Published: May 26, 2011 r2011 American Chemical Society 7456 dx.doi.org/10.1021/jp204478v | J. Phys. Chem. A 2011, 115, 7456–7460 ARTICLE pubs.acs.org/JPCA Symmetry-Switching Molecular Fe(O 2 ) n þ Clusters Giannis Mpourmpakis, Michalis Velegrakis,* Claudia Mihesan, and Antonis N. Andriotis Institute of Electronic Structure and Laser, FORTH, Heraklion 71110, Crete, Greece b S Supporting Information 1. INTRODUCTION In the course of understanding corrosion, biological and catalytic processes (e.g., CO oxidation) and in the search of new magnetic nanomaterials, much effort has been focused recently on iron oxide clusters (see, for example, ref 1 and references therein). Iron oxide clusters have been produced in the past in various laboratories. 2À10 In the majority of these studies, laser vaporization of a solid target in the presence of O 2 was employed to form mainly Fe m O n clusters with m > 1. However, iron forms a large number of complexes at various stoichiometries when it interacts with oxygen. A characteristic case is that of oxygen- rich FeO n clusters. For these small complexes, only a few experimental 2,4,6,9,10 and theoretical studies 4,11À14 have been reported, investigating their formation and stability. Only recently, Gutsev et al. 15 extended the theoretical studies of neutral and negative iron oxide clusters to larger sizes (n e 12). A common feature of the theoretical studies 1,4,11À15 is the use of density functional theory (DFT) in the spin- polarized generalized electron density gradient approximation (SGGA). As for other transition metal oxides, the study of Fe m O n clusters encounters the problems related to the drawback of the DFT/SGGA in describing incomplete d shells. The theoretical results appear to be sensitive to the approximations made in incorporating the electron correlations. For instance, even though the theoretical results 12 obtained for FeO n ,(n e 4, neutral and negative species) using molecular orbital theory at the BPW91 level of approximation are in good agreement with the experimental data, they deviate from the results obtained at the B3LYP level of approximation (hybrid-DFT). On the other hand, B3LYP reproduces accurately both experimental and theoretical results, obtained at a high level of theory, on Fe þ -containing compounds. 16 In most of the FeÀO complexes studied, the oxygen is in the atomic state, directly bonded to the Fe atoms/ions. In a few cases, however, evidence of molecular O 2 bonding to the core of the complex was reported, such as, for example, in the FeO 5 system, where an O 2 molecule was considered as an adduct to the FeO 3 core. 11,14 In the theoretical study of Gutsev et al. 15 on FeO n and FeO n À clusters (n =5À12), the authors investigated a series of structural configurations and showed that the ground states of the clusters containing an even number of O atoms exhibit superoxo- and peroxo-type bonding, whereas the clusters with an odd number of O atoms include one or more oxo-type bondings. In the present work, we report the experimental formation of gas-phase FeO n þ , n e 16 clusters. Additionally, we measure fragmentation cross sections of cluster ions colliding with a Ne secondary beam, using the crossed molecular beam scattering method. Moreover, by using first-principle calculations, we reveal the appearance of approximately isoenergetic low- and high-spin states and a structural phase transition as the number of O ligands bonded to Fe þ increases. 2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Experimental Setup. The experiments have been per- formed in a molecular beam apparatus described in detail previously. 17À20 In short, a beam of iron oxide clusters is formed in an open source without a growth channel by mixing the plasma plume produced by fundamental (1064 nm) or quadrupled (266 nm) Nd:YAG laser ablation of pure iron with the supersonic Received: May 13, 2011 Revised: May 26, 2011 ABSTRACT: Experimental and theoretical studies based on mass spectrometry, collision-induced dissociation, and ab initio calculations are performed on the formation and stability of FeO n þ clusters, as well as on their structural, electronic, and magnetic properties. In the mass spectra, clusters with an even number of oxygen atoms show increased stability, most prominently for FeO 10 þ . The extra stability of this cluster is confirmed by measurements of fragmentation cross sections through crossed molecular beam experiments. In addition, the calcula- tions indicate a structural phase transition at this size, and most importantly, the FeO n þ clusters show unique magnetic features, exhibiting isoenergetic low-spin (LS) and high-spin (HS) ground states. In the LS state, the magnetic moments of the O atoms adopt an antiferromagnetic alignment with respect to the magnetic moment of Fe þ , whereas in the HS state, the alignment is ferromagnetic. FeO 10 þ is the largest thermodynamicaly stable complex, with the highest magnetic moment among the FeO n þ clusters (13 μ B in HS).