Electron paramagnetic resonance (EPR) is used to identify photoinduced titanium-associated electron traps in TiO2 crystals (rutile). Defect production occurs at low temperature with 442 nm laser light. Spectra with S=1/2 and S=1 are assigned to singly ionized and neutral oxygen vacancies, respectively. These oxygen vacancies have their unpaired spins localized on the two neighboring 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 are also observed. Isolated substitutional Fe3+ and Cr3+ ions serve as hole traps.
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Yang et al. (2009) studied this question.