Pulsed DNP experiments have been discussed in the literature for quite a while already. While several experiments have been proposed and conducted at low magnetic fields, where instrumentation is less demanding, this is the first example of pulsed DNP experiments performed at high magnetic fields.
Tan, Kong Ooi, Chen Yang, Ralph T Weber, Guinevere Mathies, and Robert G Griffin. “Time-Optimized Pulsed Dynamic Nuclear Polarization.” SCIENCE ADVANCES 5 (2019): 8. https://doi.org/10.1126/sciadv.aav6909.
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 τp spaced with delays d. A particularly robust DNP condition using a period τm = τp + d set to ~1.25 times the Larmor period τLarmor 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.