Effective attenuation length of an electron in liquid water between 10 and 600 eV

Aqueous Ammonium Nitrate Investigated Using Photoelectron Spectroscopy of Cylindrical and Flat Liquid Jets

Ammonium nitrate in aqueous solution was investigated with synchrotron radiation based photoelectron spectroscopy using two types of liquid jet nozzles. Electron emission from a cylindrical microjet of aqueous ammonium nitrate solution was measured at two different angles relative to the horizontal polarization of the incident synchrotron radiation, 90° and 54.7° (the "magic angle"), for a range of photon energies (470-530 eV). We obtained β parameter values as a function of photon energy, based on a normalization procedure relying on simulations of background intensity with the SESSA (Simulation of Electron Spectra for Surface Analysis) package. The β values are similar to literature data for O 1s ionization of liquid water, and the β value of N 1s from NH4+ is higher than that for NO3-, by ≈0.1. The measurements also show that the photoelectron signal from NO3- exhibits a photon energy dependent cross section variation not observed in NH4+. Additional measurements using a flat jet nozzle found that the ammonium and nitrate peak area ratio was unaffected by changes in the takeoff angle, indicating a similar distribution of both ammonium and nitrate in the surface region.

Generation of sub-10-fs deep and extreme ultraviolet pulses for time-resolved photoemission spectroscopy

We present a light source capable of generating sub-10-fs deep UV (DUV) and extreme UV (EUV) pulses for use in time-resolved photoemission spectroscopy. The fundamental output of a Ti:sapphire laser was compressed using the multi-plate method and mixed with the uncompressed second harmonic in a filamentation four-wave mixing process to generate sub-10-fs DUV pulses. Sub-10-fs EUV pulses were generated via high-order harmonic generation driven by the second harmonic pulses that were compressed using Ar gas and chirped mirrors. The minimum cross correlation time between 267 and 57 nm (corresponding to 21.7 eV) was measured to be 10.6 ± 0.4 fs.

The Surface of Ionic Liquids in Water: From an Ionic Tug of War to a Quasi-Ordered Two-Dimensional Layer

The sustainable development encompasses the search for new materials for energy storage, gas capture, separation, and solvents in industrial processes that can substitute conventional ones in an efficient and clean manner. Ionic liquids (ILs) emerged and have been advanced as alternative materials for such applications, but an obstacle is their hygroscopicity and the effects on their physical properties in the presence of humidity. Several industrial processes depend on the aqueous interfacial properties, and the main focus of this work is the water/IL interface. The behavior of the aqueous ionic liquids at the water-vacuum interface is representative for their water interfacial properties. Using X-ray photoelectron spectroscopy in combination with molecular dynamics simulations we investigate four aqueous IL systems, and provide molecular level insight on the interfacial behaviour of the ionic liquids, such as ion-pair formation, orientation and surface concentration. We find that ionic liquids containing a chloride anion have a lowered surface enrichment due to the low surface propensity of chloride. In contrast, the ionic liquids containing a bistriflimide anion are extremely surface-enriched due to cooperative surface propensity between the cations and anions, forming a two-dimensional ionic liquid on the water surface at low concentrations.

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.

Inversion model for extracting chemically resolved depth profiles across liquid interfaces of various configurations from XPS data: PROPHESY

PROPHESY, a technique for the reconstruction of surface-depth profiles from X-ray photoelectron spectroscopy data, is introduced. The inversion methodology is based on a Bayesian framework and primal-dual convex optimization. The acquisition model is developed for several geometries representing different sample types: plane (bulk sample), cylinder (liquid microjet) and sphere (droplet). The methodology is tested and characterized with respect to simulated data as a proof of concept. Possible limitations of the method due to uncertainty in the attenuation length of the photo-emitted electron are illustrated.

Ångstrom-Depth Resolution with Chemical Specificity at the Liquid-Vapor Interface

The determination of depth profiles across interfaces is of primary importance in many scientific and technological areas. Photoemission spectroscopy is in principle well suited for this purpose, yet a quantitative implementation for investigations of liquid-vapor interfaces is hindered by the lack of understanding of electron-scattering processes in liquids. Previous studies have shown, however, that core-level photoelectron angular distributions (PADs) are altered by depth-dependent elastic electron scattering and can, thus, reveal information on the depth distribution of species across the interface. Here, we explore this concept further and show that the experimental anisotropy parameter characterizing the PAD scales linearly with the average distance of atoms along the surface normal obtained by molecular dynamics simulations. This behavior can be accounted for in the low-collision-number regime. We also show that results for different atomic species can be compared on the same length scale. We demonstrate that atoms separated by about 1 Å along the surface normal can be clearly distinguished with this method, achieving excellent depth resolution.

