A setup for studies of photoelectron circular dichroism from chiral molecules in aqueous solution

We present a unique experimental design that enables the measurement of photoelectron circular dichroism (PECD) from chiral molecules in aqueous solution. The effect is revealed from the intensity difference of photoelectron emission into a backward-scattering angle relative to the photon propagation direction when ionizing with circularly polarized light of different helicity. This leads to asymmetries (normalized intensity differences) that depend on the handedness of the chiral sample and exceed the ones in conventional dichroic mechanisms by orders of magnitude. The asymmetry is largest for photon energies within several electron volts above the ionization threshold. A primary aim is to explore the effect of hydration on PECD. The modular and flexible design of our experimental setup EASI (Electronic structure from Aqueous Solutions and Interfaces) also allows for detection of more common photoelectron angular distributions, requiring distinctively different detection geometries and typically using linearly polarized light. A microjet is used for liquid-sample delivery. We describe EASI’s technical features and present two selected experimental results, one based on synchrotron-light measurements and the other performed in the laboratory, using monochromatized He-II α radiation. The former demonstrates the principal effectiveness of PECD detection, illustrated for prototypic gas-phase fenchone. We also discuss the first data from liquid fenchone. In the second example, we present valence photoelectron spectra from liquid water and NaI aqueous solution, here obtained from a planar-surface microjet (flatjet). This new development features a more favorable symmetry for angle-dependent photoelectron measurements

Imaging of Chemical Kinetics at the Water-Water Interface in a Free-Flowing Liquid Flat-Jet

We present chemical kinetics measurements of the luminol oxidation chemiluminescence (CL) reaction at the interface between two aqueous solutions, using liquid jet technology. Free-flowing liquid microjets are a relatively recent development that have found their way into a growing number of applications in spectroscopy and dynamics. A variant thereof, called flat-jet, is obtained when two cylindrical jets of a liquid are crossed, leading to a chain of planar leaf-shaped structures of the flowing liquid. We here show that in the first leaf of this chain, the fluids do not exhibit turbulent mixing, providing a clean interface between the liquids from the impinging jets. We also show, using the example of the luminol CL reaction, how this setup can be used to obtain kinetics information from friction-less flow and by circumventing the requirement for rapid mixing by intentionally suppressing all turbulent mixing and instead relying on diffusion

Probing aqueous ions with non-local Auger relaxation

Non-local analogues of Auger decay are increasingly recognized as important relaxation processes in the condensed phase. Here, we explore non-local autoionization, specifically Intermolecular Coulombic Decay (ICD), of a series of aqueous-phase isoelectronic cations following 1s core-level ionization. In particular, we focus on Na+, Mg2+, and Al3+ ions. We unambiguously identify the ICD contribution to the K-edge Auger spectrum. The different strength of the ion-water interactions is manifested by varying intensities of the respective signals: the ICD signal intensity is greatest for the Al3+ case, weaker for Mg2+, and absent for weakly-solvent-bound Na+. With the assistance of ab initio calculations and molecular dynamics simulations, we provide a microscopic understanding of the non-local decay processes. We assign the ICD signals to decay processes ending in two-hole states, delocalized between the central ion and neighbouring water. Importantly, these processes are shown to be highly selective with respect to the promoted water solvent ionization channels. Furthermore, using a core-hole-clock analysis, the associated ICD timescales are estimated to be around 76 fs for Mg2+ and 34 fs for Al3+. Building on these results, we argue that Auger and ICD spectroscopy represents a unique tool for the exploration of intra- and inter-molecular structure in the liquid phase, simultaneously providing both structural and electronic information

Structural Investigation of Amyloid Oligomers via IM-MS and Infrared-Spectroscopy

A hallmark of the Alzheimer´s disease (AD) is the spontaneous transition of Abeta peptides from soluble, unstructured monomers into insoluble amyloid fibrils. Recent evidences increasingly suggest that not the mature fibrils but rather polydisperse Abeta 42 oligomers represent the toxic species. H/D exchange experiments, which rely on the solvent accessibility, have shown that especially the central hydrophobic core as well as the C terminus are highly involved in the aggregation cascade. Furthermore, the Abeta 42 peptide, which just differs in two additional C terminal amino acids, is more cytotoxic than the Abeta 40. A fundamental understanding of all involved structures is crucial for the development of effective drugs. Traditional condensed phase methods, however, only provide averaged information of the assembly. On the other hand, gas phase techniques are able to isolate and characterize one species in the presence of many others without affecting their underlying equilibrium. Especially, ion mobility mass spectrometry (IM MS) is routinely applied for the structural characterization of amyloid oligomers. It provides information on the ion´s shape and size and it can further serve as a filter for a subsequent analysis by orthogonal methods such as infrared (IR) spectroscopy. IR spectroscopy depends on intramolecular vibrations and is therefore highly sensitive towards the secondary structure, adopted by proteins and peptides. The combination of IM MS and IR spectroscopy therefore allows to obtain tertiary/quaternary as well as secondary structure information of individual oligomers. In this thesis, fragments derived from the central hydrophobic core and the C terminus of the Abeta peptide as well as the full length Abeta 40 and Abeta 42 peptides were investigated. The data show that the last alanine residue (Ala 42) play the most important role for the aggregation into β sheet rich aggregates. Both full length Abeta monomers and the Abeta 40 dimer adopt turn like conformations in the gas phase and therefore the characteristic transition into highly structured aggregates occurs through higher oligomers

