Kinetics of adsorption and desorption using Auger electron spectroscopy: Application to xenon covered (0001) graphite

Three regimes of condensation have been observed between 74 and 80 K in the adsorption and desorption of a submonolayer film of xenon. The first one corresponds to thej condensation or evaporation of a two-dimensional (2D) ‘gas’, the second one to the growth of 2D crystal in the presence of the 2D gas, and the third one to the completion of the 2D crystal on the (0001) graphite face. Zero order kinetics for both adsorption and desorption is found in the large range of coverage (0.3 < θ <0.9) where the two phases coexist on the surface. The activation energy of desorption of the 2D crystal is measured; its value (～6 kcal mole^?1) is in fair agreement with the value of the latent heat of evaporation of this phase (5.5 or 5.7 kcal mole^?1) determined previously. No activation energy of nucleation has been observed during the adsorption process. The growth rate is controlled by the incident flux only.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

A surface analysis of CuAu alloy by leed and Auger electron spectroscopy

Surface structures and compositions of the CuAu alloys have been investigated, which were prepared by depositing gold on (110) and (111) surfaces of copper and by subsequent heating. By this method the structure of alloy surfaces corresponding to different compositions can be observed by LEED. A series of the LEED patterns, streak, (1 × 2), (1 × 1)I, complex, c(3 × 1), (1 × 1)II, (2 × 2) and (1 × 1) have been observed on the (110) surface with decreasing gold composition. On the (111) surface (1 × 1) pattern, weak (2/√3 × 2/√3)R30° and (2 × 2) patterns are observed. The mean surface composition is determined by analysing the data of Auger electron spectroscopy. Most surface periodicities observed are different from those expected if one passes a mathematical plane through the crystal (unreconstructed surface).     

Methane adsorption on graphite（0001） films： A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite(0001) films. The surface energy and the interlayer relaxation of the clean graphite(0001) as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.     

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.     

LEED and Auger electron observations of the SiC(0001) surface

LEED and AES experiments of the SiC{0001} crystal surfaces show that on heat-treatment these surfaces are easily “covered” with a layer of graphite by evaporation of silicon. The graphite layer, which has a distinct crystallographic relation to the SiC crystal, is monocrystalline on the Si-face and mostly polycrystalline on the C-face. A speculation about the mechanism of the initial graphitization of the basal faces of SiC is given.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

Hydrogen adsorption on silicon (1 1 1) surfaces studied by Auger electron spectroscopy

The adsorption of atomic hydrogen on the (1 1 1) planes of silicon single crystals is studied by Auger Electron Analysis, using the decrease of the silicon Auger peak heigh as a measure of the hydrogen coverage. The zero coverage sticking coefficient is found to be 3 × 10^?4.     

Incipient oxidation of Mg(0001) face studied by electronic energy loss spectroscopy and leed

The intensity change, upon O₂ exposure of Mg(0001), of the low energy (89 eV) electron loss peaks is interpreted by the nucleation and growth of islands of chemisorbed oxygen converting progressively to the MgO (111) structure while regions between the islands order differently.     

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.     

As(0001) surfaces: Auger,loss, and photoelectron spectroscopic studies

The (0001) surfaces of arsenic single crystals have been characterized by Auger, loss, and X-ray photoelectron spectroscopy. A comparison of the results leads to an unusual suggestion for excitation, conduction, and emission modes in the low-energy region. It is proposed that the dominant 20 eV near-elastic loss is the excitation of N₁(4s) electrons to a conduction level 2 eV above the instrumental vacuum level. It is also proposed that two low-energy “Auger-like” peaks at 12 eV and 1.5 eV are electrons emitted at the surface from permitted bands in the bulk. A strong correlation among Auger and loss signals, integrated secondary-electron emission, elastic peak heights and the Kikuchi display is also reported. A study of the Kikuchi correlation of the 12 eV peak suggests that the surface Debye-Waller effect on As(0001) is a long-range, collective oscillation rather than short-range, individual atomic disorder.     

The adsorption of methyl iodide on uranium and uranium dioxide: Surface characterization using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES)

The adsorption of methyl iodide on uranium and on uranium dioxide has been studied at 25 °C. Surfaces of the substrates were characterized before and after adsorption by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The XPS binding energy results indicate that CH₃I adsorption on uranium yields a carbide-type carbon, UC, and uranium iodide, UI₃. On uranium dioxide the carbon electron binding energy measurements are consistent with the formation of a hydrocarbon, —CH₃-type moiety. The interpretation of XPS and AES spectral features for CH₃I adsorption on uranium suggest that a complex dissociative adsorption reaction takes place. Adsorption of CH₃I on UO₂ occurs via a dissociative process. Saturation coverage occurs on uranium at approximately two langmuir (1 L = 10^?6 Torr s) exposure whereas saturation coverage on uranium dioxide is found at about five langmuir.     