Improved Chiral Photoelectron Spectroscopy via Selection of Chirality-Selective Molecular Axis Orientations: A Theoretical Analysis of Non-Negative Smooth Functions

Photoelectron spectroscopy for chiral discrimination is routinely performed for low photoelectron kinetic energies (PKEs), whereas it is considered impossible for high PKEs. We demonstrate theoretically that chiral photoelectron spectroscopy for high PKEs is possible using chirality-selective molecular orientation. The photoelectron angular distribution associated with one-photon ionization by unpolarized light can be characterized by a single parameter, β. We show that most other anisotropy parameters are zero when β is 2, as is often the case in the high PKEs. Exceptionally, odd-order anisotropy parameters are increased by a factor of 20 by orientation, even for high PKEs.

Core-Level Photoelectron Angular Distributions at the Liquid-Vapor Interface

ConspectusPhotoelectron spectroscopy (PES) is a powerful tool for the investigation of liquid-vapor interfaces, with applications in many fields from environmental chemistry to fundamental physics. Among the aspects that have been addressed with PES is the question of how molecules and ions arrange and distribute themselves within the interface, that is, the first few nanometers into solution. This information is of crucial importance, for instance, for atmospheric chemistry, to determine which species are exposed in what concentration to the gas-phase environment. Other topics of interest include the surface propensity of surfactants, their tendency for orientation and self-assembly, as well as ion double layers beneath the liquid-vapor interface. The chemical specificity and surface sensitivity of PES make it in principle well suited for this endeavor. Ideally, one would want to access complete atomic-density distributions along the surface normal, which, however, is difficult to achieve experimentally for reasons to be outlined in this Account. A major complication is the lack of accurate information on electron transport and scattering properties, especially in the kinetic-energy regime below 100 eV, a pre-requisite to retrieving the depth information contained in photoelectron signals.In this Account, we discuss the measurement of the photoelectron angular distributions (PADs) as a way to obtain depth information. Photoelectrons scatter with a certain probability when moving through the bulk liquid before being expelled into a vacuum. Elastic scattering changes the electron direction without a change in the electron kinetic energy, in contrast to inelastic scattering. Random elastic-scattering events usually lead to a reduction of the measured anisotropy as compared to the initial, that is, nascent PAD. This effect that would be considered parasitic when attempting to retrieve information on photoionization dynamics from nascent liquid-phase PADs can be turned into a powerful tool to access information on elastic scattering, and hence probing depth, by measuring core-level PADs. Core-level PADs are relatively unaffected by effects other than elastic scattering, such as orbital character changes due to solvation. By comparing a molecule's gas-phase angular anisotropy, assumed to represent the nascent PAD, with its liquid-phase anisotropy, one can estimate the magnitude of elastic versus inelastic scattering experienced by photoelectrons on their way to the surface from the site at which they were generated. Scattering events increase with increasing depth into solution, and thus it is possible to correlate the observed reduction in angular anisotropy with the depth below the surface along the surface normal.We will showcase this approach for a few examples. In particular, our recent works on surfactant molecules demonstrated that one can indeed probe atomic distances within these molecules with a high sensitivity of ∼1 Å resolution along the surface normal. We were also able to show that the anisotropy reduction scales linearly with the distance along the surface normal within certain limits. The limits and prospects of this technique are discussed at the end, with a focus on possible future applications, including depth profiling at solid-vapor interfaces.

Cellular dosimetry of 197Hg, 197mHg and 111In: comparison of dose deposition and identification of the cell and nuclear membrane as important targets

PURPOSE

To examine the reliability to model cellular S-values for the Auger electron (AE) emitters, ¹¹¹In, ¹⁹⁷Hg and ^197mHg with MCNP6 and their relative dose deposition in subcellular targets.

