Oxygen adsorption on Al(111): low transient mobility

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 s how pairs of oxygen adatoms with interatomic distances mainly between one a nd three Al interatomic spacings. This clearly shows that dissociation of t he oxygen molecules results in a rather low transient mobility of the two o xygen atoms, a fact which is in contrast to previous work [Phys. Rev. Lett. 68 (1992) 624]. We also find evidence for oxygen atoms in a second metasta ble adsorption site at these temperatures. At room temperature, we find gro ups of two or more oxygen atoms in adjacent fee 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 s eparate by diffusion and their oxygen atoms attach to other pairs or groups , forming the larger groups found. The pairs and larger groups are stable d ue to an attractive interaction of oxygen atoms in adjacent fee hollow site s. (C) 2001 Elsevier Science B.V. All rights reserved.