Applications of Surface Science最新文献

Resonant photoemission has attracted considerable interest since 1977, when an enhancement of the valence band satellite in Ni at photon energies close to the 3p core level binding energy was first observed. Interpretations for the presence of this satellite, and similar satellites found in other transition metals, have included autoionization, M_2,3VV Auger processes, interband transitions, shake-up excitations and various combinations of these. We explain the energies of these photoemission satellites using a simple model, corrected for the final state effect of hole-hole Coulomb interaction and relaxation by using U_eff determined empirically from our Auger results. “Off” resonance the satellites observed are interpreted in terms of a shake-up process and at resonance as merely the M_2,3VV Auger feature occurring at the energy of the shake-up satellite. We show that the recent conclusions of Chandesris et al. [Phys. Rev. B27 (1983) 2630], which state that the correlation energy between two 3d holes is constant for Cr through to Ni, are incorrect and that the second satellite, observed at resonance in Ni is merely the manifestation of a peak observed in the M_2,3VV Auger spectra. Further we conclude that the 3 eV increase in the satellite to main photoemission peak separation in going from Ni to NiO, is unlikely to be due to an increase in U_eff as tentatively assigned by Thuler et al. [Phys. Rev. B27 (1983) 2082]. We suggest that for “off” resonance it is a shake-up process involving empty 4sp states at the bottom of the conduction band, whilst at resonance it is due simply to changes in the energy of the M_2,3VV Auger transition.

Photoemission of Adsorbed Xenon (PAX) is shown to provide valuable information on the initial growth mode of a thin, 0.2 monolayer silver film on Ru(001) at 50 K on an atomic scale. By taking advantage of the fact that the Xe(5p_{$\text{3}\text{2}\text{,}\text{1}\text{2}$}) UPS spectra of adsorbed xenon allow a clear distinction between Xe atoms being adsorbed on Ag sites, on Ru sites, and at mixed Ag-Ru sites, a model is proposed, according to which the amount of 0.2 ML silver is distributed on the Ru(001) in the form of small islands without alloying. From the 5p UPS intensities of the various Xe adsorption states the radius as well as the number of these islands (assumed to be circular) could be calculated. The number of these islands agrees remarkably well with the number of defects on the original Ru(001) substrate, which was again “titrated” by means of xenon adsorption. These defects are therefore regarded as nucleation centers for the incipient Ag island formation. This model is supported by accompanying AES, LEED, Xe desorption and work function measurements.

The dynamical scattering matrix method is used to obtain reflection intensities for the incidence of low energy electrons in the range 0.25-15.0 eV on various members of the layered transition metal dichalcogenide family: 2H-MoSe₂, 1T-ZrS₂ and 2H-NbSe₂. In particular, the results of these diffraction calculations, based on a simple pseudopotential, are compared with experimental spectra obtained using the total (net) current technique. In the calculation, an incident energy dependent absorption potential is estimated by a combination of results from dielectric response theory together with experimental values of the high energy electron dielectric loss function. Comments are made regarding the apparent coincidence of low order kinematical Bragg reflection conditions with features present in the experimental and dynamical calculation spectra. As in previous work, it is concluded that it is essentially the crystal structure that determines the intensity spectrum and that a precise description of the individual scattering events is not as necessary as might first appear.

Secondary ion mass spectroscopy has been used in the static mode to study fracture faces of ceramic materials and glasses. The static SIMS conditions minimize interlayer mixing, and equivalent etch rates down to 0.001 nm min^-1 can be used with reasonable sensitivity, e.g. count rates > 50 s^-1. A comparison between a caesium aluminosilicate ceramic (pollucite, CsAlSi₂O₆), a titania-based multiphase ceramic (Synroc C) and a borosilicate glass will be described. The multiphase ceramic Synoc C gives evidence that alkali metal cations are enhanced by factors greater than three on fracture face surfaces compared with bulk levels or polished faces. The fracture mechanism appears to be predominantly intergranular. Under the same conditions enhancement is not observed on fracture faces of the single-phase ceramic pollucite or on the borosilicate glass. The evidence suggests that selective sputtering, ion beam-induced diffusion, changes in secondary ion/neutral ratios and interlayer mixing are not responsible for the enhancement but that intergranular regions retain higher levels of Cs, Na and K. Interlayer mixing effects can be observed on the pollucite and glass samples at higher beam currents and voltages. The paper discusses the validity of SIMS results by comparison with scanning Auger and XPS evidence from similar surfaces.

