Strong Transient Absorption of Trapped Holes in Anatase and Rutile TiO₂ at High Laser Intensities

During transient absorption spectroscopic investigations we found that the intensity of the transient absorption signal of the trapped holes monitored in the microsecond time domain drastically increases at high excitation laser intensity. This increase has been related to the presence of long-lived Ti³⁺ centers formed upon high laser exposure via a surface reorganization. The Coulomb interaction of the trapped holes with long-lived Ti³⁺ centers leads to an increased absorption coefficient of the former resulting in much higher transient absorption signals below 450 nm rather than in the wavelength region above where the trapped electrons absorb. The surface reorganization induced via the excitation source can be avoided in the case of anatase if the measurements are conducted at low laser intensities, while in the case of rutile already at low excitation conditions the transient absorption enhancement of the trapped holes occurs.