| 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. |
- Lindén, Fredrik
(författare)
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Anions from the lab to ionospheres
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Anion processes have been in the focus of interest during the last decades, especially due to their importance in planetary ionospheres. Previous observations done by the Cassini Plasma Spectrometer, detected a multitude of heavy neutral and ionic molecules in the ionosphere of Saturn's moon Titan. Unfortunately, only the lighter anions could be identified as CN-, C3N- and C5N-. It has been believed that the ion-neutral reactions involving these cyano anions could explain the reaction mechanism leading to heavier negatively charged anions.In this thesis we present experimental investigations of the reaction of C3N - with C2 H2, using three different guided ion beam mass spectrometry setups. Complementary, we performed ab inito calculations at CCSD(T)/6-311++G** level of theory. These calculations provide a detailed picture of the different reaction pathways. Unfortunately our computations yielded that all these pathways possess high reaction barriers and the above-mentioned process therefore an unlikely reaction to occur in Titans atmosphere. We also performed an investigation using velocity map imaging spectrometry to study the attack mechanism of CN- on halogenated hydrocarbons. In this experiment we studied the nucleophilic substitutionThe energy distribution at low collision energies reveals isotropic scattering of I- which suggests that the process proceeds via a long-lived transition state complex, whereas at higher collision energies the backward scattering mechanism becomes more dominant. Additionally ab inito calculations were carried out, and even though the reaction energy difference between the two attack mechanisms is quite considerable (approximately 1 eV), we could not identify the ratio between the number of reactions involving an attack from the nitrogen atom leading to methylisocyanide (CH3NC) versus the one proceeding via an attack from carbon atom forming acetonitrile (CH3CN). Furthermore, I present a general semi-classical model that describes the forces and electron-transfer processes between two dielectric spheres in vacuum. The model spheres may be charged, may have different radii and different dielectric constants. Here, the model is used to calculate the so called Langevin cross sections for fusion of two spheres and charge exchange cross sections between two charged spheres. The model may be used to predict properties of reactions relevant in the chemistry of astronomical objects like dark clouds, circumstellar envelopes, protoplanetary disks and star-forming regions.
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| 5. |
- Lindén, Fredrik, 1988-
(författare)
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Mechanisms of Anion Reactions from the lab to ionospheres
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A multitude of heavy neutral and ionic molecules have been discovered by the Cassini Plasma Spectrometer in the ionosphere of Saturn's largest moon Titan. However, only three cyano anions were explicitly identified there, namely CN-, C3N- and C5N-. The identity of the heavier anions, which show an abundance maximum at m/z 1000, could, however, not be elucidated and there is, so far, no clear explanation how these were generated.We investigated the reaction of the cyanide anion with methyl iodide using a velocity map imaging spectrometer setup and ab initio calculations. The data indicate a dominant direct rebound mechanism and a high internal excitation of the neutral product. According to the ab initio calculation two possible reaction pathways were expected, but in the experiment the two channels turned out to be indistinguishable due to low resolution.We also studied the reaction between C3N- and acetylene using three different experimental setups: a triple quadrupole mass spectrometer, a tandem quadrupole mass spectrometer, and the ''CERISES'' guided ion beam apparatus.The reaction showed three primary reaction pathways leading to C2H-, CN-, and C5N-. The production of C2H- could either happen via proton transfer or via formation of an adduct. The appearance of CN- could be explained by a reaction sequence involving an intermediate adduct but also via collision induced dissociation. Even though ab initio calculations predict two exoergic pathways leading to CN- and C5N-, all products are only accessible via energy barriers above 1 eV.In addition, we investigated the reaction between C5N- and acetylene. Also in this case the experimental and theoretical studies revealed that all reaction pathways proceed via energy barriers well above 1 eV. The sole exoergic pathway leading to C7N- has an energy barrier of 1.91 eV. Since the chemistry in dark interstellar clouds and planetary ionospheres is restricted to exoergic reactions with energy barriers less than 20 meV or proceed in a barrier-less manner (Vuitton et al. Planetary and Space Science 57, 1558-1572 (2009)), none of the observed pathways are feasible growth mechanism in those environments.We also performed investigations of reactions between charged clusters with and without barriers using electrostatic models. This led to the development of both approximate and exact expressions, which describe the sphere-sphere interaction and the electron transfer from a (neutral or charged) dielectric sphere to another charged dielectric sphere. The exact solutions include sums that describe polarization effects to infinite orders. However, we have shown that these infinite sums can be simplified, and that these approximations can be applied to calculate the charge transfer cross-sections and Langevin-type cross-sections.
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| 6. |
- 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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| 7. |
- 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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| 8. |
- 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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| 9. |
- Florin, Naemi, 1993-
(författare)
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Cosmic Molecules and Clusters : Knockout Driven Reactions
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fullerenes and PAHs (polycyclic aromatic hydrocarbons) are two families of carbon based molecules. These are both present in the interstellar medium, and are there believed to play important roles in various processes, including the formation of stars in the case of PAHs. This thesis presents studies on the structures and dynamics of fullerenes and PAHs and their weakly bound clusters, that all have relevance in an astrophysical context. Here, the focus is on knockout driven reactions in which a single atom is knocked out of a molecule or a molecular cluster as a result of Rutherford-like scattering processes. These are modelled by means of classical molecular dynamics simulations.The first study investigates knockout processes where a C60 molecule is collided with helium atoms at 166 eV in the centre-of-mass-frame, similar to the velocities in interstellar shocks. Using a combination of experimental measurements and molecular dynamics simulations we find that highly reactive C59 fragments can be created sufficiently cold to stabilise and survive indefinitely inisolation.Following the first study, we model the structures and stabilities of mixed clusters of C60 and C24H12 (coronene) molecules. We find that the two molecular species do not mix very well, but that they like to be in compact formations. For larger pure coronene clusters, we find that the most stable clusters contain two interacting stacks, forming a shape that looks similar to a “handshake”. These results are consistent with earlier modelling studies. Here, we show that such stacks also show up as subclusters in large mixed clusters.Finally, we use the most stable clusters from the second study as targets in collisions with 3 keV argon atoms. We find that the simulated mass spectra strongly resemble the corresponding experimental ones. These show that many various forms of new molecular structures, both fragments and large new molecules, are being formed, as a result of the collisions. Here, the simulations give information on the reaction pathways and on the structures of these new species. There are also examples of hydrogenated, but otherwise intact, fullerene and coronene molecules being formed.The mechanisms we have studied mimic inter- and circumstellar conditions where shockwaves and stellar winds drive particles (atoms and ions) at velocities similar to those studied here. The reactions covered in this work are thus likely to take place in such environments when carbon-based molecules and grains are energetically processed.
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| 10. |
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