The adsorption and reactivity of the isoelectronic molecules CO, CH₃NC and CH₃CN on Pt(111) have been studied by high resolution electron energy loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). In this way, it has been shown that CO and CH₃NC both adsorb initially in a terminal configuration which at higher coverages coexists with a bridge-bonded form. While CO desorbs molecularly, CH₃NC decomposes at > 400 K to H₂ and HCN. At high coverages a low level of isomerization to the more stable CH₃CN is also seen at 430 K. CH₃CN adsorbs weakly and reversibly in an η²-configuration with an imine-like CN bond. The relationship of these configurations to the chemically important frontier molecular orbitals is addressed.

Appearance potential spectroscopy (APS) probes the binding energy of core levels and local conduction band states of solid surfaces. Soft X-ray APS (SXAPS) and Auger electron APS (AEAPS) respectively measure the differential X-ray fluorescence and secondary electron yields as a function of incident electron energy. We have obtained the N_4,5-level SXAPS and AEAPS spectra of La, Ce, Pr, Nd and Sm metals and their oxides. The N_4,5-level AEAPS studies of these metals have not been reported previously. The comparison of the two spectra shows a strong resemblance. Both exhibit multiplet structure below the expected 4d excitation threshold and a broad, 10–20 eV wide peak above threshold followed by small peaks of decreasing intensity. AEAPS peaks are more intense and narrower than SXAPS peaks. The data are used to gain an understanding of the decay mechanism following the excitation of the core levels in these spectroscopies. The spectra recorded from oxidized metals do not show any major changes in the spectral features except for the chemical shift of the threshold. The shifts for all metal oxides are less than 1.0 eV except for Sm which is 2.5 eV. The similarity of the metal and oxide spectra indicates that the APS spectra are dominated by atomic rather than band effects.

Photoemission, LEED and Auger spectroscopy have been used to study the carbon layer formed by ethylene decomposition on a Ni(110) surface. Of the two ordered overlayers reported in the literature only the (4 × 5) was obtained. It was found to be carbidic in nature; its photoelectron spectrum is in agreement with recent theoretical calculations.

The transmission coefficient of very low energy electrons (⩽10 eV) incident on epitaxial films on single crystal substrates is modulated by interference between reflection at the vacuum/film and film/substrate interfaces. We have observed these quantum size effect (QSE) oscillations for various systems: Cu and Ag/W(110), Cu/Ni(001), Ag/Cu(111) and Ag and Cu/Si(111)-7 × 7 for thickness from a few monolayers to ∼ 100 Å. The sensitivity of the quantum size oscillations to the abruptness of the interfaces is illustrated by a reduction in their amplitude with increasing disorder on an atomic scale at either the vacuum/film or the film/substrate interface. Using the W(110) LEED (00) beam width as an index of substrate surface roughness produced by mild ion bombardment prior to film deposition, we find that the QSE amplitude decreases linearly by 70% for an increase in monoatomic step densities from 1 to only 3% at the Cu/W interface. Quantum size oscillations for epitaxial Cu/Ni(001) were found to be characteristic of the thickness of the film with 1 ML resolution from 1 to several tens of monolayers. The experimental features are well explained by a one-dimensional theory of electron transmission for overlayer crystal based on LEED theory and including a reasonable description of the band structures of the film and the substrate.

The adsorption of nitric oxide on an Fe(110) single crystal at temperatures 90–350 K has been investigated by HeI UPS and by Auger spectroscopy. Four adsorption states are detected in UPS depending on the substrate temperature. Molecular absorption states, α₁ at 90–110 K and α₂ at 110–170 K are observed. The α₂ state is preceded by some NO dissociation. A β adsorption state exists over a very wide temperature range of 170–290 K. It consists of NO chemisorbed amidst atomic N and O from the initial dissociation. Nitric oxide was found to initially dissociate at all substrate temperatures above 110 K. The extent of dissociation increased with temperature, being complete at above 300 K (d state). No measurable molecular NO was found to exist above 330 K. The lower temperature adsorption states are shown to undergo an irreversible conversion to the next highest temperature state upon heating the substrate: α₁→α₂→β→d.

Since the early seventies, a correlation has been known to exist between changes in intramolecular bond vibrational frequencies (as measured by IAS) and core level binding energy shifts (as measured by XPS). It is possible to explain such correlations by relating both shifts to the partial atomic charges which exist on the bonding pair of atoms. For a series of related molecules, for example, the charge on a common atom may vary and may be related to the core level shifts on that atom. Equally, Liesegang and Lee have shown that changes in this charge correspond with variations to the bond stretching frequency. This model has also been recently extended to include correlations between XPS shifts and rocking vibrational mode variations. In both applications, however, the elastic properties of bonds (springs) are considered. In the former, spring stretching properties are linked to the partial charges, while in the latter case molecular flexing involving a cantilever-like vibration permits a link with partial charges and their effect on rocking frequency. Experimental data will be presented for comparative purposes to illustrate the predictions of the model.