No abstract

No abstract

The development of a single-mode dielectric resonator specifically designed for high-frequency paramagnetic resonance applications is discussed. The use of dielectric resonators is expected to give better performances in comparison to metallic cavities, as well established at low frequency. The relatively low dielectric constant of the common good-quality materials at high frequency requires the presence of a metallic shielding in order to obtain ah efficient single-mode operation. The configuration proposed in this paper concems a partially open structure in which the confinement of the radiation is guaranteed by the forbidden propagation around the dielectric region. By this way a single-mode resonator can be obtained for arbitrary values of the employed dielectrie constant. The complex resonance frequencies and the field distributions of the proposed nonradiative device are obtained in terms of the complex permittivity of the resonator and of the finite conductivity of the employed conductors. The actual field distribution and intensity can then be obtained even in the presence of a lossy sample. An efficient excitation scheme, fully compatible with the common electron paramagnetic resonance setups, has been developed exploiting the nonradiative nature of the proposed device. Preliminary measurements around 90 and 186 GHz have been then successfully realized. Finally, the specific benefits of the proposed resonator have been discussed and compared with the performances of a conventional metallic cavity.

This contribution presents a novel design of a double-resonance structure for high-field dynamic nuclear polarization operating at 95 GHz and 144 MHz, in which a miniaturized radiofrequency coil is integrated within a single-mode nonradiative dielectric resonator. After a detailed discussion of the design principles, the conversion factors of this system are determined by means of microwave and radiofrequency measurements. The obtained results, 1.68 mT/W 1/2 for the microwave conversion factor and 0.8 mT/W 1/2 for the radiofrequency conversion factor, represent the state-of-the-art among the double-resonance structures. Simultaneous electron paramagnetic resonance and liquid-state 1 H nuclear magnetic resonance experiments are performed on samples of nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl dissolved in a mixture of water and dioxane. A maximum dynamic nuclear polarization enhancement of about -16 is obtained at a microwave power of 70 mW with a radical concentration of 10 mM in nanoliter-sized sample volumes. These results are discussed in view of further improvements and applications of the proposed double-resonance structure. G. Annino (&)Istituto per i Processi Chimico-Fisici, CNR,

Measurements for the determination of the complex permittivity of liquid and solid materials based on the use of whispering gallery (WG) dielectric resonators are presented. The procedure implies the measurement of the electromagnetic parameters of the involved resonator interacting with the material under study. The field distribution in the different WG resonant modes was obtained by an analytical calculation under the mode matching method approximation. The results of this calculation, combined with the experimental data, well account for the values of the complex permittivity of materials of interest. Due to the peculiar properties of WG resonators, the method shares the advantages of the wideband systems with the sensitivity and accuracy of the resonator systems in dielectric measurements. Dielectric permittivity of alumina (Al2O3) and of cyclohexane is measured in the frequency range 18–26 GHz. The overall accuracy of the measurements is discussed and the different sources of errors analyzed. Specific attention is paid to the measurements of variations of dielectric properties to monitor some physico–chemical processes. Finally, preliminary measurements at hundreds of GHz show that the procedure is particularly useful for study of dielectric properties up to the THz frequency region.