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High-throughput photofragmentation studies of thymine and guanine were performed at 257 nm and 343 nm and for a wide range of ionisation laser intensities.

Structural isomers, such as conformers or tautomers, are of significant importance across chemistry and biology, as they can have different functionalities. In gas-phase experiments using molecular beams, formation of many different isomers cannot be prevented, and their presence significantly complicates the assignment of spectral lines. Current isomer-resolved spectroscopic techniques heavily rely on theoretical calculations or make use of elaborate double-resonance schemes. We show here that isomer-resolved spectroscopy can also be performed using a single tunable laser. In particular, we demonstrate single-color isomer-resolved spectroscopy by utilizing electrostatic deflection to spatially separate the isomers. We show that for 3-aminophenol we can spatially separate the

Physical review applied 17(4), 044044 (2022). doi:10.1103/PhysRevApplied.17.044044, Photophoretic forces are induced when light causes a net momentum exchange between a particle and a surrounding gas. Such forces have been shown to be a robust means for trapping and guiding particles in air over long distances. Here, we apply the concept of an optical funnel for the delivery of bioparticles to the focus of an x-ray free-electron laser (XFEL) for femtosecond x-ray diffractive imaging. We provide the experimental demonstration of transversely compressing a high-speed beam of aerosolized viruses via photophoretic forces in a low-pressure gas environment. Relative temperature gradients induced on the viruses by the laser are estimated via particle-velocimetry measurements. The results demonstrate the potential for an optical funnel to improve particle-delivery efficiency in XFEL imaging and spectroscopy., Published by American Physical Society, College Park, Md. [u.a.]

Abstract A major obstacle to the gas-phase study of larger (bio)molecular systems is the vaporisation step, that is, the introduction of intact sample molecules into the gas-phase. A promising approach is the use of laser-induced acoustic desorption (LIAD) sources, which have been demonstrated using both nanosecond pulsed and continuous desorption lasers. We directly compare here both approaches for the first time under otherwise identical conditions using adenine as a prototypical biological molecule, and study the produced molecular plumes using femtosecond multiphoton ionisation. We observe different desorption mechanisms at play for the two different desorption laser sources; however, we find no evidence in either case that the desorption process leads to fragmentation of the target molecule unless excessive desorption energy is applied. This makes LIAD a powerful approach for techniques that require high density and high purity samples in the gas-phase, such as ultrafast dynamics studies or diffraction experiments. Graphic abstract

The products formed following the photodissociation of UV (200 nm) excited CS

In single particle imaging experiments, beams of individual nanoparticles are exposed to intense pulses of x-rays from free-electron lasers to record diffraction patterns of single, isolated molecules. The reconstruction for structure determination relies on signal from many identical particles. Therefore, well-defined-sample delivery conditions are desired in order to achieve sample uniformity, including avoidance of charge polydispersity. We have observed charging of 220 nm polystyrene particles in an aerosol beam created by a gas-dynamic virtual nozzle focusing technique, without intentional charging of the nanoparticles. Here, we present a deflection method for detecting and characterizing the charge states of a beam of aerosolized nanoparticles. Our analysis of the observed charge-state distribution using optical light-sheet localization microscopy and quantitative particle trajectory simulations is consistent with previous descriptions of skewed charging probabilities of triboelectrically charged nanoparticles., Comment: 5 pages, 4 figures

The journal of chemical physics 154(3), 034302 (2021). doi:10.1063/5.0035045, The products formed following the photodissociation of UV (200 nm) excited CS$_2$ are monitored in a time resolved photoelectron spectroscopy experiment using femtosecond XUV (21.5 eV) photons. By spectrally resolving the electrons, we identify separate photoelectron bands related to the CS$_2$ + h�� ��� S($^1$D) + CS and CS$_2$ + h�� ��� S($^3$P) + CS dissociation channels, which show different appearance and rise times. The measurements show that there is no delay in the appearance of the S(1D) product contrary to the results of Horio et al. [J. Chem. Phys. 147, 013932 (2017)]. Analysis of the photoelectron yield associated with the atomic products allows us to obtain a S($^3$P)/S($^1$D) branching ratio and the rate constants associated with dissociation and intersystem crossing rather than the effective lifetime observed through the measurement of excited state populations alone., Published by American Institute of Physics, Melville, NY

