High field electron paramagnetic resonance spectroscopy under ultrahigh vacuum conditions—A multipurpose machine to study paramagnetic species on well defined single crystal surfaces

Abstract

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

Field and frequency modulated sub-THz electron spin resonance spectrometer

“…1 spectroscopy in order to explore for example the spin properties of novel oxide interfaces, 2-7 or of spin functional surfaces, [8][9][10][11][12][13][14][15][16] or to unravel details of the reactions taking place at the surface of catalysts [17][18][19][20][21][22][23][24][25][26][27] requires ever greater resolution and sensitivity. This implies the use of ever greater applied fields, which require in turn sample irradiation frequencies in the sub-THz range.…”

Chapter model systems in heterogeneous catalysis at the atomic level: a personal view

“…In order to improve on resolution, it was necessary to move to higher magnetic fields (W-band, 94 GHz). The reduced wavelength of the radiation introduces additional constraints which required to develop an appropriate re-sonator structure [25,26]. However, it has been possible to set up a working spectrometer (Figure 3(b)) and proving its ability to improve spectral resolution using color centers in oxides as a model system.…”

“…(b) Experimental setup of the multipurpose W-band ESR apparatus comprising scanning-tunneling (STM) (green) and IRRAS (blue); inset: closeup of the semi-confocal Fabry-resonator using the metal surface as a planar mirror, adapted with permission from Ref. [25], copyright by AIP Publishing (2014), adapted with permission from Ref. [26], copyright by American Physical Society (2016) (color online).…”

Location of Trapped Electron Centers in the Bulk of Epitaxial MgO(001) Films Grown on Mo(001) Usingin situW-band Electron Paramagnetic Resonance Spectroscopy

“…X-band EPR experiments using a TE 102 resonator were carried out in an UHV chamber equipped with standard facilities for thin-film preparation and characterization, which has been described in detail elsewhere [17]. W-band EPR experiments using a home-built Fabry-Perot resonator were performed in an UHV apparatus comprising a preparation chamber as well as a dedicated chamber for the EPR measurements, as described in detail elsewhere [18]. Microwave power and modulation amplitude were chosen to avoid saturation or overmodulation effects.…”

“…To improve the spectral resolution the same system was studied using in situ W-band (94-GHz) EPR spectroscopy [18]. In contrast to the single line observed at X-band frequency, the W-band spectrum [ Fig.…”