250 MHz crossed‐loop resonator for pulsed electron paramagnetic resonance

Abstract

ABSTRACT:The crossed-loop resonator (CLR) uses two orthogonal lumped-element resonators [one to excite the spins and one to detect the electron paramagnetic resonance (EPR)] to isolate the signal from the microwave source. It eliminates the need for a circulator. The high isolation provided by the CLR reduces the energy stored in the resonator that detects the signal, thereby reducing the intensity of the resonator ring down after the pulse, which decreases the instrument dead time. Overcoupling and synchronous switching of the Q's of the two resonators between high and low states were used to further reduce dead time and maximize the EPR signal following a pulse. Each section of a 250 MHz resonator that accommodates 1 inch diameter samples had a critically coupled Q of 950 when Q-switching was not installed. With Q-switching, free induction decays and electron spin echoes for 0.2 mM aqueous solutions of triarylmethyl radicals were obtained with dead times of a few hundred nanoseconds. Typically 50 -60 dB isolation was achieved with various samples.

Fast‐response VHF pulsed 2 KW power amplifiers

“…The sample was in a 10-mm o.d, 9-mm i.d. sample tube in the 25-mm diameter crossed-loop resonator previously described (9). Although this is a strong sample (ϳ4 ϫ 10 17 spins in the resonator), there is significant interference of instrumental artifacts with the FID for at least the first half microsecond.…”

A pulsed and continuous wave 250 MHz electron paramagnetic resonance spectrometer

“…The CLR isolates the EPR signal from source noise and decreases the dead time in pulsed EPR, as described in Ref. 40. The isolation provided by the CLR also permits placing the lownoise signal amplifier close to the output of the resonator, which cannot be done with a reflection resonator.…”

“…Rectangular cavity resonators or cylindrical cavity resonators would be extremely large at these low RFs [see the review by Hyde and Froncisz (35)]. The crossed-loop resonator (36 -39) used for pulsed EPR in our spectrometer is described in an accompanying paper (40). Sotgiu and coworkers incorporated reentrant resonators and multipole magnets, which can produce magnetic field gradients in low-frequency EPR imaging systems (41)(42)(43)(44)(45).…”

High-Frequency EPR Instrumentation

“…The frequency dependence of the signal-to-noise from the MHz range up to 250 GHz has been analyzed by Rinard et al [2,[102][103][104]. Here, we will only treat the most relevant parameters related to source and detector noise.…”