S. Krückeberg

The dissociation channels of silver clusters Agn+, 3 ≤ n ≤ 20☆

Abstract The low energy dissociation channels of silver cluster ions Agn+, 3 ≤ n ≤ 20 are determined by collision-induced dissociation (CID) in a Penning trap. While for most cluster sizes the first fragment cluster ion is produced by monomer evaporation, the fragment ion of small odd-sized clusters has two atoms less than their precursors indicating an evaporation of dimers. The results are compared to similar CID studies on gold cluster ions, photofragmentation patterns, abundance spectra for various silver-cluster production techniques and calculated binding energies.

Electronic effects in the production of smali dianionic gold clusters by electron attachment on to stored Au-n, n = 12-28

Abstract Single charged gold clusters Au n -, 12  n  28, are stored in a Penning trap, size selected and transformed into dianions, Au2- n by the application of an electron beam. At the onset of dianion production, that is that range of cluster sizes n where the smallest doubly charged clusters are observed, the measured intensity ratio of the dianions to their precursors is not a continuous function of cluster size. Instead, there is a strong odd-even effect and a comparatively intense signal of Au2-18 The observed structures are very reminiscent of similar phenomena in the abundance spectra of metal clusters as observed by Knight et al. (1984, Phys. Rev. Lett., 52, 2141), which gave rise to the electronic shell structure interpretation and the further development of the jellium model as applied to metal clusters.

PROBING CLUSTER STRUCTURES WITH SENSOR MOLECULES : METHANOL ADSORBED ONTO GOLD CLUSTERS

Abstract Structural, dynamical and electronic properties of the adducts formed by adsorbing methanol onto size-selected gold clusters are investigated using infrared multiple-photon dissociation spectroscopy of trapped Au n + CH 3 OH, n ⩽15, in conjunction with Car–Parrinello calculations. The C–O stretching vibration of the attached sensor molecule changes discontinuously as a function of cluster size, which is traced back to a change in dimensionality of the cluster structure.

Trapped metal cluster ions

An overview is given of experiments with stored metal cluster ions in a Penning trap system. The setup allows axial injection of clusters produced in an external source and a time-of-flight mass analysis of the reaction products after axial ejection. The systems options include the selection of stored ions, the manipulation of their orbits, addition of reactant and buffer gases and axial optical access for laser spectroscopic studies. As described by various examples, investigations have been made with respect to the development of trapping techniques and the characterization of metal clusters in terms of their physical and chemical properties.

Electron impact ionization/dissociation of size selected gold cluster cations

Abstract Singly charged gold clusters, Au n + in the size range n =12 to 72 have been captured and stored in a Penning trap, size selected and subjected to an electron beam. This interaction leads to further ionization as well as dissociation. The resulting abundance spectra of doubly and triply charged clusters show (a) a lower size limit for the production of multiply charged clusters from an ensemble of hot precursors, which can be understood in terms of the respective decay pathways, (b) an odd/even alternation of singly and doubly charged clusters in the size range below n =30, which inverses sign with change of charge state, and (c) magic numbers, i.e. prominent signals for particular cluster sizes, which can be identified as closed electronic shells. In contrast to similar experiments on silver clusters and in spite of the well-known ligand-stabilized species, there is no indication of a particularly stable charge-state-independent cluster size of n =55.

Dissociation pathways of doubly and triply charged gold clusters

AbstractCollision induced dissociation is applied to study the fragmentation channels of multiply charged gold clusters, Au N2+, size N= 7 –35, and Au> N3+}, N = 19–35, stored in an ion cyclotron resonance (Penning) trap. The main dissociation pathways are neutral monomer evaporation, Au> NZ+to Au> N-1Z+} + Au, for the larger and fission into a charged trimer plus the remaining cluster, Au> NZ+to{}Au> N-3(Z-1)+} +{}Au> 3+}, for the smaller clusters. In the intermediate cluster size region an odd–even alternation of the two competing decay pathways is observed. In addition, for some specific cluster sizes there are indications of neutral dimer evaporation, Au> NZ+to{}Au> N-2Z++Au> 2}, and of extremely asymmetric fission of the form Au> NZ+to Au> N-1(Z-1)+} + Au+.

Collision induced dissociation of stored gold cluster ions

The stability of gold cluster ions Aun+ (2≦n≦23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au3+, Au9+, and Au19+ in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn≦15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energies are deduced.

Production and investigation of multiply charged metal clusters in a Penning trap

Singly charged gold cluster ions from a laser-vaporization source are transferred into a Penning trap and subjected to electron bombardment. The charged reaction products are analyzed by time-of-flight mass spectrometry after axial ejection from the trap. They include singly charged cluster fragments, multiply charged clusters of the initial size and multiply charged cluster fragments. The multiply charged clusters are selected and further investigated by collision induced dissociation. Two types of reactions can be distinguished: Dissociation into several charged fragments and evaporation of neutrals. Several features of multiply charged clusters relevant for future investigations are reviewed.

Decay pathway determination of even-size dicationic silver clusters: Ag162+ and Ag182+ revisited by pre-precursor selection and sequential decay

Abstract It is demonstrated how by selection of pre-precursor cluster ions Ag 2 n +1 2+ and collision-induced dissociation an ensemble of Ag 2 n 2+ can be produced, which is not contaminated by Ag n + clusters of the same size-to-charge state ratio, n / z . By use of this technique, Ag 16 2+ and Ag 18 2+ precursor ensembles have been prepared for further investigations. They are observed to decay by neutral monomer evaporation, Ag 2 n 2+ →Ag 2 n −1 2+ +Ag, and trimer fission, Ag 2 n 2+ →Ag 2 n −3 + +Ag 3 + , and show no sign of symmetric fission.

Infrared photodesorption of methanol molecules adsorbed on a Au4+ cluster

Abstract The infrared photodissociation of mass selected cluster-adsorbate complexes Au 4 + (CH 3 OH) m ( m = 1–3) is studied in a Penning ion trap. Spectra are obtained by the excitation of vibrational modes of methanol, the desorption of neutral molecules and the detection of fragment ions by time-of-flight mass spectrometry. The photodissociation spectra are red-shifted with respect to the absorption of frequency of free methanol. The number of photons necessary for the desorption of the adsorbate molecules and the respective separation energies are estimated.