METHODS

A model cell was defined as four concentric spheres consisting of the nucleus (N), cytoplasm (Cy), cell and nuclear membranes (CM, NM) in which radionuclides distributed homogeneously. The transport of AE, conversion electrons and photons were simulated by MCNP6 to calculate cellular S values (S_N←CM, S_N←Cy, S_N←NM, S_N←N, S_CM←CM, S_NM←NM). S_N←CM, S_N←Cy and S_N←N were also calculated with MIRDcell.

RESULTS

MIRDcell and MCNP6-calculated S_N←N were in excellent agreement, but a slight discrepancy on S_N←Cy and S_N←CM was observed. The ratios of S_CM←CM or S_NM←NM vs. S_N←N were 9.7-51.0 or 10.5-37.4, 7.9-41.8 or 8.4-31.8 and 7.2-36.9 or 8.0-28.1 for ¹¹¹In, ¹⁹⁷Hg, ^197mHg, respectively. The mean S(¹⁹⁷Hg)/S(¹¹¹In) and S(^197mHg)/S(¹¹¹In) were 2.5 ± 0.5 and 2.5 ± 0.6, respectively.

CONCLUSIONS

Cellular S-values were reliably calculated with MCNP6. ¹⁹⁷Hg and ^197mHg deposit two-fold more doses than ¹¹¹In at the subcellular scale. All AE emitters deposit a higher self-dose in the CM and NM than in the N, which warrants studies on the effects of targeting the CM and NM by AE emitters.

Absolute Electronic Energetics and Quantitative Work Functions of Liquids from Photoelectron Spectroscopy

Liquid-jet photoelectron spectroscopy (LJ-PES) enabled a breakthrough in the experimental study of the electronic structure of liquid water, aqueous solutions, and volatile liquids more generally. The novelty of this technique, dating back over 25 years, lies in stabilizing a continuous, micron-diameter LJ in a vacuum environment to enable PES studies. A key quantity in PES is the most probable energy associated with vertical promotion of an electron into vacuum: the vertical ionization energy, VIE, for neutrals and cations, or vertical detachment energy, VDE, for anions. These quantities can be used to identify species, their chemical states and bonding environments, and their structural properties in solution. The ability to accurately measure VIEs and VDEs is correspondingly crucial. An associated principal challenge is the determination of these quantities with respect to well-defined energy references. Only with recently developed methods are such measurements routinely and generally viable for liquids. Practically, these methods involve the application of condensed-matter concepts to the acquisition of photoelectron (PE) spectra from liquid samples, rather than solely relying on molecular-physics treatments that have been commonly implemented since the first LJ-PES experiments. This includes explicit consideration of the traversal of electrons to and through the liquid's surface, prior to free-electron detection. Our approach to measuring VIEs and VDEs with respect to the liquid vacuum level specifically involves detecting the lowest-energy electrons emitted from the sample, which have barely enough energy to surmount the surface potential and accumulate in the low-energy tail of the liquid-phase spectrum. By applying a sufficient bias potential to the liquid sample, this low-energy spectral tail can generally be exposed, with its sharp, low-energy cutoff revealing the genuine kinetic-energy-zero in a measured spectrum, independent of any perturbing intrinsic or extrinsic potentials in the experiment. Together with a precisely known ionizing photon energy, this feature enables the straightforward determination of VIEs or VDEs, with respect to the liquid-phase vacuum level, from any PE feature of interest. Furthermore, by additionally determining solution-phase VIEs and VDEs with respect to the common equilibrated energy level in condensed matter, the Fermi level─the generally implemented reference energy in solid-state PES─solution work functions, eΦ, and liquid-vacuum surface dipole effects can be quantified. With LJs, the Fermi level can only be properly accessed by controlling unwanted surface charging and all other extrinsic potentials, which lead to energy shifts of all PE features and preclude access to accurate electronic energetics. More specifically, conditions must be engineered to minimize all undesirable potentials, while maintaining the equilibrated, intrinsic (contact) potential difference between the sample and apparatus. The establishment of these liquid-phase, accurate energy-referencing protocols importantly enables VIE and VDE determinations from near-arbitrary solutions and the quantitative distinction between bulk electronic structure and interfacial effects. We will review and exemplify these protocols for liquid water and several exemplary aqueous solutions here, with a focus on the lowest-ionization- or lowest-detachment-energy PE peaks, which importantly relate to the oxidative stabilities of aqueous-phase species.

