Structural electric dipole in small ionic nanocrystals D. Rayane * , A.R. Allouche, I. Compagnon, R. Antoine, M. Aubert-Fr econ, M. Broyer, Ph. Dugourd Laboratoire de Spectrom etrie Ionique et Mol eculaire (UMR no 5579), CNRS et Universit e Lyon I, b^ at. A. Kastler, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 17 September 2002; in final form 10 October 2002 Abstract We present the results of gas phase electric dipole measurements for a series of M 18 X 17 alkali–halide clusters. This cluster has a large structural dipole, which is due to the periodic arrangement of the ions in the nano- crystal and is directly related to the size of the particle. Experimental values are compared to the prediction of a Born Mayer model and to ab initio calculations. The results for the various salts are reproduced by our calcu- lations. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction The electric dipole moment of a molecule or a cluster is a key quantity to investigate the charge transfers, the atomic arrangement and the inter- atomic distances which govern the overall struc- ture of a nanoscopic system. For example in metal–C 60 clusters, the dipole moment is directly related to the arrangement of metallic atoms on the C 60 cage [1]. In polypeptides, the structure in a helix induces a large electric dipole which is much smaller for more compact arrangement [2]. In ionic nanocrystals, large dipoles, which are directly re- lated to the ionic arrangement, have recently been observed [3,4]. Salt nanocrystals are simple and extensively studied model systems [5–11]. They tend to adopt cubic structures similar to the bulk rock-salt structure. Ab initio as well as model calculations may be performed for the interpreta- tion of experimental measurements. In this Letter, we present the results of electric dipole measurement for a series of alkali–halide clusters with one excess electron and we focus on one size M 18 X 17 . For this size, the large electric dipole is structural and directly related to the lat- tice parameter. We therefore obtain for the first time a direct probe of this lattice parameter for a large family of alkali halides. The series of dipole values, obtained for a size intermediate between the MX molecule and the bulk, can be used to validate ab initio or empirical calculations on salt clusters. Experimental results are compared to the prediction of a Born Mayer model and to ab initio calculations. Chemical Physics Letters 367 (2003) 278–283 www.elsevier.com/locate/cplett * Corresponding author. E-mail address: driss.rayane@lasim.univ-lyon1.fr (D. Ray- ane). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(02)01696-2