| 1. |
- Diesen, Elias, et al.
(författare)
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Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy
- 2021
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 127:1
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Tidskriftsartikel (refereegranskat)abstract
- We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.
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| 2. |
- Falowska-Pietrzak, Olga, 1995-
(författare)
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Absorbed dose and fluence measurements and simulations at EuXFEL undulator systems
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The European X-Ray Free Electron Laser generates X-ray pulsesas GeV electrons progress in the beam pipe through periodic arrangements of magnets called undulators. An undesired scattered radiation (stray radiation) is also present outside the beam pipe. It may damage not only undulator magnets, but also diagnostic and correction equipment located near undulators. It results in decreased performance of the EuXFEL. As the severity of magnet damage differs for different particles and energies, it is of great importance to properly characterize the stray radiation field in the vicinity of the undulators. In this thesis, results of Gafchromic film dose measurements near EuXFEL undulators are presented. Films were placed at the entrance of the undulator segments, along the undulator permanent magnets and at the front of phase shifters located in the intersections separating two undulator segments. In addition, Monte Carlo simulations of absorbed dose and fluence were performed with Geant4 toolkit to characterize the composition and energy spectra of the radiation field near undulators. Comparison of film measurements and simulations show that theradiation field near the upstream undulator cells may be a result of high-energy electrons interactions with the beam pipe. Almost 99 % of the dose absorbed to the permanent magnets comes from electrons and positrons created through photon interactions with matter. However, the majority of particles are photons. In addition,simulations show that neutrons are also presented within the magnets. Significant doses were measured further away from thepermanent magnets, close to phase shifters installed in the intersections.
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| 3. |
- Paul, Raka
(författare)
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Mutual Neutralization in collisionswith oppositely charged fullerene(C60) ions
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, results from Mutual Neutralization experiment carried out atthe DESIREE facility, involving merged interacting ion beams, of oppositelycharged C60 ions are presented. This reaction may be of importance for explain-ing reliable estimates of the ionization balance in astrophysical environments,which defines the chemistry in molecular gas clouds. To interpret the resultsfrom the experiment, and for estimating the reaction cross section as a functionof collision energy, the Landau-Zener model is employed, with the electronicexcited states of neutral C60 calculated using Time-Dependent Density Func-tional Theory. The importance of applying an interacting potential that takesinto account mutual polarization effects and the finite sizes of the collisionpartners, in the Landau-Zener model is highlighted. We find that the total MNcross section is similar for different coupling elements. A good agreement isobserved between the Landau-Zener model and the experiment, and the modelis applied to calculate the total Mutual Neutralization cross section as a functionof center-of-mass collision energy, for a range of (0.0 - 4.98) eV excitationenergy. We find the cross section is almost an order of magnitude larger thanwhat has been reported for H+ + H− collisions and comparable with modelresults for C+60 + e− → C60 reactions. Thus, it suggests that Mutual Neutraliza-tion processes indeed may be important for the charge balance in interstellarmolecular clouds where electrons are not the main carriers of negative charges.©Raka Paul, Stockholm 2023Distributor: Department of Physics, Stockholm University
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