University thesis:
Dissertation, Universität Freiburg, 2021
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Description:
Abstract: This thesis deals with inter- and intramolecular dynamics in molecular, hydrogen-bonded clusters and superfluid helium droplets. The experiments were carried out at the free-electron-laser (FEL) FERMI in Trieste and at the ETH Zurich. For these experiments, new data evaluation and analysis methods were developed which are presented and described in the thesis. Additionally, the experimental results and following interpretation will be presented. The experiments are focused on four separate main topics. In the first experiment, a plasma was generated by irradiating ammonia clusters with intense laser pulses in the extreme ultraviolet (XUV) range. With the help of a time-delayed ultraviolet laser, an XUV-UV pump-probe experiment was carried out and time-resolved photoelectron and photoion spectroscopy were used to characterize the plasma dynamics in a three-phase model. The kinetic energy of protons in the ion time-of-flight spectrometer was used in connection with a correlation analysis to show that a highly ionized ammonia cluster detaches its outer shell in a concerted Coulomb explosion. This process was shown to take place on a timescale of a few hundred femtoseconds and leads to an almost complete charge neutralization of the clusters. Subsequently, the slightly charged clusters expanded<br>further under the pressure of hydrodynamic forces. It was also shown that during the hot plasma phase, excited hydrogen is generated in large quantities. Two different channels for the generation of excited hydrogen were identified, the first being a direct recombination of protons with free electrons, and the second being through dissociation of highly excited molecules.<br>In the second experiment, water clusters were irradiated with XUV pulses in order<br>to induce electron solvation. Hydrated electrons were observed upon irradiating with<br>133 nm, 80nm and high intensity 52nm wavelength XUV radiation. The vertical binding energy and formation dynamics were probed using photoelectron spectroscopy on the electrons created by a time-delayed UV laser pulse. The characteristic dynamics observed in the intensity, vertical binding energy, and full width half maximum of the photoelectron signal created through ionization of the hydrated electrons was interpreted in terms of the different steps of electron hydration. These steps are the thermalization and localization of the electron, molecular rearrangement forming bound state hydrated electron, and finally<br>recombination of the hydrated electron. Molecular rearrangement dynamics were identified through the isotope effect when comparing H2O and D2O. In the interpretation of the data, important information on the processes of photolysis and radiolysis in water clusters was gained.<br>The third experiment investigated the dynamics of interatomic Coulombic decay (ICD) processes in multiply excited helium droplets. It was observed that short-term movement of the nuclei of the excited helium atoms has a large impact on the ICD process. A model was developed for the description of the process and substantiated with time-dependent density functional theory (TDDFT) calculations. Time-resolved photoelectron spectra from ICD between two excited helium atoms and between a single excited helium atom and a doped sodium atom were compared with the theoretical calculations from the model.<br>In a fourth experiment, it was discovered that the above threshold ionization (ATI) is<br>strongly enhanced in helium droplets compared to gas-phase helium when ionizing multiply excited helium droplets using 400nm or 800nm laser pulses. By varying the excitation density in the droplet, and the mean droplet size, we showed that the enhancement is due to a collective effect of the excitations in the droplet. The observed enhancement is found to be incompatible with the prevalent theoretical models describing enhanced ATI generation in condensed systems. We introduced a semi-empirical model based on collective absorption and emission of electrons that qualitatively reproduces the experimental observation