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Institute
This thesis presents the production of polyanionic clusters within two ion storage devices:
Considering a Penning trap, the accessible range of polyanionic aluminium clusters has been expanded up to the 10th charge state. In particular, abundance curves for clusters with 5 to 9 excess electrons have been measured for the first time and analysed with respect to their lifetime-dependent appearance sizes. These sizes reveal a nearly quadratic dependency on the charge state for experimentally accessible lifetimes.
Additionally, the production of polyanionic clusters has been enabled in a radiofrequency ion trap. Therefore, the transition from a harmonic to a digital 2- and 3-state guiding signal has been investigated with respect to the ion storage. The passing of electrons through the trap during field-free periods of the guiding signal led to the first production of polyanionic clusters within a radiofrequency ion trap.
Lead-cluster investigations
(2017)
In this thesis, investigations on lead clusters stored in a Penning trap are presented. The measurements are performed at the ClusterTrap setup at the Institute of Physics of the University of Greifswald. A Penning trap with a superconducting magnet (B=12 Tesla) makes up the central part of the experiment. In this trap, singly positively or negatively charged lead clusters (a group of lead atoms) are stored, their amplitudes of motion are cooled, and a specific cluster size is selected. Thus, clusters of only a single size are prepared for experimental investigation. After interactions with electrons and/or photons, the trap content is extracted and analyzed by time-of-flight mass spectrometry.
In the first experiment, the size-selected clusters are excited by a frequency-doubled Nd:YAG laser, which leads to fragmentation processes. The preferred fragmentation pathway, which is observed to be break-off of a seven-atom neutral cluster is unusual for metal clusters, which typically evaporate monomers. Furthermore, the already known magic cluster sizes are observed.
In a subsequent experiment, positively charged lead clusters with 31 atoms are irradiated with laser light and fragmentation processes are time resolved investigated. The assumption that lead clusters fragment by break-off of neutral heptamers is confirmed.
In the following experiment, an electron beam is guided through the Penning trap to ionize pulsed-in argon atoms. While the positive argon ions leave the trap, the secondary electrons are trapped together with the selected lead clusters. This allows the electrons to attach to the singly charged lead clusters, which leads to multiply negatively charged lead clusters. The relative abundance of multiply-charged clusters is measured with respect to the cluster size, from which the appearance sizes of di- and trianions can be calculated. In addition to the attachment of electrons, fragmentation products similar to those of the photoexcitation measurements are observed. Furthermore, the cluster sizes 10 and 12 are observed regardless of the investigated precursor size, together with clusters of the precursor size reduced by 10 and 12. This is a first hint for a fission process of doubly negatively charged lead clusters into two singly charged products. In a following measurement, doubly charged lead clusters are produced and photoexcited. The observed abundance spectra confirm this assumption.
This thesis describes investigations of metal clusters stored in an ion-cyclotron resonance (ICR) trap, as well as corresponding trap research and development. Charged clusters are produced and investigated in the experimental setup Cluster-Trap, comprising a cluster-ion source, an ICR trap and a time-of-flight (ToF) mass spectrometer. In the framework of its move to the new building of the Institute of Physics, new components have been added to the ClusterTrap setup. A radio-frequency ion trap is now used for cluster ion preparation prior to the performance of cluster experiments in the ICR trap. A quadrupole ion deflector allows an optimized usage of the ICR trap, as well as simultaneous use of several ion sources and detectors. The implementation of a potential lift at the ToF mass spectrometer enables a more flexible operation of the setup with ion energies up to several hundreds of electron volts. The new components have been tested and characterized, and the experimental procedures have been adapted. An important aspect of cluster investigations is the manipulation of trapped ions by application of appropriate excitation fields. For the ICR trap, a vector representation model has been developed for quick analysis of radial excitation fields, applied to the quarter-segmented ring electrode of an ICR trap. Its application has been demonstrated for asymmetric radial quadrupolar excitation of stored cluster ions, confirming the observation of unintended ion ejection from the trap. Investigation of multiply negatively charged metal clusters at ClusterTrap has been continued. By the "electron-bath" technique, i.e. simultaneous storage of cluster mono-anions and electrons in the ICR trap, high charge states are produced up to a limit which arises from restrictions for ion trapping. A modification of the electron bath, which bypasses this limit, has been introduced and demonstrated by the first-time production and detection of aluminum cluster anions carrying five excess electrons (penta-anions). Results of the penta-anion production as a function of the trapping voltage relate to the Coulomb potentials of the cluster anions involved, in agreement with previous findings. The observed poly-anionic clusters are meta-stable and their abundance as a function of the cluster size is determined by their lifetimes. Observed poly-anion abundances are described by a thermionic-emission approach, by means of the Richardson-Dushman formula. The height of the Coulomb potential in the formula is decreased to match experimental data, thus accounting for electron tunneling. Poly-anions are observed only above a minimum cluster size, the appearance size. To determine this limit from experimental results, a new data evaluation method has been introduced, which considers the poly-anion lifetimes and respective abundances of a range of cluster sizes. As a result, the experimental appearance size is larger than the smallest poly-anionic cluster observed, in contrast to previous approaches.