Electron spin relaxation of P1 centers in synthetic diamonds with potential as B₁ standards for DNP enhanced NMR

The microwave magnetic field, B₁, in the non-resonant structures typically used for DNP-enhanced NMR is relatively small, so calibration via continuous wave (CW) power saturation requires a sample with longer spin lattice relaxation times than the samples used as CW standards in X-band cavities. HPHT diamonds have strong, easily observed EPR signals from P1 centers (nitrogen defects), and are indefinitely stable. This makes HPHT diamonds attractive as secondary standards for calibration of electron B₁ field strength in a variety of experimental arrangements. The concentrations of P1 centers is also typically in the 30-200 ppm range, or equivalently 10-60 mM, and therefore the EPR relaxation observed is relevant to DNP enhanced NMR employing free radical polarizing agents at similar concentrations. Pulsed and CW saturation relaxation measurements T₁ and T₂ are compared at X-band. Under CW conditions the relevant T₁T₂ product of time constants in our samples at room temperature is found to be dominated by electron-electron spin diffusion, and the product is large enough that saturation will be possible with the B₁ of typical DNP systems. The similarity of T₁ and T₂ values obtained by pulse measurements at X-band and Q-band suggests that the X-band results can be extrapolated to the higher EPR frequencies used for DNP experiments. The electron spin dynamics observed here in HPHT diamond samples identify them as useful model systems to better delineate the interplay of electron spin relaxation, magic angle spinning, and inhomogeneous microwave irradiation as they affect DNP enhancement.