Electron Impact with the Liquid-Vapor Interface

Reaction dynamics in the liquid-vapor interface is one of the crucial physical sciences but is still starving for in-depth exploration. It is challenging to selectively detect the interfacial species or the yields of chemical reaction therein, meanwhile shielding or reducing the interference from the vapor and liquid bulk. Mass spectrometry is a straightforward method but is also frustrated in such a selective detection. Using a liquid microjet in combination with a pulsed electron beam, a linear time-of-flight mass spectrometer, and a quadrupole mass filter, we recently innovated time-delayed mass spectrometry for investigations of the liquid-vapor interface. In this Account, we illustrate how this unique method succeeds in disentangling different sources, i.e., the vapor and liquid-vapor interface, of the ionic yields of the electron impacts with a liquid beam of alcohol in vacuum. These achievements are basically attributed to the application of an onion-peeling strategy in the ion detection. Concretely, the microsecond time scale of molecular volatilization can be resolved well by tuning the delay time between the nanosecond pulses of incident electron bunch and ion attractor. First, the specific orientation of the interfacial molecule, i.e., a well-known fact about the hydrophobic hydrocarbon groups pointing outside the liquid surface of alcohol, is validated again. More importantly, the dynamic features of time-delayed mass spectra, in particular, for the ionic yields from the liquid-vapor interface, are rationalized explicitly. Moreover, we demonstrate evidence of in situ molecular dimers in the liquid-vapor interface of 1-propanol. As the first example of electron-induced reaction in the liquid-vapor interface, dimethyl ether can be synthesized in the liquid methanol interface due to local interfacial acidification by high-energy electron impacts. On the contrary, the low energy electron can lead to local basicity through dissociative electron attachment (DEA). Besides the primary low-energy electrons, the low-energy secondary and inelastically scattered electrons in the higher-energy impacts of the primary electrons can also participate in the DEA process. In contrast to the gas- or solid-phase DEAs, that in the liquid-vapor interface shows distinct differences in both the types and efficiencies of anionic products. With these and efforts in the future, we develop a molecular-level understanding of how the chemical reactions happen in the liquid-vapor interface.

Probing the molecular structure of aqueous triiodide via X-ray photoelectron spectroscopy and correlated electron phenomena

Liquid-microjet-based X-ray photoelectron spectroscopy was applied to aqueous triiodide solutions, I₃^-_(aq.), to investigate the anion's valence- and core-level electronic structure, ionization dynamics, associated electron-correlation effects, and nuclear geometric structure. The roles of multi-active-electron (shake-up) ionization processes - with noted sensitivity to the solute geometric structure - were investigated through I₃^-_(aq.) solution valence, I 4d, and I 3d core-level measurements. The experimental spectra were interpreted with the aid of simulated photoelectron spectra, built upon multi-reference ab initio electronic structure calculations associated with different I₃^-_(aq.) molecular geometries. A comparison of the single-to-multi-active-electron ionization signal ratios extracted from the experimental and theoretical core-level photoemission spectra suggests that the ground state of the solute adopts a near-linear average geometry in aqueous solutions. This contrasts with the interpretation of time-resolved X-ray solution scattering studies, but is found to be fully consistent with the rest of the solution-phase I₃^-_(aq.) literature. Comparing the results of low- and high-photon-energy photoemission measurements, we further suggest that the aqueous anion adopts a more asymmetric geometry at the aqueous-solution-gas interface than in the aqueous bulk.

Photoionization of the aqueous phase: clusters, droplets and liquid jets

This perspective article reviews specific challenges associated with photoemission spectroscopy of bulk liquid water, aqueous solutions, water droplets and water clusters. The main focus lies on retrieving accurate energetics and photoelectron angular information from measured photoemission spectra, and on the question how these quantities differ in different aqueous environments. Measured photoelectron band shapes, vertical binding energies (ionization energies), and photoelectron angular distributions are influenced by various phenomena. We discuss the influences of multiple energy-dependent electron scattering in aqueous environments, and we discuss different energy referencing methods, including the application of a bias voltage to access absolute energetics of solvent and solute. Recommendations how to account for or minimize the influence of electron scattering are provided. The example of the hydrated electron in different aqueous environments illustrates how one can account for electron scattering, while reliable methods addressing parasitic potentials and proper energy referencing are demonstrated for ionization from the outermost valence orbital of neat liquid water.

This perspective article reviews specific challenges associated with photoemission spectroscopy of bulk liquid water, aqueous solutions, water droplets and water clusters.

