Novel loop-gap probe head for time-resolved electron paramagnetic resonance at 9.5 GHz

A novel probe head with a two-loop-one-gap resonator for X-band (9.5 GHz) time-resolved transient electron paramagnetic resonance measurements with direct detection is described. The low quality factor Q of the resonator allows time resolution in the order of 10 ns while the high filling factor guarantees a sensitivity which is at least comparable to that of conventional cavity resonators. A novel feature of the resonator is the coupling mechanism which exploits the field distribution in the microwave shield. The type of the resonator and the microwave coupling scheme chosen provide high mechanical stability of the setup. This is particularly important for pulsed laser excitation and gas stream cooling of the sample. The advantages of the probe head are demonstrated by relaxation measurements on the photoexcited triplet state of zinc-tetratolylporphyrin, for which the high time resolution allows the determination of fast anisotropic relaxation. This relaxation behavior can be attributed to the dynamic Jahn-Teller effect. (C) 1998 American Institute of Physics. [S0034-6748(98)00810-7].