Individual paramagnetic defect centers in diamond nanocrystals were investigated by means of low-temperature high-resolution optical spectroscopy. Narrow fluorescence excitation spectral lines were found, thus indicating transitions between individual spin sub-levels. Spectral diffusion was explained by cross-relaxation among spin sub-levels and by the presence of excited electrons in the conduction band of diamond. The relaxation times were in the ms range.Single Spin States in a Defect Center Resolved by Optical Spectroscopy. F.Jelezko, I.Popa, A.Gruber, C.Tietz, J.Wrachtrup, A.Nizovtsev, S.Kilin: Applied Physics Letters, 2002, 81[12], 2160-2