Evaporation and Molecular Beam Scattering from a Flat Liquid Jet

An experimental setup for molecular beam scattering from flat liquid sheets has been developed, with the goal of studying reactions at gas-liquid interfaces for volatile liquids. Specifically, a crossed molecular beam instrument that can measure angular and translational energy distributions of scattered products has been adapted for liquid jet scattering. A microfluidic chip is used to create a stable flat liquid sheet inside vacuum from which scattering occurs, and both evaporation and scattering from this sheet are characterized using a rotatable mass spectrometer that can measure product time-of-flight distributions. This article describes the instrument and reports on the first measurements of evaporation of dodecane and Ne from a Ne-doped dodecane flat jet, as well as scattering of Ne from a flat jet of pure dodecane.

Electron-induced fragmentation of water droplets: Simulation study

The transport of free electrons in a water environment is still poorly understood. We show that additional insight can be brought about by investigating fragmentation patterns of finite-size particles upon electron impact ionization. We have developed a composite protocol aiming to simulate fragmentation of water clusters by electrons with kinetic energies in the range of up to 100 eV. The ionization events for atomistically described molecular clusters are identified by a kinetic Monte Carlo procedure. We subsequently model the fragmentation with classical molecular dynamics simulations, calibrated by non-adiabatic quantum mechanics/molecular mechanics simulations of the ionization process. We consider one-electron ionizations, energy transfer via electronic excitation events, elastic scattering, and also the autoionization events through intermolecular Coulombic decay. The simulations reveal that larger water clusters are often ionized repeatedly, which is the cause of substantial fragmentation. After losing most of its energy, low-energy electrons further contribute to fragmentation by electronic excitations. The simultaneous measurement of cluster size distribution before and after the ionization represents a sensitive measure of the energy transferred into the system by an incident electron.

Time-resolved photoelectron spectroscopy: the continuing evolution of a mature technique

Time-resolved photoelectron spectroscopy (TRPES) has become one of the most widespread techniques for probing nonadiabatic dynamics in the excited electronic states of molecules. Furthermore, the complementary development of ab initio approaches for the simulation of TRPES signals has enabled the interpretation of these transient spectra in terms of underlying coupled electronic-nuclear dynamics. In this perspective, we discuss the current state-of-the-art approaches, including efforts to push femtosecond pulses into vacuum ultraviolet and soft X-ray regimes as well as the utilization of novel polarizations to use time-resolved optical activity as a probe of nonadiabatic dynamics. We close this perspective with a forward-looking prospectus on the new areas of application for this technique.

Low-energy electron distributions from the photoionization of liquid water: a sensitive test of electron mean-free paths

The availability of accurate mean free paths for slow electrons (<50 eV) in water is central to the understanding of many electron-driven processes in aqueous solutions, but their determination poses major challenges to experiment and theory alike. Here, we describe a joint experimental and theoretical study demonstrating a novel approach for testing, and, in the future, refining such mean free paths. We report the development of Monte-Carlo electron-trajectory simulations including elastic and inelastic electron scattering, as well as energy loss and secondary-electron production to predict complete photoelectron spectra of liquid water. These simulations are compared to a new set of photoelectron spectra of a liquid-water microjet recorded over a broad range of photon energies in the extreme ultraviolet (20–57 eV). Several previously published sets of scattering parameters are investigated, providing direct and intuitive insights on how they influence the shape of the low-energy electron spectra. A pronounced sensitivity to the escape barrier is also demonstrated. These simulations considerably advance our understanding of the origin of the prominent low-energy electron distributions in photoelectron spectra of liquid water and clarify the influence of scattering parameters and the escape barrier on their shape. They moreover describe the reshaping and displacement of low-energy photoelectron bands caused by vibrationally inelastic scattering. Our work provides a quantitative basis for the interpretation of the complete photoelectron spectra of liquids and opens the path to fully predictive simulations of low-energy scattering in liquid water.

Our study reveals the detailed influence of elastic and inelastic mean-free paths on the complete photoelectron spectra of liquid water, including the low-energy electron distributions and the reshaping of the primary photoelectron bands.

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⁻ 5p binding energy. For NaI_(aq), an almost identical I⁻ 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 interactions.

