| 1. |
- Kristiansson, Moa, 1989-
(författare)
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Precision measurements on negative ions
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, experiments with negative ions performed at the DESIREE facility are presented. DESIREE consists of two electrostatic storage rings operated at 13 K. The low temperature in combination with a low pressure allows for storage times of the negative ion beams to be up to several hours. This unique feature is to advantage in the development of a new method for high-precision measurements of electron affinities and binding energies of excited states of negative ions. In this method, ions are stored in the one of the two storage rings of DESIREE, and a high-power laser is applied to photodetach ions in the excited state. These excited states are often very long-lived for negative ions and the photodetachment signal from these states are a major source of background, which dominates the uncertainties, in threshold spectroscopy experiments. By removing the ions in the excited state, threshold photodetachment spectroscopy can be performed with almost no background below the photodetachment threshold. This allows the threshold energy to be determined with a very high precision.The new method is demonstrated by performing measurements of the electron affinities of 16O and 18O. The obtained uncertainty in the electron affinity of 16O is a tenfold improvement in accuracy compared to previous measurements and the resulting electron affinity is the most accurately measured electron affinity of any element to date. In addition to the electron affinity, the fine-structure splitting of the ground state is also measured for both isotopes. From the measured electron affinities, the isotope shift in the 18O and 16O electron affinity is determined with a high accuracy. This shift is a quantity not only relevant in atomic physics but also of great interest in nuclear physics.In addition to the high-precision electron affinity measurements, the possibility to store ions for a long time is utilized to perform measurements of lifetimes of excited states in negative ions. Measurements are presented where excited states in Ir-, Bi-, and CH- are studied. Lifetimes ranging from 100 ms up to 5000 s are observed.
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| 2. |
- Anderson, Emma K., 1986-
(författare)
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DESIREE: Instrumentation Developments and Hot Metal Cluster Decays
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents instrumentation developments and measurements performed at the Double ElectroStatic Ion Ring ExpEriment, DESIREE, at Stockholm University. DESIREE operates at cryogenic temperatures ~13 K, with very low background pressures of ~10-14mbar, allowing the observation of stored ions to long times of tens of seconds and longer.Investigations into improving the count rate capability of the DESIREE detectors are presented. Microchannel plate (MCP) detectors are used for position sensitive particle counting in the DESIREE detector assemblies. In a cryogenic environment the operational resistance of MCPs is orders of magnitude higher than at room temperature and this limits the possible count rates. Novel low-resistance MCP detectors were investigated and resulted in the replacement of the MCPs in the DESIREE detector assemblies.DESIREE was used to measure spontaneous decays of hot, small cluster anions. The decays of small silver, copper and gold cluster anions are presented and compared to statistical model calculations. An experiment that is able to measure the proportion of spontaneous decay due to fragmentation or electron detachment in dimer anions of silver and copper is presented and significant, previously overlooked, contributions from electron detachment to the decay is identified. Furthermore, measurements of the stability and decay of small carbon cluster dianions are presented. These experiments utilised the aforementioned low-resistance MCPs.
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| 3. |
- Gatchell, Michael, 1988-
(författare)
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Molecular Hole Punching : Impulse Driven Reactions in Molecules and Molecular Clusters
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When molecules are excited by photons or energetic particles, they will cool through the emission of photons, electrons, or by fragmenting. Such processes are often thermal as they occur after the excitation energy has been redistributed across all degrees-of-freedom in the system. Collisions with atoms or ions may also lead to ultrafast fragmentation in Rutherford-like scattering processes, where one or several atoms can literally be knocked out of the molecule by the incoming projectile before the energy can be completely redistributed. The resulting fragmentation pathways can in such knockout processes be very different from those in thermal processes.This thesis covers extensive studies of collisions between ions/atoms and isolated Polycyclic Aromatic Hydrocarbon (PAH) molecules, isolated fullerene molecules, or clusters of these. The high stabilities and distinct fragmentation channels make these types of molecules excellent test cases for characterizing knockout-driven fragmentation and the reactions that these processes can lead to. I will present experimental measurements for a wide range of energies and compare them with my own molecular dynamics simulations and quantum chemical calculations. In this thesis, I present an in-depth study of the role of knockout in the energetic processing of molecules and clusters. The competition between knockout and thermally driven fragmentation is discussed in detail.Knockout-driven fragmentation is shown to result in exotic fragments that are far more reactive than the intact parent molecules or fragments from thermal processes. When such reactive species are formed within molecular clusters efficient molecular growth can take place on sub-picosecond timescales. The cluster environments are crucial here because they protect the newly formed molecules by absorbing excess energy. This is a possible pathway for the growth of large PAHs, fullerenes, and similar carbonaceous complexes found in, for instance, the interstellar medium.
