.3 ± 0.2 eV is attributed to non-stoichiometric surface oxygen. Exposure to air produces quite different results, with a dominant O 1s feature at 531.5 ± 0.2 eV and dominant $\text{Co}\text{2}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ features centered at 781.3 ± 0.2 eV and 797.1 ± 0.2 eV. These three values are very close to those reported here for bulk Co(OH)₂. Ion etching of the air-exposed surface removes this dominant surface product rapidly revealing some oxide and finally metal.     

Adsorption of ethylenediamine on clean and oxygen covered Fe/Ni(100) surfaces studied by XPS

The interaction of ethylenediamine with Fe/Ni(100) surfaces oxidized to various extents has been studied in the temperature range 260–450 K by means of X-ray photoelectron Spectroscopy. The use of ~ 1 monolayer of Fe enables us to characterize the oxidation states of the topmost layer atoms unambiguously, based on the XPS spectra using a conventional spectrometer. On clean and c(2 × 2)-O surfaces the ethylenediamine can dissociate the N-H bond at 260 K. On heating the adlayer to 340 K the dissociation was further developed. On the surfaces whose Fe atoms were oxidized to FeO/Ni(100) and further, only molecularly adsorbed species were present at 260 K and desorbed partly without dissociation of the N-H bond after heating to 340 K.     

Disordered clean surfaces and adsorbates investigated with atom-surface scattering

In this paper two different physical situations are considered which can be treated with the same method: a fluid adsorbate (disordered in the x, y plane) and a clean surface with random steps (disordered in the z direction). The hard corrugated wall model is used in the eikonal approximation; the differences between the two cases arise only from the different statistical properties of the two physical situations. The differential scattering probability is evaluated. For the fluid adsorbate the latter splits into a coherent (purely specular) contribution and an incoherent one (which is, in fact, weakly inelastic and related to classical diffusion on the surface). For stepped “rough” surfaces only incoherent scattering is present and the differential scattering probability for hexagonal lattices is given.     

Experimental aspects of angle-resolved photoemission from clean and adsorbate-covered metal surfaces

Instrumental techniques for the observation of angle-resolved photoelectron spectra from surfaces are reviewed. Results from the work of various groups on clean metals, particularly tungsten, copper, nickel and palladium, are described. Spectra induced by adsorption of hydrogen, oxygen, carbon monoxide and benzene on some of these metals are discussed.     

Chemisorption and decomposition reactions of oxygen-containing organic molecules on clean Pd surfaces studied by UV photoemission

We have used uv photoeinission (primarily at a photon energy hv = 40.8 eV) to study chemisorption and decomposition reactions of small oxygen-containing organic molecules on clean polycrystalline Pd surfaces at 120 and 300 K. These molecules include methanol (CH₃OH), dimethyl ether (CH₃OCH₃) formaldehyde (H₂CO), acetaldehyde [H(CH₃)CO], and acetone [(CH₃)₂CO]. Chemisorption bonding of these molecules to the Pd surface occurs primarily through the lone-pair orbitais associated with the oxygen atoms, as evidenced by chemical bonding shifts of these orbitais toward larger electron binding energy relative to the other adsorbate valence orbitals. At 300 K all the molecules studied decompose on the surface, resulting in chemisorbed CO. Since chemisorbed (as well as condensed) phases of some of these molecules (CH₃OH and H(CH₃)CO) are observed at low temperature, the decomposition to CO is a thermally-activated reaction. The observed orbital shifts associated with chemisorption bonding are used to make rough estimates of interaction strengths and chemisorption bond energies (within the framework of Mulliken's theory of electron donor-acceptor complexes as applied to chemisorption by Grimley). The resulting heats of chemisorption are consistent with the observed surface reactions.     

Adsorption of oxygen on clean single crystal faces of aluminium

Auger electron spectroscopy and work function measurements have been used to study the interaction of clean Al(111) and Al(100) faces with oxygen at low pressure near room temperature. The results for the two faces differ strongly. Thus, the sticking probability of the (111) face decreases rapidly with coverage, while the work function increases slightly, by 0.1 eV at 200 L. In contrast, the sticking probability of the (100) faces goes through a maximum, whereas the work function decreases almost linearly with coverage, the total decrease at 200 L being 0.5–0.8 eV. The shape of the Al L_{2, 3}VV spectrum from oxidized Al(100) is independent of coverage, and it is in fact very similar to previously reported spectra from oxidized polycrystalline aluminium. The corresponding spectrum from Al(111) exhibits large changes with oxygen coverage and shows a previously unreported double peak at ~60 eV. The results are explained on the assumption that oxygen adsorbs randomly on the (111) face, and that thin (~5 Å) islands of Al₂O₃-like oxide form on the (100) face.     

Adsorption of oxygen on silver single crystal surfaces

The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10^?5Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10^?3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of (n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.     

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.     

Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

Electronic surface states investigated by means of photoemission spectroscopy

Ultraviolet photoemission spectroscopy (UPS) as an investigative method for surface states is presented. The measured energy distribution curve (EDC) furnishes information on the electronic density of states distribution in the valence band, nearest to the core levels and surface states. The measured angle resolved EDC's provide the possibility to determine the energy-momentum E(k) dependence of electrons in the bulk of the crystal and on the surface. Application of the synchrotron-storage ring system as a source of ultraviolet radiation in the energy range from 10 to 300 eV opens new avenues to investigate structure of electronic states. It features the following possibilities: (1) To distinguish the contribution of the surface and bulk states to the obtained EDC by measuring the change of the EDC when varying the exciting energy hv around the minimum of the escape depth (E = 80 eV) and outside of this region. (2) To discriminate in the valence band the contribution of the d-electrons from that of the s-p electrons due to a different change of the photoemission cross section of the d and s-p electrons with a change of hv. (3) To recognize the localized and delocalized contribution to the density of states in the valence band and to determine E(k) for these electrons by measurements of the angle resolved EDC. (4) To obtain information on the initial and final state distribution using the constant initial states (CIS) and/or constant final states (CFS) techniques. The potential of the photoemission technique will be illustrated by the results of the electronic structure investigation of some metals and semiconductors.     

Interaction of oxygen and nitrogen with clean transition metal surfaces

The interaction of O and N with evaporated films of Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Fe, Ni, Cu, and Ag has been analysed in the range 10^?3 to 10^?10 mbar and 77 to 370 K with respect to the processes prevailing in various metal-gas systems. The shape of typical energy-reaction path diagrams estimated from thermodynamical data and from the dependence of the sticking probability on pressure, temperature, and coverage provide a suitable framework for a systematic characterization of the surface reactions observed.     

Magnetism of surfaces by spin polarized photoemission

Spin polarized photoemission is used as a method to study the magnetic peoperties of solid surfaces. Here we consider those systems where the spin polarization is believed to be conserved during the photoemission process. The surface sensitivity of the experiment will be therefore mainly determined by the relative magnitudes of the escape depth of the photoelectrons and of the magnetic coherence length. We analyze both the temperature and magnetic-field dependence of the degree of spin polarizationP of the electrons emitted from single crystals of magnetite, Fe₃O₄, and thin films (D=2–3000 Å) of Fe, Co and Ni. These data yield information on 1) the temperature dependence of the magnetization of the surface, 2) the relative sizes of and, 3) changes of the magnetic moment of atoms in the surface layers, and 4) the surface magnetic anisotropy. We show that this technique emerges as a novel tool to study magnetic systems with free surfaces and to test the predictions of recent calculations of the magnetic properties of surfaces. Emphasis is given to the fact that only relative changes ofP and not its absolute values are relevant for an analysis of the data.     

Structural phase transitions at clean and metal-covered Si(111) surfaces investigated by RHEED spot analysis

Structural phase transitions between various kinds of superlattice structures formed on a Si(111) surface have been investigated by spot analysis of reflection high-energy electron diffraction (RHEED). Reversible transitions induced by temperature changes and irreversible ones induced by metal depositions were observed. Detailed discussions on the dynamics of the phase transitions are made by quantitative analyses of integrated spot intensity and profile. For a phase transition of 7′7  1′1 structures on a clean Si(111) surface, a hysteresis with temperature difference of 5°C. between in heating and cooling processes was found in the spot intensity change, indicating a first-order transition. Hysteresis was hardly recognized, on the other hand, for transitions of Au-induced superstructures (5×2-Au or ×-Au)  1×1-Au. The spot profiles were found to be broadened during the transition of Si(111)-×-Au  1×1-Au, which was a signature of a continuous transition, while the profiles remained unchanged during the transitions of the 7×7  1×1 and 5×2-Au  1×1-Au phases. Structural conversions induced by In adsorption on the Si(111) surface kept at constant temperatures were also analyzed. The conversions at room temperature were totally dependent on the initial substrate surface structures; the 7×7 surface did not show any structural conversion with In adsorption, while the ×-In surface successively converted to a 2×2 and a × phase with coverage increase. The structural transitions at elevated temperatures were sensitively dependent on the temperatures. Sequences of transitions among the 7×7, 4×1, ×, 1×, and ×4 were quantitatively revealed as changes in RHEED spot intensity.     

The electronic structure of KMnO4 investigated by resonant photoemission

The valence band of potassium permanganate (KMnO₄) was investigated by means of resonant photoemission spectroscopy (ResPES) at the Mn2p, Mn3p and O1s edge. These data confirm the previous conclusions of a strong deviation from a purely ionic charge distribution in this compound. The ResPES data are in agreement with previous results about the character of the individual valence band states. A simple cluster model is used to explain qualitatively the different structures seen in the valence band spectra and their dependence on the photon energy.     

Magnetic properties of thin epitaxial films investigated by spin-polarized photoemission

The surface sensitivity of the spin-polarized photoemission experiment was exploited to study two-dimensional magnetism. The magnetization of thin films of Fe, Co, and V in the monolayer (ML) range, grown on Cu(001) and Ag(001) single crystals, was measured as a function of perpendicularly applied field and temperature. Bcc Fe films and fcc Fe and Co films exhibit ferromagnetism down to the single monolayer range, while no evidence for ferromagnetism is found for V on Ag(001). All Co films are magnetized in plane and have a Curie temperature far above room temperature. A thickness dependence of the anisotropy and Curie temperature is observed for the two phases of Fe. Remanent magnetization perpendicular to the surface is found at 30 K for fcc Fe films thicker than 2 ML and for bcc Fe between 3 and 4 ML. The magnetic effects caused by coating and by interdiffusion are discussed in the light of measurements of Cu/Fe/Cu sandwiches and of overlayers obtained by simultaneous evaporation of Fe and Cu. The fcc Fe films are shown to be suitable for thermomagnetic writing.