Abstract: The combination of femtosecond laser time-resolved two-photon photoemission (TR-2PP) and ultra high vacuum (UHV) surface science techniques provides the possibility to study electronic structures and interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO2 single-crystalline surfaces. The experimental apparatus used in our measurements (e.g. laser, Interferometer, UHV electronics) are briefly discussed. The 2PP spectra of the molecules (H2O, CH3OH, etc.) adsorbed TiO2 surfaces display a broad unoccupied electronic state at 2.2 to 2.4 eV above the Fermi level, which is corresponding to the photo-excitation from the bandgap states of the reduced Ti ions to the molecules induced excited states on the surface, along with a reverse charge transfer process. Density functional theory (DFT) shows this to be a wet-electron state analogous to that reported in homogeneous systems. The femto-second pump-probe measurements show the wet-electrons at the H2O/TiO2 interface decay within 15 fs nonadiabatically; and the wet-electrons at CH3OH/TiO2 exhibit complex decay dynamics along with a pronounced deuterium isotope effect (CH3OD), which can be interpreted as a proton- coupled electron transfer (PCET) mechanism.