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| 4. |
- Navarro Navarrete, Jose Eduardo, 1988-
(författare)
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Precise measurements of electron affinities using Laser Photodetachment Threshold Spectroscopy
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this work we have performed Laser Photodetachment Threshold Spectroscopy (LPTS) to determine the electron affinities of the atomic species $^{133}$Cs and $^{16}$O with high precision. The negative ion of Cesium has been produced and investigated at the Gothenburg University Negative Ion and Laser LAboratory (GUNILLA) while the Oxygen anion have been studied in the Double ElectroStatic Ion Ring ExpEriment (DESIREE) at Stockholm University. Pilot studies have also been carried out to measure the electron affinity of complex molecular systems in DESIREE. Specifically, we have studied the beautifully symmetric fullerene C$_{60}^-$ negative ion and its spontaneous and laser induced decays. A preliminary analysis suggests that DESIREE's internal temperature of $\unit{13}{\kelvin}$ may allow an efficient cooling of these initially created hot molecules on time scales of a few seconds. This is a key aspect in measuring detachment thresholds as cooling suppresses hot band contributions associated with vibrationally excited states. Therefore, photodetachment techniques of the types successfully used for atomic species can be implemented for molecules. The preliminary results appear to settle a matter of disagreement in the literature concerning the most precise method to measure the EA of C$_{60}$, that is, LPTS or Photodetachment Electron Spectroscopy (PES).
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| 5. |
- Wolf, Michael, 1989-
(författare)
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Energetic processing of complex molecules in the gas phase
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Collisions between molecules and gas phase targets often lead to various intriguing processes. Such collisions may induce fragmentation of molecules that can be divided into different subsets depending on the projectile, target, and collision energy. One major part of the present research is the exploration of astrophysical relevant collision mechanisms. In collisions between polycyclic aromatic hydrocarbon (PAH) molecules or fullerenes with, for example, helium, nuclear stopping can lead to the prompt knockout of a carbon atom from the molecule. Such a vacancy in the molecular carbon backbone can be highly reactive, and lead to the formation of larger molecules. The energy dependencies of such processes are important for the understanding of astrochemical molecular growth processes, which in turn may lead to the formation of larger and more complex molecules in space. In addition, hydrogenation of PAHs changes their structures and internal properties, including their resistance against fragmentation. To better understand the effects of hydrogenation on the fragmentation of PAHs, low energy photofragmentation experiments are presented along with the collision experiments, and a detailed comparison is made between the effects of these different types of energy transfer processes.Besides astrophysically relevant research, studies on the response of biomolecules to collisions with gas phase targets are presented. Here, the energy dependence for formation of the protonated n-butyl β-ionone Schiff base through electrocyclization of the protonated n-butylamine Schiff base of all-trans-retinal in collisions is presented. The latter is a model compound for all-trans-retinal, the chromophore of the light sensitive opsin proteins, and such studies are essential for the understanding of the operation of mammal vision.While our collision studies are very successful, they are sometimes also limited by the experimental timescale. Therefore, we have constructed an experimental setup for ion storage and fragmentation analysis. The goal of this new experiment is to store internally hot fragments to investigate their behavior on extended timescales and as functions of internal excitation energies.
