Stefan Gilb

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.

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.

Electronic photodissociation spectroscopy of Aun−⋅Xe (n=7–11) versus time-dependent density functional theory prediction

Electronic (one-photon) photodepletion spectra were recorded for gold cluster anions complexed with one xenon atom over the photon energy range 2.1-3.4 eV. Clusters were generated by pulsed laser vaporization and probed under collisionless molecular beam conditions. The spectra obtained are highly structured with the narrowest features--assigned to individual electronic transitions--having bandwidths of less than 40 meV. Time-dependent density functional theory predictions of optically allowed transitions for the most stable--planar--isomers of the corresponding bare metal cluster anions are generally consistent with the experimental observation.

Electronic photodissociation spectroscopy of isolated IrX62− (X=Cl,Br)

Photodissociation spectra of free doubly charged anions IrX62− (X=Cl,Br) were measured in the photon energy range from 1.5 to 2.9 eV. Both data sets show the same features as the spectra of the respective aqueous solutions. Compared to solution, the gas phase absorption bands of IrBr62− are redshifted by 0.01–0.15 eV. For IrCl62− no such shift could be observed. Photodissociation of IrBr62− results in the formation of Br−, IrBr4−, and IrBr5−. Fluence dependent measurements of fragment formation as well as parent ion depletion, allowed inferences regarding the dissociation pathway and the inner barrier height for the dissociation process which was estimated to be 1.6±0.2 eV. From measurements of the kinetic energy released upon fragmentation into monoanions, we estimate the outer barrier height to be 2.2±0.2 eV.

Electronic photodissociation spectroscopy of Au4+⋅Arn, n=0–4: Experiment and theory

We report experimental and theoretical studies of the electronic absorption spectra of mass selected Au4+⋅Arn (n=0–4) clusters in the photon energy range of 2.14–3.35 eV. Photodissociation spectra were recorded by monitoring ion depletion upon photon absorption, yielding absolute photodissociation cross sections. The experimental spectra were interpreted by comparing them with calculations of the optical response in the framework of time-dependent density functional theory using cluster structures calculated both by density functional theory (B3-LYP functional) and by ab initio calculations at the RI-MP2 level.

A high resolution dual mass gate for ion separation in laser desorption/ionization time of flight mass spectrometry

A dual mass gate is described which allows for high resolution separation of ions under realistic laser desorption/ionization and matrix assisted laser desorption/ionization time of flight mass spectrometry conditions at 10 kV kinetic energy. The device consists of two interleaved comb units sequentially positioned in the ion beam path. This allows significant improvement in resolution relative to one unit by separation of “low pass” and “high pass” functions. Principal limits as given by mechanical dimensions of the device and electrical properties of the switches are also discussed.

Tunneling electron loss from isolated platinum tetrahalide dianions

Electron detachment from doubly charged anions PtClxBr4−x2− to form PtClxBr4−x− (x=0–4) was probed for in a Fourier-transform ion-cyclotron-resonance mass spectrometer at 288 K. The corresponding decay rates were measured as a function of collision gas pressure. In the zero-pressure limit, nonzero unimolecular decay rate constants were observed for x=4 and 3 (0.273±0.035 and 0.056±0.019 s−1 for 288 K, respectively). These show a strong temperature dependence suggesting the new phenomenon of blackbody radiation induced tunneling electron loss. Comparison to electron tunneling rates calculated by means of the Wentzel–Kramers–Brillouin approximation using Coulomb barrier properties and electron detachment energies derived from ab initio quantum chemical calculations provides a qualitative rationalization of the observed trends.

Photodissociation spectroscopy of Ag4+(N2)m, m=0–4

We have determined photodissociation spectra of Ag4+(N2)m=0–4 in the photon energy range from 2.1 to 3.75 eV. The cluster complexes were generated by laser vaporization and their absolute photodissociation cross sections were measured in depletion using a modified reflectron time-of-flight mass spectrometer. The band spectra show significant nonmonotonic variations with increasing N2 coverage. The experimental data are contrasted with quantum chemical calculations of ground and electronically excited states using density functional as well as post-Hartree–Fock methods. Experiment and theory are in qualitative agreement allowing tentative assignment of the depletion spectra and insight into the relevant bonding interactions.