Spectral dependence of UV light penetration into powder TiO₂ anatase

The penetration depth of UV monochromatic light into deposited TiO₂ anatase powder was investigated. The light penetration depth was considered in units of numbers of surface hole, N_s, and electron, N_O2, centers activated during irradiation. The numbers of hole centers N_s were obtained from photostimulated oxygen isotope exchange reaction registered by means of mass spectrometry. The numbers of electron centers N_O2 were obtained from subsequent thermo-programmed desorption spectroscopy of photoadsorbed oxygen O₂^-. It was shown that the depth D of UV light penetration into UV absorbing TiO₂ powder coat depends on the wavelength and intensity I₀ of incident UV light and obeys to the equation DI₀=[Formula presented]lg(I₀/I_m) obtained from the Beer–Lambert–Bouguer law with effective extinction coefficient k_eff and minimal value of light intensity I_m, below which the photoactivation is considered negligible. Commonly, the value I_m has an order of a few microwatts/cm² for activation of electron and hole type centers except for a case of 334 nm falling into the band of the first direct band-to-band transition of anatase, where relatively high threshold value I_m=0.063 mW/cm² for photoactivation of hole centers was revealed. Effective extinction coefficient k_eff may be considered proportional to the absorption coefficient of TiO₂ in the region of fundamental absorption, whereas, in the non-fundamental absorption region, k_eff exceeds significantly that of TiO₂ due to, probably, dominating scattering processes. At small intensities of the incident light, there is a strong sensitivity of the penetration depth (and activated surface) to the intensity.