Surface Science Letters Oxygen adsorption on Al111): low transient mobility M. Schmid * , G. Leonardelli, R. Tschelieûnig, A. Biedermann, P. Varga Institut f ur Allgemeine Physik, Technische Universitat Wien, Wiedner Hauptstrasse, 8-10/134, A-1040 Wien, Austria Received 17 November 2000; accepted for publication 21 February 2001 Abstract Adsorption of oxygen on Al111) is studied by scanning tunneling microscopy at 80 and 300 K. After adsorption at 130±195 K, STM images taken at 80 K show pairs of oxygen adatoms with interatomic distances mainly between one and three Al interatomic spacings. This clearly shows that dissociation of the oxygen molecules results in a rather low transient mobility of the two oxygen atoms, a fact which is in contrast to previous work [Phys. Rev. Lett. 68 1992) 624]. We also ®nd evidence for oxygen atoms in a second metastable adsorption site at these temperatures. At room tem- perature, we ®nd groups of two or more oxygen atoms in adjacent fcc hollow sites, but no single oxygen atoms. We therefore explain the room-temperature results by part of the oxygen pairs remaining or becoming nearest neighbors, whereas others separate by diusion and their oxygen atoms attach to other pairs or groups, forming the larger groups found. The pairs and larger groups are stable due to an attractive interaction of oxygen atoms in adjacent fcc hollow sites. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Aluminum; Single crystal surfaces; Adsorption kinetics; Adatoms; Surface diusion 1. Introduction Adsorption of a diatomic molecule on a close- packed pure metal surface can be considered one of the most simple processes in surface science, a simple model of ``real-world'' phenomena where more complex compounds and spatially and chem- ically inhomogeneous surfaces abound. Adsorp- tion of O 2 on Al111) is such a simple process, nevertheless it can be considered far from under- stood. It is known that the adsorption process liberates a lot of energy approximately 5 eV per O atom [1]), and one of the unsolved questions is to what extent this energy is transformed into kinetic energy of the resulting O adatoms and how this energy is dissipated. It was concluded from a scanning tunneling microscopy STM) study that after adsorption at room temperature RT) mainly single adatoms are present at the surface. The homogeneous distri- bution of these species identi®ed as single adatoms, not forming any pairs, led to the conclusion that the individual O atoms created by dissociation of O 2 ``¯y apart at least 80  A before their excess en- ergy is dissipated'' [2,3]. This phenomenon became known as ``hot adatoms'' or ``high transient mo- bility''. This experimental study has inspired several attempts to tackle the problem theoretically. In an early eective-medium study, an average separa- tion of the O adatoms of 16  A was found, whereas Surface Science 478 2001) L355±L362 www.elsevier.nl/locate/susc * Corresponding author. Tel.: +43-1-58801-13452; fax: +43- 1-58801-13499. E-mail address: schmid@iap.tuwien.ac.at M. Schmid). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)00967-0