A novel crossed-molecular-beam experiment for investigating reactions of state- and conformationally selected strong-field-seeking molecules

The structure and quantum state of the reactants have a profound impact on the kinetics and dynamics of chemical reactions. Over the past years, significant advances have been made in the control and manipulation of molecules with external electric and magnetic fields in molecular-beam experiments for investigations of their state-, structure- and energy-specific chemical reactivity. Whereas studies for neutrals have so far mainly focused on weak-field-seeking species, we report here progress towards investigating reactions of strong-field-seeking molecules by introducing a novel crossed-molecular-beam experiment featuring an electrostatic deflector. The new setup enables the characterisation of state- and geometry-specific effects in reactions under single-collision conditions. As a proof of principle, we present results on the chemi-ionisation reaction of metastable neon atoms with rotationally state-selected carbonyl sulfide (OCS) molecules and show that the branching ratio between the Penning and dissociative ionisation pathways strongly depends on the initial rotational state of OCS.Comment: 8 pages, 6 figure

Development and characterization of high-repetition-rate sources for supersonic beams of fluorine radicals

We present and compare two high-pressure, high-repetition-rate electric-discharge sources for the generation of supersonic beams of fluorine radicals. The sources are based on dielectric-barrier-discharge (DBD) and plate-discharge units attached to a pulsed solenoid valve. The corrosion-resistant discharge sources were operated with fluorine gas seeded in helium up to backing pressures as high as 30 bars. We employed a (3 + 1) resonance-enhanced multiphoton ionization combined with velocity-map imaging for the optimization, characterization, and comparison of the fluorine beams. Additionally, universal femtosecond-laser-ionization detection was used for the characterization of the discharge sources at experimental repetition rates up to 200 Hz. Our results show that the plate discharge is more efficient in F2 dissociation than the DBD by a factor between 8 and 9, whereas the DBD produces internally colder fluorine radicals

Development and characterization of high-frequency sources for supersonic beams of fluorine radicals

We present and compare two high-pressure, high-frequency electric-discharge sources for the generation of supersonic beams of fluorine radicals. The sources are based on dielectric-barrier-discharge (DBD) and plate-discharge units attached to a pulsed solenoid valve. The corrosion-resistant discharge sources were operated with fluorine gas seeded in helium up to backing pressures as high as 30 bar. We employed a (3+1) resonance-enhanced multiphoton ionization combined with velocity-map imaging for the optimization, characterization and comparison of the fluorine beams. Additionally, universal femtosecond-laser-ionization detection was used for the characterization of the discharge sources at experimental repetition rates up to 200 Hz. Our results show that the plate discharge is more efficient in F$_{2}$ dissociation than the DBD by a factor of 8-9, whereas the DBD produces internally colder fluorine radicals.Comment: 6 pages, 7 figure

Cold and intense OH radical beam sources

We present the design and performance of two supersonic radical beam sources: a conventional pinhole-discharge source and a dielectric barrier discharge (DBD) source, both based on the Nijmegen pulsed valve. Both designs have been characterized by discharging water molecules seeded in the rare gases Ar, Kr, or Xe. The resulting OH radicals have been detected by laser-induced fluorescence. The measured OH densities are (3.0 ± 0.6) × 10(11) cm(-3) and (1.0 ± 0.5) × 10(11) cm(-3) for the pinhole-discharge and DBD sources, respectively. The beam profiles for both radical sources show a relative longitudinal velocity spread of about 10%. The absolute rotational ground state population of the OH beam generated from the pinhole-discharge source has been determined to be more than 98%. The DBD source even produces a rotationally colder OH beam with a population of the ground state exceeding 99%. For the DBD source, addition of O2 molecules to the gas mixture increases the OH beam density by a factor of about 2.5, improves the DBD valve stability, and allows to tune the mean velocity of the radical beam

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