Titanium dioxide (TiO2) is a prototypical light-harvesting semiconductor and shows potential for application to photocatalysis. The physical and chemical properties of TiO2 are known to be strongly influenced by the presence of common surface defects and adsorbates, which modify the availability of excess electrons. Since excited state charge carriers mediate reactions at metal oxide surfaces, it is important to understand the effects of defects on the surface's electronic structure.
We present ultra-fast, two-photon photoemission measurements evidencing a resonant photoexcitation process between occupied Ti 3d defect states and an unoccupied state, centred ~2.7 eV above the Fermi level, at the rutile TiO2(110) surface following chemical reduction [1,2]. The lifetime of the excited state is found to be fewer than 15 fs. The intensity of the Ti 3d resonance is shown to be greatly increased by the creation of bridging hydroxyl groups.
At the anatase TiO2(101) surface, the availability and localisation of excess electrons is dictated by the defect concentration. Since oxygen vacancies quickly migrate to the sub-surface region above 200 K, a thermally-regulated equilibrium exists. We present evidence that hydroxyl creation at this industrially relevant surface increases the concentration of excess electrons at the surface and hence may increase its potential to facilitate some chemical reactions .
 Y. Zhang, D. T. Payne, C. L. Pang, H. H. Fielding, and G. Thornton. J Phys Chem Lett, vol. 6, no. 17 (2015).
 D. T. Payne, Y. Zhang, C. L. Pang, H. H. Fielding, and G. Thornton. Surf Sci, vol. 652 (2016).
 D. T. Payne, Y. Zhang, C. L. Pang, H. H. Fielding, and G. Thornton, Top Catal, vol. 2, no. 101 (2016).