Metastable Impact Electron Spectroscopy Spectra Research Articles

Adsorption studies of trichloroethylene (TCE) on MgO(100)/Mo(100)

Employing ultraviolet photoelectron spectroscopy (UPS, He I), the more surface sensitive metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) measurements of the adsorption properties of the pollutant trichloroethylene (TCE) on thin MgO(100) films, grown on a Mo(100) single crystal, have been investigated. From TPD spectra of different coverages it is concluded that TCE interacts only weakly with MgO, which is attributed to physisorption. For increasing coverages a change from one peak to two peaks in the TPD spectra, one at higher, the second at lower temperatures with respect to the single peak is detected. Additionally, the observation of a local minimum for the work function (WF) for both MIES and UPS spectra is presented. Such a local minimum has been reported previously for the adsorption of metals with outer s valence electrons on transition metal substrates and adsorption of metals with outer s valence electrons on metal oxide films. Herein, we present the first WF minimum observed for a system of organic molecules adsorbed on an insulating surface. Two different models are discussed in order to understand the presented results.

Sodium Interacting with Amorphous Water Films at 10 and 100 K

In the present study, we compare the adsorption of Na on amorphous D(2)O ice films, held at 10 and 100 K. OH, D(2)O, and Na are easily distinguished by their characteristic signatures in metastable impact electron spectroscopy (MIES). It is found that at 10 K substrate temperature the donation of 3sNa charge to the ice film, which is regarded as a precursor for water deprotonation, is significantly reduced relative to 100 K. This observation is discussed on the basis of recent theoretical work, suggesting that a rearrangement of the water molecules at the outermost water surface is the prerequisite for hydration/solvation of the 3sNa electron in the water ice bulk. The MIES spectra, showing spectral features from both OH and D(2)O, can be interpreted as reflecting the composition of the Na-water complexes in the near surface region. The relative intensity of the OH and D(2)O features is the same for 10 and 100 K. This finding suggests that two different sites for Na adsorption exist, one on the perfect water network and the other at OH dangling bond sites whereby, at 10 K, only the latter one leads to deprotonation of D(2)O. Finally, charge exchange phenomena observed when applying electron spectroscopies to ice films are discussed.

Application of the metastable impact electron spectroscopy (MIES), in combination with UPS and TPD, to the study of processes at ice surfaces

We report studies of the interaction (80–200 K) of atoms and molecules with ice films, deposited on tungsten, by combining photoelectron spectroscopy, UPS with HeI and II, metastable impact electron spectroscopy (MIES), and temperature programmed desorption (TPD). The focus is on the following issues: (1) Interaction of Na Atoms with films of solid NH 3 and CH 3OH: the delocalization of the 3sNa electrons and their role in the deprotonation of ice molecules (seen for CH 3OH, but not for NH 3) is studied. DOS (density of states) information from density functional theory (DFT) is compared with the MIES spectra. It is concluded that the 3s electron, delocalized from its Na core and trapped between the core and surrounding molecules, triggers the CH 3OH deprotonation; for NH 3, Na dimers appear to play an important role in the delocalization process. (2) Interaction of salt (NaCl) and acid (HCOOH) molecules with H 2O ice: neither NaCl nor HCOOH penetrate into the ice film during adsorption below 90 K. However, ionic dissociation of NaCl occurs as a consequence of the NaCl–ice interaction. At 105 K the solvation of ionic species becomes significant. The desorption of H 2O from the mixed film takes place between 145 and 170 K; those species bound to Na + and Cl − are removed last. No dissociation of HCOOH is observed as a consequence of the interaction with ice. Partial solvation of formic acid species takes place above about 120 K whereby these species become embedded into the rim of the water film. However, no deeper penetration of formic acid molecules into the water film can be detected before H 2O desorption becomes significant. The TPD spectra of HCOOH display structure, around 160 K, close to or coinciding with the sublimation temperature of water, and around 180 K, caused by species chemisorbed on the tungsten substrate supporting the ice film. The interaction of both NaCl and HCOOH with ice is discussed on the basis of qualitative free energy profiles.