Optica 8(1), 15 - 23 (2021). doi:10.1364/OPTICA.410851, Single particle imaging at x-ray free electron lasers (XFELs) has the potential to determine the structure and dynamics of single biomolecules at room temperature. Two major hurdles have prevented this potential from being reached, namely, the collection of sufficient high-quality diffraction patterns and robust computational purification to overcome structural heterogeneity. We report the breaking of both of these barriers using gold nanoparticle test samples, recording around 10 million diffraction patterns at the European XFEL and structurally and orientationally sorting the patterns to obtain better than 3-nm-resolution 3D reconstructions for each of four samples. With these new developments, integrating advancements in x-ray sources, fast-framing detectors, efficient sample delivery, and data analysis algorithms, we illuminate the path towards sub-nanometer biomolecular imaging. The methods developed here can also be extended to characterize ensembles that are inherently diverse to obtain their full structural landscape., Published by OSA, Washington, DC

Communications Physics 3(1), 97 (2020). doi:10.1038/s42005-020-0362-y, The emergence of high repetition-rate X-ray free-electron lasers (XFELs) powered by superconducting accelerator technology enables the measurement of significantly more experimental data per day than was previously possible. The European XFEL is expected to provide 27,000 pulses per second, over two orders of magnitude more than any other XFEL. The increased pulse rate is a key enabling factor for single-particle X-ray diffractive imaging, which relies on averaging the weak diffraction signal from single biological particles. Taking full advantage of this new capability requires that all experimental steps, from sample preparation and delivery to the acquisition of diffraction patterns, are compatible with the increased pulse repetition rate. Here, we show that single-particle imaging can be performed using X-ray pulses at megahertz repetition rates. The results obtained pave the way towards exploiting high repetition-rate X-ray free-electron lasers for single-particle imaging at their full repetition rate., Published by Springer Nature, London

We present a comparison between a numerical model and the experimental results on the generation of a first-order Bessel-Gaussian optical funnel for guiding of airborne microparticles.

Imaging biological molecules in the gas-phase requires novel sample delivery methods, which generally have to be characterized and optimized to produce high-density particle beams. A non-destructive characterization method of the transverse particle beam profile is presented. It enables the characterization of the particle beam in parallel to the collection of, for instance, x-ray-diffraction patterns. As a rather simple experimental method, it requires the generation of a small laser-light sheet using a cylindrical telescope and a microscope. The working principle of this technique was demonstrated for the characterization of the fluid-dynamic-focusing behavior of 220 nm polystyrene beads as prototypical nanoparticles. The particle flux was determined and the velocity distribution was calibrated using Mie-scattering calculations.

Spatial separation of water dimer from water monomer and larger water-clusters through the electric deflector is presented. A beam of water dimer with $93~\%$ purity and a rotational temperature of $1.5~$K was obtained. Following strong-field ionization using a $35~$fs laser pulse with a wavelength centered around $800~$nm and a peak intensity of $10^{14}~\text{W}/\text{cm}^2$ we observed proton transfer and $46~\%$ of the ionized water dimer broke apart into a hydronium ion $\text{H}_3\text{O}^+$ and neutral OH.

Atomic‐resolution‐imaging approaches for single molecules, such as coherent X‐ray diffraction at free‐electron lasers, require the delivery of high‐density beams of identical molecules. However, even very cold beams of biomolecules typically have multiple conformational states populated. We demonstrate the production of very cold $(T_{rot}≈2.3 K)$ molecular beams of intact dipeptide molecules, which were then spatially separated into the individual populated conformational states. This is achieved using the combination of supersonic expansion laser‐desorption vaporization with electrostatic deflection in strong inhomogeneous fields. This represents the first demonstration of a conformer‐separated and rotationally cold molecular beam of a peptide, which enables the investigation of conformer‐specific chemistry using inherently non‐conformer‐specific techniques. It furthermore represents a milestone toward the direct structural imaging of individual biological molecules with atomic resolution by ultrafast diffractive‐imaging methods.

