Rapid diffusion of dipolar order enhances dynamic nuclear polarization

We have observed that the bulk proton dipolar reservoir is cooled to a spin temperature of 15.5 mu K in a dynamic nuclear polarization (DNP) experiment, following microwave irradiation for 800 s. This is significantly cooler than the 35 mK spin temperature of the proton Zeeman reservoir following DNP. Equilibration of the two reservoirs results in a 50% increase in the NMR signal intensity, corresponding to a proton Zeeman spin temperature of 23 mK. In order to achieve this polarization directly, it was necessary to irradiate the sample with microwaves for 1500 s. The experiment was performed on a 40 mM solution of 4-amino-TEMPO in a 40:60 water/glycerol mixture. Cooling of the dipolar reservoir occurs during polarization transport through the magnetic field gradient around the paramagnetic impurity, and is rapidly communicated to the bulk by dipolar spin diffusion. As dipolar spin diffusion is significantly faster than Zeeman spin diffusion, the bulk dipolar reservoir cools faster than the Zeeman reservoir. This process can be exploited to rapidly polarize the nuclear spins, by repeatedly cooling the dipolar system and transferring the polarization to the Zeeman reservoir.