| 1. |
- Berholts, Marta, et al.
(författare)
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Photofragmentation of gas-phase acetic acid and acetamide clusters in the vacuum ultraviolet region
- 2017
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 147:19
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Tidskriftsartikel (refereegranskat)abstract
- Photofragmentation of gas-phase acetamide and acetic acid clusters produced by a supersonic expansion source has been studied using time-of-flight mass spectrometry and the partial ion yield (PIY) technique combined with tunable vacuum-ultraviolet synchrotron radiation. Appearance energies of the clusters and their fragments were experimentally determined from the PIY measurements. The effect of clusterization conditions on the formation and fragmentation of acetic acid clusters was investigated. Ab initio quantum mechanical calculations were performed on both samples' dimers to find their neutral and ionized geometries as well as proton transfer energy barriers leading to the optimal geometries. In the case of the acetamide dimer, the reaction resulting in the production of ammoniated acetamide was probed, and the geometry of the obtained ion was calculated.
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| 2. |
- Cheng, Yu Chen, et al.
(författare)
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Imaging multiphoton ionization dynamics of CH3I at a high repetition rate XUV free-electron laser
- 2021
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Ingår i: Journal of Physics B-Atomic Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 54:1
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Tidskriftsartikel (refereegranskat)abstract
- XUV multiphoton ionization of molecules is commonly used in free-electron laser experiments to study charge transfer dynamics. However, molecular dissociation and electron dynamics, such as multiple photon absorption, Auger decay, and charge transfer, often happen on competing time scales, and the contributions of individual processes can be difficult to unravel. We experimentally investigate the Coulomb explosion dynamics of methyl iodide upon core-hole ionization of the shallow inner-shell of iodine (4d) and classically simulate the fragmentation by phenomenologically introducing ionization dynamics and charge transfer. Under our experimental conditions with medium fluence and relatively long XUV pulses (similar to 75 fs), we find that fast Auger decay prior to charge transfer significantly contributes to the charging mechanism, leading to a yield enhancement of higher carbon charge states upon molecular dissociation. Furthermore, we argue for the existence of another charging mechanism for the weak fragmentation channels leading to triply charged carbon atoms. This study shows that classical simulations can be a useful tool to guide the quantum mechanical description of the femtosecond dynamics upon multiphoton absorption in molecular systems.
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| 3. |
- Ganguly, Smita, et al.
(författare)
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The origin of enhanced O2+ production from photoionized CO2 clusters
- 2022
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Ingår i: Communications Chemistry. - 2399-3669.
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Tidskriftsartikel (refereegranskat)abstract
- CO2-rich planetary atmospheres are continuously exposed to ionising radiation driving major photochemical processes. In the Martian atmosphere, CO2 clusters are predicted to exist at high altitudes motivating a deeper understanding of their photochemistry. In this joint experimental-theoretical study, we investigate the photoreactions of CO2 clusters (≤2 nm) induced by soft X-ray ionisation. We observe dramatically enhanced production of O2- from photoionized CO2 clusters compared to the case of the isolated molecule and identify two relevant reactions. Using quantum chemistry calculations and multi-coincidence mass spectrometry, we pinpoint the origin of this enhancement: A size-dependent structural transition of the clusters from a covalently bonded arrangement to a weakly bonded polyhedral geometry that activates an exothermic reaction producing O+2. Our results unambiguously demonstrate that the photochemistry of small clusters/particles will likely have a strong influence on the ion balance in atmospheres.
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| 4. |
- Inhester, Ludger, et al.