Significant differences are observed in liquid-water's lowest electron binding energy with increasing solute concentration in archetypal aqueous solutions. For NaI_(aq) and TBAI_(aq), the energy changes extend to +0.3 eV and −0.7 eV, respectively.

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 processes.

A generalised liquid-phase photoelectron spectroscopy approach is reported, allowing accurate, absolute energy scale ionisation energies of liquid water and aqueous solutions, as well as liquid water's work function to be reported.

Probing Interfacial Effects on Ionization Energies: The Surprising Banality of Anion-Water Hydrogen Bonding at the Air/Water Interface

Liquid microjet photoelectron spectroscopy is an increasingly common technique to measure vertical ionization energies (VIEs) of aqueous solutes, but the interpretation of these experiments is subject to questions regarding sensitivity to bulk versus interfacial solvation environments. We have computed aqueous-phase VIEs for a set of inorganic anions, using a combination of molecular dynamics simulations and electronic structure calculations, with results that are in excellent agreement with experiment regardless of whether the simulation data are restricted to ions at the air/water interface or to those in bulk aqueous solution. Although the computed VIEs are sensitive to ion-water hydrogen bonding, we find that the short-range solvation structure is sufficiently similar in both environments that it proves impossible to discriminate between the two on the basis of the VIE, a conclusion that has important implications for the interpretation of liquid-phase photoelectron spectroscopy. More generally, analysis of the simulation data suggests that the surface activity of soft anions is largely a second or third solvation shell effect, arising from disruption of water-water hydrogen bonds and not from significant changes in first-shell anion-water hydrogen bonding.

Mean Free Paths and Cross Sections for Electron Scattering from Liquid Water

Electron collision with liquid water is theoretically investigated and reported in this article. The range of projectile energy considered is 10-5000 eV, covering all major channels, viz., ionization, inelastic, elastic, and total scattering. The liquid phase electron charge density and static potential are generated and used in the calculation under a spherical complex optical potential formalism to achieve the goals. For the ionization channel, the complex scattering potential-ionization contribution method is used. The agreement with available theoretical data is satisfactory. The study on the total electron scattering from liquid water, using a common method for elastic and inelastic cross sections, is new and requires further attempts to support the reported data.

Photoelectron Spectroscopy of Liquid Water with Tunable Extreme-Ultraviolet Radiation: Effects of Electron Scattering

We report the first systematic photoelectron measurements of the three outer-valence bands of liquid water as a function of the ionizing photon energy in the near-threshold region. We use extreme-ultraviolet (XUV) radiation tunable between ∼17.1 and 35.6 eV, obtained through monochromatization of a high-harmonic source. We show that the absolute values of the apparent vertical ionization energies and their respective peak widths show a decreasing trend of their magnitudes with increasing photon energy close to the ionization threshold. We find that the observed effects do not only depend on the electron kinetic energy but are also different for the various outer-valence bands. These observations are consistent with, but not fully explained by, the effects of inelastic electron scattering.

Core level photoelectron spectroscopy of heterogeneous reactions at liquid-vapor interfaces: Current status, challenges, and prospects

Liquid-vapor interfaces, particularly those between aqueous solutions and air, drive numerous important chemical and physical processes in the atmosphere and in the environment. X-ray photoelectron spectroscopy is an excellent method for the investigation of these interfaces due to its surface sensitivity, elemental and chemical specificity, and the possibility to obtain information on the depth distribution of solute and solvent species in the interfacial region. In this Perspective, we review the progress that was made in this field over the past decades and discuss the challenges that need to be overcome for investigations of heterogeneous reactions at liquid-vapor interfaces under close-to-realistic environmental conditions. We close with an outlook on where some of the most exciting and promising developments might lie in this field.

The effect of relative humidity on CaCl2 nanoparticles studied by soft X-ray absorption spectroscopy

