Identification of the dinitrogen⟨001⟩split interstitial H1a in diamond

The structure of the defect responsible for the H1a absorption feature in diamond has been much debated. In this work, the results of uniaxial stress-splitting studies are compared to piezospectroscopic stress-splitting parameters calculated for the $⟨001⟩$ dinitrogen split interstitial $({\text{N}}_{2\text{I}})$ defect. The stress-splitting data shows that the H1a absorption band arises from an $A\ensuremath{\rightarrow}E$ transition at a center with ${D}_{2d}$ symmetry. Furthermore, the experimentally and theoretically determined stress-splitting parameters are in excellent agreement, supporting the assignment of the H1a band to the ${\text{N}}_{2\text{I}}$ defect.