J. Stultz

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.

CHARACTERIZATION OF COADSORBED MOLECULAR SPECIES IN A MULTILAYER SOLVENT ENVIRONMENT ON INSULATING SURFACES

By employing metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS, HeI) together with work function measurements and temperature-programmed desorption (TPD), the condensation of multilayer solvent systems, such as water and methanol on ultrathin Mo(100)-supported MgO films, has been investigated. In a first step, the techniques MIES/UPS and TPD are used to characterize the condensation of the pure solvent systems. Data collected in a coverage regime from submono- to multilayers at substrate temperatures between 100 and 500 K are surveyed in order to provide information about the buildup of the multilayer systems on an atomic level. Besides investigating the chemistry of pure phases, the present work provides insight into the chemistry of coadsorbed molecular species in a multilayer solvent environment. It is shown that thin films of amorphous water prepared at 100 K are a good candidate for studying aqueous multilayer chemistry under well-defined conditions, i.e. ultrahigh vacuum, by MIES. We are taking advantage of the unique properties of amorphous water films for investigating the interaction of water with coadsorbates, such as sodium and methanol. A deliberately chosen surface preparation technique in combination with the surface-sensitive electron-spectroscopic technique MIES enables the discrimination of various stages in the complex coadsorbate multilayer solvent reaction. Sodium has been chosen as a representative of a class of highly reactive and readily soluble species, and methanol as a surfactant.

Interaction of methanol and water on MgO(100) studied by ultraviolet photoelectron and metastable impact electron spectroscopies

The coadsorption of methanol (CH3OH) and water (D2O) on the MgO(100)/Mo(100) surface at 100 K has been studied by metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy [UPS (HeI)], and by thermal programmed desorption (TPD). Methanol wets the MgO surface and adsorbs nondissociatively within the first monolayer with the hydroxyl group oriented toward the substrate. In coadsorption experiments, methanol wets a water precovered MgO surface; however, adding water to a methanol precovered MgO surface does not yield a water-only surface. Essentially, a constant fraction of the preadsorbed methanol remains within the topmost surface layer during the buildup of the water multilayer. Temperature-dependent measurements show that the adsorption and desorption dynamics of both water and methanol are governed by intermolecular interactions rather than adsorbate–substrate interactions.

TWO- TO THREE-DIMENSIONAL TRANSITION INTERMEDIATE : GROWTH OF BENZENE ON RU(001) AND MO(100)

Abstract The complex phase transitions of physisorbed benzene on Ru(001) and Mo(100) have been investigated using temperature programmed desorption (TPD), metastable impact electron spectroscopy (MIES) and work function measurements. The phase transition from benzene molecules whose planes are parallel to the surface to molecules with their planes approximately perpendicular to the surface is governed by a reversible two-dimensional gas–solid transition in the first physisorbed benzene layer. Furthermore, an additional phase of loosely bound benzene molecules, which can be considered as a two- to three-dimensional transition intermediate, is identified.

Mg clusters on MgO surfaces: characterization by MIES and electronic structure ab initio calculations

MgO films (2 nm thick) were grown on Mo and W substrates while metastable impact electron (MIES) and photoelectron spectra (UPS(HeI)) were collected in situ. Apart from the valence band (VB) emission no additional spectral features have been detected with electron spectroscopies. After exposing the oxide surface to Mg (substrate temperature between 100 K and RT) an additional peak, not seen with UPS, located within the band gap, shows up in MIES. it is located at about 2 eV above the top of the VE with a FWHM of about I eV at the lowest exposures. Electronic structure calculations based on the density functional theory (DFT) are presented for the adsorption of up to Four Mg atoms to the defective MgO surface. Based on these calculations we attribute the additional emission to the formation of small Mg clusters. The calculations suggest that the nucleation starts at extended defects, steps in particular