Optical studies of the 1.40-eV Ni center in diamond

Synthetic diamonds grown using a solvent catalyst that contains Ni show the 1.401-1.404 eV luminescence and absorption system. Isotope splitting data on the zero-phonon lines establish that the optical center contains one Ni atom. Uniaxial stress and Zeeman measurements are reported. We show that the optical transition is between a doubly degenerate orbital ground state (²E), which is split by spin-orbit interaction, and a nondegenerate orbital excited state (²A) at a defect with trigonal symmetry. An effective spin of 1/2 is consistent with the Zeeman and the stress results. We propose that the center is a Ni⁺, with electronic configuration (3d⁹,S=1/2), distorted along a 〈111〉 direction.