Patrick Weis

The structures of small gold cluster anions as determined by a combination of ion mobility measurements and density functional calculations

A combined experimental and theoretical study of small gold cluster anions is performed. The experimental effort consists of ion mobilitymeasurements that lead to the assignment of the collision cross sections for the different cluster sizes at room temperature. The theoretical study is based on ab initiomolecular dynamics calculations with the goal to find energetically favorable candidate structures. By comparison of the theoretical results with the measured collision cross sections as well as vertical detachment energies (VDEs) from the literature, we assign structures for the small Au n − ions (n<13) and locate the transition from planar to three-dimensional structures. While a unique assignment based on the observed VDEs alone is generally not possible, the collision cross sections provide a direct and rather sensitive measure of the cluster structure. In contrast to what was expected from other metal clusters and previous theoretical studies, the structural transition occurs at an unusually large cluster size of twelve atoms.

Structures of small gold cluster cations (Aun+, n<14): Ion mobility measurements versus density functional calculations

We have performed ion mobility measurements on gold cluster cations Aun+ generated by pulsed laser vaporization. For clusters with n<14, experimental cross sections are compared with theoretical results from density functional calculations. This comparison allows structural assignment. We find that room temperature gold cluster cations have planar structures for n=3–7. Starting at n=8 they form three dimensional structures with (slightly distorted) fragments of the bulk phase structure being observed for n=8–10.

Fragmentation of C+60 and higher fullerenes by surface impact

Fragmentation of various fullerenes was studied by surface impact on highly oriented pyrolytic graphite at collision energies Ecol of 150–1050 eV/molecule. The projectiles C+60, C+70, C+76, C+84, and C+94 were formed by laser desorption of chromatographically separated samples, while large carbon clusters C+94, C+110, C+164 were produced by laser‐induced coalescence reactions. Except at the highest impact energies, the fragment distributions consist of even numbered C+n species with abundance maxima similar to those observed in fullerene synthesis. With increasing Ecol, we observe a size evolution in the fragment distributions characteristic of a sequential fragmentation process. Simulated fragment distributions based on statistical rate theory and a sequential C2 loss mechanism reproduce the experimental data well up to a maximum Ecol. They are used to determine the mean energy transfer during surface impact as a function of collision energy as well as its dependence on several experimental parameters su...

Structures of small silver cluster cations (Agn+, n<12): ion mobility measurements versus density functional and MP2 calculations

Abstract We have performed ion mobility measurements on silver cluster cations Ag n + generated by pulsed laser vaporization. For clusters with n n =3–4. Starting at n =5 they form three dimensional structures. The structures are compared with predictions by Bonacic-Koutecky et al. and the respective results obtained in a previous study for gold clusters.

Metastability of isolated platinum and palladium tetrahalide dianions and the role of electron tunneling

Abstract In the present study we have isolated MX 4 2− (M=Pd, Pt; X=Cl, Br) in a room temperature Penning trap under virtually collision-free, ultra high vacuum conditions and have probed for metastable decay on a time scale of up to 1000 s. While PdCl 4 2− , PdBr 4 2− and PtBr 4 2− do not react appreciably on this time scale, PtCl 4 2− undergoes tunneling electron loss to form PtCl 4 − with a half-life of 2.5 s. We provide kinetic data and ab-initio quantumchemical calculations of relevant initial and final states as well as Coulomb barriers. All four species are metastable towards halide loss on the basis of these calculations.

Determining the size-dependent structure of ligand-free gold-cluster ions

Ligand-free metal clusters can be prepared over a wide size range, but only in comparatively small amounts. Determining their size-dependent properties has therefore required the development of experimental methods that allow characterization of sample sizes comprising only a few thousand mass-selected particles under well-defined collision-free conditions. In this review, we describe the application of these methods to the geometric structural determination of Au+n and Au−n with n = 3–20. Geometries were assigned by comparing experimental data, primarily from ion-mobility spectrometry and trapped ion electron diffraction, to structural models from quantum chemical calculations.

Structures of mixed gold-silver cluster cations (AgmAun+,m+n<6): Ion mobility measurements and density-functional calculations

The collision cross sections of Ag(m)Au(n)+ (m+n)<6 cluster ions were determined. For bimetallic clusters, we observe a significant intracluster charge transfer leaving most of the ions positive charge on the silver atoms. The mixed trimeric ions Ag2Au+ and AgAu2+ are triangular like the pure gold and silver trimers. Most of the tetrameric clusters are rhombus shaped, with the exception of Ag3Au+, which has a Y structure with the gold atom in the center. Among the pentamers we find distorted X structures for all systems. For Ag2Au3+ we find an additional isomer which is a trigonal bipyramid. These findings are in line with predictions based on density-functional theory calculations, i.e., all these structures either represent the global minima or are within less than 0.1 eV of the predicted global minimum.

Unusually stable magic number clusters of serine with a surprising preference for homochirality

Abstract The amino acid serine forms unusually stable octameric clusters upon electrospray ionization of relatively concentrated solutions. A strong preference for homochiral clusters is observed, while mixed clusters of D- and L-serine are significantly underrepresented. The present essay reviews the experimental results as well as the structures suggested so far. Surprisingly, there is good agreement on the experimental facts among different research groups, but a vivid discussion with respect to the octamer structure is going on. In order to provide a firm basis for a discussion of these issues, we briefly address the questions of chiral recognition of amino acids and cluster formation in more general terms. Another important issue in this respect is the generation of zwitterionic amino acids in the gas phase. This paper suggests a new structure that seems to be in good agreement with the experimental evidence gathered so far. The review terminates with some thoughts on the implications of the homochirality of serine octamers for homochirogenesis and the origin of life.

Ab initio studies of small sodium–sodium halide clusters, NanCln and NanCln−1 (n≤4)

The geometries, energetics, and some first and second order molecular properties(dipole moments, force constants, polarizabilities) of small stoichiometric (NaCl) n and nonstoichiometric (Na n Cl n−1), n≤4 clusters were studied at the SCF and various correlated levels.

Enhanced coalescence upon laser desorption of fullerene oxides

C60Ox, x=1–3 were prepared by exposing C60 solutions to ozone. After high performance liquid chromatography purification, these materials were studied by laser desorption time of flight mass spectroscopy. These mass spectra suggest that fullerene oxides undergo laser desorption induced coalescence more efficiently than pure C60. A correlation between the fragmentation of the desorbed parent species and the observed coalescence products both in yield and distribution suggests that efficient gas phase coalescence involves at least one reactive species, such as C58, produced by fragmentation of the desorbed fullerenes or fullerene derivatives.