| 1. |
- Aartsen, M. G., et al.
(författare)
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Efficient propagation of systematic uncertainties from calibration to analysis with the SnowStorm method in IceCube
- 2019
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP PUBLISHING LTD. - 1475-7516. ; :10
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Tidskriftsartikel (refereegranskat)abstract
- Efficient treatment of systematic uncertainties that depend on a large number of nuisance parameters is a persistent difficulty in particle physics and astrophysics experiments. Where low-level effects are not amenable to simple parameterization or re-weighting, analyses often rely on discrete simulation sets to quantify the effects of nuisance parameters on key analysis observables. Such methods may become computationally untenable for analyses requiring high statistics Monte Carlo with a large number of nuisance degrees of freedom, especially in cases where these degrees of freedom parameterize the shape of a continuous distribution. In this paper we present a method for treating systematic uncertainties in a computationally efficient and comprehensive manner using a single simulation set with multiple and continuously varied nuisance parameters. This method is demonstrated for the case of the depth-dependent effective dust distribution within the IceCube Neutrino Telescope.
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| 2. |
- Björneholm, Olle, et al.
(författare)
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Deeper Insight into Depth-Profiling of Aqueous Solutions Using Photoelectron Spectroscopy
- 2014
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:50, s. 29333-29339
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Tidskriftsartikel (refereegranskat)abstract
- X-ray photoelectron spectroscopy (XPS) is widely used to probe properties such as molecular stoichiometry, microscopic distributions relative to the surface by so-called depth-profiling, and molecular orientation. Such studies usually rely on the core-level photoionization cross sections being independent of molecular composition. The validity of this assumption has recently been questioned, as a number of gas-phase molecules have been shown to exhibit photon-energy-dependent nonstochiometric intensity oscillations arising from EXAFS-like modulations of the photoionization cross section. We have studied this phenomenon in trichloroethanol in both gas phase and dissolved in water. The gas-phase species exhibits pronounced intensity oscillations, similar to the ones observed for other gas-phase molecules. These oscillations are also observed for the dissolved species, implying that the effect has to be taken into account when performing depth-profiling experiments of solutions and other condensed matter systems. The similarity between the intensity oscillations for gas phase and dissolved species allows us to determine the photoelectron kinetic energy of maximum surface sensitivity, approximate to 100 eV, which lies in the range of pronounced intensity oscillations.
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| 3. |
- Bloß, Dana, et al.
(författare)
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X-ray radiation damage cycle of solvated inorganic ions
- 2024
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 15:1
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Tidskriftsartikel (refereegranskat)abstract
- X-ray-induced damage is one of the key topics in radiation chemistry. Substantial damage is attributed to low-energy electrons and radicals emerging from direct inner-shell photoionization or produced by subsequent processes. We apply multi-electron coincidence spectroscopy to X-ray-irradiated aqueous solutions of inorganic ions to investigate the production of low-energy electrons (LEEs) in a predicted cascade of intermolecular charge- and energy-transfer processes, namely electron-transfer-mediated decay (ETMD) and interatomic/intermolecular Coulombic decay (ICD). An advanced coincidence technique allows us to identify several LEE-producing steps during the decay of 1s vacancies in solvated Mg2+ ions, which escaped observation in previous non-coincident experiments. We provide strong evidence for the predicted recovering of the ion’s initial state. In natural environments the recovering of the ion’s initial state is expected to cause inorganic ions to be radiation-damage hot spots, repeatedly producing destructive particles under continuous irradiation.
