Time-resolved Microwave Induced Optical Nuclear Polarization

It was shown that a higher nuclear polarization can be achieved by transferring the high population differences of the electronic sublevels of photoexcited triplet states in molecular crystals to the nuclear spin system /I/. This transfer occurs in the level anticrossing area of an external magnetic field BQ /2/ or with simultaneous irradiation of light and microwave, a method which was termed microwave induced optical nuclear polarization (MI-ONP) /3/. The main advantage of the MI-ONP as compared to the classical method for obtaining high nuclear polarizations by transferring the Boltzmann polarization of the unpaired electrons of doublet ground states, e.g. free radicals, to the nuclei, termed solid effect, originates from the short lifetimes of the excited triplet states of the order of a few milliseconds to a few seconds. When polarizing the nuclei by their interaction with doublet ground states, the unpaired electrons are always present within the sample and may have undesirable effects in most experiments to be performed with the polarized nuclei. On the other hand, when polarizing the nuclei by their interaction with the unpaired electrons of photo-excited triplet states, the unpaired electrons vanish with the lifetime of the triplet states and leave a diamagnetic crystal with polarized nuclei which preserve their polarization for many hours due to the in most cases very long nuclear spin lattice relaxation time T^ of spins I = 1/2 in diamagnetic solids at very low temperatures.