Photoelectron spectroscopy has been employed to probe the electronic structure of TiO2(100)c(2x2)K. In sharp contrast to earlier work, no features indicating the presence of Ti3+ cations are found in the photoelectron spectra. This unexpected result is rationalised on the basis of ab initio periodic slab calculations. These calculations demonstrate that in this system K is fully ionised (K --> K+), and that the majority of the charge transferred across the TiO2/K interface resides on Ti cations, reducing them formally from Ti4+ to Ti3+. However, the excess charge is not restricted to near surface Ti ions, but rather can occupy any Ti site throughout the bulk, and so is unobservable by photoelectron spectroscopy, due to the negligible concentration of Ti3+ cations in the surface region.
