High resolution spatial map imaging of a gaseous target

Out-of-focus spatial map imaging of magnetically deflected sodium ammonia clusters

This paper introduces out-of-focus spatial map imaging (SMI) as a detection method for magnetic deflection of molecular/cluster beams, using Nam(NH3)n to illustrate its capabilities. This method enables imaging of the complete spatial distribution, simplifying measurements and allowing for cluster-size-resolved analysis by shifting away from traditional in-focus SMI conditions. Incorporating out-of-focus SMI with TOF-MS and velocity map imaging into a single setup allows for direct assessment of clusters' magnetic moments without needing to pre-select velocities. Key findings include a slower relaxation for Na(NH3)4 compared to Na(NH3)3 and Na(NH3)5, unexpectedly high deflection for larger clusters up to Na(NH3)9, hinting at changes in cluster dynamics as the first solvation shell closes. The study also covers the first measurements of Na2(NH3)1 and Na3(NH3)n, showing distinct deflection behaviors and underscoring the improved capabilities of the new detection method.

Access to the full three-dimensional Brillouin zone with time resolution, using a new tool for pump-probe angle-resolved photoemission spectroscopy

Here, we report the first time- and angle-resolved photoemission spectroscopy (TR-ARPES) with the new Fermiologics "FeSuMa" analyzer. The new experimental setup has been commissioned at the Artemis laboratory of the UK Central Laser Facility. We explain here some of the advantages of the FeSuMa for TR-ARPES and discuss how its capabilities relate to those of hemispherical analyzers and momentum microscopes. We have integrated the FeSuMa into an optimized pump-probe beamline that permits photon-energy (i.e., kz)-dependent scanning, using probe energies generated from high harmonics in a gas jet. The advantages of using the FeSuMa in this situation include the possibility of taking advantage of its "fisheye" mode of operation.

Compact realization of all-attosecond pump-probe spectroscopy

The ability to perform attosecond-pump attosecond-probe spectroscopy (APAPS) is a longstanding goal in ultrafast science. While first pioneering experiments demonstrated the feasibility of APAPS, the low repetition rates (10 to 120 Hz) and the large footprints of existing setups have so far hindered the widespread exploitation of APAPS. Here, we demonstrate two-color APAPS using a commercial laser system at 1 kHz, straightforward post-compression in a hollow-core fiber, and a compact high-harmonic generation (HHG) setup. The latter enables the generation of intense extreme-ultraviolet (XUV) pulses by using an out-of-focus HHG geometry and by exploiting a transient blueshift of the driving laser in the HHG medium. Near-isolated attosecond pulses are generated, as demonstrated by one-color and two-color XUV-pump XUV-probe experiments. Our concept allows selective pumping and probing on extremely short timescales in many laboratories and permits investigations of fundamental processes that are not accessible by other pump-probe techniques.

Imaging Frontside and Backside Attack in Radical Ion-Molecule Reactive Scattering

We report on the reactive scattering of methyl iodide, CH₃I, with atomic oxygen anions O^-. This radical ion-molecule reaction can produce different ionic products depending on the angle of attack of the nucleophile O^- on the target molecule. We present results on the backside and frontside attack of O^- on CH₃I, which can lead to I^- and IO^- products, respectively. We combine crossed-beam velocity map imaging with quantum chemical calculations to unravel the chemical reaction dynamics. Energy-dependent scattering experiments in the range of 0.3-2.0 eV relative collision energy revealed that three different reaction pathways can lead to I^- products, making it the predominant observed product. Backside attack occurs via a hydrogen-bonded complex with observed indirect, forward, and sideways scattered iodide products. Halide abstraction via frontside attack produces IO^-, which mainly shows isotropic and backward scattered products at low energies. IO^- is observed to dissociate further to I^- + O at a certain energy threshold and favors more direct dynamics at higher collision energies.

Imaging the Atomistic Dynamics of Single Proton Transfer and Combined Hydrogen/Proton Transfer in the O- + CH3I Reaction

We report on reactive scattering studies of the proton transfer and combined hydrogen/proton transfer in the O^- + CH₃I reaction. We combine state-of-the-art crossed-beam velocity map imaging and quantum chemistry calculations to understand the dynamics for the formations of the CH₂I^- + OH and CHI^- + H₂O products. The experimental velocity- and angle-differential cross section show for both products and at all collision energies (between 0.3 and 2.0 eV) that the product ions are predominantly forward scattered. For the CHI^- + H₂O channel, the data show lower product velocities, indicative of higher internal excitation, than in the case of single proton transfer. Furthermore, our results suggest that the combined hydrogen/proton transfer proceeds via a two-step process: In the first step, O^- abstracts one H atom to form OH^-, and then the transient OH^- removes an additional proton from CH₂I to form the energetically stable H₂O coproduct.

