A Novel Crossed-Molecular-Beam Setup: Investigating state- and conformer-specific effects in bimolecular reactions

Chemical reactions are profoundly impacted by the properties of the interacting molecules, including their structure and quantum state. Crossed-beam setups have been proven to be a great tool for the investigation of such fundamental interactions occurring in bimolecular reactions under single-collision conditions. The information gained from these studies depends strongly on the experimental control which can be achieved over the reactants' properties. In recent years, tremendous progress has been made in that regard by manipulating molecules in supersonic beams with external electric and magnetic fields. However, the experimental techniques used were mainly limited to the control and manipulation of weak-field-seeking reactants.
This thesis presents the design, construction and characterisation of a novel crossed-molecular-beam setup suited for investigating bimolecular reactions of controlled strong-field-seeking molecules. The distinctive feature of the new setup is an electrostatic deflector integrated into one of the molecular beams, which exploits the interaction of polar molecules with its strongly inhomogeneous electric field. This enables the spatial separation of molecules in different rotational states as well as the selection of specific molecular conformations. This allows in particular the study of bimolecular reactions with isolated conformers, whose tendency to interconvert has provided strong experimental challenges in the past. The co-reactants to the prepared molecules are provided by a home-built discharge valve, which allows the generation of radicals and metastable-rare-gas atoms. A time-of-flight mass spectrometer allows the detection and identification of the reaction products as well as the determination of mass-specific relative integral cross sections. Differential cross sections of specific reaction products can be obtained by a mass-gated velocity-map-imaging detector.
The capabilities of the new setup are demonstrated with pioneering experiments of state- and conformationally-resolved chemi-ionisation reactions. Studying the reactive collisions of metastable neon atoms with rotational-state selected carbonyl sulfide (OCS) molecules revealed that the branching ratio between the reaction pathways resulting in the Penning ion (OCS$^+$) or the dissociative-ionisation product S$^+$ strongly depends on the initial rotational state of OCS. Also the investigation of the conformationally-resolved chemi-ionisation reaction of metastable neon with trans- and cis-hydroquinone showed a clear difference in the reactivity towards forming the Penning ion or the dissociative-ionisation products, which seems to be dependent on the initial conformational- and rotational state of hydroquinone.
The novel crossed-molecular-beam setup presented here should be applicable for the investigation of a broad range of different reactions of polyatomic molecules and by that will provide a useful tool for unravelling the fundamental details of geometry-specific effects in bimolecular reactions.