ELSEVIER Surface Science 383 (1997) 216-225 surface science Cs adsorption on oxide films (A1203, MgO, SiOz) M. Brause, D. Ochs, J. Gtinster 1, Th. Mayer, B. Braun, V. Puchin 2, W. Maus-Friedrichs, V. Kempter * Physikalisches Institut der Technischen Universitdt Clausthal, Leibnizstrasse 4, D-38678 Clausthal-Zellerfeld, Germany Received 4 February 1997; accepted for publication 3 March 1997 Abstract Films of alumina, magnesia, and silica were exposed to cesium atoms at room temperature. Cesium adsorption as a function of exposure time was studied with metastable impact electron spectroscopy (MIES) and photoelectron spectroscopy (UPS; He 1). On silica, cesium atoms readily chemisorb in the initial stages of exposure. The bonding is apparently due to the interaction with active surface oxygen such as non-bridging oxygen atoms. We suggest that on alumina and magnesia, the chemisorption of cesium, in the form of an ionic state, takes place at surface defects, presumably edge sites such as steps, kinks, comers, etc. In all cases studied, prolonged exposure leads to additional Cs adsorption. Apparently, patches with metallic properties (but no uniform adlayer) develop on the surface; they disappear after the cesium supply is interrupted. © 1997 Elsevier Science B.V. Keywords: Alkali metals; Aluminum oxide; Chemisorption; Clusters; Magnesium oxides; Metallic films; Photon absorption spectroscopy; Physical adsorption; Silicon oxides; Visible/ultraviolet absorption spectroscopy 1. Introduction The study of the adsorption of alkali atoms on metals has a long tradition, reaching back to Taylor and Langrnuir [1] (see Ref. [2] for a recent review). At low coverages a fairly general picture appears to exist: the alkali atom transfers its valence electron to the substrate. The alkali ions form a dipolar layer which lowers the work func- tion and is responsible for the repulsive dipole- dipole interaction between the adatoms. At higher coverages, the delicate balance between the adsor- * Corresponding author. Fax: +49 5323 723600; e-mail: kempter@physik.tu-clausthal.de 1 Present address: NIRIM, Tsukuba, Namiki 1-1, Ibaraki 305, Japan. 2 On leave from the University of Latvia, Riga, Latvia. 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0039-6028(97)00174-X bate-adsorbate and adsorbate-substrate inter- actions leads to a wealth of overlayer structures. The adsorption of alkali atoms on semiconduc- tors is less well understood. In general, semicon- ductors display dangling bonds at the surface which protrude into the vacuum, similar to the surface states of metals. However, even for the extensively studied Si(100)-(2 x l) substrate, sub- stantial disagreement exists concerning the over- layer structure and the nature of the alkali bonding to the substrate (for some recent papers on these issues, see Refs. [2-9]. Studies of the interaction of alkali atoms with insulators such as oxide surfaces are limited (see Ref. [ 101 for a summary). Nevertheless, metal par- ticles and adlayers on oxide surfaces are relevant to crystal growth, catalysis, gas sensor applica- tions, bonding in composites, etc. Many catalysts