Cluster growth of Cu on graphite: XPS,Auger and electron energy loss studies

Auger, XPS and EELS techniques have been used to investigate the core levels, the d-valence band and the electronic transitions of different UHV deposited Cu clusters on graphite. The decreasing of the Cu particle size produces core levels and valence band shifts towards higher binding energies. Lower extra-atomic screening of the conduction electrons near the excited atom and shift of the d-band towards the isolated atom levels are claimed to explain these effects. The EELS results suggest that, for smallest clusters, no structural change but only a lattice parameter contraction of the f.c.c. cage occur.     

Cs adsorption on a polar NbC(111) surface: photoemission and auger electron spectroscopy studies

The Cs adsorption process on a NbC(111) surface has been studied with core-level photoemission spectroscopy (PES) and Auger electron spectroscopy (AES). Coverage-dependent Cs 4d core-level PES shows that the polarization-depolarization transition of the Cs overlayer occurs in the coverage region of 0.5 ≤ θ ≤ 0.8 ML where the work function shows a minimum value. The charge transfer in the initial stage of adsorption is investigated using valence-related AES, and it is found that the transfer of the Cs 6s charge to the substrate occurs in the polarized phase.     

Experimentally derived Auger transition probabilities in X-ray excited Auger electron spectroscopy (XAES)

The capability of Auger transition probabilities experimentally derived from X-ray excited Auger electron spectra in XPS were tested. The relative sensitivity factor (RSF) method has been employed in the quantification by AES (Electron excited Auger electron spectroscopy). However, the difference between experimentally derived RSF and theoretically calculated ones has been found in some reports. One of the great reason of the difference may be caused by the calculated values of the Auger transition yield which has been commonly employed without the consideration of the allotment of coupling scheme in the transition selected in the quantification, for instance, the allotment of each six coupling KL₁L₁, KL₁L₂, KL₁L₃, KL₂L₂, KL₂L₃, and KL₃L₃ in KLL transition. The employment of derived Auger transition probabilities reduce the difference between theoretically calculated RSF and experimentally derived one.     

Auger electron spectroscopy (AES) studies of argon ion induced desorption of carbon from tantalum

AES studies of argon ion induced desorption of carbon from tantalum were performed. The carbon adlayer was allowed to adsorb from a well characterized residual gas atmosphere, that was unvarying within 20%. The argon ions impact on the surface at an angle of 60° from the surface normal with energies between 0.2 to 1.0 keV. The total desorption cross section values measured under these conditions are 0.07–1.1 × 10^?15 cm².     

Phase transitions in physisorbed films on cadmium (0001): comparison with graphite (0001)

Physical adsorption of simple molecules has been studied experimentally on cadmium (0001). Adsorption isotherms of Kr and CF₄ at 77.3 K show a first-order phase transition in the first layer, characteristic of adsorption on uniform substrates.

The nature of the phase transition on cadmium (0001) in the first layer is investigated for Kr and CF₄ by a thermodynamic approach in comparison with results obtained earlier on graphite (0001).

From the comparison between the two uniform substrates, we show that (1) cadmium (0001) has a less attractive force field than graphite (0001); and (2) like graphite (0001), cadmium (0001) is a very uniform surface suitable for studying two-dimensional phases according to the Gibbsian formulation.     

Hydrogen adsorption on Gd(0001)

The electronic structure of hydrogen adsorbate-induced states on Gd(0001) was investigated by means of photoelectron spectroscopy with linearly polarized radiation. The E vector of the incoming photon beam is rotatable. Clean and well-ordered rare-earth (0001) surfaces exhibit a highly localized surface state near the Fermi edge. After the adsorption of hydrogen, the surface state disappears and an additional sharp feature at about 4 eV binding energy is observed. For this latter state, the ratio of the radial matrix elements as well as the relative phase shifts were determined to be R=R_p/R_f=2.4±0.3 and δ_f−δ_p=310±10°, respectively. The removal of the Gd surface state by hydrogen adsorption was investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). The removal of the surface state exhibits domain-like behavior, with surface steps acting as domain boundaries. The tunneling spectra reveal that hydrogen adsorption causes a dramatic reduction in the differential conductivity near the Fermi level.