HREELS study of the intercalation of K atoms in C 60 monolayers C. Silien * , P.A. Thiry, Y. Caudano Laboratoire de Spectroscopie Mol eculaire de Surface, Facult es Universitaires Notre-Dame de la Paix, Department of Physics, 61 rue de Bruxelles, B-5000 Namur, Belgium Received 24 September 2003; accepted for publication 30 March 2004 Available online 15 April 2004 Abstract Extra- and intramolecular vibrations of K-doped C 60 monolayers adsorbed on Ag(1 1 1) and Cu(1 1 1) have been studied by high-resolution electron energy loss spectroscopy (HREELS) in reflection geometry. Intramolecular modes are used to assess the electronic occupancy of C 60 , which is increased by intercalating K atoms. Regardless of the charge donated by the substrate, extramolecular modes have characteristic frequencies for K 1 C 60 ,K 2 C 60 , and K 4 C 60 mono- layers. When the K concentration exceeds four, the K-doped monolayers exhibit different structures on Ag(1 1 1) and Cu(1 1 1). Ó 2004 Elsevier B.V. All rights reserved. Keywords: Fullerenes; Carbon; Alkali metals; Silver; Copper; Electron energy loss spectroscopy (EELS); Phonons 1. Introduction Since the discovery of C 60 [1], its vibrational excitations have been intensively studied, owing to the role they are assumed to play in the super- conductivity of fullerene materials [2]. In addi- tion, these excitations are efficient probes of the molecular environment and provide insight into the molecule symmetry and charge state, as well as into the composition and structure of derived materials. Consequently, their study extends our understanding of fullerene compounds, such as adsorbed and doped monolayers. Due to the high symmetry of C 60 , the vibra- tional intramolecular fingerprint reduces to four T 1u infrared-active modes and to 10 Raman-active modes, which comprise the well-known A g ð1Þ, breathing, and A g ð2Þ, pentagonal pinch, vibra- tions. Intramolecular modes lie between 200 and 2000 cm 1 and are used here to assess the charge transfer, i.e. the filling of the C 60 LUMO-derived states, upon K-doping. Because C 60 materials are typical molecular solids where adjacent molecules are only slightly interacting, extra- and intra- molecular vibrations occur in well-defined energy ranges, the frequencies of the extramolecular ex- citations being below 200 cm 1 . The phonons of fullerite, i.e. the pure C 60 crystal, have been studied thoroughly by using far-infrared and Raman spec- troscopies [3], inelastic neutron scattering (INS) [4–6], and inelastic He scattering [7,8]. In addition * Corresponding author. Tel.: +32-81-725487; fax:+32-81- 724718. E-mail address: christophe.silien@fundp.ac.be (C. Silien). 0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.03.065 Surface Science 558 (2004) 174–180 www.elsevier.com/locate/susc