On the Peculiar EPR Spectra of P1 Centers at High Magnetic Fields

The most common lattice defect in high-pressure high-temperature (HPHT) diamonds is the nitrogen substitution center. This is a paramagnetic defect with a single unpaired electron spin coupled to a ¹⁴𝑁 nuclear spin forming an 𝑆 = ½, 𝐼 = 1 spin system. While P1 centers have been studied by EPR spectroscopy for decades, only recently did their behavior at ultra-high (> 12 T) magnetic fields become of interest. This is because P1 centers were recently found to be very efficient polarizing agents in Dynamic Nuclear Polarization (DNP) experiments, which are typically carried out at high magnetic fields. The P1 ultra-high field EPR spectra show multiple peaks which the lower fields spectra do not. In this paper, we present an account of the EPR spectra of P1 centers at ultra-high fields and show that the more complex spectra at 12–20 T are the result of significant state mixing in the 𝑚𝑆=+½ electron spin manifold. The state mixing is a result of fulfilling the cancellation condition, meaning the e–¹⁴𝑁 hyperfine interaction equals twice the Larmor frequency of the ¹⁴𝑁 nuclear spin. We illustrate the influence of the cancellation condition on the EPR spectra by comparing EPR spectra acquired at 6.9 and 13.8 T. While the former are similar to the consensus spectra observed at lower fields, the latter are very different. We present numerical simulations that quantitatively account for the experimental spectra at both 6.9 and 13.8 T. Finally, we use Electron Electron Double Resonance (ELDOR) measurements to show that the cancellation condition results in increased spectral diffusion in the 13.8 T spectrum. This work sheds light on the spin properties of P1 centers under DNP-characteristic conditions, which will be conducive to their efficient utilization in DNP.