EPR and 14N electron-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 ¹⁴N and ¹³C 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 ¹⁴N 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 ¹⁴N (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 ¹⁴N is -6.7(3) MHz. [N-N]⁺ can be created in some natural diamonds by illumination with photons of energy greater than 3.0 eV. Studies on the number of [N-N]⁺ centers remaining after the optical excitation is switched off indicate that there is a wide distribution of lifetimes, presumably resulting from a large variation in the separation between a [N-N]⁺ center and its electron trap and/or donor. Below about 25 K, the decay rate is independent of temperature, indicating tunnelling between the donor and/or trap and the [N-N]⁺ center; at higher temperatures thermally activated hopping also contributes. In powered diamond [N-N]⁺ can be observed without illumination whereas in the single crystal it was only observed after illumination, suggesting that a defect created near the surface could be acting as a trap and/or donor. It appears that [N-N]⁺ can be created via electron capture by [N-N]²⁺ or by ionization of [N-N]⁰, or possibly both, depending on the traps and donors available.