Oxygen molecules on Ag(0 0 1): superstructure, binding site and molecular orientation St  ephane Messerli a , Silvia Schintke a , Karina Morgenstern a, * , Jouko Nieminen b , Wolf-Dieter Schneider a a Institut de Physique de la Mati  ere Condens ee, Universit e de Lausanne, CH-1015 Lausanne, Switzerland b Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland Received 12 May 2000; received in ®nal form 14 August 2000 Abstract Oxygen molecules on Ag(0 0 1), adsorbed at about 60 K, are found to form two-dimensional c2  4 compact is- lands. We determine binding site and orientation of the molecules within the superstructure by comparing experimental and calculated scanning tunneling images in combination with molecular dynamics simulations. The molecule adsorbs in the thermodynamically stable fourfold hollow site with its molecular axis in the direction of short periodicity of the superstructure. Rehybridization of 1p and 2p orbitals on adsorption is at the origin of the observed image con- trast. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction The detailed understanding of the interaction of oxygen with metal surfaces is of utmost impor- tance in technological areas such as heterogeneous catalysis, oxidation, and corrosion [1]. These pro- cesses depend crucially on initial binding site and orientation of the oxygen molecule. The oxygen adsorption on silver surfaces has intensely been studied [2±17]. It is of interest for the chemical industry because of the relevance of silver in the epoxidation process of ethylene [18]. Most of the studies on the O 2 ±Ag system were performed on the (1 1 0) surface [3±7], where the formation of short chains has been observed at low tempera- tures (65 K) by scanning tunneling microscopy (STM) [6]. Ag(0 0 1) has attracted little attention [8±11] until recently [12±14]. For this surface a stable chemisorbed molecular state is reported at 120±130 K [9,10]. Molecular orbital (MO) calcu- lations predict the fourfold hollow site with the two oxygen atoms pointing to the bridge sites to be the most stable binding site [11]. In this work, we determine binding site and orientation of a molecule by comparing measured and calculated STM images for O 2 on Ag(0 0 1). We ®nd the formation of two-dimensional molec- ular oxygen islands, exhibiting a c2  4 super- structure, which was not detected before [9,13]. Within these islands the molecules occupy fourfold hollow sites and are oriented with their molecular axis in the direction of short periodicity. The oxygen molecule is imaged as a protrusion at 6 October 2000 Chemical Physics Letters 328 (2000) 330±336 www.elsevier.nl/locate/cplett * Corresponding author. Present address: Inst. f. Experimen- talphysik, FB Physik, FU Berlin, Arnimallee 14, D-14195 Berlin, Germany. Fax: +49-30-838-1355. E-mail address: karina.morgenstern@physik.fu-berlin.de (K. Morgenstern). 0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 0 9 5 1 - 9