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

Conformer-specific polar cycloaddition of dibromobutadiene with trapped propene ions

Identifying a concerted or stepwise mechanism in Diels-Alder reactions is experimentally challenging. Here the authors demonstrate the coexistence of both mechanisms in the reaction of 2,3-dibromobuta-1,3-diene with propene ions, using a conformationally controlled molecular beam reacting with trapped ions and ab initio computations Diels-Alder cycloadditions are efficient routes for the synthesis of cyclic organic compounds. There has been a long-standing discussion whether these reactions proceed via stepwise or concerted mechanisms. Here, we adopt an experimental approach to explore the mechanism of the model polar cycloaddition of 2,3-dibromo-1,3-butadiene with propene ions by probing its conformational specificities in the entrance channel under single-collision conditions in the gas phase. Combining a conformationally controlled molecular beam with trapped ions, we find that both conformers of the diene, gauche and s-trans, are reactive with capture-limited reaction rates. Aided by quantum-chemical and quantum-capture calculations, this finding is rationalised by a simultaneous competition of concerted and stepwise reaction pathways, revealing an interesting mechanistic borderline case

Demonstration and STEM Analysis of Ferroelectric Switching in MOCVD-Grown Single Crystalline Al0.85_{0.85}Sc0.15_{0.15}N

Wurtzite-type Al$_{1-x}$Sc$_x$N solid solutions grown by metal organic chemical vapour deposition are for the first time confirmed to be ferroelectric. The film with 230 nm thickness and x = 0.15 exhibits a coercive field of 5.5 MV/cm at a measurement frequency of 1.5 kHz. Single crystal quality and homogeneous chemical composition of the film was confirmed by X-ray diffraction spectroscopic methods such as time of flight secondary ion mass spectrometry. Annular bright field scanning transmission electron microscopy served to proof the ferroelectric polarization inversion on unit cell level. The single crystal quality further allowed to image the large-scale domain pattern of a wurtzite-type ferroelectric for the first time, revealing a predominantly cone-like domain shape along the c-axis of the material. As in previous work, this again implies the presence of strong polarization discontinuities along this crystallographic axis, which could be suitable for current transport. The domains are separated by narrow domain walls, for which an upper thickness limit of 3 nm was deduced, but which could potentially be atomically sharp. We are confident that these results will advance the commencing integration of wurtzite-type ferroelectrics to GaN as well as generally III-N based heterostructures and devices.Comment: 5 Figure