We have designed, constructed, and commissioned a cryogenic helium buffer-gas source for producing a cryogenically cooled molecular beam and evaluated the effect of different cell geometries on the intensity of the produced molecular beam, using ammonia as a test molecule. Planar and conical entrance and exit geometries are tested. We observe a threefold enhancement in the ${\mathrm{NH}}_{3}$ signal for a cell with planar entrance and conical-exit geometry, compared to that for a typically used ``boxlike'' geometry with planar entrance and exit. These observations are rationalized by flow field simulations for the different buffer-gas cell geometries. The full thermalization of molecules with the helium buffer gas is confirmed through rotationally resolved resonance-enhanced multiphoton ionization spectra yielding a rotational temperature of 5 K.

The dissociation dynamics of the $\tilde{A}$-state of ammonia have been studied using a resonant multiphoton ionisation probe in a photoelectron spectroscopy experiment. The use of a resonant intermediate in the multiphoton ionisation process changes the ionisation propensity, allowing access to different ion states when compared with equivalent single photon ionisation experiments. Ionisation through the $E'$ $^1$A$_1'$ Rydberg intermediate means we maintain overlap with the ion state for an extended period allowing us to monitor the excited state population for several hundred femtoseconds. The vibrational states in the photoelectron spectrum show two distinct timescales, 200 fs and 320 fs, that we assign to the non-adiabatic and adiabatic dissociation processes respectively. The different timescales derive from differences in the wavepacket trajectories for the two dissociation pathways that resonantly excite different vibrational states in the intermediate Rydberg state. The timescales are similar to those obtained from time resolved ion kinetic energy release measurements, suggesting we can measure the different trajectories taken out to the region of conical intersection.

We probe the dynamics of dissociating $CS^2$ molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.

Die Abbildung isolierter Moleküle mit atomarer Auflösung, z. B. in Experimenten zur kohärenten Röntgenbeugung an Freie‐Elektronen‐Lasern, benötigt Molekülstrahlen identischer Moleküle mit hoher Dichte. Allerdings besetzen selbst Biomoleküle in kalten Molekülstrahlen typischerweise mehrere Konformerzustände. Hier wird die Produktion von sehr kalten $(T_{rot}≈2.3 K)$ Molekülstrahlen aus intakten Dipeptiden demonstriert, in denen anschließend die einzelnen besetzten Konformerzustände räumlich getrennt werden. Dies wird durch die Kombination von Überschallexpansion mit Laserdesorptionsverdampfung und elektrostatischer Ablenkung in starken inhomogenen Feldern erreicht. Dies ist die erste Demonstration von konformerselektierten und rotationskalten Molekülstrahlen von Peptiden und ermöglicht in Zukunft die Untersuchung von konformerspezifischer Chemie durch nicht‐Konformer‐spezifische Techniken. Des Weiteren ist dies ein Meilenstein, um direkte Strukturabbildungen von isolierten biologischen Molekülen mit atomarer Auflösung mittels ultraschneller Beugungsexperimente zu ermöglichen.

The design and characterization of a new laser-desorption molecular beam source, tailored for use in x-ray-free-electron-laser and ultrashort-pulse-laser imaging experiments, is presented. It consists of a single mechanical unit containing all source components, including the molecular-beam valve, the sample, and the fiber-coupled desorption laser, which is movable in five axes, as required for experiments at central facilities. Utilizing strong-field ionization, we characterize the produced molecular beam and evaluate the influence of desorption laser pulse energy, relative timing of valve opening and desorption laser, sample bar height, and which part of the molecular packet is probed on the sample properties. Strong-field ionization acts as a universal probe and allows to detect all species present in the molecular beam, and hence enables us to analyze the purity of the produced molecular beam, including molecular fragments. We present optimized experimental parameters for the production of the purest molecular beam, containing the highest yield of intact parent ions, which we find to be very sensitive to the placement of the desorbed-molecules plume within the supersonic expansion.

