Nature of Ar bonding to small Co_n^+ clusters and its effect on the structure determination by far-infrared absorption spectroscopy

Far-infrared vibrational spectroscopy by multiple photon dissociation has proven to be a very useful technique for the structural fingerprinting of small metal clusters. Contrary to previous studies on cationic V, Nb and Ta clusters, measured vibrational spectra of small cationic cobalt clusters show a strong dependence on the number of adsorbed Ar probe atoms, which increases with decreasing cluster size. Focusing on the series Co_4^+ to Co_8^+ we therefore use density-functional theory to analyze the nature of the Ar-Co_n^+ bond and its role for the vibrational spectra. In a first step, energetically low-lying isomer structures are identified through first-principles basin-hopping sampling runs and their vibrational spectra computed for a varying number of adsorbed Ar atoms. A comparison of these fingerprints with the experimental data enables in some cases a unique assignment of the cluster structure. Independent of the specific low-lying isomer, we obtain a pronounced increase of the Ar binding energy for the smallest cluster sizes, which correlates nicely with the observed increased influence of the Ar probe atoms on the IR spectra. Further analysis of the electronic structure motivates a simple electrostatic picture that not only explains this binding energy trend, but also why the influence of the rare-gas atom is much stronger than in the previously studied systems.Comment: 12 pages including 10 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Experimental vibrational spectra of gas-phase tantalum cluster cations

Contains fulltext : 98817.pdf (author's version ) (Open Access

Argon physisorption as structural probe for endohedrally doped silicon clusters

Contains fulltext : 98812.pdf (publisher's version ) (Open Access

The adsorption of CO on group 10 (Ni, Pd, Pt) transition-metal clusters

The adsorption of a single CO molecule on clusters of the Group 10 transition metals is characterized by infrared multiple photon dissociation (IR-MPD) spectroscopy. The cationic, neutral, and anionic carbonyl complexes contain between 3 and up to 25 metal atoms. The C\u2013O stretching frequency \u3bd(CO) shows that while both nickel and platinum clusters adsorb CO only in atop positions, palladium clusters exhibit a variety of binding sites. These findings can be rationalized by considering the increasing role relativistic effects play in the electronic structure of the cluster complexes going down the group. Conclusions for the cluster-support interactions for size-selected supported particles are drawn from the charge dependence of \u3bd(CO) for the gas-phase species.Peer reviewed: YesNRC publication: Ye

Not so loosely bound rare gas atoms : finite-temperature vibrational fingerprints of neutral gold-cluster complexes

We present an experimental and theoretical study of the structure of small, neutral gold clusters\u2014Au3, Au4 and Au7\u2014'tagged' by krypton atoms. Infrared (IR) spectra of AuNcenterdotKrM complexes formed at 100 K are obtained via far-IR multiple photon dissociation in a molecular beam. The theoretical study is based on a statistical (canonical) sampling of the AuNcenterdotKrM complexes through ab initio molecular dynamics using density-functional theory in the generalized gradient approximation, explicitly corrected for long-range van-der-Waals (vdW) interactions. The choice of the functional is validated against higher-level first-principle methods. Thereby finite-temperature theoretical vibrational spectra are obtained that are compared with the experimental spectra. This enables us to identify which structures are present in the experimental molecular beam for a given cluster size. For Au2, Au3 and Au4, the predicted vibrational spectra of the Kr-complexed and pristine species differ. For Au7, the presence of Kr influences the vibrational spectra only marginally. This behavior is explained in terms of the formation of a weak chemical bond between Kr and small gold clusters that localizes the Kr atom at a defined adsorption site, whereas for bigger clusters the vdW interactions prevail and the Kr adatom is delocalized and orbits the gold cluster. In all cases, at temperatures as low as T = 100 K, vibrational spectra already display a notable anharmonicity and show, in comparison with harmonic spectra, different position of the peaks, different intensities and broadenings, and even the appearance of new peaks.Peer reviewed: YesNRC publication: Ye

STRUCTURE DETERMINATION OF SILICON CLUSTERS IN THE GAS PHASE: A VIBRATIONAL SPECTROSCOPY AND DFT INVESTIGATION

Fielicke, A.; Kirilyuk, A.; Ratsch, C.; Behler, J.; Scheffler, M.; von Helden, G.; Meijer, G. Phys. Rev. Lett.Gruene, P.; Fielicke, A.; Meijer, G.; Janssens, E.; Ngan, V. T.; Nguyen, M. T.; Lievens, P. Chem. Phys. Chem.Author Institution: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin (Germany); Department of Physics and Astronomy, University of Leuven, Celestijenlaan 200 D, 3001 Leuven (Belgium); Department of Chemistry, University of Leuven, Celestijenlaan 200 F, 3001 Leuven (Belgium)Metal clusters are often considered convenient models for studying the interactions occurring at active sites in heterogeneous catalysts and semiconductor clusters have implications to the electronics industry. The properties of small clusters often differ from those of bulk material and cluster characteristics can vary drastically with particle size. Hence, it is essential to have systematic studies over a wide range of cluster sizes. We employ infrared multiple photon dissociation (IR-MPD) spectroscopy to obtain vibrational spectra of clusters tagged with a noble gas atom in the gas phase. This spectroscopic technique makes use of the Free Electron Laser for Infrared eXperiments (FELIX) and has been shown to be an effective method to probe the structure of transition metal clusters.} \textbf{2004}, \textit{93}, 023401.} Comparing the observed infrared spectra to those theoretically predicted for multiple isomers of a single cluster enables accurate structural assignments. Recent results on the structures of silicon clusters will be presented and the effect of doping silicon clusters with transition metal atoms} \textbf{2008}, in press.} will be discussed

Structures of neutral Au7, Au19 and Au20 clusters in the gas phase.

The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two- dimensional structure for neutral Au-7 and a pyramidal structure for neutral Au-20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au-20 cluster is directly reflected in the vibrational spectrum of Au-19

Structural Identification of Caged Vanadium Doped Silicon Clusters

The geometry of cationic silicon clusters doped with vanadium, SinV+ (n = 12-16), is investigated by using infrared multiple photon dissociation of the corresponding rare gas complexes in combination with ab initio calculations. It is shown that the clusters are endohedral cages, and evidence is provided that Si16V+ is a fluxional system with a symmetric Frank-Kasper geometry.status: publishe