Relation between surface resistance, infrared-, surface enhanced infrared- and surface enhanced Raman-spectroscopy’s of C 2 H 4 and CO on copper M. Hein b , P. Dumas c , M. Sinther a , A. Priebe a , P. Lilie b , A. Bruckbauer b , A. Pucci a, * , A. Otto b, * a Kirchhoff-Institut fu ¨ r Physik, Universita ¨ t Heidelberg, Germany b Institut fu ¨ r Physik der kondensierten Materie, Universita ¨t Du ¨ sseldorf, Germany c LURE, Centre Universitaire Paris Sud, Paris Received 12 September 2005; accepted for publication 14 December 2005 Available online 19 January 2006 Abstract The common basis of the observations made with the four experimental methods is the transient electron transfer from the metal to the lowest unoccupied electronic states of the adsorbed molecules. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Vibrations of adsorbed carbon monoxide and ethylene; Cu; Raman- and infrared-spectroscopy; Surface defects 1. Introduction The concept of dynamic electron transfer between the metal and the adsorbate is an old one, already used in the Newns–Anderson (NA) model of bonding [1] as first order of approximation. The concept has been successfully introduced into the theory of vibrational damping [2], sur- face enhanced Raman scattering (SERS) [3], DC surface resistance [4,5] and broadband infrared absorption of adsorbates [6] by Persson. Its possible role in overcoming the infrared (IR) surface selection rule of internal vibra- tions with dipolar moment normal to the surface has sel- dom been mentioned [7]. Devlin and Consani expected that an electron transfer from the metal to an adsorbate would deliver a transition moment normal to the surface but excite total symmetric, Raman active vibrations of the adsorbate. In this way Raman active modes of a ligand are observed in the IR spectra of metal–ligand complexes. This however is not the whole story, see below. In Section 2, we present a short review of surface resis- tance and IR background during co-adsorption of CO and C 2 H 4 on Cu(111) [8], which allowed to test the dynamic electron transfer model. Section 3 shows results of infrared reflection absorption spectroscopy (IRRAS) of ethylene (C 2 H 4 ) on copper films deposited in ultra high vacuum (UHV) at 300 and 50 K. Section 4 starts with IRRAS of Cu(1 1 0) during co- adsorption of CO and C 2 H 4 . Then follow SERS and infra- red transmission results of the co-adsorption system CO and C 2 H 4 on cold-deposited Cu films. 2. Surface resistance and infrared reflection absorption spectroscopy at smooth surfaces The restriction of the surface resistance to the first ad- sorbed layer of molecules has been demonstrated for in- stance in [9,10] for various adsorbates on annealed silver films. Following the microscopic theory of Persson [5], 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.12.040 * Corresponding authors. E-mail addresses: a.pucci@urz.uni-heidelberg.de (A. Pucci), otto@uni- duesseldorf.de (A. Otto). www.elsevier.com/locate/susc Surface Science 600 (2006) 1017–1025