Time-optimized pulsed dynamic nuclear polarization

Pulsed dynamic nuclear polarization (DNP) techniques can accomplish electron-nuclear polarization transfer efficiently with an enhancement factor that is independent of the Zeeman field. However, they often require large Rabi frequencies and, therefore, high-power microwave irradiation. Here, we propose a new low-power DNP sequence for static samples that is composed of a train of microwave pulses of length t p spaced with delays d. A particularly robust DNP condition using a period t m = t p + d set to ~1.25 times the Larmor period t Larmor is investigated which is a time-optimized pulsed DNP sequence (TOP-DNP). At 0.35 T, we obtained an enhancement of ~200 using TOP-DNP compared to ~172 with nuclear spin orientation via electron spin locking (NOVEL), a commonly used pulsed DNP sequence, while using only ~7% microwave power required for NOVEL. Experimental data and simulations at higher fields suggest a field-independent enhancement factor, as predicted by the effective Hamiltonian.Swiss National Science Foundation (Grant no. 165285)German Research Council (Emmy Noether Grant project no. 321027114)National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant EB-002804)National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant EB-002026