A novel loop-gap resonator probehead for EPR and ENDOR at X-band

Abstract

Abstraet. An EPR and ENDOR probehead with a loop-gap resonator for X-band is described. The novel feature of the construction is that an iris-type coupling of the resonator is used instead of the conventional antenna coupling. The ENDOR coil combines the role of creating the radio frequency field and that of a shield for the microwave loop-gap structure. Hence, in order to accommodate the iris and waveguide, a pair of RF coils is used in conjunction with a reduced waveguide with dielectric filling. This arrangement simplifies matching the resonator to the microwave bridge, and standard EPR cryostats can be used making sample manipulation more convenient.One of the main possibilities of increasing the effŸ of Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) experiments, still open for development, is to design alternative resonator eonfigurations. The major innovations in this area have been to reduce the dimensions of the resonance mode region with the highest microwave (MW) field, B~, as compared to those of conventional, cylindrical of rectangular, EPR cavities. Thereby, the filling factor can be improved for small samples increasing the signal-to-noise (S/N) ratio. Moreover, this also increases the B1/~/-P (Pis MW power) ratio at typically lower Q, allowing for easier saturation in continuous-wave (cw) EPR, a necessary condition for cw ENDOR, and faster MW pulsing for pulsed EPR.Several types of sueh "concentrating" structures have been proposed: the loopgap resonator (LGR) [1-6] and the bridged loop-gap resonator (BLGR) [7-10]; the slotted tube resonator (STR) [11][12][13]; the stripline resonator [14,15]; the transmission-line resonator [16]; and the dielectric resonator [17]. The probehead also includes the MW-screening shield, typically cylindrical in shape, which encloses these structures. Therefore, these resonator constructions may be consid-