J. Ziegler

External-ion accumulation in a Penning trap with quadrupole excitation assisted buffer gas cooling

Abstract A pulsed ion beam from an external source is injected into a Penning trap and accumulated by repeatedly lowering during ion capture to prevent the ions already captured from escaping. For the same reason the newly captured ions have to be cooled, which achieved by buffer gas collisions. To prevent radial on loss, the ions are exposed to azimuthal quadrupole excitation. By choosing the appropriate frequency (range) this method (selective quadrupole excitation assisted capture and centering (SQUEACE) allows a mass selection during the capture process and leads to a centering of those ions in the Penning trap. The multiple ion bunch capture results in a significant improvement in signal-to-noise ratio and a decrease in experiment duration.

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.

A Penning trap mass spectrometer for the study of cluster ions

A Penning trap system has been set up for storing and investigating cluster ions over time ranges from microseconds up to minutes. This enables studies of cluster reactions with extremely low cross sections and the observation of their time dependence in a new regime. The ions are created externally by laser vaporization, cooled by adiabatic expansion of a supersonic beam, and injected into the Penning trap. Detection of reaction products is achieved by combining the advantages of two complementary approaches, viz. the high resolution of Fourier transform mass spectrometry and the high sensitivity of single‐ion counting with a time‐of‐flight mass spectrometer. The performance of the apparatus is illustrated by results of recent cluster experiments.

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 trapping condition and a new instability of the ion motion in the ion cyclotron resonance trap

Abstract In analogy to the critical mass, m crit , a critical voltage, U crit , (and a general trapping parameter, π trap ) is defined, above which the ion motion in an ion cyclotron resonance (ICR) trap is unstable and the ions are lost from the trap. The theoretical values for the critical voltage are confirmed by experimental results. Singly charged gold cluster ions, Au n − , of several sizes, n = 50, 60, 76, 100, 110, and 145 (the latter corresponding to an ion mass of 28 560 u), were injected into an ICR trap, stored, and detected by axial ejection and single ion counting using a microchannel plate detector. During the storage period the trapping voltage, U , was varied for extended durations (from a few to over one hundred milliseconds), to probe its effect on the trapping efficiency. In addition to the expected instability above the critical trapping condition ( m m crit = U U crit = π trap = 1 ), a new instability was observed below the critical value. This is explained in terms of a resonance effect occurring for a trapping parameter β trap = U U crit = m m crit = 8/9 , which corresponds to the smallest possible integer ratio of the eigenfrequencies of ion motion, i.e. ω + = ω z = 2 ω − (where ω + , ω z and ω − stand for the reduced cyclotron, the trapping and the magnetron frequency, respectively).

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.

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>nN3+}, N = 19–35, stored in an ion cyclotron resonance (Penning) trap. The main dissociation pathways are neutral monomer evaporation, Au>nNZ+to Au>nN-1Z+} + Au, for the larger and fission into a charged trimer plus the remaining cluster, Au>nNZ+to{}Au>nN-3(Z-1)+} +{}Au>n3+}, 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>nNZ+to{}Au>nN-2Z++Au>n2}, and of extremely asymmetric fission of the form Au>nNZ+to Au>nN-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.

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.