Time resolved photoelectron spectroscopy (TRPES) shows exquisite sensitivity to both the electronic and nuclear dynamics of molecules. TRPES is also, in principle, a windowless technique that can provide a measurement for any molecular structure as all states can be ionised. With the advent of high harmonic generation (HHG) sources, this windowless character is beginning to be realised, as pulses of ultrashort duration and sufficient energies to ionise the important reaction intermediates and products are now available. The limited use of HHG sources so far is due to the relatively low flux attainable from most HHG sources. This means experiments have either had to combine the HHG pulse with a strong field pump, or limit their measurements to relatively simple dynamics in diatomic molecules. The use of strong fields in the pump enhances the excitation probability at the expense of selectivity such that the range of dynamics and systems that can be studied is not ideal. Here we present UV pump-XUV probe TRPES experiment that allows for the unambiguous assignment of the full reaction pathway in a polyatomic molecule. The experiments measure the dissociation dynamics of CS 2 following UV excitation through to the formation of multiple dissociation products.

Water exists as two nuclear-spin isomers, para and ortho, determined by the overall spin of its two hydrogen nuclei. For isolated water molecules, the conversion between these isomers is forbidden and they act as different molecular species. Yet, these species are not readily separated, and no pure para sample has been produced. Accordingly, little is known about their specific physical and chemical properties, conversion mechanisms, or interactions. The production of isolated samples of both spin isomers is demonstrated in pure beams of para and ortho water in their respective absolute ground state. These single-quantum-state samples are ideal targets for unraveling spin-conversion mechanisms, for precision spectroscopy and fundamental symmetry-breaking studies, and for spin-enhanced applications, for example laboratory astrophysics and astrochemistry or hypersensitized NMR experiments.

Wasser existiert in Form zweier Kernspinisomere, para und ortho, definiert durch den Gesamtspin der zwei Wasserstoffatomkerne. In isolierten Wassermolekülen ist die Umwandlung zwischen den beiden Isomeren quantenmechanisch verboten, und sie können als zwei verschiedene Molekülspezies betrachtet werden. Ihre Trennung ist nicht trivial, und reine Proben von para-Wasser sind bisher nicht erzeugt worden. Daher sind die physikalischen und chemischen Eigenschaften, Umwandlungsmechanismen und Wechselwirkungen der beiden Isomere kaum bekannt. Hier demonstrieren wir die Isolierung beider Spinisomere in reinen Molekülstrahlen von para- und ortho-Wasser im jeweiligen absoluten Grundzustand. Die Moleküle in einzelnen Quantenzuständen sind ideale Proben für Studien zu Umwandlungsmechanismen der Isomere, für Präzisionsspektroskopie, zur Untersuchung fundamentaler physikalischer Symmetriebrechung sowie für kernspinbasierte Anwendungen, z. B. Laborexperimente zur Astrophysik und -chemie oder hochempfindliche NMR-Experimente.

A numerical simulation infrastructure capable of calculating the flow of gas and the trajectories of particles through an aerodynamic lens injector is presented. The simulations increase the fundamental understanding and predict optimized injection geometries and parameters. Our simulation results were compared to previous reports and also validated against experimental data for 500 nm polystyrene spheres from an aerosol-beam-characterization setup. The simulations yielded a detailed understanding of the radial phase-space distribution and highlighted weaknesses of current aerosol injectors for single-particle diffractive imaging. With the aid of these simulations we developed new experimental implementations to overcome current limitations.

A novel methodology for measuring gas flow from small orifices or nozzles into vacuum is presented. It utilizes a high-intensity femtosecond laser pulse to create a plasma within the gas plume produced by the nozzle, which is imaged by a microscope. Calibration of the imaging system allows for the extraction of absolute number densities. We show detection down to helium densities of 4 × 10 16 cm−3 with a spatial resolution of a few micrometers. The technique is used to characterize the gas flow from a convergent-nozzle aerosol injector [Kirian et al., Struct. Dyn. 2, 041717 (2015)] as used in single-particle diffractive imaging experiments at free-electron laser sources. Based on the measured gas-density profile, we estimate the scattering background signal under typical operating conditions of single-particle imaging experiments and estimate that fewer than 50 photons per shot can be expected on the detector.

