Electron Spin Echoes with a Loop-Gap Resonator*

We report our preliminary findings on the use of a split-ring (I) or loop-gap resonator in electron spin echo (ESE) spectroscopy at 9 GHz. The loop-gap (LG) resonator of Froncisz and Hyde (2) was reported to have several features which could make it more advantageous than the overcoupled (3) TEro2 microwave cavity traditionally used in ESE experiments (4). Among these are the large uniform microwave magnetic field H, per incident microwave power, broadband (low Q), small sample volume, high filling factor, and high field homogeneity over the sample (2). These advantages could make it possible with short pulse lengths to rotate an entire nitroxide spin probe spectrum (30 G) due to the large HI. In addition, conventional ESE experiments could be carried out with a 1 W klystron rather than a traveling wave tube amplifier (TWTA), thus significantly reducing the cost of an ESE spectrometer. Here we compare our findings on the LG resonator to an overcoupled TEtoz cavity typical of that used in ESE spectroscopy. A two-slit loop-gap resonator constructed out of brass and plated with silver was used rather than Macor (Corning Glass) since field modulation is not required in the ESE experiments, and it has superior tolerance to temperature extremes. The dimensions of the resonator were 6.6 mm o.d., 1.2 i.d., 0.1 mm slit width, and 4.5 mm in length. The slits were filled with Teflon, resulting in resonance at 8.85 GHz. The resonator was inductively coupled to the microwave bridge by a loop through the 12.7 mm i.d. outer shield around the resonator. The loaded Q of the LG re:sonator at critical coupling was approximately 400. The TElo2 cavity was a Varian V-4532 with a Gordon coupler (5) and 1.0 cm X 0.72 cm X 0.127 mm thick rectangular coupling hole of silver-plated brass. This configuration allowed overcoupling and gave the cavity a loaded Q of approximately 400. Both the LG resonator and TE102 cavity were overcoupled such that the voltage reflection coefficient was approximately 0.85 (a VSWR of 12) so the calculated (6) overcoupled Q was 65. The details of the ESE spectrometer are described elsewhere (4). Comparisons between a TEIoz cavity and LG resonator were made using the standard 90”-t-180”-t-echo pulse sequence. To assure that the first pulse was a x/2, and not a higher multiple, the minimum power regime needed to produce an