K. Lützenkirchen

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.

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.

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).

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.

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.

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.

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.