Pulsed Dynamic Nuclear Polarization

In the last two decades continuous-wave (CW) microwave irradiation obtained from gyrotron microwave sources has been utilized extensively in the development and applications of new experimental approaches to high frequency dynamic nuclear polarization (DNP). Despite the abundant successes of this approach, it is well established experimentally and understood theoretically that at higher magnetic fields, where the resolution of the NMR spectra is optimal, the enhancement factors in CW DNP experiments decrease. Potentially this issue can be mitigated by using time domain or pulsed DNP techniques, which theoretically have field-independent enhancement factors. In this contribution, we discuss the pulsed DNP experiments that have been developed to date, along with the theory and the applicability of the sequences. As we will see pulsed techniques are fundamentally different from the CW-DNP methodology and require a different array of instrumentation, spin physics, and radicals. Hence, in addition to the underlying theory, we discuss the specifications of the microwave sources, DNP probes, and optimal radicals for pulsed DNP. The review ends with a summary of the current and future applications of pulsed DNP and conjectures as to the development of the pulsed methods for experiments at increasingly higher magnetic fields.