A new endstation for extreme-ultraviolet spectroscopy of free clusters and nanodroplets

In this work, we present a new endstation for the AMOLine of the ASTRID2 synchrotron at Aarhus University, which combines a cluster and nanodroplet beam source with a velocity map imaging and time-of-flight spectrometer for coincidence imaging spectroscopy. Extreme-ultraviolet spectroscopy of free nanoparticles is a powerful tool for studying the photophysics and photochemistry of resonantly excited or ionized nanometer-sized condensed-phase systems. Here, we demonstrate this capability by performing photoelectron-photoion coincidence experiments with pure and doped superfluid helium nanodroplets. Different doping options and beam sources provide a versatile platform to generate various van der Waals clusters as well as He nanodroplets. We present a detailed characterization of the new setup and show examples of its use for measuring high-resolution yield spectra of charged particles, time-of-flight ion mass spectra, anion-cation coincidence spectra, multi-coincidence electron spectra, and angular distributions. A particular focus of the research with this new endstation is on intermolecular charge and energy-transfer processes in heterogeneous nanosystems induced by valence-shell excitation and ionization.

Maximal kinetic energy and angular distribution analysis of spatial map imaging: Application to photoelectrons from a single quantum state of H2O

Dynamical or spatial properties of charged species can be obtained using electrostatic lenses by velocity map imaging (VMI) or spatial map imaging (SMI), respectively. Here, we report an approach for extracting dynamical and spatial information from patterns in SMI images that map the initial coordinates, velocity vectors, and angular distributions of charged particles onto the detector, using the same apparatus as in VMI. Deciphering these patterns required analysis and modeling, involving both their predictions from convolved spatial and velocity distributions and fitting observed images to kinetic energies (KEs) and anisotropy parameters (βs). As the first demonstration of this capability of SMI, the ensuing photoelectrons resulting from (2 + 1) resonant ionization of water in a selected rotational state were chosen to provide a rigorous basis for comparison to VMI. Operation with low acceleration voltages led to a measured SMI pattern with a unique vertical intensity profile that could be least-squares fitted to yield KE and β, in good agreement with VMI measurement. Due to the potential for improved resolution and the extended KE range achievable by this new technique, we expect that it might augment VMI in applications that require the analysis of charged particles and particularly in processes with high KE release.

A new imaging-based method for alignment of multiple laser beams

A new method for multiple laser beams alignment, useful in a wide range of spectroscopies, is proposed and demonstrated. The method, based on the coupling of spatial map imaging (SMI) with velocity map imaging (VMI), aided beams visualization, through interrogation of the ionization signal of different species in a time-of-flight mass spectrometer. This approach is very effective for alignment and for evaluating the spatial overlap of laser beams with a molecular beam. This was demonstrated by monitoring the resonant two-photon ionization spectrum of 2-phenylethylamine (PEA) monomer and its hydrated (PEA-H₂O) cluster and the ionization-loss stimulated Raman spectrum of the cluster, via VMIs accumulation, as a function of the exciting laser wavelength. The former permitted immediate classification of the features in the spectrum, corresponding to the molecular ion or the cluster. The proposed methodology will be useful in other challenging multiple laser beam experiments for spectroscopic studies and is expected to improve extensively their outcome.

Proton transfer dynamics modified by CH-stretching excitation

Gaining insight how specific rovibrational states influence reaction kinetics and dynamics is a fundamental goal of physical chemistry. Purely statistical approaches often fail to predict the influence of a specific state on the reaction outcome, evident in a great number of both experimental and theoretical studies. Most detailed insight in atomistic reaction mechanisms is achieved using accurate collision experiments and high level dynamics calculations. For ion-molecule reactions such experiments are scarce. Here we show the influence of symmetric CH-stretching vibration on the rate and dynamics of proton transfer in the reaction of F- + CH3I. We find a pronounced shift in the reaction dynamics for excited reactions from indirect to preferred direct dynamics at higher collision energy. Moreover, excited reactions occur at larger impact parameters. Finally, we compare vibrational excitation with collision energy and find that vibration is overall more efficient in promoting reactivity, which agrees with recent theoretical calculations.

Imaging Reaction Dynamics of F-(H2O) and Cl-(H2O) with CH3I

The dynamics of microhydrated nucleophilic substitution reactions have been studied using crossed beam velocity map imaging experiments and quasiclassical trajectory simulations at different collision energies between 0.3 and 2.6 eV. For F^–(H₂O) reacting with CH₃I, a small fraction of hydrated product ions I^–(H₂O) is observed at low collision energies. This product, as well as the dominant I^–, is formed predominantly through indirect reaction mechanisms. In contrast, a much smaller indirect fraction is determined for the unsolvated reaction. At the largest studied collision energies, the solvated reaction is found to also occur via a direct rebound mechanism. The measured product angular distributions exhibit an overall good agreement with the simulated angular distributions. Besides nucleophilic substitution, also ligand exchange reactions forming F^–(CH₃I) and, at high collision energies, proton transfer reactions are detected. The differential scattering images reveal that the Cl^–(H₂O) + CH₃I reaction also proceeds predominantly via indirect reaction mechanisms.

