It was recalled that the ground state of the neutral vacancy in diamond was diamagnetic and had therefore not been studied by using electron paramagnetic resonance techniques. Observations were made here of the electron paramagnetic resonance of the 5A2 excited state of the neutral vacancy by illuminating an electron-irradiated natural IaB crystal with ultra-violet light. The use of electron paramagnetic resonance and electron-nuclear double resonance methods showed that the center had tetrahedral symmetry, and an effective electron spin of 2. The results of 13C electron nuclear double resonance measurements of the nearest- and next-nearest neighbor atoms were interpreted by using a simple molecular orbital calculation. The unpaired electron population was localized mainly in the C dangling orbitals. The calculations suggested that relaxation of the nearest-neighbor C atoms away from the vacancy was greater for the 5A2 excited state of V0 than for the 4A2 ground state of V-. It was noted that the electron paramagnetic resonance signal had an unusual line shape. This arose from a combination of small shifts in the positions of transitions between different Ms states, and the anisotropic population of the various states. An electronic g-shift of 0.0010 was accounted for in terms of spin-orbit coupling which mixed the 2T2 and 5A2 states (which were separated by about 1eV). The effect of illumination with monochromatic photons, upon the electron paramagnetic resonance, was studied for both the ground state, V-, and the excited state, V0. It was proposed that V0 was created by the ionization of V- rather than by the internal excitation of V0. The lifetime of the 5A2 excited state was measured at liquid He temperatures by monitoring the decay of the electron paramagnetic resonance signal when the ultra-violet light was removed. At temperatures above about 100K, the electron paramagnetic resonance line-width varied in an exponential fashion; with an activation energy of about 0.04eV.

J.A.Van Wyk, O.D.Tucker, M.E.Newton, J.M.Baker, G.S.Woods, P.Spear: Physical Review B, 1995, 52[17], 12657-67