Ca- and Cl-containing nanoparticles are common in atmosphere, originating for example from desert dust and sea water. The properties and effects on atmospheric processes of these aerosol particles depend on the relative humidity (RH) as they are often both hygroscopic and deliquescent. We present here a study of surface structure of free-flying CaCl₂ nanoparticles (CaCl₂-NPs) in the 100 nm size regime prepared at different humidity levels (RH: 11-85%). We also created mixed nanoparticles by aerosolizing a solution of CaCl₂ and phenylalanine (Phe), which is a hydrophobic amino acid present in atmosphere. Information of hydration state of CaCl₂-NPs and production of mixed CaCl₂ + Phe nanoparticles was obtained using soft X-ray absorption spectroscopy (XAS) at Ca 2p, Cl 2p, C 1s, and O 1s edges. We also report Ca 2p and Cl 2p X-ray absorption spectra of an aqueous CaCl₂ solution. The O 1s X-ray absorption spectra measured from hydrated CaCl₂-NPs resemble liquid-like water spectrum, which is heavily influenced by the presence of ions. Core level spectra of Ca²⁺ and Cl^- ions do not show a clear dependence of % RH, indicating that the first coordination shell remains similar in all measured hydrated CaCl₂-NPs, but they differ from aqueous solution and solid CaCl₂.

The electronic structure of the aqueous permanganate ion: aqueous-phase energetics and molecular bonding studied using liquid jet photoelectron spectroscopy

Permanganate aqueous solutions, MnO4-(aq.), were studied using liquid-micro-jet-based soft X-ray non-resonant and resonant photoelectron spectroscopy to determine valence and core-level binding energies. To identify possible differences in the energetics between the aqueous bulk and the solution-gas interface, non-resonant spectra were recorded at two different probing depths. Similar experiments were performed with different counter ions, Na+ and K+, with the two solutions yielding indistinguishable anion electron binding energies. Our resonant photoelectron spectroscopy measurements, performed near the Mn LII,III- and O K-edges, selectively probed valence charge distributions between the Mn metal center, O ligands, and first solvation shell in the aqueous bulk. Associated resonantly-enhanced solute ionisation signals revealed hybridisation of the solute constituents' atomic orbitals, including the inner valence Mn 3p and O 2s. We identified intermolecular coulombic decay relaxation processes following resonant X-ray excitation of the solute that highlight valence MnO4-(aq.)-H2O(l) electronic couplings. Furthermore, our results allowed us to infer oxidative reorganisation energies of MnO4˙(aq.) and adiabatic valence ionisation energies of MnO4-(aq.), revealing the Gibbs free energy of oxidation and permitting estimation of the vertical electron affinity of MnO4˙(aq.). Finally, the Gibbs free energy of hydration of isolated MnO4- was determined. Our results and analysis allowed a near-complete binding-energy-scaled MnO4-(aq.) molecular orbital and a valence energy level diagram to be produced for the MnO4-(aq.)/MnO4˙(aq.) system. Cumulatively, our mapping of the aqueous-phase electronic structure of MnO4- is expected to contribute to a deeper understanding of the exceptional redox properties of this widely applied aqueous transition-metal complex ion.

Identifying the Molecular Orientation and Clusters in the Liquid-Vapor Interface of 1-Propanol by Time-Delayed Mass Spectrometry

Structural inhomogeneity of the liquid-vapor interface, such as the spatial orientation of molecular specific groups and the non-uniform distribution of hydrogen-bonded (HB) clusters, is crucial for understanding the physicochemical processes therein. Although the molecular orientation at the outermost layer was authenticated, to date, direct experimental evidence of the in situ existence of different-sized HB clusters, as a long-standing theoretical argument, is still lacking. Here we report time-delayed electron-impact tandem mass spectrometry, and its powerful ability to identify the local structures of the liquid-vapor interface of 1-propanol is demonstrated not only by mapping the molecular orientations both in the outermost layer and in the subsurface but also by validating the existence of the HB molecular dimers in the subsurface by detecting their protonated ions. We further distinguish two different sources of the protonated dimer: the gas-phase protonation of the neutral dimer that evaporates in advance and the time-lag evaporation of the protonated dimer produced in the subsurface. This methodology is a brand-new way to explore the microstructures and the electron-driven chemical reactions in different local regions of the liquid-vapor interface.

Attosecond spectroscopy of liquid water

Electronic dynamics in liquids are of fundamental importance, but time-resolved experiments have so far remained limited to the femtosecond time scale. We report the extension of attosecond spectroscopy to the liquid phase. We measured time delays of 50 to 70 attoseconds between the photoemission from liquid water and that from gaseous water at photon energies of 21.7 to 31.0 electron volts. These photoemission delays can be decomposed into a photoionization delay sensitive to the local environment and a delay originating from electron transport. In our experiments, the latter contribution is shown to be negligible. By referencing liquid water to gaseous water, we isolated the effect of solvation on the attosecond photoionization dynamics of water molecules. Our methods define an approach to separating bound and unbound electron dynamics from the structural response of the solvent.

