250 MHz crossed-loop electron paramagnetic resonator for pulsed resonance

The crossed-loop resonator (CLR) uses two orthogonal lumped-element resonators lone 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 0 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. (C) 2002 Wiley Periodicals, Inc.