A laser-induced acoustic desorption source, developed for use at central facilities, such as free-electron lasers, is presented. It features prolonged measurement times and a fixed interaction point. A novel sample deposition method using aerosol spraying provides a uniform sample coverage and hence stable signal intensity. Utilizing strong-field ionization as a universal detection scheme, the produced molecular plume is characterized in terms of number density, spatial extend, fragmentation, temporal distribution, translational velocity, and translational temperature. The effect of desorption laser intensity on these plume properties is evaluated. While translational velocity is invariant for different desorption laser intensities, pointing to a non-thermal desorption mechanism, the translational temperature increases significantly and higher fragmentation is observed with increased desorption laser fluence., 8 pages, 7 figures

New ab initio surface hopping simulations of the excited state dynamics of CS$_2$ including spin-orbit coupling are compared to new experimental measurements using a multiphoton ionisation probe in a photoelectron spectroscopy experiment. The calculations highlight the importance of the triplet states even in the very early time dynamics of the dissociation process and allow us to unravel the signatures in the experimental spectrum, linking the observed changes to both electronic and nuclear degrees of freedom within the molecule.

The green fluorescent protein (GFP) is employed extensively as a marker in biology and the life sciences as a result of its spectacular fluorescence properties. Here, we employ femtosecond time-resolved photoelectron spectroscopy to investigate the ultrafast excited state dynamics of the isolated GFP chromophore anion. Excited state population is found to decay bi-exponentially, with characteristic lifetimes of 330 fs and 1.4 ps. Distinct photoelectron spectra can be assigned to each of these timescales and point to the presence of a transient intermediate along the decay coordinate. Guided by ab initio calculations, we assign these observations to twisting about the C–C–C bridge followed by internal conversion to the anion ground state. The dynamics in vacuo are very similar to those observed in solution, despite the difference in absorption spectra between the two media. This is consistent with the protein environment restricting rotation about the C–C–C bond in order to prevent ultrafast internal conversion and preserve the fluorescence.

SPIE Nanoscience + Engineering, San Diego, California, 17 Aug 2016 - 21 Aug 2016 ; Proceedings of SPIE 9022, 99220K(2016). doi:10.1117/12.2239171, Optical trapping of light-absorbing particles in a gas environment is usually dominated by laser-induced thermal or photophoretic forces, which can be orders of magnitude higher than the force due to radiation pressure. Particle guiding with photophoretic forces over large distances in open air was recently realised by an optical pipeline, formed by a vortex laser beam of doughnut-like intensity profile, with a high-intensity ring of light that surrounds a dark core. We are adapting the optical pipeline concept for the purpose of guiding aerosolized particles into the intense focus of a x-ray free-electron laser (XFEL), in order to enable high-efficiency femtosecond x-ray coherent diffractive imaging (CDI). XFEL-based CDI allows single-shot nanometer-resolution imaging, and multi-shot Angstrom-resolution tomography in the case of reproducible nanoparticles, at a time resolution better than 10 femtoseconds. Remarkably, by imaging at timescales shorter than atomic motion, the crucial resolution-limiting effects of radiation damage may be overcome for radiation-sensitive targets such as viruses and biomolecules. Following on our previous work, we are developing an optical first-order Bessel-like beam with a variable-diameter hollow core and an axial-to-lateral aspect ratio up to ~2000, that can be used to guide particles with a spatial precision of less than a few µm over centimetre-long distances. We present the ways to control the beam divergence aiming to focus the stream of particles by thermal forces and forces of radiation pressure, analyse the forces acting on the particle in the beam, and uncover the beam structure and intensity to apply for a real-time experiment with XFEL. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only., Published by SPIE, Bellingham, Wash.

