PROBEHEAD WITH INTERCHANGEABLE TUNABLE BRIDGED LOOP-GAP RESONATOR FORPULSED ZERO-FIELD OPTICALLY DETECTED MAGNETIC-RESONANCE EXPERIMENTS ON PHOTOEXCITED TRIPLET-STATES

Conventional zero-field optically detected magnetic resonance (ODMR) i s normally performed by using a slow-wave helix for microwave excitati on with a quality factor Q approximate to 1. With available microwave sources this low Q factor leads to long microwave pulse lengths for co herent pulse experiments (pi-pulse duration of about 300 ns for 20 W m icrowave excitation power). For our zero-field experiments we took adv antage of the bridged loop-gap microwave resonator configuration with relatively high Q factor. Without the possibility of tuning the Zeeman energy level splitting as in electron paramagnetic resonance (ERR), i n zero-field ODMR the resonator has to cover a wide range of frequenci es. We are able to tune our probehead in the range of 1.9-8 GHz with a loaded Q factor of up to 800 by using interchangeable bridged loop-ga p resonators of various designs. Thereby, the pulse lengths, compared to the slow-wave helix, could be reduced by nearly one order of magnit ude (t(resonator)(pi)=45 ns employing the same microwave power of 20 W ). Experimental data are presented for triplet states of photoexcited acridine and benzophenone molecules at different resonance frequencies for their \T-x]-\T-z] transitions (nu=2.472 GHz and nu=5.226 GHz), re spectively. (C) 1997 American Institute of Physics.