Stretching vibration is a spectator in nucleophilic substitution

How chemical reactions are influenced by reactant vibrational excitation is a long-standing question at the core of chemical reaction dynamics. In reactions of polyatomic molecules, where the Polanyi rules are not directly applicable, certain vibrational modes can act as spectators. In nucleophilic substitution reactions, CH stretching vibrations have been considered to be such spectators. While this picture has been challenged by some theoretical studies, experimental insight has been lacking. We show that the nucleophilic substitution reaction of F^- with CH₃I is minimally influenced by an excitation of the symmetric CH stretching vibration. This contrasts with the strong vibrational enhancement of the proton transfer reaction measured in parallel. The spectator behavior of the stretching mode is supported by both quasi-classical trajectory simulations and the Sudden Vector Projection model.

Note: A well-confined pulsed low-energy ion beam: Test experiments of Ar. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2018;89:066104. [PMID: 29960543 DOI: 10.1063/1.5026822]

Here we report a pulsed low-energy ion beam source for ion-molecule reaction study, in which the ions produced by the pulsed electron impact are confined well in the spatial size of each bunch. In contrast to the ion focusing method to reduce the transverse section of the beam, the longitudinal section in the translational direction is compressed by introducing a second pulse in the ion time-of-flight system. The test experiments for the low-energy argon ions are performed. The present beam source is ready for applications in the ion-molecule reaction dynamics experiments, in particular, in combination with the ion velocity map imaging technique.

Perspective: Advanced particle imaging

Since the first ion imaging experiment [D. W. Chandler and P. L. Houston, J. Chem. Phys. 87, 1445-1447 (1987)], demonstrating the capability of collecting an image of the photofragments from a unimolecular dissociation event and analyzing that image to obtain the three-dimensional velocity distribution of the fragments, the efficacy and breadth of application of the ion imaging technique have continued to improve and grow. With the addition of velocity mapping, ion/electron centroiding, and slice imaging techniques, the versatility and velocity resolution have been unmatched. Recent improvements in molecular beam, laser, sensor, and computer technology are allowing even more advanced particle imaging experiments, and eventually we can expect multi-mass imaging with co-variance and full coincidence capability on a single shot basis with repetition rates in the kilohertz range. This progress should further enable "complete" experiments-the holy grail of molecular dynamics-where all quantum numbers of reactants and products of a bimolecular scattering event are fully determined and even under our control.

Imaging the dynamics of ion–molecule reactions

A range of ion–molecule reactions have been studied in the last years using the crossed-beam ion imaging technique, from charge transfer and proton transfer to nucleophilic substitution and elimination.

A simple photoionization scheme for characterizing electron and ion spectrometers

We present a simple diode laser-based photoionization scheme for generating electrons and ions with well-defined spatial and energetic (≲2 eV) structures. This scheme can easily be implemented in ion or electron imaging spectrometers for the purpose of off-line characterization and calibration. The low laser power ∼1 mW needed from a passively stabilized diode laser and the low flux of potassium atoms in an effusive beam make our scheme a versatile source of ions and electrons for applications in research and education.

Materials Properties and Solvated Electron Dynamics of Isolated Nanoparticles and Nanodroplets Probed with Ultrafast Extreme Ultraviolet Beams

We present ultrafast photoemission measurements of isolated nanoparticles in vacuum using extreme ultraviolet (EUV) light produced through high harmonic generation. Surface-selective static EUV photoemission measurements were performed on nanoparticles with a wide array of compositions, ranging from ionic crystals to nanodroplets of organic material. We find that the total photoelectron yield varies greatly with nanoparticle composition and provides insight into material properties such as the electron mean free path and effective mass. Additionally, we conduct time-resolved photoelectron yield measurements of isolated oleylamine nanodroplets, observing that EUV photons can create solvated electrons in liquid nanodroplets. Using photoemission from a time-delayed 790 nm pulse, we observe that a solvated electron is produced in an excited state and subsequently relaxes to its ground state with a lifetime of 151 ± 31 fs. This work demonstrates that femotosecond EUV photoemission is a versatile surface-sensitive probe of the properties and ultrafast dynamics of isolated nanoparticles.

Velocity Map Imaging Study of Charge-Transfer and Proton-Transfer Reactions of CH3 Radicals with H3(.)

The velocity map imaging method has been applied to crossed beam studies of charge transfer and proton transfer between methyl (CH3) radicals formed by pyrolysis and H3(+) cations over the collision energy range from 1.2 to 3.4 eV. Vibrational excitation in the H3(+) reactants plays an important role both in promoting endoergic charge transfer and in supplying energy to the products of the proton-transfer reaction. Excited H3(+) reactants with vibrational energy in excess of the barrier lead to energy-resonant charge transfer via long-range collisions. A small fraction of collisions that take place at low impact parameters appear to form charge-transfer products with higher levels of internal excitation. The proton-transfer reaction exhibits direct, stripping-like dynamics. Consistent with the kinematics of proton transfer, incremental kinetic energy supplied to the reactants is strongly directed into product relative kinetic energy, as predicted by the concept of "induced repulsive energy release".