Measurement of electron spin-lattice relaxation times in radical doped butanol samples at 1 K using the NEDOR method

摘要

The electron spin-lattice relaxation time (T_(1e)) of TEMPO- and trityl-doped butanol samples at 2.5 T and temperatures between 0.95 K and 2.17 K was studied by pulsed nuclear magnetic resonance (NMR) using the nuclear-electron double resonance (NEDOR) method. This method is based on the idea to measure the NMR lineshift produced by the local field of paramagnetic impurities, whose polarization can be manipulated. This is of technical advantage as measurements can be performed under conditions typically used for the dynamic nuclear polarization (DNP) process - in our case 2.5 T and temperatures around 1 K- where a direct measurement on the electronic spins would be far more complicated to perform. As T_(1e) is a crucial parameter determining the overall efficiency of DNP, the effect of the radical type, its spin concentration, the temperature and the oxygen content on T_(1e) has been investigated. For radical concentrations as used in DNP (several 10~(19) spins/cm~3) the relaxation rate (T_(1e)~(-1)) has shown a linear dependence on the paramagnetic electron concentration for both radicals investigated. Experiments with perdeuterated and ordinary butanol have given no indication for any influence of the host materials isotopes. The measured temperature dependence has shown an exponential characteristic. It is further observed that the oxygen content in the butanol samples has a considerable effect on the electron relaxation time and thus influences the nuclear relaxation time and polarization rate during the DNP. The experiments also show a variation in the NMR linewidth, leading to comparable time constants as determined by the lineshift. NEDOR measurements were also performed on irradiated, crystal grains of ~6LiD. These samples exhibited a linewidth behavior similar to that of the cylindrically shaped butanol samples.