Volume 205, number 1 CHEMICAL PHYSICS LETTERS 2 April 1993 Shell structure of small indium clusters below IV=200 atoms B. Baguenard, M. Pellarin, C. Bordas, J. Lermt, J.L. Vialle and M. Broyer zyxwvutsrqponmlkjihgfed Loboratoire de Spectrom&rie ioniqueet M okculaire ‘, Universitk LyonI, Bcitiment 205 43 BoulevardduII Novembre 1918.69622 Viiieurbanne Cedex, France Received 4 December 1992; in final form 21 January 1993 Neutral iodium clusters InN (N< 200) are laser photoionized near threshold and analyzed by time-of-flight mass spectrometry. Strong shell effects are observed in cluster mass spectra. For In, cIusters below N= 125, individual ionization potentials are deduced. They are compared with theoretical predictions calculated within the framework of the spherical-jellium model. The agreement is qualitatively quite good and the observed shell e&ts correspond to the successive major electronic shell closures. Deviations from electronic shell model predictions are much less important for indium clusters than for aluminum ones. A ten- tative explanation of this difference is given. 1. Introduction The structure of small alkali clusters is now well understood. In the size domain N< 1500 atoms, it is dominated by the electronic shell closings which may be, at least qualitatively, calculated in the framework of the jellium model which takes into account only the N valence electrons. This electronic shell struc- ture has been found to apply for all the group IA metal clusters in LiN [ 1,2], NaN [ 3-51, I&, [ 61, Cs, [ 71 under most experimental conditions. The ques- tion arises if this concept may be extended to other metal clusters. Trivalent metals are good candidates because their atomic structure (s2p) remains rela- tively simple. However, even for these elements, the situation becomes less clear than for alkali ones. Up to now, only aluminum clusters have been exten- sively studied. First, Whetten and co-workers [ 8,9 ] found evidence for jellium-like features in the size range 10 <NC 250 assuming that each atom contrib- utes three quasi-free electrons. However, the elec- tronic shell mode1 was not able to explain all the ob- served structures. Then, Pellarin et al. [ 10,l I ] obtained a regular shell structure beyond N= 250 up to N= 1400, tentatively interpreted in terms of elec- tronic shell structure within the framework of semi- ’ UR.4 CNRSNo. 171. classical theory (classical star orbit). Recently, Martin et al. [ 121 showed that these shell features extended up to N= 10000 atoms and could be ex- plained by the coverage of successive triangular faces on close-packed octahedral clusters. For other trivalent metal clusters, the experimen- tal results are quite scarce, except for some prelim- inary experiments on indium clusters [ 8,9]. We present in this Letter an extensive study of indium clusters in the size domain Ng 200. Photoionization spectra have been obtained and ionization potentials are deduced. Although some similarity with alumi- num clusters exists, many differences are clearly es- tablished. The present results can be interpreted in the framework of the jellium model and indium clus- ters seem to be more jellium-like than aluminum clusters. 2. Experimental The indium clusters are produced by the laser va- porization technique. Our cluster source is designed according to the one described by Milani and de Heer [ 13 ] and is presented in detail in ref. [ 1 I 1. The tem- perature of the block source can be lowered down to about 100 K by liquid nitrogen cooling. The second harmonic of a Nd:YAG laser (532 nm) is focused 0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved. 13