Hydrogen bonding interactions between biological chromophores and their surrounding protein and solvent environment significantly affect the photochemical pathways of the chromophore and its biological function. A common first step in the dynamics of these systems is excited state proton transfer between the non-covalently bound molecules, which stabilises the system against dissociation and principally alters relaxation pathways. Despite such fundamental importance, studying excited state proton transfer across a hydrogen bond has proven difficult, leaving uncertainties about the mechanism. Through time-resolved photoelectron imaging measurements we demonstrate how the addition of a single hydrogen bond and the opening of an excited state proton transfer channel dramatically changes the outcome of a photochemical reaction, from rapid dissociation in the isolated chromophore, to efficient stabilisation and ground state recovery in the hydrogen bonded case, and uncover the mechanism of excited state proton transfer at a hydrogen bond, which follows sequential hydrogen and charge transfer processes.

The spectroscopy and ultrafast relaxation dynamics of excited states of the radical anion of a representative charge-transfer acceptor molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, have been studied in the gas phase using time-resolved photoelectron spectroscopy. The photoelectron spectra reveal that at least two anion excited states are bound. Time-resolved studies show that both excited states are very short-lived and internally convert to the anion ground state, with the lower energy state relaxing within 200 fs and a near-threshold valence-excited state relaxing on a 60 fs time scale. These excited states, and in particular the valence-excited state, present efficient pathways for electron-transfer reactions in the highly exergonic inverted region which commonly displays rates exceeding predictions from electron-transfer theory.

A 100 kHz high harmonic source with record high >10$^{11}$ photons/s in single harmonics between 55-73 eV is presented. The unique capabilities are underlined by using it for coincidence experiments and measurements on magnetic samples.

A demonstration of a new beamline for measurement of ultrafast photo induced processes using direct excitation and a high energy probe. The setup of the beamline and initial measurements on photodissociation of CS2 are shown.

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10sup11/supphotons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CHsub3/subI are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

The photoelectron signal of the singly deprotonated fluorescein anion is found to be highly dependent on the relative polarization between pump and probe pulses, and time-resolved photodetachment anisotropy (TR-PA) is developed as a probe of the rotational dynamics of the chromophore. The total photoelectron signal shows both rotational and vibrational wavepacket dynamics, and we demonstrate how TR-PA can readily disentangle these dynamical processes. TR-PA in fluorescein presents specific opportunities for its development as a probe for rotational dynamics in large biomolecules as fluorescein derivatives are commonly incorporated in complex biomolecules and have been used extensively in time-resolved fluorescence anisotropy measurements, to which TR-PA is a gas-phase analogue.

A major challenge in high-resolution x-ray free-electron laser-based coherent diffractive imaging is the development of aerosol injectors that can efficiently deliver particles to the peak intensity of the focused X-ray beam. Here, we consider the use of a simple convergent-orifice nozzle for producing tightly focused beams of particles. Through optical imaging we show that 0.5 μm particles can be focused to a full-width at half maximum diameter of 4.2 μm, and we demonstrate the use of such a nozzle for injecting viruses into a micro-focused soft-X-ray FEL beam.

The understanding of molecular structure and function is at the very heart of the chemical and molecular sciences. Experiments that allow for the creation of structurally pure samples and the investigation of their molecular dynamics and chemical function have developed tremendously over the last few decades, although "there's plenty of room at the bottom" for better control as well as further applications. Here, we describe the use of inhomogeneous electric fields for the manipulation of neutral molecules in the gas-phase, \ie, for the separation of complex molecules according to size, structural isomer, and quantum state. For these complex molecules, all quantum states are strong-field seeking, requiring dynamic fields for their confinement. Current applications of these controlled samples are summarised and interesting future applications discussed., Comment: Accepted by Int. Rev. Phys. Chem

Water exists as two nuclear-spin isomers, para and ortho, determined by the overall spin of its two hydrogen nuclei. For isolated water molecules, the conversion between these isomers is forbidden and they act as different molecular species. Yet, these species are not readily separated, and no pure para sample has been produced. Accordingly, little is known about their specific physical and chemical properties, conversion mechanisms, or interactions. The production of isolated samples of both spin isomers is demonstrated in pure beams of para and ortho water in their respective absolute ground state. These single-quantum-state samples are ideal targets for unraveling spin-conversion mechanisms, for precision spectroscopy and fundamental symmetry-breaking studies, and for spin-enhanced applications, for example laboratory astrophysics and astrochemistry or hypersensitized NMR experiments.

