The CO binding behavior to gold particles supported on MgO thin films has been analyzed with scanning tunneling microscopy (STM) and infrared spectroscopy (IRAS). The ad-particles accommodate excess electrons that originate either from a charge transfer through the thin oxide film or from a local interaction with electron-rich oxide defects that act as Au nucleation centers. The enhanced electron density in the Au aggregates affects both the spatial distribution and the vibrational properties of adsorbed CO species. Whereas preferential CO attachment to the chemically unsaturated and electron-rich boundary sites of the Au islands is deduced from the STM data, a continuous downshift of the CO stretching frequency with decreasing particle size is observed in IRAS. Both results are interpreted in the light of CO adsorption to negatively charged metal aggregates and used to draw general conclusions on the interplay between charge and adsorption properties of confined metal systems.

The adsorption of N2O on the surface of MgO(001) thin films has been studied at low temperature (60 K) using infrared reflection−absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The observed infrared spectrum consists of several components indicating different adsorption sites for N2O. The different IR peaks can be related to particular thermal desorption features by combining the TPD spectra with temperature-dependent infrared measurement. By comparing spectra from films with different roughness and different supports, a band at 2236 cm−1 can be assigned to N2O adsorbed on the MgO terraces, while peaks at the higher frequency are assigned to molecules adsorbed on low-coordinated sites.

Abstract:A new ultrahigh vacuum EPR spectrometer operating at 94 GHz to investigate paramagnetic centers on single crystal surfaces is described. It is particularly designed to study paramagnetic centers on well-defined model catalysts using epitaxial thin oxide films grown on single crystals. The EPR setup is based on a commercial Bruker E600 spectrometer, which is adapted to ultrahigh vacuum conditions using a home made Fabry Perot resonator. The key idea of the resonator is to use the planar metal single crystal required to grow the single crystalline oxide films as one of the mirrors of the resonator. EPR spectroscopy is solely sensitive to paramagnetic species, which are typically minority species in such system. Hence, additional experimental characterization tools are required to allow for a comprehensive investigation of the surface. The apparatus includes a preparation chamber hosting equipment, which is required to prepare supported model catalysts. In addition, surface characterization tools such as LEED/Auger, TPD and IRAS spectroscopy are available to characterize the surfaces. A second chamber used to perform EPR spectroscopy at 94 GHz has a room temperature scanning tunneling microscope (STM) attached to it, which allows for real space structural characterization. The heart of the UHV adaptation of the EPR experiment is the sealing of the Fabry-Perot resonator against atmosphere. To this end it is possible to use a thin sapphire window glued to the backside of the coupling orifice of the Fabry Perot resonator.With the help of a variety of stabilization measures reducing vibrations as well as thermal 2 drift it is possible to accumulate data for a time span, which is for low temperature measurements only limited by the amount of liquid helium. Test measurements show that the system can detect paramagnetic species with a density of approximately 5x10 11 spins/cm 2 , which is comparable to the limit obtained for the presently available UHV-EPR spectrometer operating at 10 GHz (X-band). Investigation of electron trapped centers in MgO (001) films show that the increased resolution offered by the experiments at W-band allows to identify new paramagnetic species, that cannot be differentiated with the currently available methodology.3