Ultrafast dynamics of small quantum systems studied using electron-ion coincidence spectroscopy

Studying how small quantum systems, like molecules and clusters, interact with X-rays is crucial to understanding the ultrafast processes that occur in nature on incredibly short timescales, ranging from femtoseconds to picoseconds. X-rays excite small quantum systems to unstable core hole states, leading to a cascade of phenomena, including Auger decay, nuclear rearrangement, and dissociation. The dissociation of molecules is influenced by the initial site of X-ray excitation, as well as the properties of the Auger populated states, such as charge localization and internal energy. In clusters, the dissociation process depends on intermolecular interactions, cluster size, and geometry. The interplay between electronic and nuclear dynamics in core-excited/ionized molecules and clusters is a critical factor that needs to be assessed.
This thesis investigates X-ray-induced fragmentation of molecular adamantane and CO₂ clusters using synchrotron radiation. The kinematics of molecular and cluster fragmentation is measured using advanced techniques, such as 3D momentum imaging of the ion fragments and multiparticle coincidence spectroscopy. Site-selective fragmentation of the carbon cage of the adamantane molecule is studied using Auger-electron Photoion coincidence spectroscopy, revealing the influence of the core-hole site on the Auger decay and dissociation process. Statistical data analysis treatment is developed and implemented to remove background contamination in the coincidence data using experimental random coincidences. The results highlight that the fragmentation of adamantane cation and dication is a complex dynamical process with competing relaxation pathways involving cage opening, hydrogen migration, and carbon-carbon bond breaking. Additionally, the thesis investigates the photoreactions of core-ionized CO₂ clusters, reporting a significantly increased production of O₂⁺ compared to isolated CO₂ molecules. Through quantum chemistry calculations and multi-coincidence 3D momentum imaging, the study determined that the enhanced production of O₂⁺ is due to a size-dependent structural transition of the clusters. The study also proposes two relevant photoreactions involving intermolecular interactions.
This thesis highlights the complexity of core-hole dynamics in molecular and cluster chemistry and emphasizes the need for meticulous experimental and theoretical investigations of the underlying mechanisms. It also discusses the relevance of the results in the context of X-ray-induced astrochemistry. Indeed, the experiments presented are conducted in vacuum chambers in a controlled environment and can crudely replicate the conditions found in astrophysical environments. From the adamantane study, we conclude that X-ray absorption emphatically results in dissociation into smaller hydrocarbons and low photostability can play a part in the absence of diamondoids in the interstellar medium. From the CO₂ clusters study, we found an enhancement in the O₂⁺ yield, which can significantly influence the ion balance in CO₂-rich atmospheres like Mars.

Have you ever wondered how X-rays from the Sun can activate unique chemistry in the atmosphere of Mars? Or how the absence of certain hydrocarbons in astronomical observations can be explained by their interaction with ionizing radiation? These are the fascinating questions that are explored in this PhD thesis using X-rays and state-of-the-art electron-ion spectrometers. Are you ready to explore the tiny world of molecules and clusters at the quantum level? Buckle up because things are about to get small and fast!
When a molecule absorbs an X-ray photon, it excites the molecule to a highly unstable state. This excited molecule then relaxes to reach a stable state, releasing charged particles like electrons and ions in the process. By analyzing the properties of these particles, we can track the molecule's evolution over time. However, this process happens very quickly, on a timescale of femtoseconds, so specialized experimental methods are required to capture these ultrafast dynamics. This thesis focuses on studying the ultrafast behavior of photoexcited molecules and nano-clusters by detecting the charged particles they emit simultaneously. This technique is called ‘coincidence detection’. The study investigates the implications of these photoreactions, both from a fundamental physics perspective and in specific astrochemistry scenarios.
Planetary atmospheres contain many molecular clusters, formed by the condensation of molecules into small nanoparticles that eventually grow into clouds. CO₂ clusters are expected to exist in CO₂-rich atmospheres such as Mars, where the Sun's X-rays can activate unique photochemical reactions. In Paper I, we studied CO₂ clusters that were ionized by X-rays using complete momentum imaging of ions. We investigated the high production of O₂⁺ ions using both experimental and theoretical methods. Hydrocarbons are a significant part of the space between stars, but a particular kind of hydrocarbon, diamondoids, has not been observed in space. In Papers II and III, we investigate the photochemistry of adamantane, which is the smallest diamondoid, using electron-ion coincidence detection. Our observations reveal that adamantane breaks down into smaller hydrocarbons upon absorbing X-ray photons and could be responsible for the absence of diamondoids in astronomical observations. We also analyze the mechanisms behind the loss of hydrogen in adamantane and compare it to other hydrocarbons.
The scientific questions discussed in this thesis require advanced experimental methods. Another part of this thesis, therefore, describes the commissioning of a new electron-ion spectrometer for use in future experiments at the MAXIV laboratory (paper IV). Data analysis routines for electron-ion coincidence data are also developed and discussed for the different spectrometers used in this work.