MAGNETIC-RESONANCE MEASUREMENTS ON THE (5)A(2) EXCITED-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)A(2) excited state of the neutral vacancy by EPR when illuminating an electron-irradiated na tural IaB diamond with ultraviolet light. EPR and electron nuclear dou ble-resonance (ENDOR) measurements show that the center has tetrahedra l symmetry and the effective electron spin S=2. C-13 ENDOR measurement s on the nearest- and next-nearest neighbor atoms have been interprete d by a simple molecular orbital calculation. The unpaired electron pop ulation is predominately localized in the carbon dangling orbitals. Ou r calculations suggest that the relaxation of the nearest-neighbor car bon atoms away from the vacancy is greater for the (5)A(2) excited sta te of V-0 than the (4)A(2) ground state of V-. The EPR signal has an u nusual line shape which arises from a combination of small shifts in t he positions of transitions between different M(s) states and the anis otropic population of the different states. The electronic g shift of 0.0010(1) can be accounted for by spin-orbit coupling mixing the T-3(2 ) and (5)A(2) states, which are separated by approximately 1 eV. The e ffect of illumination with monochromatic photons on the EPR has been s tudied of both ground state V- and excited state V-0. We propose that the excited V-0 is created by ionization of V- rather than internal ex citation in V-0. The lifetime of the (5)A(2) excited state was measure d at helium temperatures by monitoring the decay of the EPR signal whe n the ultraviolet light was removed. Above about 100 K the EPR linewid th varied in an exponential fashion with an activation energy of about 40 meV.