Ultrafast coherent spectroscopy of photocatalytic TiO₂surfaces

Understanding the photoexcited electron dynamics in TiO₂ is vital to extend its applications in photocatalysis through a thorough understanding of its electronic structure. In our previous study of reduced TiO₂ by two-photon photoemission using UV excitation, we found a pair of nearly degenerate unoccupied states, which we attributed to transitions between Ti-3d bands of t_2g and e_g symmetry. We extended our study to multiphoton photoemission(mPP) spectroscopy in the near-IR-VIS spectral region. I will report on the dynamics of electrons excited from defect states of reduced TiO₂(110) and Au/TiO₂ through its conduction band by mPP with a tunable ultrashort(~20 fs) laser pulses. mPP spectra reveal resonances where TiO₂ defect state electrons absorb energy to reveal new electronic energy levels. We performed interferometric time resolved-mPP experiments on these resonances to study the photoinduced polarization and electron relaxation dynamics. Surprisingly, the experiment reveals measurable coherent polarization and population relaxation dynamics on tens of femtosecond time scales.  Decoration of the surfaces with Au overlayer introduces new resonances. Our study contributes to a comprehensive knowledge of these transitions relevant to the photocatalytic processes on TiO₂.