Marta Berholts will defend her PhD thesis "Fragmentation of ionic and hydrogen-bonded molecules induced by synchrotron radiation" on 7 September 2018.
Prof. Edwin Kukk, Department of Physics and Astronomy, University of Turku
Prof. Ergo Nõmmiste, Institute of Physics, University of Tartu
Dr. Kuno Kooser, Department of Physics and Astronomy, University of Turku
Prof. Jan-Erik Rubensson, Department of Physics and Astronomy, Uppsala University
Assoc. Prof. Matjaž Žitnik, Department of Low and Medium Energy Physics, Jožef Stefan Institute
Assoc. Prof. Paolo Piseri, Department of Physics, University of Milan
In this work, we investigated experimentally and computationally the fragmentation processes of ionic and hydrogen-bonded molecules following valence photoionization. Four samples were studied: ionic 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and hydrogen-bonded molecular clusters of acetamide (CH3CONH2), acetamide-2,2,2-d3 (CD3CONH2), and acetic acid (CH3COOH). The goal of the work was to investigate the influence of the photon energy on the stability of the samples and their fragmentation mechanisms. Tunable synchrotron radiation or gas discharge lamp radiation in the vacuum ultraviolet region was used to ionize the molecules in the gas phase. Clusters were studied by mass spectrometry, while ionic liquid was additionally studied by photoelectron spectroscopy. Partial ion yield technique was used to elucidate the energetics of various photofragmentation pathways. Both types of compounds were shown to be unstable toward near threshold ionization and therefore subjected to extensive fragmentation. Different fragmentation mechanisms were identified and common trends in dissociation behaviour of ionic and hydrogen-bonded compounds were observed. A significant finding is that not only photon energy influences the outcome of the valence ionization (at higher photon energy new fragmentation channels open up), but also the conditions at which the samples are introduced into the gas phase. In the case of acetic acid clusters, different clusterization conditions resulted in different mass spectra. A conclusion is drawn that photofragmentation channels of a molecule depend on its internal energy. Lower internal energy suppresses atomic rearrangements that might be required for a certain fragment formation and instead a new fragment is formed that does not require any rearrangement of the system. We observed such behaviour for acetic acid trimer that starts producing unprotonated dimers at stronger expansion conditions (lower internal energy), while there was no unprotonated dimers produced at weaker expansion conditions (higher internal energy).
This thesis was done under joint supervision of the University of Tartu (Institute of Physics, Faculty of Science and Technology, Estonia) and the University of Turku (Department of Physics and Astronomy, Faculty of Science and Engineering, Finland).