The vibrational properties of well-defined, two-dimensional silica films grown on Ru(0001) are characterized by high-resolution electron energy loss spectroscopy (HREELS). It is an interesting model system because it can adopt both crystalline and vitreous states. A transformation between these states induced by thermal annealing does hardly change the vibrational spectrum despite the redistribution of ring sizes. This holds good for the two intense phonon modes as well as for a variety of weaker modes observed by HREELS. The HREELS spectra allow the characterization of the structural arrangement of the oxygen atoms on the Ru(0001) surface underneath the silica bilayer. The density of oxygen at the interface can be controlled by the oxygen partial pressure during annealing, resulting in a characteristic change of the corresponding signals, which can be assigned to different oxygen structures based on density functional theory calculations. By comparison with quantum mechanical calculations and spectroscopic results from the literature, we assign most of the remaining weak signals observed here to the dipole-inactive modes of the bilayer film, structural imperfections such as patches of monolayer structure, and additional silica particles on top of the bilayer film.

Electron-assisted hydroxylation of single-crystalline silica bilayer films grown on Ru(0001) is studied by high-resolution electron energy loss spectroscopy (HREELS) and electron paramagnetic resonance (EPR) spectroscopy. The HREELS results reveal the formation of several hydroxyl species whose number and speciation depend on the defect structure of the film. For incomplete bilayer films, which exhibit nanometer-sized holes in the bilayer, the level of hydroxylation is significantly larger than for complete films. HREEL spectra taken in off-specular geometry provide evidence for the presence of hydroxyl groups with a transition dipole moment almost parallel to the surface for complete and incomplete bilayer films. Hydroxylation with isotopically labeled water (H2 18O) reveals a clear difference between the two casesOH species on the incomplete film almost exclusively contain oxygen from water, while the more ideal film exhibits OH groups with oxygen atoms stemming from both water and the silica film. These observations not only indicate that the degree of hydroxylation is significantly enhanced for the incomplete film but also that the reaction mechanism for hydroxylation at defect sites of this film is different. To gain insight into the reaction mechanism of electron-assisted hydroxylation, in situ EPR spectroscopy of electron-bombarded adsorbed ice layers was combined with infrared (IR) spectroscopy and mass spectrometry. We show that the electron bombardment removes a significant part of the water layer and produces different reactive paramagnetic species, namely, O2D, D, and solvated electrons, which may be trapped at low temperatures. The interaction of the silica film with such species may lead to splitting Si–O bonds even for covalently saturated silica structures as found in the ideal bilayer film and thus provide insight into possible reaction mechanisms.

Paramagnetic defects of thin CaO(001)/Mo(001) films are characterized by EPR spectroscopy under ultrahigh vacuum conditions.

No abstract