A CW and pulse EPR spectrometer operating at 122 and 244 GHz using a quasi-optical bridge and a cryogen-free 12 T superconducting magnet

Abstract

A high-field continuous-wave (CW) and pulse electron paramagnetic resonance spectrometer operating at 122 and 244 GHz is desc¡The instrument is based on a millimeter-wave bridge built from quasi-optical components. To improve the sensitivity, a cryo-cooled detector/mixer is used. The magnetic field is generated using a cryogen-free superconducting 12 T magnet (warm bore, 88 mm) equipped with a helium-flow cryostat for sample cooling. The advantages of this spectrometer are described and first results (obtained in CW mode) on different types of samples at 122 and 244 GHz are presented. The extensions to pulse operation as weI1 as double resonance techniques (electron-electron and electron-nuclear) are briefly discussed.

High-Frequency EPR Instrumentation

“…This principle was applied in the pulsed EPR bridge at 275 GHz by Schmidt et al [39,82]. Similar schemes were employed in the multi-frequency bridges operating at 120 and 240 GHz developed by van Tol et al [41,81] and Reijerse et al [42]. The functional block diagram of these systems is illustrated in Fig.…”

“…The ease of operation and the possibility to integrate these devices in heterodyne as well as homodyne configurations lead to the expectation that these may be the sources of the future. Several groups already have integrated these designs in their HF-EPR spectrometers [38,41,42,56].…”

“…The use of quasi-optics and corrugated waveguides in combination with induction-mode detection, as described above, will greatly increase the achievable sensitivity. It has been demonstrated that non-resonant probe heads using tapered (corrugated) waveguides can achieve similar concentration sensitivity as a singlemode resonator [42,60,81]. The decoupling of the cross-polar components to nonresonant probe heads can be further improved by using an adjustable roof mirror underneath the sample holder, correcting phase and polarization mismatches caused by the sample itself.…”

“…In addition, phase drifts due to temperature variations in the probe head insert are compensated. The quasi-optical heterodyne spectrometers developed by van Tol et al [41,81] and Reijerse et al [42] are based on unity-gain optics. This enables a multi-frequency operation since the optics are frequency independent.…”

“…To control these high powers and maintain a very short system dead-time, advanced quasioptical techniques have proven to be essential [36,37]. Also pulsed EPR at frequencies higher than 150 GHz (using quasi-optical spectrometers) is being pursued in an increasing number of laboratories [38][39][40][41][42].…”