Microwave Induced Nuclear Polarization

The classical method to increase the nuclear spin polarization is the "solid effect" mainly developed by Abragam and Jeffries, where the Boltzmann polarization of unpaired electrons present in the sample is transferred to the nuclei by inducing forbidden transitions which flip one nucleus and one electron simultaneously (1,2). A different method is the optical nuclear polarization (ONP) which we discovered in 19&7• In this method the nuclear spins are polarized by their interaction with the unpaired electrons of an excited triplet state in a molecular crystal (3) . It could be shown that the selection rules of spin-orbit coupling combined with the mixing of states in the triplet manifold by the hyperfine coupling results in different population (s;) and decay rates (kj) and hence in different equilibrium populations Nj of the electronic nuclear sublevels (4). This type of ONP produces an appreciable nuclear spin polarization p n only in the level anticrossing (LAC) region of the external magnetic field.