Electron paramagnetic resonance was used to identify photo-induced titanium-associated electron traps in TiO2 (rutile) crystals. Defect production occurred at low temperature with 442nm laser light. Spectra with S = 1/2 and S = 1 were assigned to singly ionized and neutral oxygen vacancies, respectively. These oxygen vacancies have their unpaired spins localized on the two neighbouring titanium ions aligned along the c axis. A Ti3+ ion next to a Si4+ ion, a Ti3+ self-trapped electron, and a self-trapped hole shared by two adjacent oxygen ions were also observed. Isolated substitutional Fe3+ and Cr3+ ions serve as hole traps.

Photo-Induced Electron Paramagnetic Resonance Study of Electron Traps in TiO2 Crystals: Oxygen Vacancies and Ti3+ Ions. Yang, S., Halliburton, L.E., Manivannan, A., Bunton, P.H., Baker, D.B., Klemm, M., Horn, S., Fujishima, A.: Applied Physics Letters, 2009, 94[16], 162114