Electron spectroscopy (UPS(HeI and II) and metastable impact electron spectroscopy (MIES)) applied to molecular surfaces: the interaction of atoms and molecules with solid water

We report studies of the interaction of atoms and molecules with solid molecular surfaces, water in particular, by combining photoelectron spectroscopy, UPS with HeI and II, and metastable impact electron spectroscopy (MIES). In MIES charge exchange processes of the Auger-type taking place between metastable He atoms and the surface under study are utilized to gain information on their electronic structure. The MIES spectra give a rather direct image of the surface DOS. We concentrate on the following processes taking place on water films produced at 80 K: (1) Interaction of Na Atoms with Amorphous Solid H 2O Films: emphasis was on the role of the 3sNa electrons in the water dissociation process. In order to make a detailed comparison with density functional theory (DFT), DOS (density of states) information is compared with the MIES spectra. Our results are consistent with the theoretical prediction that the 3s-electron is delocalized from the Na-core and trapped (solvated) between the Na-core and water molecules of the surrounding water shell. (2) Ionization and Solvation of NaCl Interacting with Amorphous Solid Water: at 90 K there is no interpenetration of H 2O and NaCl. However, ionic dissociation of NaCl takes place when H 2O and NaCl are in direct contact. At 105 K the solvation of the ionic species Cl − and Na + becomes significant. The desorption of H 2O from the mixed film takes place between 145 and 170 K; those species bound ionically to Na + and Cl − are removed last. (3) The Interaction of PBTs (persistent, bio-accumulative, and toxic substances), chlorobenzene and chlorophenyl, with amorphous solid water: the organic layers produced at 80 K were annealed up to 200 K under in situ control of MIES and UPS. The different behaviour of the interfaces for the three studied cases is traced back to the different mobilities of the molecules with respect to that of water. The interaction between H 2O and the benzene derivatives is discussed on the basis of qualitative free energy profiles.

Identification of Defect Sites on MgO(100) Surfaces

Methodologies are reported that allow the identification of extended defects on MgO(100) surfaces. The techniques used include low-energy electron diffraction (LEED), metastable impact electron spectroscopy (MIES), D2O and CO temperature-programmed desorption (TPD), and MIES of adsorbed Xe. LEED and MIES data for MgO(100), TPD spectra of D2O and CO, and MIES spectra for adsorbed Xe are significantly altered as a function of the density of the extended defects. NO is shown to be an ineffective probe molecule for titration of defect sites on MgO(100).

Atomic and electronic structures of barium oxide on Si(0 0 1) studied by metastable impact electron spectroscopy (MIES) and coaxial impact collision ion scattering spectroscopy (CAICISS)

The BaO thin film was grown on the Si(001) surface and its surface properties were investigated using metastable impact electron spectroscopy (MIES) and coaxial impact collision ion scattering spectroscopy (CAICISS). The BaO surface reduced by annealing at 600°C in ultrahigh vacuum is covered with a metallic Ba overlayer and exhibits the work function as small as 0.5eV. The semiconducting property recovers upon oxygenation of this surface as revealed from the MIES spectra. It is found that film quality is degraded considerably after several reduction/oxygenation cycles. Probably, this is because a template BaSi2 structure is broken gradually as a consequence of the solid-state reaction between BaO and BaSi2, which is induced by the migration of chemisorbed oxygen to the interface.

Interaction of NH3 with oxygen-pretreated Ni at 600 K

The aim of the present study was to investigate the reaction between ammonia and pre-adsorbed oxygen on Ni at 600 K using metastable impact electron spectroscopy (MIES) and UPS techniques. For a given oxygen-pretreated nickel surface, the changes in the MIES spectra along with ammonia adsorption can be decomposed into three different steps: a first domain without significant changes, a second domain corresponding to drastic changes in the intensity of a signal at 15.8 eV; and a third domain where no more change is detected. The first domain may correspond to the ammonia chemisorption on the surface, whereas the changes in the second domain would be attributed to surface dehydroxylation. The existence of NHx groups on the surface has been detected by UPS; they result from ammonia chemisorption. The changes in the MIES spectra have also been studied versus the initial oxygen uptake on Ni. It emerged that the first domain is larger with increasing initial oxygen exposure: this may be explained by the poorer surface reactivity of NiO, contrary to the oxygen-adsorbed phase. Copyright  2002 John Wiley & Sons, Ltd.

