Dynamic Polarization by Thermal Mixing between Two Spin Systems

A new dynamic nuclear polarization method is described, producing polarizations larger than the maximum polarization arising from a "solid effect." The main effect is a cooling of a spin system $I$ in zero applied or effective field, where its Hamiltonian consists of its dipolar spin-spin interactions only; this cooling is followed by an adiabatic magnetization which produces the polarization. The cooling of the spin system $I$ results from its thermal mixing with a second spin system $S$, having a large quadrupole interaction. Two types of experiments are considered: In zero applied field the thermal mixing is achieved in a frame rotating with respect to spins $S$ and fixed with respect to spins $I$, by irradiating the sample with an rf field of frequency close to the quadrupole frequency of the spins $S$. When a dc field is applied, the mixing takes place in a frame rotating at different frequencies with respect to spins $I$ and $S$, by using two rf fields of frequencies close, respectively, to the resonance frequency of each spin system. The theory is based entirely upon the concept of spin temperature, both in laboratory and rotating frames. Experiments performed on paradichlorobenzene confirm the main features of the theory and provide a furthur verification of the spin-temperature hypothesis. A maximum proton polarization eight times larger than the polarization arising from a full solid effect has been obtained.