Magnetic-resonance measurements on the5A2excited state of the neutral vacancy in diamond

The ground state of the neutral vacancy in diamond is diamagnetic and therefore has not been studied by electron paramagnetic resonance (EPR). We report the observation of EPR from the ${}_{}{}^5{}_{}{}^{}$ ${\mathit{A}}_2$ excited state of the neutral vacancy by EPR when illuminating an electron-irradiated natural IaB diamond with ultraviolet light. EPR and electron nuclear double-resonance (ENDOR) measurements show that the center has tetrahedral symmetry and the effective electron spin S=2. ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ENDOR measurements on the nearest- and next-nearest neighbor atoms have been interpreted by a simple molecular orbital calculation. The unpaired electron population is predominately localized in the carbon dangling orbitals. Our calculations suggest that the relaxation of the nearest-neighbor carbon atoms away from the vacancy is greater for the ${}_{}{}^5{}_{}{}^{}$ ${\mathit{A}}_2$ excited state of ${\mathit{V}}^0$ than the ${}_{}{}^4{}_{}{}^{}$ ${\mathit{A}}_2$ ground state of ${\mathit{V}}^{\mathrm{-}}$. The EPR signal has an unusual line shape which arises from a combination of small shifts in the positions of transitions between different ${\mathit{M}}_{\mathit{S}}$ states and the anisotropic population of the different states. The electronic g shift of 0.0010(1) can be accounted for by spin-orbit coupling mixing the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{T}}_2$ and ${}_{}{}^5{}_{}{}^{}$ ${\mathit{A}}_2$ states, which are separated by approximately 1 eV. The effect of illumination with monochromatic photons on the EPR has been studied of both ground state ${\mathit{V}}^{\mathrm{-}}$ and excited state ${\mathit{V}}^0$. We propose that the excited ${\mathit{V}}^0$ is created by ionization of ${\mathit{V}}^{\mathrm{-}}$ rather than internal excitation in ${\mathit{V}}^0$. The lifetime of the ${}_{}{}^5{}_{}{}^{}$ ${\mathit{A}}_2$ excited state was measured at helium temperatures by monitoring the decay of the EPR signal when the ultraviolet light was removed. Above about 100 K the EPR linewidth varied in an exponential fashion with an activation energy of about 40 meV.