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| 4. |
- Gopakumar, Geethanjali, 1992-
(författare)
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An X-ray Based Spectroscopic Study of Structure Influencing Electrons : Fragmentation, Ultrafast Charge Dynamics and Surface Composition
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The structure of biomolecules, such as proteins, is intimately connected to the function of the molecules. These structures are often studied using X-ray diffraction. However, the interaction of the X-ray photons with the molecule can excite or ionize its electrons which in turn can causes changes in the molecular structure. Getting a better understanding of the radiation damage induced by the X-rays, will lead to higher resolution molecular imaging. In proteins, sulfur bridges stabilize the structure, but sulfur is relatively more susceptible to X-ray photon-induced dynamics. The first section of the thesis presents results obtained by fragment mass spectroscopy using an ion trap on the X-ray induced dynamics leading to breakage in the smallest unit containing sulfur bridge in proteins, cystine. The fragmentation of the bridge is seen to depend on the photon energy used.Molecular damage is not always undesirable. The radiation-induced damage of DNA of cancer cells is an aspired outcome of radiation therapy treatment. Along with the direct damaging effect of the radiation, the surrounding water and metal ions also play a role in indirectly destroying the DNA structure. The X-rays ionize the metal ions and water molecules, which relaxes via different processes, producing water radicals and slow electrons. Both are agents of the destruction of DNA strands. The second section of the thesis reports on results obtained by electron spectroscopy on ultrafast electron dynamics originating from the relaxation of core-excited and ionized aqueous ions, which can result in slow electrons and water radicals. To understand the damages in the system of aqueous ions and biomolecules, one needs to understand the interaction between the organic-inorganic species. Using surface sensitive X-ray photoelectron spectroscopy on such a sample mixture of potassium chloride and amino acids is explained in the last section of the thesis. It is seen that changing the chemical environment in the solution (pH), affects both the protonation of the functional group of amino acids and the surface distribution of solvated counter ions. The interaction between organic biomolecules and inorganic ions can be ensured by controlling the chemical environment. This thesis puts forward the study of electrons that influences the molecular structure using various X-ray based spectroscopy techniques.
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| 5. |
- Gopakumar, Geethanjali, 1992-, et al.
(författare)
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Non-local decay of highly charged aqueous inorganic ions produced by Auger decay
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- High-Z atoms are more important for biological radiation damage by photons than their low abundance due to their higher photoionization cross-section. Using the inorganic Mg 2+ and Al3+ ions in water as model systems, we have studied decay processes following deep core-level ionization. Local Auger decay rapidly produces highly charged Mg 4+and Al5+ ions, the decay of which must involve non-local processes involving the surrounding water, such as electron transfer mediated decay (ETMD). Using electron spectroscopy we observe two distinct ETMD decay steps for Al, corresponding to decay from Al5+ to Al4+, and then from Al4+ to Al3+. The ETMD energetics is discussed using both a simple model and calculations. Contrary to expectations, we do not observe any ETMD for Mg, and possible reasons for this are discussed.
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| 6. |
- Gopakumar, Geethanjali, 1992-, et al.
(författare)
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Probing Aqueous Ions with Non-local Auger Relaxation
- 2022
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 24:15, s. 8661-8671
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Tidskriftsartikel (refereegranskat)abstract
- The decay of core holes is often regarded as a local process, but in some systems, it involves the autoionization of neighbouring atoms or molecules. Here, we explore such non-local autoionization (Intermolecular Coulombic Decay, ICD) of surrounding molecules upon 1s ionization of aqueous-phase Na+, Mg2+ and Al3+ ions. The three ions are isoelectronic but differ in the strength of the ion-water interactions which is manifested in experimental Auger electron spectra by varying intensities. While for strongly interacting Mg2+ and Al3+ the non-local decay is observed, for weakly bound Na+ no signal was measured. Combined with theoretical simulations we provide a microscopic understanding of the non-local decay processes. We assigned the ICD to decay processes ending with two-hole states delocalized between the central ion and neighbouring water. The ICD process is also shown to be highly selective with respect to water molecular orbitals. The ICD lifetime was estimated to be around 40 fs for Mg and 20 fs for Al. Auger spectroscopy thus represents a novel tool for exploring molecules in the liquid phase, providing simultaneously structural and electronic information.
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| 7. |
- Gopakumar, Geethanjali, et al.
