AN ELLIPSOMETRIC STUDY OF POTASSIUM ADSORPTION OF TIO2(110)

Spectroscopic ellipsometry in the energy range 1.8-4.0 eV, LEED and TPD have been used to study the adsorption of potassium on TiO2(110) at 140 K. A c(2 x 2) ordered overlayer is associated with a strongly interacting potassium monolayer which desorbs at 750 K. Subsequent adsorption produces more weakly bound multilayers which desorb at 300-350 K. The ellipsometric parameters delta-DELTA and delta-PSI, measured at a constant energy of 2.0 eV, exhibit large fluctuations as a function of coverage during the initial adsorption of potassium, and quite different than the changes expected for the adsorption of a free electron metal. These fluctuations are associated with changes in the optical response of the titania by electron transfer to states within the indirect band gap. The spectroscopic measurements reveal that sub-monolayers of potassium induce a particularly large change in the optical response of the substrate in the region of the direct band gap. The effect is strikingly similar to field induced reflectivity changes at electrochemical and thin film interfaces of TiO2(100) and TiO2(110) generated during potential modulation spectroscopy. It is ascribed to an increase in the band gap energy due to polarisation of the lattice at the surface which, in the case of potassium adsorption, is induced by electron transfer. The onset of free electron behaviour in the potassium multilayers is evident in the spectroscopic measurements from the adsorption of the second layer of potassium. In addition to the apparent Drude behaviour of the adsorbing potassium, an optical transition is observed at 2.7 eV and ascribed to the excitation of the potassium surface plasmon.