Adsorption of H 2O on potassium films

Abstract

The interaction of H 2O with K films of 30–110 Å thickness was studied at 125 K under ultra-high vacuum conditions utilizing photoemission spectroscopies (XPS, UPS) and measurements of work function as analytical techniques. The K films were grown in situ on a Pt(111) substrate. The interaction of H 2O with the K films was found to be complex and included the following processes: adsorption, dissociation and formation of KOH, dissolution and hydration. This reaction proceeded throughout the bulk of the initial K film at 125 K until it was completely transformed into a mixed KOH H 2O layer. At this stage a thick ice layer could be grown on top of the mixed KOH H 2O film. The conversion of K into KOH H 2O was also characterized by a decreasing intensity of plasmon excitation peaks, as observed in connection with K 2p spectra, and by an up-shift of K 2p and K 3p core level binding energies. H 2O molecules in the ice layer, dissolved H 2O and OH species could be distinguished by their O1s core level and valence orbital binding energies. The dissolved H 2O was stoichiometrically linked to KOH and identified as hydration water. Its desorption temperature was considerably higher (280 K) than that of adsorbed H 2O ( ~ 160 K). The KOH layer was thermally stable up to 580 K. The clean K film and the KOH layer had work functions of 2.2 and 1.5 eV, respectively. The KOH layer showed an at least ten-fold increase in secondary electron emission below 6 eV kinetic energy (UPS).