C. Walther

Multiple-collision induced dissociation of trapped silver clusters Agn+ (2⩽n⩽25)

The dissociation energies of singly charged silver cluster cations, Agn+ (2⩽n⩽25), are determined by multiple-collision induced dissociation (MCID) in a Penning trap. The fragment yield is analyzed in terms of a linearized impulsive collision theory for the energy transfer in the multicollisional process and the delayed decay as predicted by the Rice–Ramsperger–Kassel (RRK) model. Previous photofragmentation experiments performed in the size range (9⩽n⩽21) are found to be in good agreement with the present results. Theoretical predictions agree for most clusters sizes.

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.

Decay pathways and dissociation energies of copper clusters, Cun+ (2⩽n⩽25), Cun2+ (15⩽n⩽25)

The fragmentation pathways and dissociation energies of copper cluster cations, Cun+ and Cun2+, are determined by multiple-collision induced dissociation. For singly charged clusters, an odd–even staggering is observed throughout the investigated size range, 2⩽n⩽25, where the odd-size clusters have a higher dissociation energy than the average value of their even-size neighbors. The odd–even effect decreases with increasing cluster size. In small clusters it manifests itself by dimer evaporation of the odd-size clusters with n=3,5,11 and possibly n=7, while for all other cluster sizes dissociation by neutral monomer evaporation is observed. The clusters of size n=3, 9, 15, and 21 show particularly high dissociation energies and thus indicate electronic shell closures for n=2, 8, 14, and 20 atomic valence electrons. These results are compared with recent density functional theory calculations. The investigations on singly charged clusters are complemented by studies on doubly charged Cun2+, n=15–25. These ...

The interaction of gold clusters with methanol molecules: Infrared photodissociation of mass-selected Aun+(CH3OH)m

Structural and energetic properties of the adducts formed by adsorbing methanol onto size-selected gold clusters are investigated by infrared photodissociation of trapped Aun+(CH3OH)m, n=1–10,15 and m=1–3. The excitation of vibrational modes of methanol leads to the desorption of neutral molecules which is monitored by time-of-flight mass spectrometry. Spectra are obtained by measuring the fragment ion intensity as a function of photon energy. The C–O stretching vibration of adsorbed methanol changes discontinuously with cluster size. By comparison with Car–Parrinello calculations this change is traced back to the dimensionality of the gold clusters. The number of photons necessary for the desorption of methanol molecules provides an estimate of the respective separation 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+.

TIME RESOLVED PHOTOFRAGMENTATION OF AU+15 CLUSTERS

Abstract Au + 15 ions were produced by laser vaporization and transferred into a Penning trap. They were illuminated by a 10 ns dye-laser pulse at photon energies covering the range from 2.7 to 4.4 eV. Besides the determination of fragmentation patterns and photoabsorption cross sections the monomer evaporation of the excited clusters was observed time resolved on a microsecond to millisecond timescale yielding a monomer separation energy of 1.95(5) eV. In addition, the dissociation energy for Au + 14 could be determined as 1.57(5) eV by time resolved investigation of the sequential decay Au + 15 → Au + 14 + Au → Au + 13 + 2 Au.