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Momentum-resolved high-resolution electron energy loss spectroscopy has been used to probe the phonon dispersion of periodically rippled graphene grown on Ru(0001). Optical phonons in this system show anomalies with respect to other graphene/metal interfaces. Novel modes with weak intensity are observed at ∼86 and ∼150 meV. Moreover, the dispersion relation of the highest optical branches in the Γ-K direction strongly deviates from the expected behavior. These unusual features in the phonon dispersion should be related to the corrugated and chiral structure of the graphene overlayer on Ru(0001).

Temperature effects on the alkali-induced CO dissociation on Ni(1 1 1) were studied by high-resolution electron energy loss spectroscopy. Measurements showed that CO adsorption on an alkali-precovered Ni(1 1 1) surface is partially dissociative in a temperature range up to 400 K and completely dissociative for higher sample temperatures. Depending on temperature an alkali critical precoverage for CO dissociation was found to exist, in agreement with theoretical calculations. Moreover, loss measurements demonstrated that Na and CO desorption are non-concurrent, in contrast with results obtained using thermal desorption spectroscopy for similar systems.

Nanostructured titanium dioxide (ns-TiO2) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO2 nanocrystals embedded in an amorphous TiO2 phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO2 films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.

We present the results of high-resolution electron energy loss experiments on (CO/O)/Ni(1 0 0) and (CO/H)/Ni(1 0 0) systems. Oxygen and hydrogen interact differently with Ni(1 0 0) surface, nevertheless, both species do not affect to a great extent the vibrational properties of CO. A phase of CO molecules weakly bonded to the surface and unaffected by coadsorbed oxygen and hydrogen, is found. Coverage of 0.5 ML of predosed oxygen chemically passivates the Ni(1 0 0) surface and inhibits any CO adsorption at room temperature. CO sites are unambiguously determined for each predosed Ni(1 0 0) surface.

The vibrational properties of CO on Ni(1 0 0) were investigated by high resolution electron energy loss spectroscopy. Loss measurements were performed as a function of temperature and CO exposures. At room temperature, regardless of CO coverage, we found stretching energies at 250 meV and 240 meV, assigned to CO at atop and bridge sites, respectively. At low temperature and for CO exposures lower than 0.6 L, the loss spectra showed a single stretching peak at 240 meV which is assigned to CO at bridge sites. For higher doses, a new intense peak at 260 meV appeared in the loss spectra. The rise of this new loss strongly influenced the intensity of the peak at 240 meV suggesting the occurrence of dipole couplings between adjacent CO molecules. This unusual high stretching frequency leads us to assign the peak at 260 meV to the stretching vibration of CO molecules which are weakly bonded to the Ni surface. We suggest that the formation of this phase is due to an electronic effect arising from a reduced back donation of electrons into the empty π orbital of CO.

The electronic and vibrational properties of oxygen and potassium co-adsorbed on the Ni(1 1 1) surface at room temperature were studied by high resolution electron energy loss and X-ray photoelectron spectroscopies. The sample surface was prepared by means of two different procedures, namely by adsorbing oxygen on a pre-dosed potassium ad-layer and by reversing this sequential order. In both experiments we observed that a K coverage as high as 0.25 ML ( i.e. p (2 × 2) phase) is the condition necessary for measuring a weak K–O bond, while a K coverage greater than 0.30 ML (almost full layer), gives rise to a strong K–O bond which is evidenced by the loss feature at 27 meV. The loss spectrum of the K–O- c (4 × 2) phase shows that both species interact mainly with the substrate.

The co-adsorption of oxygen and carbon monoxide on the Ni(1 1 1) surface was investigated by high resolution electron energy loss spectroscopy. We found that CO molecules adsorbed at 150 K on the oxygen pre-covered surface have two distinct internal vibrations, i.e. at 236 and 258 meV. The vibrations at 236 and 258 meV are the fingerprints of molecules adsorbed at threefold hollow sites and at on-top sites, respectively. At 150 K it was not possible to selectively populate one kind of the allowed sites. On the contrary, above 200 K the adsorption of carbon monoxide on a O-p(2 × 2) surface led to the occupation of only on-top sites. As regards 0.25 and 0.33 ML of oxygen pre-coverages, the heating of the co-adsorbed (CO + O) layer from 150 to 200 K gave rise to a site change of some CO molecules, moving from hollow to on-top sites.

The electronic excitations of a well ordered K-(√3 x √3)R30° on Al(111) were investigated by using the angular resolved energy loss spectroscopy technique. Measurements were performed at low temperature (LT) where K atoms in the (√3 × √3)R30° occupy the on-top sites and at room temperature (RT) where K atoms in (√3 x √3)R30° are in substitutional sites. We found that the structural changes induce significant variation in the loss spectra. The excitation spectrum in the RT phase is ascribed to single-particle transitions involving states of the Al(111) surface with a (√3 x √3)R30° vacancy structure; a new peak at 7.2 eV is interpreted as a multipole plasmon mode coming from the charge density profile predicted by calculations for the alloy surface phase. On the contrary, spectra in the LT exhibit single as well as collective excitations coming from states derived from the K adsorbate.