The intrinsic photophysics of indigo has been studied using gas-phase time-resolved photoelectron imaging of the indigo carmine dianion (InC(2-)). The action spectrum reveals that the gas-phase absorption spectrum arising from the S(1)-- S(0) transition in InC(2-) has a similar solvent shift to that of neutral indigo. Femtosecond spectroscopy shows that the S(1) state decays on a 1.4 ps timescale. Through isotopic substitution, the primary mechanism on the S(1) excited state can be assigned to an intra-molecular proton transfer, which is the same as that which has been observed in solution. However, the excited state lifetime is significantly shorter in vacuum. These similarities and differences are discussed in terms of recent theoretical investigations of the S(1) excited state of indigo.

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

Many properties of isolated multiply-charged anions (MCAs) are dictated by the strong intra-molecular Coulomb interactions that are present. The most striking property of MCAs is a long-range repulsive Coulomb barrier (RCB) that arises from the repulsive interaction between an electron and an anion which must be overcome to form a MCA. Excited states provide a route to probing this RCB and the focus of this Perspective is on recent photoelectron experiments, including angularly and temporally resolved, that have provided detailed physical insight into the RCB surfaces, their anisotropy, and their use to monitor molecular dynamics in real-time. An outlook provides some future prospects that studies on MCAs provide in terms of monitoring structural, charge-migration, and solvation dynamics.

We demonstrate the spatial separation of the cis- and trans-conformers of 3-fluorophenol in the gas phase based on their distinct electric dipole moments. For both conformers we create very polar samples of their lowest-energy rotational quantum states. A >95 % pure beam of trans-3-fluorophenol and a >90 % pure beam of the lowest-energy rotational states of the less polar cis-3-fluorophenol were obtained for helium and neon supersonic expansions, respectively. This is the first demonstration of the spatial separation of the lowest-energy rotational states of the least polar conformer, which is necessary for strong alignment and orientation of all individual conformers., 5 pages, 5 figures

The photoelectron imaging of the indigo carmine dianion is used to demonstrate the effects of resonance excitation, pulse duration and pulse intensity on the photoelectron spectra and angular distributions of a dianion. Excitation of the S1 state leads to an aligned distribution of excited state dianions. The photoelectron angular distribution following subsequent photodetachment within a femtosecond laser pulse is primarily determined by the repulsive Coulomb barrier. Extending the timescale for photodetachment to nanoseconds leads to dramatic changes in both the spectral and angular distributions. These observations are explained in terms of statistical detachment of electrons, either from the monoanion, or from the ground state of the dianion following a number of photon cycles through the S1 ← S0 transition. At high intensity, new electron emission channels open up, leading to emission below the repulsive Coulomb barrier. This has been assigned to strong-field induced detachment and the effect of an electric field on the Coulomb barrier is discussed in terms of the photoelectron spectra and angular distributions.

A photoelectron imaging study of the gas-phase dianion of pyrromethene-556 is presented. The photoelectron spectra and angular distributions following resonant excitation of the S1 excited state with nanosecond and femtosecond laser pulses are compared, and the influence of the repulsive Coulomb barrier (RCB) in both cases evaluated. Photoelectron angular distributions show the effect of molecular alignment due to an allowed electronic excitation and can be understood qualitatively based on the calculated RCB surface using the Local Static Approximation. Comparison between femtosecond and nanosecond excitation reveals marked differences in the photoelectron spectra. While femtosecond experiments confirm that tunneling through the RCB is adiabatic, nanosecond experiments show a broad photoelectron feature peaking near the RCB maximum. This is explained in terms of the lifetime of internal conversion, which has been determined by time-resolved photoelectron spectroscopy to be ∼120 ps: as this is faster than the nanosecond laser pulses, multiple photons can be absorbed through the S1 ← S0 transition which leads to large amounts of internal energy and enables electron detachment directly above the RCB. Fragmentation and detachment from the monoanion are also inferred by the presence of photoelectrons emitted at very low kinetic energy. Our results highlight the difficulty in interpreting photoelectron spectra of polyanions in which a resonant state is excited.