Eur. Phys. J. D 35, 199–205 (2005) DOI: 10.1140/epjd/e2005-00089-5 T HE EUROPEAN P HYSICAL JOURNAL D Electron attachment to strongly polar clusters Formamide molecule and clusters M. Seydou 1, 2 , A. Modelli 3 , B. Lucas 1 , K. Konate 2 , C. Desfran¸ cois 1 , and J.P. Schermann 1, a 1 Laboratoire de Physique des Lasers, UMR7538, Institut Galil´ ee, Universit´ e Paris 13, 93405 Villetaneuse, France 2 DER de Physique, Facult´ e des Sciences et techniques, Bamako, BPE3206, Mali 3 Dipartimento di Chimica, “G. Ciamician”, Universita di Bologna, via Selmi 2, 40126 Bologna, Italy and Centro Interdipartimentale di Ricerca in Scienze Ambientali (CIRSA), via S. Alberto163, 48100 Ravenna, Italy Received 11 March 2005 Published online 14 June 2005 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. Electron localization is studied in formamide cluster anions. The isolated formamide molecule has a large dipole moment and its clusters can give birth to multipole-bound anions as well as valence anions. The vertical valence electron affinity of the isolated molecule is determined by electron transmission spectroscopy. The anion formation process is studied as a function of cluster size with Rydberg electron transfer spectroscopy. DFT calculations of the neutral and negatively-charged cluster structures show that the anion excess electron localizes on a single molecule. The adiabatic valence electron affinity of isolated formamide is deduced from the observation of the cluster size threshold for valence attachment. PACS. 34.80.Gs Molecular excitation and ionization by electron impact – 36.40.Mr Spectroscopy and geometrical structure of clusters – 87.15.By Structure and bonding 1 Introduction Different low-energy electron attachment processes com- pete in polar molecular systems. Excess electrons can enter molecular orbitals in conventional (valence) anions or re- main located nearly totally outside the molecular frame in multipole-bound anions. Multipole-bound anions can only be created if the dipole and/or quadrupole moments of the parent systems exceed critical values [1,2]. Clusters of molecules with individual dipole moments below the crit- ical value ca. 2.5 D can also bind excess electrons either internally as in solvated electrons or in diffuse orbitals as surface states or multipole-bound states [3–5]. When the valence electron affinity of the constituting polar molecule is negative, transient valence anion formation appears as a resonance in the free-electron scattering cross-section of the neutral monomer [6]. When polar molecules are em- bedded in a cluster, the total multipole (dipole and/or quadrupole) moments of the most stable configurations can exceed or be lower than the critical values [7] and ex- cess electrons can be bound or not, then leading to magic numbers in the anion mass-spectra [8,9]. When the clus- ter size further increases, solvation effects become more and more important and valence electron binding enters into competition with electron multipole binding. Above a threshold size value N th , the cluster valence electron affin- a e-mail: scherman@galilee.univ-paris13.fr ity becomes positive and a smooth anion mass distribution is observed. In the most widely studied case of anion water clus- ters, the mass-spectra exhibit magic numbers (N =2, 6, 7) and become smooth above N th = 10 [10–12]. In am- monia clusters, there are no magic numbers and a smooth mass-spectrum is observed for cluster sizes above N th = 33 [10,12]. Both water and ammonia have a dipole moment much smaller than the critical value for electron binding. The theoretical interpretation of the observed mass-spectra is then very difficult and the structure of water anions is still the subject of elaborated experimen- tal [5,9] and theoretical studies [2,8]. In the present work, our attention is focused on valence electron attachment to the isolated formamide molecule which possesses a dipole moment of 3.72 D, much larger than the critical value, and to its homogeneous clusters. In gas-phase collisions, the isolated formamide molecule can temporarily attach a free electron of appropriate en- ergy and angular momentum into a vacant MO, the pro- cess being referred to as a shape resonance [13]. Electron transmission spectroscopy (ETS) [14] is one of the most suitable means for detecting the formation of such short- lived anions. Because electron attachment is rapid with re- spect to nuclear motion, the temporary anion is formed in the equilibrium geometry of the neutral molecule. The im- pact electron energy at which electron attachment occurs is properly denoted as vertical attachment energy (VAE)