High-resolution electron energy-loss spectra of the coadsorption of potassium and CO on the Ni(100) surface show the role of temperature in driving chemical species along the surface towards the formation of ordered islands made of alkali atoms and CO species. At low potassium precoverages, the formation of K–CO islands occurs by heating the sample to 420 K. On the contrary, for a potassium coverage near 0.38 ML, K–CO patches of ordered phases grow spontaneously even at 220 K. The interpretation of the present data relies essentially on two literature works on the K–CO/Ni(100) system, one of them presenting calorimetric measurements and the other reoprting the theoretical temperature behaviour of the chemical species on the nickel surface.

The ordered c(2×2) phase of Sn atoms on the Ni(100) surface was studied by constant-energy-loss electron-energy-loss holography (CEL-EELH). Three-dimensional images at an atomic level around adsorbed Sn atoms of the surface were deduced. The results clearly favour a chemisorption geometry with Sn atoms incorporated into the first Ni layer, and demonstrate the ability of the CEL-EELH technique to give atomic images similar to those obtained by synchrotron radiation holography.

The temperature effects on sub-monolayers of V deposited at the TiO2(001) surface have been studied by ultraviolet and X-ray photoelectron spectroscopies, UPS and XPS, from ∼ 300 up to 623 K. V coverages, Θv, between 0.2 and 0.7 monolayers (ML) were deposited by an e-beam evaporator at 300 K. The V 2p32 core line region exhibits two well-defined components whose relative intensity depends on Θv. These two components, assigned to different oxidation states of V, are correlated with two features, with a dominant V 3d character, detected within the TiO2(001) band gap of the UPS valence band spectra. UPS and XPS measurements performed after in-situ thermal treatments show unambiguous and reproducible changes of these spectral components. After annealing at 623 K only the higher binding energy component is present in the V 2p32 spectra; the Ti 2p core lines recover the typical symmetry of the clean and stoichiometric TiO2(001) surface and the higher binding energy feature only is detected in the TiO2 band gap. These data suggest that, within the volume probed by XPS and UPS, Ti ions have a mainly d0 configuration, while V has a single and stable open-shell configuration, as revealed by the significant intensity detected within the TiO2 band gap. These annealing-induced changes are due, as suggested by the O 1sTi 2p core line intensity trend, to an oxygen diffusion from the TiO2 bulk to the surface. Finally, a detailed analysis of the data indicates that different VTiO2(001) interfaces exhibit different behaviours after annealing treatments, depending on Θv. For Θv = 0.7 ML, V interdiffuses into the TiO2 sub-surface layers, whereas for Θv = 0.2 ML it remains at the surface. This finding is consistent with a rearrangement of V atoms. which under annealing occupy first the energetically most favorable surface sites (Θv = 0.2 ML) before interdiffusing into the TiO2 lattice (Θv = 0.7 ML).

Single-particle and collective excitations of KNi(111) were studied by high resolution electron energy loss spectroscopy (HREELS). Loss spectra were taken at 200 K as a function of K coverage over the 0–4 eV range of loss energy. Two peaks and a shoulder are characteristic of the submonolayer regime while, as the second atomic layer of potassium is formed, a single distinct feature dominates the loss spectrum. We assign the low-energy features in the submonolayer coverage regime to single-particle excitations: the loss at 0.5–0.6 eV is interpreted as due to electronic transitions from alkali s states to the Fermi level, while the one at about 1.68 eV (for a K coverage of 0.25 ML) is assigned to transitions from initial hybridized alkali s-Ni dz2 states, to the Fermi level. We measured the dispersion curve ω(q‖) of the single feature of the two layers of potassium. It follows the theoretical predictions for the surface plasmon of a thick K overlayer showing an initial downward dispersion versus q‖. The present results also give a hint of the nature of the alkali metal-to-substrate interactions from submonolayer regimes up to two layers of K on the Ni surface.

The coadsorption of CO and K on Ni(111) has been studied by high-resolution electron energy-loss spectroscopy. We used three precoverages, one of them (0.25 ML) corresponding to an ordered surface overlayer (p(2 × 2)) and the other two (0.3 and 0.47 ML), to an incommensurate and a disordered surface layer, respectively. Different local CO adsorption arrangements are suggested for each K coverage. The vibrational spectra show features at 27, 180 and 210 meV. The loss at 27 meV is related to a vibration of the reconstructed Ni(111) surface due to the p(2 × 2)-K layer and is damped by 3 L of CO. For the other two K precoverages, that loss exists regardless of the amount of CO. The losses at 180 and 210 meV are assigned to the CO stretching vibration corresponding, respectively, to the occupation of only one or both sites of the same p(2 × 2)-K cell. The CO saturation of the p (2 × 2)- K Ni (111) surface gives rise to a short-range interaction among CO molecules as well as between each K atom and CO molecules, which causes an overall shift of the CO stretching frequency towards higher loss energies.

