Chlorine chemisorption on Cu(0 0 1) by surface X-ray diffraction: Geometry and substrate relaxation He ´lio C.N. Tolentino a, * , Maurizio De Santis a , Yves Gauthier a , Veronique Langlais a,b a Institut Ne ´el, CNRS&UJF, 25 Avenue des Martyrs, BP166, 38042 Grenoble, France b Universitat Autonoma de Barcelona, Departament de Fisica, 08193 Bellaterra, Barcelona, Spain Received 16 March 2007; accepted for publication 27 April 2007 Available online 10 May 2007 Abstract We report on the precise location of Cl atoms chemisorbed on a Cu(0 0 1) surface and the interlayer relaxations of the metal surface. Previous studies have shown that chlorine dissociates on Cu(0 0 1) to form a c(2 · 2) chemisorbed layer with Cl atoms occupying four- fold hollow sites. A Cu–Cl interlayer spacing of 1.60 A ˚ and a slightly expanded Cu–Cu first interlayer spacing of 1.85 A ˚ (1.807 A ˚ for bulk Cu) was determined by LEED. The resulting Cu–Cl bond length, 2.41 A ˚ , is very similar to the SEXAFS value of 2.37 A ˚ . Contradictory results were obtained by angle-resolved photoemission extended fine structure: while confirming the Cu–Cl interlayer spacing of 1.60 A ˚ , no first Cu–Cu interlayer relaxation has been observed. On the other hand, a small corrugation of the second Cu layer was pointed out. We carried out a detailed structural determination of the Cu(0 0 1)–c(2 · 2)-Cl system using surface X-ray diffraction technique with syn- chrotron radiation. We find a Cu–Cl interlayer spacing of 1.584(5) A ˚ and confirm the expansion of the first Cu–Cu interlayer, with an average spacing of 1.840(5) A ˚ . In addition, we observe a small corrugation of the second Cu layer, with Cu atoms just below Cl atoms more tightly bound to the surface layer, and even a second Cu–Cu interlayer expansion. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Surface structure; X-ray diffraction; Molecular beam epitaxy; Chemisorption 1. Introduction Halogens play an important role in many technological processes, like in surface-induced morphology, corrosion, etching and restructuring processes [1–4]. As they interact strongly with all metal surfaces, they efficiently poison their top layer and thus inhibit various catalytic reactions. On the other hand, they may also be employed as promoters in some particular situations. In the context of self- structuring surfaces, they are interesting candidates for halogen-induced reconstructions, owing to their high adsorption energy. This is precisely the case for Cu(0 0 1) where step faceting mechanism takes place [1]. A large number of experimental and theoretical studies have been devoted to the halogen chemisorption on metal surfaces and the way it affects their geometric and electronic struc- tures. However, there are only few studies of halogen on metals dealing with an accurate quantitative determination of the adsorbate geometry and its induced effects on the substrate-surface structure. In the case of chlorine on Cu(0 0 1) surface, these studies have included techniques such as quantitative low energy electron diffraction (LEED) [5], surface extended X-ray absorption fine struc- ture spectroscopy (SEXAFS) [3,6,7], X-ray standing waves (XSW) [8] and angle-resolved photoemission extended fine structure (ARPEFS) [9]. In all these studies, the halogen is found to adsorb as a simple overlayer for halogen cover- ages up 0.5 monolayer. It has been shown that Cl 2 dissociates on Cu(0 0 1) to form a c(2 · 2) chemisorbed layer with Cl atoms occupying four-fold hollow sites. A Cu–Cl interlayer spacing of 1.60(2) A ˚ and a slightly expanded Cu–Cu first interlayer 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.04.257 * Corresponding author. E-mail address: helio.tolentino@grenoble.cnrs.fr (H.C.N. Tolentino). www.elsevier.com/locate/susc Surface Science 601 (2007) 2962–2966