A new ultrahigh vacuum (UHV) electron paramagnetic resonance (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 metal 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 a 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 low energy electron
diffraction (LEED)/Auger spectroscopy, temperature programmed desorption
(TPD), and infrared reflection absorption spectroscopy (IRAS) are available to
characterize the surfaces. A second chamber used to perform EPR spectroscopy
at 94 GHz has a room temperature scanning tunneling microscope 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 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 5 × 1011 spins/cm2, 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
shows 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