Quantitative electronic structure and work function changes of liquid water induced by solute

Recent advancement in quantitative liquid jet photoelectron spectroscopy enables the accurate determination of the absolute scale electronic energetics of liquids and species in solution. The major objective of the present work is the determination of the absolute lowest ionization energy of liquid water, corresponding to the 1b1 orbital electron liberation, which is found to vary upon solute addition, and depends on the solute concentration. We discuss two prototypical aqueous salt solutions, NaI aq and tetrabutylammonium iodide, TBAI aq , with the latter being a strong surfactant. Our results reveal considerably different behavior of the liquid water 1b1 binding energy in each case. In the NaI aq solutions, the 1b1 energy increases by about 0.3 eV upon increasing the salt concentration from very dilute to near saturation concentrations, whereas for TBAI the energy decreases by about 0.7 eV upon formation of a TBAI surface layer. The photoelectron spectra also allow us to quantify the solute induced effects on the solute binding energies, as inferred from concentration dependent energy shifts of the I amp; 8722; 5p binding energy. For NaI aq , an almost identical I amp; 8722; 5p shift is found as for the water 1b1 binding energy, with a larger shift occurring in the opposite direction for the TBAI aq solution. We show that the evolution of the water 1b1 energy in the NaI aq solutions can be primarily assigned to a change of water s electronic structure in the solution bulk. In contrast, apparent changes of the 1b1 energy for TBAI aq solutions can be related to changes of the solution work function which could arise from surface molecular dipoles. Furthermore, for both of the solutions studied here, the measured water 1b1 binding energies can be correlated with the extensive solution molecular structure changes occurring at high salt concentrations, where in the case of NaI aq , too few water molecules exist to hydrate individual ions and the solution adopts a crystalline like phase. We also comment on the concentration dependent shape of the second, 3a1 orbital liquid water ionization feature which is a sensitive signature of water water hydrogen bond interaction

Accurate vertical ionization energy and work function determinations of liquid water and aqueous solutions

The absolute scale electronic energetics of liquid water and aqueous solutions, both in the bulk and at associated interfaces, are the central determiners of water based chemistry. However, such information is generally experimentally inaccessible. Here we demonstrate that a refined implementation of the liquid microjet photoelectron spectroscopy PES technique can be adopted to address this. Implementing concepts from condensed matter physics, we establish novel all liquid phase vacuum and equilibrated solution metal electrode Fermi level referencing procedures. This enables the precise and accurate determination of previously elusive water solvent and solute vertical ionization energies, VIEs. Notably, this includes quantification of solute induced perturbations of water s electronic energetics and VIE definition on an absolute and universal chemical potential scale. Defining and applying these procedures over a broad range of ionization energies, we accurately and respectively determine the VIE and oxidative stability of liquid water as 11.33 0.03 eV and 6.60 0.08 eV with respect to its liquid vacuum interface potential and Fermi level. Combining our referencing schemes, we accurately determine the work function of liquid water as 4.73 0.09 eV. Further, applying our novel approach to a pair of exemplary aqueous solutions, we extract absolute VIEs of aqueous iodide anions, reaffirm the robustness of liquid water s electronic structure to high bulk salt concentrations 2 M sodium iodide , and quantify reference level dependent reductions of water s VIE and a 0.48 0.13 eV contraction of the solution s work function upon partial hydration of a known surfactant 25 mM tetrabutylammonium iodide . Our combined experimental accomplishments mark a major advance in our ability to quantify electronic structure interactions and chemical reactivity in liquid water, which now explicitly extends to the measurement of absolute scale bulk and interfacial solution energetics, including those of relevance to aqueous electrochemical processe

Radiation damage by extensive local water ionization from two-step electron-transfer-mediated decay of solvated ions

Biomolecular radiation damage is largely mediated by radicals and low-energy electrons formed by water ionization rather than by direct ionization of biomolecules. It was speculated that such an extensive, localized water ionization can be caused by ultrafast processes following excitation by core-level ionization of hydrated metal ions. In this model, ions relax via a cascade of local Auger-Meitner and, importantly, non-local charge- and energy-transfer processes involving the water environment. Here, we experimentally and theoretically show that, for solvated paradigmatic intermediate-mass Al3+ ions, electronic relaxation involves two sequential solute-solvent electron transfer-mediated decay processes. The electron transfer-mediated decay steps correspond to sequential relaxation from Al5+ to Al3+ accompanied by formation of four ionized water molecules and two low-energy electrons. Such charge multiplication and the generated highly reactive species are expected to initiate cascades of radical reactions