MIES and UPS study of O-predosed nickel surface reduction by H2 deposition

The present study was aimed at investigating the reduction by hydrogen of an oxygen-predosed nickel surface at high temperature (530 K) using metastable impact electron spectroscopy (MIES) and UPS (with He(I)) experimental techniques. Both techniques have allowed us to follow the evolution of the electronic structure of the valence band during H 2 deposition. The work function variation seems to be related directly to the oxygen coverage of the surface and exhibits two stages: an induction period during which no reduction reaction occurs; a rapid reaction period corresponding to the reduction of oxygen. The reaction mechanism entails the formation of hydroxyl compounds but, owing to their thermal desorption at lower temperature in the form of water, no evident OH features have been observed in the MIES and UPS spectra. The 3d band intensity, measured at 0.5 eV under the Fermi level using UPS, increased linearly during H 2 deposition. This evolution could correspond to that of bare Ni atoms in the three or four surface layers. On the other hand, evolution of the MIES signal taken at 5.7 eV under the Fermi level provided us with more accurate information about the evolution of the outermost layer of the predosed nickel surface during H 2 exposure.

Mg interaction with TiO 2(1 0 0): MIES and UPS(HeI) results

The interaction of Mg with TiO 2(1 0 0)-(1×3) surfaces at room temperature was studied with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy. In the early stage of Mg exposure a characteristic initial decrease of the surface work function by about 2 eV and a concomitant rise of the yield for formation of Ti 3+ 3d-cations is observed; charge transfer from Mg adsorbates to Ti cations takes place. Ionically bonded Mg species seem to play a key role as nucleation sites. In the MIES spectra Mg-induced emission is found in the band gap of titania. This emission involves Mg3s electrons that are involved in bonds formed between the ionically bonded Mg + species and additional Mg atoms. Initially, the Mg layer shows 2D growth. From the exposure dependence of the Mg3s-induced MIES signal a critical coverage of about 0.35 ML for the transition from 2D to 3D growth is derived.

Interaction of O2 with LaB6(0 0 1) surfaces as studied with MIES and UPS

Clean and O2 adsorbed LaB6(001) surfaces were investigated by metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS). Two structures appear close to the Fermi level EF and 3 eV below it in the MIE spectrum of the clean LaB6(001) surface (work function; 2.3 eV), and can be assigned to the La(5d) and dangling bond of B(2p), respectively. The peak near EF originates mainly from auto-detachment process between He*(1s2s) and La(5d). The results suggest that the clean LaB6(001) surface is terminated with La. Particular emphasis was paid to changes of these spectral features as a function of oxygen exposure. Both structures disappear at 1.5 L. It is concluded that O2 is dissociated at the La-terminated surfaces. Two oxygen-induced peaks appear in the MIES and UPS(HeII) spectra at 5.8 and 7.5 eV binding energy. It is concluded that O2 is dissociated at the La-terminated surfaces. Their origin is discussed on the basis of two mechanisms for the interaction of O2 with LaB6 surfaces.

Characterization of surface defects on MgO thin films by ultraviolet photoelectron and metastable impact electron spectroscopies

Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy are used in this study to investigate low-defect and defective MgO(100) thin films. Unlike low-defect films, defective films exhibit a new spectroscopic feature located ∼2 eV above the top of valence band. Exposing the defective film to oxygen quenches the emission of electrons from F centers created on the surface and in the subsurface regions. Extended defects, unseen in the MIES spectra of the clean surface, are detectable using NO titration. MIES and thermal programmed desorption indicate that at ∼100 K NO adsorbs dissociatively on defects, forming N2O. Only a small fraction of the MgO surface becomes covered with N2O at ∼100 K for the low-defect MgO film indicating that N2O molecules preferentially adsorb on the extended defects. The saturation coverage of N2O increases appreciably for the defective sample.

