Adsorption of water on epitactic NiO(1 0 0)

The water adsorption and desorption on NiO(1 0 0) films on a Ag(1 0 0) substrate was studied with XPS, ultra-violet photoelectron spectra (UPS) and thermally programmed desorption (TPD). Two different adsorption states for the water in the monolayer can be distinguished. In the TPD spectra one signal with a narrow peak at 200 K and a second broad desorption peak at approximately 220 K are observed. The TPD signal at 200 K shows the typical behavior for a first order desorption kinetic. The quantity of the water adsorbed in this state strongly depends on the degree of surface order. spectra measured on defect rich surfaces the 200 K desorption peak is significantly reduced. The onset of the broad TPD signal at 220 K shifts strongly to higher temperatures with decreasing water coverage, like a second order desorption kinetic or the desorption of repulsively interacting adsorbent. The UPS shows only molecular water and no signal from OH groups, so a second order desorption can be excluded.