REACTION OF WATER WITH VACUUM-CLEAVED CAO(100) SURFACES - AN X-RAY PHOTOEMISSION SPECTROSCOPY STUDY

The reaction of water vapor with UHV-cleaved CaO( 100) surfaces at 300 K was studied using surface-sensitive synchrotron-based photoemission spectroscopy and LEED. The surface reacts with water at vapor pressur es less than or equal to 5 x 10(-10) Torr, resulting in the formation of surface hydroxyls. The reaction is fast for the formation of the fi rst 0.8 ML (monolayer) of hydroxyls, with an apparent sticking coeffic ient of approximate to 0.9. The reaction slows down dramatically after that, with the sticking coefficient being reduced to approximate to 3 x 10(-5). The highest hydroxyl coverages (greater than or equal to 1 ML) obtained in our experiments were found on surfaces exposed to p(H2 O) = 3 x 10(-4) to 1 x 10(-3) Torr. Bulk hydration of CaO occurs at th ese water exposure pressures and, in separate experiments, when the su rfaces are immersed in liquid water. Air-exposed surfaces contain larg e amounts of adsorbed molecular water, which can be released in the UH V chamber, causing rapid degradation of freshly cleaved CaO(100) surfa ces and some degradation of the vacuum conditions. Annealing CaO sampl es to 573 K for a few hours removes molecular water and restores the v acuum to the 5 x 10(-11) Torr range, but surfaces cleaved from the ann ealed samples still degrade slowly. It is not known if this slow degra dation is caused by the residual water molecules in the UHV system or by surface diffusion of hydroxyls that remain on the surfaces of the a nnealed sample. (C) 1998 Published by Elsevier Science B.V. All rights reserved.