Surface defects on MgO thin films: Their detection using metastable impact electron spectroscopy and interaction with probe molecules

AbstractMgO thin films having different defect densities are explored in this study using metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), and scanning tunneling microscopy (STM). Surface point defects on MgO exhibit themselves in both the MIES and UPS spectra as a feature approximately 2 eV above the valance band, whereas extended defects are only observed spectroscopically as a broadening of the O 2p band. The interaction of NO and N2O with the MgO surface as a function of surface defect density is explored. Upon adsorption on MgO thin films at 100K, both NO and N2O show the development of three features which coincide with a standard gas phase N2O spectrum. The saturation coverage of N2O from NO adsorption increases with increasing defect density, indicating that defect sites are mainly responsible for N2O formation. STM images confirm the increase of thin film defect density upon thermal quenching.

Study of the electronic structure of TiO2(110) and Cs/TiO2(110) with metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (HeI)

We report the metastable impact electron spectroscopy (MIES) (He∗1s2s) and ultraviolet photoemission spectroscopy (UPS) (Hei) spectra for clean TiO2(110) and Cs/TiO2(110). The cesium coverage is varied at room temperature between zero and saturation. Simulations of the MIES spectra for clean and cesiated TiO2(110), based on presently available information on the electronic structure of the clean and cesiated surfaces, facilitate the interpretation of the spectra. The MIES results for low Cs coverages are compatible with fully ionic adsorption of Cs whereby the 6s electron of Cs is transferred to the substrate. This causes reduction of Ti4+ cations as detected with UPS. For larger coverages the charge transfer is incomplete as judged from the formation of temporary negative He−∗(1s2s2) ions during the interaction of the metastable He atoms with the cesiated surface. The ionicity, i.e. the charge transfer per adsorbed Cs atom, decreases from practically unity to about 12% at saturation. At saturation a rearrangement of the oxygen in the surface layer takes place with time. The results suggest that the Ti and Cs ions compete for the bonding to the oxygen.

Mg clusters on MgO surfaces: study of the nucleation mechanism with MIES and ab initio calculations

We combined experimental studies using ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) with abinitio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected insitu, and the growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using density functional theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS). The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV atom-1. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

Study of Au multiple-twinned particle micelles by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (HeI) and metastable impact electron spectroscopy

Multiple-twinned particle (MTP) micelles, comprised of Au MTP and self-assembled propionic-acid-thiolates, deposited onto highly oriented pyrolitic graphite and naturally oxidized Si(111) surfaces have been studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) (HeI) and metastable impact electron spectroscopy (MIES). Strong emission from the ionization of the molecular orbitals of the carboxyl (COOH) functional group is identified in the MIES and UPS spectra of the MTP-micelle on both substrates. The results suggest that the outermost functional group of the MTP-micelle is the carboxyl group, and that the Au MTPs are covered with propionic-acid-thiolates. By joint application of XPS, UPS (HeI) and MIES it is demonstrated that the thiolates desorb completely from the substrate (when heating to ∼400° C) while Au remains there. In the case of the silica substrate the α-(√3 × √3)R30° -Au structure is observed with low energy electron diffraction after annealing to 1000°C.

Electron- and photon-stimulated metallization and oxidation of the CaF 2(111) surface

Calcium difluoride bulk crystals cleaved along (111) planes in air and in ultra-high vacuum have been irradiated with 2.5 keV electrons and 1486 eV photons. Radiation induced changes in the surface electronic structure were monitored as a function of irradiation dosage with ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The radiation induced decomposition of the surface is directly evidenced in MIES spectra where upon increased dosage a continuous band appears at the Fermi energy while UPS performed with 21.2 eV photons is found not to be sensitive for the detection of metallized areas at the surface. For crystals cleaved in air after irradiation an additional structure 3 eV above the fluorine 2p valence band edge is found in UPS and MIES spectra that is attributed to oxidized metal patches at the surface. This assignment is confirmed by spectroscopy on Ca films on silicon surfaces dosed with oxygen.