(författare)
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Chemical Understanding of the Limited Site-Specificity in Molecular Inner-Shell Photofragmentation
- 2018
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 9:5, s. 1156-1163
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Tidskriftsartikel (refereegranskat)abstract
- In many cases fragmentation of molecules upon inner-shell ionization is very unspecific with respect to the initially localized ionization site. Often this finding is interpreted in terms of an equilibration of internal energy into vibrational degrees of freedom after Auger decay. We investigate the X-ray photofragmentation of ethyl trifluoroacetate upon core electron ionization at environmentally distinct carbon sites using photoelectron-photoion-photoion coincidence measurements and ab initio electronic structure calculations. For all four carbon ionization sites, the Auger decay weakens the same bonds and transfers the two charges to opposite ends of the molecule, which leads to a rapid dissociation into three fragments, followed by further fragmentation steps. The lack of site specificity is attributed to the character of the dicationic electronic states after Auger decay instead of a fast equilibration of internal energy.
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| 5. |
- Leroux, Juliette, et al.
(författare)
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Mapping the electronic transitions of protonation sites in peptides using soft X-ray action spectroscopy
- 2023
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 25:37, s. 25603-25618
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Tidskriftsartikel (refereegranskat)abstract
- Near-edge X-ray absorption mass spectrometry (NEXAMS) around the nitrogen and oxygen K-edges was employed on gas-phase peptides to probe the electronic transitions related to their protonation sites, namely at basic side chains, the N-terminus and the amide oxygen. The experimental results are supported by replica exchange molecular dynamics and density-functional theory and restricted open-shell configuration with single calculations to attribute the transitions responsible for the experimentally observed resonances. We studied five tailor-made glycine-based pentapeptides, where we identified the signature of the protonation site of N-terminal proline, histidine, lysine and arginine, at 406 eV, corresponding to N 1s & RARR; & sigma;*(NHx+) (x = 2 or 3) transitions, depending on the peptides. We compared the spectra of pentaglycine and triglycine to evaluate the sensitivity of NEXAMS to protomers. Separate resonances have been identified to distinguish two protomers in triglycine, the protonation site at the N-terminus at 406 eV and the protonation site at the amide oxygen characterized by a transition at 403.1 eV.
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| 6. |
- Oghbaiee, Shabnam, et al.
(författare)
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Site-dependent nuclear dynamics in core-excited butadiene
- 2022
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Ingår i: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 24:47, s. 28825-28830
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Tidskriftsartikel (refereegranskat)abstract
- Symmetry breaking and competition between electronic decay and nuclear dynamics are major factors determining whether the memory of the initial core-hole localisation in a molecule is retained long enough to affect fragmentation. We investigate the fate of core holes localised at different sites in the free 1,3 trans butadiene molecule by using synchrotron radiation to selectively excite core electrons from different C 1s sites to π* orbitals. Fragmentation involving bonds localised at the site of the core hole provides clear evidence for preferential bond breaking for a core hole located at the terminal carbon site, while the signature of localisation is weak for a vacancy on the central carbon site. The origin of this difference is attributed to out-of-plane vibrations, and statistical evaporation of protons for vacancies located at the central carbon sites.
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| 7. |
- Oostenrijk, Bart
(författare)
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Charge and proton dynamics in molecules and free clusters : from atomic to nanometer scale
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The origin of properties in complex systems can often be traced to mechanisms involving charge and energy transfer in only a few embedded molecules. The detailed study of the time evolution of these mechanisms in their original environment is a challenging task. In this thesis we develop experimental tools and methods to enable the study of charge and energy transfer. A new ion and electron momentum imaging spectrometer, along with advanced data treatment methods has been succesfully designed, built and tested. The developed spectrometer is optimized for the measurement of the ion and electron momentum correlation that results from the fragmentation of complex systems, from molecules to molecular clusters.We have conducted photodissociation studies on such complex systems, using the newly developed experimental tools.The use of modern X-ray sources allows to localize the initial energy and charge to sites and/or elements in the system, from where the transfer is initiated.The energy and charge transfer is investigated in molecules by the local (multi-)photon absorption at a controlled site. Among other studies, we investigate the origin of the site-dependence of the fragmentation, be it the population of electronic excited states, conformational isomerization, fast hydrogen evaporation and migration, or secondary breakup. The influence of these processes on the fragmentation are investigated in two ways: through the C1s ionization of chemically distinct carbon sites (ethyl trifluoroacetate), and through the C1s excitation of a model system for conjugated (π) hydrocarbons (1,3 trans butadiene).The migration of charge and transfer of energy in embedded molecular systems is studied by the use of molecular clusters as model systems. The photo-induced energy and charge transport can be facilitated by intermolecular electronic decay, hydrogen migration, proton transfer, the Grotthus mechanism and nuclear rearrangement. The role of these processes in the stabilization and fragmentation of clusters is investigated in clusters of molecules containing N-H and O-H groups that form hydrogen bonds. Among other findings, we conclude that water is an effective stabilizer in multiply-charged hydrated ammonia clusters, which can play an important role in the nucleation process and photochemistry in atmospheric nanoparticles.
