Eur. Phys. J. D 50, 165–169 (2008) DOI: 10.1140/epjd/e2008-00210-4 T HE EUROPEAN P HYSICAL JOURNAL D The valence band of free K clusters studied by photoelectron and Auger spectroscopies A. Rosso 1,2,a , G. ¨ Ohrwall 1,2 , S.E. Canton 3 , S. Legendre 2 , I.L. Bradeanu 2 , S. Svensson 2 , O. Bj¨ orneholm 2 , and M. Tchaplyguine 1 1 MAX-lab, Lund University, Box 118, 22100 Lund, Sweden 2 Department of Physics and Material Science, Uppsala University, Box 530, 75121 Uppsala, Sweden 3 Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden Received 21 May 2008 / Received in final form 24 July 2008 / Published online 5 November 2008 c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2008 Abstract. The valence states of free neutral potassium clusters produced by a gas aggregation source were probed by synchrotron radiation based photoelectron spectroscopy. The first ionization energy (IE) of the clusters was determined to be 10% larger than the work function of bulk potassium. Using electrostatic concepts and the IE, the mean size of the clusters was estimated to be ≈2000 atoms. Further information about the valence band was provided by investigation of the Auger process initiated by the ionization of the 3p level with a subsequent emission of an Auger electron from the valence band (M2,3VV). Plasmon satellites were observed in Auger spectra of free metal clusters. PACS. 36.40.-c Atomic and molecular clusters – 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals – 79.60.-i Photoemission and photoelectron spectra 1 Introduction The study of metal clusters gives the opportunity to map the evolution of the electronic structure from dis- crete levels to quasi-continuous bands by increasing the cluster size [1]. The valence states of free metal clusters can be probed by ultraviolet photoelectron spectroscopy (UPS) [1]. For sufficiently high photon energy the valence spectrum provides a picture of the occupied density of states weighted by the cross section and final state ef- fects [2]. It gives direct information about quantities such as the first ionization energy (IE) of the clusters, which can be compared with the work function (W f ) of bulk metals. Auger electron spectroscopy (AES) yields knowl- edge about the valence and core electronic states. The kinetic energy of the Auger electron is mainly defined by the energy difference between the initial core-ionized state and the two-hole final state. For the so-called core-valence- valence (CVV) Auger processes, in which the electrons in- volved are from the valence band, the spectral shape of the Auger feature is related to the occupied states in the valence band. In addition, collective effects like plasmon excitation can be observed in Auger spectra [2]. These techniques, UPS and AES, combined with synchrotron radiation are thus efficient tools to study the electronic structure of large metal clusters [3,4]. a e-mail: aldana.rosso@maxlab.lu.se Free alkali metal atoms possess only one valence elec- tron, which excludes the possibility of a normal Auger decay for the outermost core orbital. However, the Auger decay channel is open for alkali metal clusters, since each atom contributes with one valence electron. For exam- ple, Auger decay has been observed already in sodium dimers where the 3s-derived molecular orbital has two electrons [5]. In this work, we continue our study of free potassium clusters [6] and focus on the structure of the valence band. We determine the IE of the clusters using the information contained in by valence spectrum. The cluster mean size is estimated using the IE and a theo- retical model based on electrostatic considerations. Ad- ditional information about the valence band is obtained from the study of the M 2,3 VV Auger decay. Plasmon satel- lites have been observed and the role of other relaxation mechanisms, such as plasmon-gain satellites, is also dis- cussed. 2 Experimental Free neutral potassium clusters were produced with a gas aggregation source [4,7]. The main components of the clus- ter source were an oven to vaporize the metal, and a cryo- stat. The cryostat temperature, controlled employing a liquid nitrogen based cooling system, was ≈200 K. Ar- gon was blown continuously into the cryostat to create