Figure 7.

(a) A typical DNP–MAS experimental pulse sequence. Continuous microwave irradiation is used to saturate the electrons in the sample, and the enhanced ¹H polarization is transferred via a CP-MAS experiment to the ¹³C nuclei for observation. A train of saturating pulses on the ¹H channel is used to ensure that all of the nuclear polarization arises from DNP. (b) ¹³C-urea spectra obtained from a sample doped with the biradical BT2E at a concentration of 5 mM (or 10 mM electrons), 90 K, ω_r/2π = 3.5 kHz. The top row shows the enhanced signal (with microwave irradiation on) as a function of the irradiation time τ_DNP. The bottom row shows the ¹³C-urea signal in the absence of DNP (microwave irradiation off). The observed enhancement is 175 ± 25. (c) Buildup curve of the enhanced nuclear polarization (filled circles) with a time constant of 5 s, coinciding with the ¹H T₁ curve, measured without microwave irradiation (empty circles).