Ag-adsorption on MgO: investigations with MIES and UPS P. Stracke, S. Krischok, V. Kempter * Institut f  ur Physik und Physikalischen Technologien, Technische Universit at Clausthal, Leibnizstraûe 4, D-38678 Clausthal-Zellerfeld, Germany Received 6 September 2000; accepted for publication 2 November 2000 Abstract MgO ®lms 2 nm thick) were grown on W1 1 0) while metastable impact electron MIES) and photoelectron UPSHeII)) spectra were collected in situ; apart from the valence band emission no additional spectral features could be detected. The oxide surface was exposed to Ag substrate temperature at 300 K). The additional Ag-induced emission found in the MIES spectra is attributed to the formation of Ag islands. In the very early stage of island growth a characteristic initial decrease of the surface work function is observed; it is therefore proposed that in the initial phase of the Ag exposure, surface defects V s -center-type and extended defects) play an important role as nucleation sites for the island formation. Initially, the islands show 2D growth; above a critical coverage of 0.08 monolayers this is followed by preferential 3D growth. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Silver; Magnesium oxides; Growth 1. Introduction Several reasons exist why considerable attention is devoted to the addition of metal atoms to metal oxide surfaces. Metal addition to oxides leads to an enhanced reactivity via electron transfer to a variety of adsorbed molecules leading to the for- mation of radical anion species. Moreover, the interaction between the metal, in particular clus- ters, and metal oxide supports plays a key role in a number of technologically important applications including catalysis [1,2], metal±ceramics compos- ites, gas sensors and microelectronics [3]. The study of the interaction between metal atoms and oxide surfaces is of importance in understanding phenomena like segregation of metal atoms at oxide surfaces [4,5], diusion of metal atoms on insulators [6], the formation of metal-induced point defects on oxides [7,8], and the formation of nanoparticles in semiconductors and insulators [9]. Surprisingly little is known about the electronic structure of metal/oxide interfaces, in particular about the type of the metal adsorption sites, the electronic character of the ®rst-layer metal spe- cies, and regarding the nature of the interaction with nearest-neighbor metal atoms [3,10]. Elec- tronic structural studies of the early stage of metal growth on oxides are complicated because nucle- ation often occurs at defect sites, vacancies or extended defect sites, such as steps, kinks, etc. Surface Science 473 2001) 86±96 www.elsevier.nl/locate/susc * Corresponding author. Tel.: +49-053-2372-2363; fax: +49- 053-2372-3600. E-mail address: volker.kempter@tu-clausthal.de V. Kemp- ter). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602800)00956-0