Relaxation of nuclear dipolar energy.

Under typical conditions for dynamic nuclear polarization (DNP)-temperature about 1 K or below and magnetic field about 3 T or higher-the polarization agent causes nuclear dipolar order to relax up to four orders of magnitude faster than nuclear polarization. However, as far as we know, this ultra-fast dipolar relaxation has thus far not been explained in a satisfactory way. We report similar ultra-fast dipolar relaxation of proton spins in naphthalene due to the photo-excited triplet spin of pentacene and propose a three-step mechanism that explains such ultra-fast dipolar relaxation by ground state electron spins as well as by photo-excited triplet spins: nuclear spin diffusion transfers nuclear dipolar order-that is nuclear dipolar energy-spatially to near the electron spins. Flip-flop transitions between nuclear spins near the electron spins convert this dipolar energy into electron-nuclear interaction energy. Finally electron spin-lattice relaxation or decay of the triplet spin transfers the latter type of energy to the lattice. We will show that this mechanism quantitatively explains the observed dipolar relaxation rate. The proposed mechanism is expected to contribute to dipolar relaxation in any spin system containing more than one spin species. It tends to create a stationary state, in which all dipolar interactions are combined in a single energy reservoir described by a single spin temperature. As an example we suggest that the addition of a relaxation agent in samples used for DNP may significantly accelerate the relaxation of the dipolar energy of the polarization agent, and as a result could possibly reduce the contribution of thermal mixing (TM) to DNP.