Construction and Application of the Endstation for Controlled Molecular Experiments in Chemical Dynamics

Abstract

Direct imaging chemical reactions with atomic-scale spatial resolution and femtosecond-to-attosecond temporal precision is long seeking goal to better understand molecular chemistry. This vision, called "molecular movie'' is a direct step for understanding of molecule dynamics and reactivity during isomerization, bond breaking and reformation, and photocatalytic reactions. Its significance lies not only in elucidating the fundamental laws of molecular dynamics, but also in advancing our understanding of functional regulation in biological systems, atmospheric chemistry, and key reactions in interstellar environments. The requirements to achieve this goal is to establish experimental techniques that combine ultrahigh spatiotemporal resolution with highly sensitive detection of ions and electrons. Meanwhile, complex molecular systems often consist of multiple conformers and clusters, which frequently cause experimental signals to represent ensemble averages, thereby blurring the distinct dynamics of individual species. Therefore, development of experiment providing precisely described molecular ensemble and ability to probe these samples with high spatiotemporal resolution is crucial step towards the understanding the intrinsic nature of complex dynamical processes.

This thesis addresses these challenges with both experimental technique development and molecular system dynamics studies. First, a new experimental setup, an endstation named eCOMO, was successfully constructed, tested, and brought into operation. The system provides a purified molecular beam source integrated with a double-sided VMI spectrometer and high spatiotemporal-resolution Timepix3 cameras. In addition, a novel methodology was developed for precise evaluation of the VMI spectrometer resolution, enabling a systematic analysis of the origins and influencing factors of spatial and temporal resolution under diverse experimental conditions. Then, the apparatus was employed to perform a series of experimental investigations. In the case of OCS molecules, a UV-pump/IR-probe experiment revealed damped oscillation patterns for OCS+ signal, with clear intensity dependence, thereby revealing a previously unobserved potential predissociation pathway in ultrafast ionization dynamics. Furthermore, using the pyrrole-water cluster samples, a combined soft x-ray-pump/IR-probe scheme enabled site-selective ionization of the nitrogen atom in pyrrole. Multi-channel analysis of the resulting processes revealed charge transfer, proton migration, and fragmentation pathways under solvation conditions, offering deeper insights into water-biomolecule interactions.

In summary, through the integration of instrument development, methodological innovation, and experimental studies on representative molecules and clusters, this thesis reveals and analyzes several novel phenomena related to molecular dynamics and solvation effects. The developed setups and obtained results provide a robust platform and valuable foundation for advancing the realization of the "molecular movie'' vision and for future ultrafast dynamical studies in chemistry, biology, and broader interdisciplinary domains.