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| 6. |
- Haag, Nicole, 1981-
(författare)
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Probing biomolecular fragmentation
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with fragmentation of complex molecular ions, especially biomolecules, in gas phase collision experiments. The aim is to investigate the relations between energy deposition and fragmentation and to shed light on the mechanisms behind energy and charge transfer processes in collisions involving the building blocks of life. Further, the question how a solvent environment influences the dissociation behavior is elucidated. In the first part of the thesis, results from different collision experiments with biomolecular ions are presented, focusing on electron capture induced dissociation of hydrated nucleotides and small peptides. The investigated processes may be relevant for the understanding of radiation damage and the optimization of sequencing methods used in protein research. Our results clearly demonstrate that effects due to surrounding solvent molecules are substantial. While the dissipation of internal energy by evaporation of the loosely bound solvent molecules may protect the biomolecule, the influence which this environment has on the electronic structure may lead to an enhancement or suppression of certain dissociation channels. The second part of the thesis focuses on recent instrumental developments. Here, the aim was to optimize and complement the techniques used in the experiments above and to have versatile tools available for different kinds of gas phase collision studies involving complex molecular ions. Therefore, we have constructed an electrospray ion source platform for the preparation of intense beams, with options of accumulation and cooling of mass selected ions, allowing for a large variety of experiments. This device is also intended to serve as an ion source for the new storage ring facility DESIREE (DoubleElectroStatic Ion Ring ExpEriment), which is currently under construction at Stockholm University. In these unique storage rings, oppositely charged ions may interact at very low relative velocities in a cryogenically cooled and ultrahigh vacuum environment.
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| 7. |
- Johansson, Henrik A. B., 1979-
(författare)
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Ionization and Fragmentation of Complex Molecules and Clusters : Biomolecules and Polycyclic Aromatic Hydrocarbons
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work deals with ionization and fragmentation of biomolecules and polycyclic aromatic hydrocarbon (PAH) molecules. They are studied in the gas phase both as isolated molecules and as weakly bound clusters. The purpose of the experimental and theoretical investigations are to elucidate charge and energy transfer and related redistribution processes, as well as fragmentation behaviors. The first part of this thesis presents results from studies on biomolecular ions, in particular nucleotides and peptides, which are primarily examined in electron capture induced dissociation processes. These investigations are relevant for the better understanding of radiation damage to DNA and processes involved in the sequencing of proteins. It is found that the immediate environment have a decisive influence on the fragmentation behaviors. Evaporation of surrounding molecules protect the biomolecules, but their effect on the electronic structure may also enhance or suppress different fragmentation channels. In the second part of the thesis, results from studies on PAH molecules are presented. Experimentally, their properties are mainly probed through collisions with atomic ion projectiles having kilo-electronvolt kinetic energies. As a widespread pollutant on Earth, and as a family of abundant molecules in space, PAHs are not only relevant from an environmental and health perspective, but they are also important for the understanding of the universe. The present results relate to the stabilities of these molecules, both in isolated form and in clusters, when heated or multiply ionized. It is found to be easier to remove several electrons from clusters of PAH molecules than from isolated PAHs, and fission processes determine their ultimate stabilities. Heated low-charge state clusters of PAHs undergo long evaporation sequences once these have started. For isolated and heated PAHs, internal structural rearrangements are demonstrated to be important in the fragmentation processes.
