Adsorption, Desorption and Diffusion of Potassium on Metal and Oxide Surfaces

Abstract

The adsorption-desorption and diffusion of potassium atoms on Ru(0001), Re(0001) and Cr²O³(0001) surfaces were studied using work function change (?), temperature programmed desorption (TPD) and optical second harmonic generation (SHG) measurements. Work function and TPD data indicated that on both metallic surfaces the bonding of potassium atoms is typical to alkali metals, namely, highly ionic nature at low coverages that gradually change to neutral layers at coverages approaching monolayer and above. This is revealed by the shift of the desorption peak from near 1000 K at 0.05 monolayer (ML) down to 350 K at 1ML.

Work function change is maximal near 0.5 ML in the case of the metallic surface, where the balance between number of positive dipoles and neutral islands leads to decrease of near 4 eV. At higher coverages starts to increase, approaching the bulk potassium work function at coverages of 1 ML and above. In the case of the oxide surface, on the other hand, at the completion of the first layer, the potassium atoms are still partially charged. This is indicated by the high desorption temperature of 600 K at 1ML. Higher coverages result in the formation of 2D and 3D islands desorbing at 450 K and 350 K, respectively.

Diffusion of the potassium atoms is studied by the SH diffraction method from coverage grating. Small coverage dependence of the diffusion rate is found on the metal substrates with a barrier for diffusion of 5 kcal/mole on Re(0001) at 1ML. On the oxide surface, the diffraction method followed the descent of potassium atoms from the edges of the 3D islands to the underlying layer of potassium on top of the oxide surface. A barrier for this process of 11 kcal/mole suggests that the effective corrugation on the oxide surface is significantly higher than on the metal substrates, leading to the higher barrier. The origin for these differences between metallic and oxide surfaces in terms of the interaction with alkali metals is discussed.