(författare)
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Probing aqueous ions with non-local Auger relaxation
- 2022
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 24:15, s. 8661-8671
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Tidskriftsartikel (refereegranskat)abstract
- Non-local analogues of Auger decay are increasingly recognized as important relaxation processes in the condensed phase. Here, we explore non-local autoionization, specifically Intermolecular Coulombic Decay (ICD), of a series of aqueous-phase isoelectronic cations following 1s core-level ionization. In particular, we focus on Na+, Mg2+, and Al3+ ions. We unambiguously identify the ICD contribution to the K-edge Auger spectrum. The different strength of the ion-water interactions is manifested by varying intensities of the respective signals: the ICD signal intensity is greatest for the Al3+ case, weaker for Mg2+, and absent for weakly-solvent-bound Na+. With the assistance of ab initio calculations and molecular dynamics simulations, we provide a microscopic understanding of the non-local decay processes. We assign the ICD signals to decay processes ending in two-hole states, delocalized between the central ion and neighbouring water. Importantly, these processes are shown to be highly selective with respect to the promoted water solvent ionization channels. Furthermore, using a core-hole-clock analysis, the associated ICD timescales are estimated to be around 76 fs for Mg2+ and 34 fs for Al3+. Building on these results, we argue that Auger and ICD spectroscopy represents a unique tool for the exploration of intra- and inter-molecular structure in the liquid phase, simultaneously providing both structural and electronic information.
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| 8. |
- Gopakumar, Geethanjali, 1992-, et al.
(författare)
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The Surface Composition of Amino Acid - Halide Salt Solutions is pH-Dependent
- 2022
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Ingår i: Environmental Science: Atmospheres. ; :3
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Tidskriftsartikel (refereegranskat)abstract
- In atmospheric aerosol particles, the chemical surface composition governs both heterogeneous chemical reactions with gas-phase species and the ability to act as nuclei for cloud droplets. The pH in aerosol droplets is expected to affect these properties, but it is very challenging to measure the pH in individual droplets, precluding the investigation of its influence on the particle's surface composition. In this work, we use photoelectron spectroscopy to explore how the surface composition of aqueous solutions containing inorganic salt and amino acids changes as a function of pH. We observe a change by a factor of 4-5 of the relative distribution of inorganic ions at the surface of a liquid water jet, as a function of solution pH and type of amino acid in the solution. The driving forces for the surface enhancement or depletion are ion pairing and the formation of charged layers close to the aqueous surface. Our findings apply to any aqueous interface at which organic species with charged functional groups are present.
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| 9. |
- Gopakumar, Geethanjali, et al.
(författare)
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The surface composition of amino acid - halide salt solutions is pH-dependent
- 2022
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Ingår i: Environmental Science: Atmospheres. - : Royal Society of Chemistry (RSC). - 2634-3606. ; 2:3, s. 441-448
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Tidskriftsartikel (refereegranskat)abstract
- In atmospheric aerosol particles, the chemical surface composition governs both heterogenous chemical reactions with gas-phase species and the ability to act as nuclei for cloud droplets. The pH in aerosol particles is expected to affect these properties, but it is very challenging to measure the pH in individual droplets, precluding the investigation of its influence on the particle's surface composition. In this work, we use photoelectron spectroscopy to explore how the surface composition of aqueous solutions containing inorganic salt and amino acids changes as a function of pH. We observe a change by a factor of 4-5 of the relative distribution of inorganic ions at the surface of a liquid water jet, as a function of solution pH and type of amino acid in the solution. The driving forces for the surface enhancement or depletion are ion pairing and the formation of charged layers close to the aqueous surface.
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| 10. |
- Gopakumar, Geethanjali, et al.
(författare)
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X-ray-induced attosecond ion-water electron dynamics of aqueous ions
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The foundation of many physical and chemical processes is the transfer of charge from one entity to another. In many cases, the charge transfer is mediated by electron transfer and due to the comparatively low mass of electrons, these processes tend to take place within a few femtoseconds or several attoseconds. We investigate the charge transfer from Na+, Mg2+ and Al3+ in an aqueous environment to neighbouring water molecules. In order to achieve this, we use the core-hole clock method and Auger spectroscopy upon 1s ionization of the respective ions. The charge transfer times range from several 100 as below the 1s ionization threshold to only 20 as far above the 1s ionization. The decrease in charge transfer times as a function of the photon energy seems to be continuous. Despite the ions being isoelectronic in our study, we nd differences in their charge transfer behaviour.
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