The molecular axis orientation of CO and NO molecules adsorbed at room temperature on Ni(111) at low coverage has been studied by the XPD technique. The CO and NO uptake versus exposure has been followed by XPS. Our results show that CO molecules are perpendicular to the surface, while NO molecules are strongly tilted or lying down.

Electron energy loss spectra of M 23 core levels of Cu and Ni in the reflection mode are reported. The analysis of the oscillations up to about 300 eV above the edges shows strict analogies to EXAFS. A comparison with X-ray results above Cu and Ni K edges shows that high accuracy in the radial distribution function (RDF) can be obtained also by ELS spectra taken above shallow core levels. The present technique has several advantages over conventional absorption of X-ray and transmission EELFS measurements. In particular thin film samples are not required and measurements of surface effects, such as relaxation and adsorbate structure determination, become feasible.

The structural properties of a carbidic carbon layer on the Ni(100) surface have been investigated both experimentally by surface extended energy loss fine structure (SEELFS) and theoretically by the linear-combination-of-Gaussian-type-orbitals model-potential local-spin-density (LCGTO-MP-LSD) method. Both experimental and theoretical results indicate that the most stable site for carbon chemisorption is the fourfold hollow site. The Ni-C distance derived from SEELFS analysis was found to be 1.75 ± 0.05 A, in good agreement with that obtained by theoretical calculation (1.79 A). The calculated vibrational frequency of the adsorbed carbon atom is found to be 407 cm−1 in excellent agreement with the experimental value of 410 cm−1.

Clean and oxygen-covered Fe(110) has been studied by surface extended energy-loss fine-structure technique. From the oscillations above the Fe M 23 edge the radial distribution functions, for 6 and 100 langmuir of oxygen at 300 K, are obtained. A larger distance between iron atoms is observed for 6 L of oxygen exposure and the O-Fe distance is found. At 100 L coverage the iron surface seems to reconstruct to the hexagonal structure of the FeO(111).

Cyanogen (NCCN) adsorbs molecularly at 100 K on Pd(100). For monolayer coverage, the Penning spectrum shows a narrow, intense peak at 6.8 eV binding energy, as well as weaker peaks at 5.8 and 7.8 eV also present at lower coverages which are identified with the MO's of molecular NCCN. Above 250 K, dissociative chemisorption is observed. Predominantly resonance ionization is obtained from He ∗ scattering from chemisorbed CN, however there is a distinct contribution from Penning processes. The dominance of resonance ionization over Penning de-excitation is surprising when compared with results for CO and NO at 300 K. It is likely that CN as well as NCCN is adsorbed with the molecular axis parallel to the Pd surface.

Autoionization features have been observed in Nb and Mo surfaces above the N23VV Auger lines. Similarly to 3d transition metals, they are interpreted as due to a coherent decay of the excited 4p electron and to a subsequent interference effect with the direct photoexcitation channel from the valence band. If the radial dependence of the 4d wavefunction does not vary strongly across the band, the line shape of the autoionization electron emission is expected to be similar to the core-energy loss result from 4p core levels. The correspondence between secondary electron emission and energy loss backscattering yield has been studied for both the clean and oxygen-exposed surfaces. A close similarity is found between the electron-induced phenomena observed above the N23VV Auger lines and the N23 core absorption.

Angle-resolved secondary electron emission excited by electrons has been used to map out the unoccupied bands of graphite single crystal. Highly dispersive structures are observed in the low kinetic energy region (0–30 eV) of the electron yield collected along the Γ-M and Γ-K lines of the two-dimensional Brillouin zone. The experimental bands are in very good agreement with recent band structure calculations. The different behaviour of the observed dispersion lines along Γ-M and Γ-K supports that effects due to single crystal of graphite are detected. Particularly, the splitting of the 2s2pxy band (∼ 3 eV kinetic energy) along the Γ-M direction has been clearly observed for the first time. The use of angular resolved spectroscopy of secondary electrons for investigating the band structures of two-dimensional systems above the vacuum level has been established as a powerful and straightforward technique.

Carbidic carbon layers, obtained on Ni (110) and (100) single crystals by exposing the samples to CO at pressures in the range 1 × 10 −6 to 3 × 10 −5 Torr at 520 K, have been studied by Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). The carbidic carbon covered surfaces have been gradually heated, and we observed new temperature-dependent carbon Auger lineshapes, which we tentatively attribute to new intermediate carbon species. At higher temperatures the carbidic carbon evolves towards graphitic carbon on Ni (110), while on Ni (100) graphitic species could not be obtained in the above pressure range.