Interpreting the observed changes of M 1 computed from the NMR line requires a model of the magnetic interactions governing the dynamics. In the problem at hand, these include the spin dipole-dipole interactions and the spin coupling with the probe through radiation feedback. However, because the spins are on resonance with the probe, radiation feedback induces no frequency shift, which is thus caused by the presence of the dipolar field only. Consequently, in these experiments, the first moment can indeed be related to the spin polarization, therefore magnetization, in the sample. During the buildup experiment that led to the spectra of Fig. , the electron polarization is in a steady state, so that the shift of the resonance line, measured by the first moment, varies linearly with the nuclear ( 1 H) z magnetization. In the DNP sample, the shift of the nuclear first moment is due to the local dipolar fields generated by both the nuclei and the paramagnetic impurities. Thus, M 1 is the sum of the respective contributions of the nuclear and electrons, M nn 1 = 2 3 µ 0 Iγ 2 h hξN h P h and M ne 1 = µ 0 Sγ h γ e hξN e P e , where I, S are the nucleus and electron spins, N e and N h their densities, and P h and P e their concentrations. The shape factor ξ = 3 8πN h i 1-3 cos 2 θ ij r 3 ij can be calculated for different sample shapes, and takes the simple values (ξ = 0, -1/2 for a sphere or an infinitely long cylinder). In contrast, the computation of the second moment cannot be related to the line narrowing observed upon DNP polarization (Fig. , where the line width drops from ∼ 16 kHz to ∼ 4300 Hz). Indeed, the width and shape of the resonance line strongly depend on the radiation feedback from the probe that maintains a phase coherence between the spins, so that the expressions derived in the literature for the second and higher moments for the case of dipolar interactions do not apply. Thus, the decrease of the line width with increasing magnetization (Figure ) is a first manifestation of the qualitative change of dynamics induced by radiation feedback from the probe upon hyperpolarization, and illustrates its interference with dipole dipole interactions between nuclear spins.