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| 8. |
- Schmidt-May, Alice Frederike, 1988-
(författare)
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State-Resolved Mutual Neutralization Experiments on Atomic Hydrogen Anions : H- with Li+, O+, N+, and C
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents experimental studies of mutual-neutralization reactions between H- and the cations 7Li+, 16O+, 12C+ and 14N+ at the double electrostatic ion-beam storage ring DESIREE. By overlapping two keV ion beams, with corresponding speeds of ca. 1000 km/s, and matching their velocities with applied electric fields, the center-of-mass collision energies are reduced to a few tens of meV. Time- and position-sensitive detectors allow the measurement of the separation of the two formed neutral products, which depends on the kinetic-energy release in the charge-transfer reaction. The branching fractions into the different quantum states are extracted by binning or modelling of the measured product-separation distributions.Due to the difficulty of merged-beams experiments with high mass ratio between the ions, 1H- is often replaced with 2H-. We were able to merge beams with a mass ratio of up to 16 between the ions, which allowed us to use 1H- in all studies presented here. For H-+7Li+ and H-+16O+, we included both hydrogen isotopes, and found a significant isotope effect. The lighter hydrogen isotope leads to an increased population in lower excited states in both cases.Our data constitute important benchmarks for theoretical calculations that are needed in the modelling of stellar photospheres.We compare our experimental findings to different theoretical models and discuss their strengths and shortcomings.In order to achieve these scientific results, the thesis work involved the development of data preparation and analysis for a frame- and an event-based detector, as well as the development of experimental methods.
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| 9. |
- Eklund, Gustav, 1990-
(författare)
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Experiments on mutual neutralization using storage rings with merged ion beams
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents work on merged-beams experiments using the DESIREE facility, which consists of two electrostatic ion-storage rings operated at cryogenic temperatures of about 13 K. The two storage rings have a common straight section, where oppositely charged ions can interact at sub-eV collision energies. The work consists of development of the experimental method, tools for data analysis, and simulations for interpretation of the experimental results. This development has led to the first results from merged-beams experiments at DESIREE. The experiments are performed on mutual neutralization, a collision process where an electron is transferred from a negatively charged ion to a positively charged ion, resulting in two neutral products. This reaction is investigated for three systems: Li+ with D−, Na+ with D−, and Mg+ with D−. The reactions studied are of astrophysical interest, in particular for modeling of stellar atmospheres. The detection of the reaction products is done by using a position- and time-sensitive detector, and from this information it is possible to determine the quantum states involved in the reaction. The branching fractions of the reaction channels are measured and compared to theoretical modeling.
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| 10. |
- Gudmundsson, Magnus, 1979-
(författare)
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Heavy particle interference and diffraction in fast electron transfer collisions
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents experimental results from the synchrotron cooler and storage ring CRYRING on charge transfer processes in fast electron transfer collisions using high-resolution cold target recoil-ion momentum spectroscopy. The main focus of these studies was to investigate a key concept of quantum mechanics: the wave-particle duality. One series of experiments has been dedicated to the study of heavy particle de Broglie wave interference due to scattering on a molecular ’double slit’. This is a fundamental manifestation of the wave properties of matter. Quantum interference oscillations were observed in the target orientation dependent cross section of single- and double-electron capture from H2 to 1.3 MeV protons and to 1.2 and 2.0 MeV He2+ ions. Another study, included in this work, is a series of angular differential cross section measurements for single-electron capture to 1.3-12.5 MeV kinetic energy protons from He that enabled us to systematically investigate the classically allowed non-radiative electron capture process in fast collisions predicted by L. H. Thomas in 1927. The cross section for this process is expected to have a nonrelativistic, asymptotic dependence on the projectile velocity, vp, of vp−11. This prediction (from 1927) was verified experimentally for the first time through the present measurements. Using the above mentioned experimental data in addition to measurements of double electron capture by 6.0 MeV He2+ from He, we have also studied the dominating, central part of the angular differential cross section, dσ/dΩ, where the peak shapes and widths surprisingly are very similar regardless of projectile energy and the number of captured electrons. We explain this with a diffraction model for the electron capture and calculate the corresponding diffracting electron capture ‘apertures’ from the shapes and widths of the measured cross sections and the projectile de Broglie wavelengths. We have on one hand established very strong experimental support for the picture suggested by Thomas in 1927 in which electrons and protons are described as classical particles. On the other hand, the diffraction picture describes the shapes of the central peaks in dσ/dΩ quite well, and nicely explains appearances of second and a third maxima in the angular differential cross section. It is hard to see how these seemingly contradicting results can be explained through complementary classical and quantum descriptions of the same underlying physical processes.
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