Interaction of small molecules with Mg and MgO surfaces studied with MIES and UPS(HeI)

Metastable Impact Electron Spectroscopy (MIES) in combination with UPS(HeI) was used to study the interaction of molecular (CO 2, CO, N 2O) adsorbates with magnesium and magnesia (MgO) surfaces at room temperature (RT). Spectra obtained for Mg on Si(100) surfaces and for MgO surfaces are discussed. CO does not chemisorb on magnesium at RT. Dosing the Mg surface with N 2O leads to the dissociation of the N 2O molecule in N 2 and O and to the subsequent oxidation of the surface. Mg surfaces exposed to CO 2 show the formation of CO 3 2−. From the comparison of MIES and UPS spectra the following reaction mechanism is suggested: (i) dissociation of CO 2 in front of Mg; (ii) formation of a Mg O bond: and (iii) CO 2 chemisorption at low-coordinated oxygen ion sites of the surface. Spectra for MgO exposed to CO 2 show only weak CO 3 2− features. This can be explained by CO 3 2− formation at defect sites only. Dosing MgO with CO or N 2O leads to no new features in the MgO spectrum.

Characterization of LiF and CaF 2 surfaces using MIES and UPS (HeI)

Metastable impact electron spectroscopy (MIES) and UPS (HeI) in combination with ab initio calculations (CRYSTAL-code) were applied to study surface and bulk defects in LiF and CaF 2. The investigated stoichiometric, defective and doped surfaces are LiF, LiF doped with Mg, and CaF 2. The experimental information obtained on the electronic structure of stoichiometric and defective surfaces of LiF (100), LiF on W (110) and CaF 2 (111) is discussed on the basis of the ab initio calculations. MIES spectra show features from Li agglomerates on the surface of electron bombarded LiF. The electronic structure of the LiF:Mg single crystal shows additional features above the valence band maximum caused by the Mg doping. These are identified as caused by the formation of (Mg O) surface bonds. Exposure of CaF 2 surfaces to X-rays (1486 eV) shows also formation of metallic regions on the surface.

The surface electronic structure of stoichiometric and defective LiF surfaces studied with MIES and UPS in combination with ab-initio calculations

UPS (He I) and metastable impact electron spectroscopy (MIES) spectra of the LiF(100) single crystal surface and stoichiometric LiF films are presented. The spectra are interpreted on the basis of ab-initio electronic structure calculations. Defective surfaces, produced by electron dosing, were studied in the same manner. The MIES spectra reveal that the electron dosing produces metallic patches on the surface, but no uniform Li adlayer. The calculation show that the F-center contribution to the electron emission is very close in energy to that from the metallic patches; thus, the two contributions cannot be distinguished by the present experimental techniques.

Study of the surface electronic structure of MgO bulk crystals and thin films

The electronic structures of the surfaces of MgO single crystals, oxidized Mg polycrystals and oxidized Mg films grown by molecular beam epitaxy on Si(100) surfaces were studied using several techniques. These include metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (He I)), and X-ray photoelectron spectroscopy (XPS). Spectra of oxidized Mg layers on Si(100) show additional features to those obtained for cleaved MgO crystals. These spectral features are attributed to dissociative adsorption of oxygen at bulk oxygen sites. Weak heating of the oxidized Mg layers removes these features and the electronic spectra for all three studied systems become similar. However, the experimental MIES and UPS spectra, both arising mainly from the ionization of the O 2p orbitals, have different structures. They are interpreted on the basis of ab initio Hartree-Fock and density functional calculations of the electronic structures of the ideal MgO(100) surface. It is shown, that the differences in the spectra can be understood by taking into account that UPS spectra reflect the density of electronic states within several surface layers, whereas MIES probes the surface states which are the most extended into the vacuum.