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| 8. |
- Oostenrijk, Bart, et al.
(författare)
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Fission of charged nano-hydrated ammonia clusters - microscopic insights into the nucleation processes
- 2019
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 21:46, s. 25749-25762
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Tidskriftsartikel (refereegranskat)abstract
- While largely studied on the macroscopic scale, the dynamics leading to nucleation and fission processes in atmospheric aerosols are still poorly understood at the molecular level. Here, we present a joint experimental-theoretical study of a model system consisting of hydrogen-bonded ammonia and water molecules. Experimentally, the clusters were produced via adiabatic co-expansion. Double ionization ionic products were prepared using synchrotron radiation and analyzed with coincidence mass- and 3D momentum spectroscopy. Calculations were carried out using ab initio molecular dynamics to understand the fragmentation within the first similar to 500 fs. Further exploration of the potential energy surfaces was performed at a DFT level of theory to gain information on the energetics of the processes. Water was identified as an efficient nano-droplet stabilizer, and is found to have a significant effect even at low water content. On the molecular level, the stabilizing role of water can be related to an increase in the dissociation energy between ammonia molecules and the water enriched environment at the cluster surface. Furthermore, our results support the role of ammonium as a charge carrier in the solution, preferentially bound to surrounding ammonia molecules, which can influence the atmospheric nucleation process.
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| 9. |
- Oostenrijk, Bart, et al.
(författare)
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The role of charge and proton transfer in fragmentation of hydrogen-bonded nanosystems : The breakup of ammonia clusters upon single photon multi-ionization
- 2018
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 20:2, s. 932-940
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Tidskriftsartikel (refereegranskat)abstract
- The charge and proton dynamics in hydrogen-bonded networks are investigated using ammonia as a model system. The fragmentation dynamics of medium-sized clusters (1-2 nm) upon single photon multi-ionization is studied, by analyzing the momenta of small ionic fragments. The observed fragmentation pattern of the doubly- and triply-charged clusters reveals a spatial anisotropy of emission between fragments (back-to-back). Protonated fragments exhibit a distinct kinematic correlation, indicating a delay between ionization and fragmentation (fission). The different kinematics observed for channels containing protonated and unprotonated species provides possible insights into the prime mechanisms of charge and proton transfer, as well as proton hopping, in such a nanoscale system.
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| 10. |
- Rading, Linnea, et al.
(författare)
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A versatile velocity map ion-electron covariance imaging spectrometer for high-intensity XUV experiments
- 2018
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Ingår i: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 8:6
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Tidskriftsartikel (refereegranskat)abstract
- We report on the design and performance of a velocity map imaging (VMI) spectrometer optimized for experiments using high-intensity extreme ultraviolet (XUV) sources such as laser-driven high-order harmonic generation (HHG) sources and free-electron lasers (FELs). Typically exhibiting low repetition rates and high single-shot count rates, such experiments do not easily lend themselves to coincident detection of photo-electrons and -ions. In order to obtain molecular frame or reaction channel-specific information, one has to rely on other correlation techniques, such as covariant detection schemes. Our device allows for combining different photo-electron and -ion detection modes for covariance analysis. We present the expected performance in the different detection modes and present the first results using an intense high-order harmonic generation (HHG) source.
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