EPR andN14electron-nuclear double-resonance measurements on the ionized nearest-neighbor dinitrogen center in diamond

The nearest-neighbor substitutional nitrogen center [N-N${]}^0$ (A center), is one of the most common defects in natural diamond. [N-N${]}^0$ is diamagnetic and therefore cannot be studied by electron paramagnetic resonance (EPR). However, the [N-N${]}^{+}$ center is paramagnetic, and we report detailed EPR and electron-nuclear double-resonance (ENDOR) studies on this center. The ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ hyperfine coupling matrices show that approximately 100% of the unpaired electron population is in the lowest-energy antibonding orbital formed between the two nitrogen atoms, which are equivalent. Using orthogonality and simple geometric considerations the ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ hyperfine interaction is used to make an estimate of the length of the N-N bond in the [N-N${]}^{+}$ center. The result appears consistent with more sophisticated calculations on the [N-N${]}^0$, and single substitutional nitrogen centers [N-C${]}^0$. For several defects incorporating substitutional ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ (including [N-N${]}^{+}$) the quadrupole interaction is proportional to the fraction of unpaired electron population on the nitrogen atom. A simple molecular orbital calculation explains this finding, and determines that the quadrupole interaction for a single unpaired electron in a 2p orbital on ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ is -6.7(3) MHz.