The structure of the Ni(1 0 0)c(2 · 2)–N 2 surface: a chemical-state-specific scanned-energy mode photoelectron diffraction determination D.I. Sayago a , M. Kittel a , J.T. Hoeft a , M. Polcik a,1 , M. Pascal b , C.L.A. Lamont b , R.L. Toomes c , J. Robinson c , D.P. Woodruff c, * a Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D 14195 Berlin, Germany b Department of Chemical and Biological Sciences, Centre for Applied Catalysis, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK c Department of Physics, University of Warwick, Coventry CV4 7AL, UK Received 28 February 2003; accepted for publication 3 May 2003 Abstract Using the chemical shift in the N 1s photoemission peak from the two inequivalent N atoms of N 2 adsorbed on Ni(1 0 0) we have performed a scanned-energy mode photoelectron diffraction (PhD) structure determination of the Ni(1 0 0)c(2 · 2)–N 2 weak chemisorption system. The N 2 is found to adsorb atop surface Ni atoms with the N–N axis perpendicular to the surface at a Ni–N nearest-neighbour distance of 1.81 ± 0.02  A. This is very significantly shorter than the value (2.25  A) found in an earlier published study. An independent density-functional theory slab calculation yields a value of 1.79  A, in excellent agreement with the results of the current experiment. Analysis of the PhD modulations of the N 1s photoemission satellite peak show that these are consistent with this comprising separable components localised at the two N atoms as has previously been assumed in an earlier investigation based on (angle- scan) X-ray photoelectron diffraction. Both experiment and theory indicate a small extension of the N–N distance due to the adsorption (0.03 ± 0.03  A and 0.02  A respectively). Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Photoelectron diffraction; Chemisorption; Surface structure, morphology, roughness, and topography; Nickel; Nitrogen molecule; Low index single crystal surfaces 1. Introduction While the dinitrogen molecule, N 2 , is known to chemisorb on a number of transition metal sur- faces, the number of spectroscopic and structural studies of this adsorbate is far less than of the isoelectronic CO molecule. Indeed, there appear to be only two quantitative structural studies, a low energy electron diffraction (LEED) determination * Corresponding author. Tel.: +44-2476-523378; fax: +44- 2476-692016. E-mail address: d.p.woodruff@warwick.ac.uk (D.P. Wood- ruff). 1 Also at Institute of Physics of the Czech Academy of Sciences, Cukrovarnicka 10, 162 53 Prague, Czech Republic. 0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00645-9 Surface Science 538 (2003) 59–75 www.elsevier.com/locate/susc