Electron spin effects in static DNP with broadband excitation

Dynamic nuclear polarization (DNP) is a method of enhancing the nuclear magnetic resonance (NMR) signal, usually in the solid state, and has gained huge popularity in the past 15 years or so, with the ability to perform DNP at high fields (>3 T). In DNP, the large electron spin polarization is transferred to nuclear spins, using on-resonance (or nearly on-resonance) microwave (MW) irradiation on the electrons, creating what is known as nuclear hyperpolarization. In most cases, monochromatic continuous-wave (CW) MW irradiation is used (i.e., MW irradiation at a single frequency), and the frequency of the MW irradiation is chosen such that the highest DNP enhancement is achieved. One method of further increasing the nuclear enhancement is frequency/field modulation. Frequency/field modulation and pulsed DNP techniques were first introduced for low-field DNP (<3 T), where they proved to be very effective methods for increasing the DNP enhancement compared to CW irradiation. Later, researchers creating polarized proton and deuteron targets used frequency modulation to enhance polarization of their targets at magnetic fields of ∼2.5 T. Since 2010, several groups have demonstrated that frequency modulation or trains of chirp pulses can enhance the DNP performance severalfold in static (nonspinning) DNP experiments for NMR at high fields (>3 T). In this article, we review the theory behind frequency modulation in DNP at high fields and discuss the practical aspects of the frequency-modulated DNP experiment under static conditions.