Adsorption of water on epitactic NiO(100)

The water adsorption and desorption on NiO(1 0 0) films on a Ag(1 0 0) subs trate was studied with XPS, ultra-violet photoelectron spectra (UPS) and th ermally programmed desorption (TPD). Two different adsorption states for th e water in the monolayer can be distinguished. In the TPD spectra one signa l with a narrow peak at 200 K and a second broad desorption peak at approxi mately 220 K are observed. The TPD signal at 200 K shows the typical behavi or for a first order desorption kinetic. The quantity of the water adsorbed in this state strongly depends on the degree of surface order. In TPD spectra measured on defect rich surfaces the 200 K desorption peak i s significantly reduced. The onset of the broad TPD signal at 220 K shifts strongly to higher temperatures with decreasing water coverage, like a seco nd order desorption kinetic or the desorption of repulsively interacting ad sorbent. The UPS shows only molecular water and no signal from OH groups, s o a second order desorption can be excluded. (C) 2001 Elsevier Science B.V. All rights reserved.