26 March 1999 Ž . Chemical Physics Letters 302 1999 595–601 Coordination of Ti cation embedded in argon clusters Michalis Velegrakis a, ) , George E. Froudakis a,b , Stavros C. Farantos a,b a Institute of Electronic Structure and Laser, Foundation for Research and Technology – Hellas, P.O. Box 1527, 711 10 Heraklion, Crete, Greece b Department of Chemistry, UniÕersity of Crete, 711 10 Heraklion, Crete, Greece Received 15 December 1998; in final form 12 January 1999 Abstract A laser vaporisation source and a time-of-flight mass spectrometer are used for the production and detection of Ti q Ar n Ž . clusters. Parallel, density functional theory DFT calculations assist in extracting the structures of the first members of the series. The mass spectrum is dominated by one intense line of Ti q Ar , for which the DFT calculations predict a regular 6 octahedron. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction Metal-doped noble gas clusters of the type MX n Ž . M s metal and X s noble gas atom constitute pro- totype systems in modelling metal–ligand interac- tions, ranging from the gas-phase diatomic com- plexes up to the bulk phase. Therefore, studies on these systems have evolved to a topic of consider- w x able interest 1–3 . If the metal exhibits a s orbital electronic configuration, the noble gas atoms tend to arrange around the central metal core. In this case, the cluster structure is often dictated by geometrical factors such as the atomic radii ratio of the metal and ligand. Such arrangements have often been observed w x in the mass spectra of MX cluster systems 4–7 n w x and have also been successfully explained 6–9 . The situation is substantially different if the metal is a transition metal. Here, the transition element is an open-shell system and forces the noble gas atoms ) Corresponding author. Fax: q30 81 391318; e-mail: vele@iesl.forth.gr to occupy certain sites depending on the filled or empty d-orbitals. This has been documented in mass w x spectrometric work 10–14 for a series of transition metal ions exhibiting a d k electronic configuration. Using arguments adopted from ligand field theory Ž . LFT , it was possible to predict the stability and the structure of some transition metal ion-doped noble gas clusters. However, for transition metal ions, whose lower electronic state is of the d ky1 s 1 kind, a number of nearly degenerate low-lying electronic states exist. Furthermore, in the complexes of these ions with atomic or molecular ligands, mixing of these states is expected and therefore the application of simple LFT arguments in predicting the structure and the stability of these clusters is not straightfor- ward. w x Partridge et al. 15,16 have performed extended ab initio calculations for the spectroscopic constants of the ground and some low-lying electronic states for all of the first-row transition metal cations with He, Ne and Ar. These studies have shown that the interaction forces are predominately electrostatic, i.e. charge-induced dipole interactions. For metal ions 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00162-1