ENDOR with a loop-gap resonator

Abstract

The loop-gap resonator is a lumped circuit resonant microwave structure that has been shown to offer significant improvements in the sensitivity of electron spin resonance measurements (I-4) particularly in the detection of dispersion signals (3). These results suggested that the new resonators might offer a number of advantages in electron nuclear double-resonance spectroscopy (5). We describe here the design additions and considerations that have enabled us to perform ENDOR experiments using loop-gap resonators, and present the first results demonstrating the feasibility of this approach. The results presented here employ a Varian E-109 spectrometer and an ENDOR apparatus described elsewhere (5, 6). We have used a two-gap resonator (Fig. 1) constructed from chemically silvered machinable Macor and mounted on a quartz tube. Coupling is achieved with an inductive coupler that can be mechanically displaced relative to the resonator loop. The various dimensions are as follows: r = 1.75 mm; R = 7.54 mm; t = 0.30 mm; W = 1 mm; 2 = 5 mm. These dimensions provide a structure which resonates at -9.4 GHz at room temperature. Field modulation is supplied by Helmholtz coils affixed to the outside of the shield (c in Fig. 1). The entire structure can be suspended in a liquid helium immersion Dewar (Janis Corp.). The radiofrequency field required for the ENDOR experiment is provided by a pair of coaxial cables that enter the shield at the opposite end from the inductive loop (Fig. la) and that terminate in a single turn loop (e in Fig. 1) positioned on the outside of the resonator. This loop lies in the plane orthogonal to that defined by the two gaps, at electric field null points, and traverses the face of the resonator between the latter and the inductive coupling loop. As predicted (2), unwanted resonances are introduced by this addition to the resonator enclosure, and care must be taken in the positioning of the rf co-ax and wire loop in order to minimize these effects. Furthermore, if the length of rf co-ax shield introduced into the resonator enclosure exceeds h/4, resonances occur which can annihilate the proper loop-gap resonance. With the ENDOR loop in place, the resonant frequency at room temperature is unchanged, -9.4 GHz, and the unloaded Q is 1250. The ESR and ENDOR signals obtained by using this structure have been compared in several ways with those of a TE 102 immersion cavity. The cavity contains orthogonal hairpin loops, one for magnetic field modulation (a single turn) and one for rf field