Multi-photon Photoemission Dynamics in TiO$_{2}$

TiO$_{2}$ is a material of interest in photocatalytic and photovoltaic applications. Until recently, however, the ability to probe the electron dynamics of this system has been limited to optical experiments. By probing the rutile TiO$_{2}$(110) surface using two-photon photoemission (2PP) with a tunable ultrashort ($\sim$20 fs) laser pulse we investigated the dynamics of electrons excited to its conduction band. Previous 2PP experiments on protic solvent covered TiO$_{2}$ surfaces using 400 nm (3.1 eV) light revealed the presence of an unoccupied surface adsorbate-induced “wet electron” state. By expanding such measurements at higher photon energy we have found a pair of new nearly degenerate unoccupied states located at 2.7 and 2.8 eV above the Fermi level. Based on the calculated electronic structure and optical transition moments, as well as related spectroscopic evidence, we assign these resonances to transitions between Ti-3d bands of nominally t$_{2g}$ and e$_{g}$ symmetry, which are split by crystal field. A detailed understanding of the t$_{2g}$-e$_{g}$ transitions is essential for the characterization of electron dynamics and adsorbate induced resonances in photocatalytic processes on TiO$_{2}$.