Solution chemistry in the surface region of aqueous solutions

Solution chemistry is commonly regarded as the physical chemistry of reactions and chemical equilibria taking place in the bulk of a solvent, and between solutes in solution, and solids or gases in contact with the solution. Our knowledge about such reactions and equilibria in aqueous solution is very detailed such as their physico–chemical constants at varying temperature, pressure, ionic medium and strength. In this paper the solution chemistry in the surface region of aqueous solutions, down to ca. 10 Å below the water–air interface, will be discussed. In this region, the density and relative permittivity are significantly smaller than in the aqueous bulk strongly affecting the chemical behaviour of solutes. Surface sensitive X-ray spectroscopic methods have recently been applicable on liquids and solutions by use of liquid jets. This allows the investigation of the speciation of compounds present in the water–air interface and the surface region, a region hardly studied before. Speciation studies show overwhelmingly that neutral molecules are accumulated in the surface region, while charged species are depleted from it. It has been shown that the equilibria between aqueous bulk, surface region, solids and/or air are very fast allowing effective transport of chemicals over the aqueous surface region.

Photoelectron spectroscopy of large water clusters ionized by an XUV comb

Detailed knowledge about photo-induced electron dynamics in water is key to the understanding of several biological and chemical mechanisms, in particular for those resulting from ionizing radiation. Here we report a method to obtain photoelectron spectra from neutral water clusters following ionization by an extreme-ultraviolet (XUV) attosecond pulse train, representing a first step towards a time-resolved analysis. Typically, a large background signal in the experiment arises from water monomers and carrier gas used in the cluster source. We report a protocol to quantify this background in order to eliminate it from the experimental spectra. We disentangle the accumulated XUV photoionization signal into contributions from the background species and the photoelectron spectra from the clusters. This proof-of-principle study demonstrates feasibility of background free photoelectron spectra of neutral water clusters ionized by XUV combs and paves the way for the detailed time-resolved analysis of the underlying dynamics.

Electron Scattering in Liquid Water and Amorphous Ice: A Striking Resemblance

The lack of accurate low-energy electron scattering cross sections for liquid water is a substantial source of uncertainty in the modeling of radiation chemistry and biology. The use of existing amorphous ice scattering cross sections for the lack of liquid data has been discussed controversially for decades. Here, we compare experimental photoemission data of liquid water with corresponding predictions using amorphous ice cross sections, with the aim of resolving the debate regarding the difference of electron scattering in liquid water and amorphous ice. We find very similar scattering properties in the liquid and the ice for electron kinetic energies up to a few hundred electron volts. The scattering cross sections recommended here for liquid water are an extension of the amorphous ice cross sections. Within the framework of currently available experimental data, our work answers one of the most debated questions regarding electron scattering in liquid water.

Photoemission from Free Particles and Droplets

Intriguing properties of photoemission from free, unsupported particles and droplets were predicted nearly 50 years ago, though experiments were a technical challenge. The last few decades have seen a surge of research in the field, due to advances in aerosol technology (generation, characterization, and transfer into vacuum), the development of photoelectron imaging spectrometers, and advances in vacuum ultraviolet and ultrafast light sources. Particles and droplets offer several advantages for photoemission studies. For example, photoemission spectra are dependent on the particle's size, shape, and composition, providing a wealth of information that allows for the retrieval of genuine electronic properties of condensed phase. In this review, with a focus on submicrometer-sized, dielectric particles and droplets, we explain the utility of photoemission from such systems, summarize several applications from the literature, and present some thoughts on future research directions.

Alternative Approach for the Determination of Mean Free Paths of Electron Scattering in Liquid Water Based on Experimental Data

Mean free paths of low-energy electrons in liquid water are of importance for modeling many physicochemical processes, but neither theoretical predictions nor experimental results have converged for these parameters. We therefore introduce an approach to determine elastic and inelastic mean free paths (EMFP, IMFP) based on experimental data. We show that ab initio calculations of electron scattering with water clusters converge with cluster size, thus providing access to condensed-phase scattering. The results are used in Monte Carlo simulations to extract EMFP and IMFP from recent liquid-microjet experiments that determined the effective attenuation length (EAL) and the photoelectron angular distribution (PAD) following oxygen 1s-ionization of liquid water. For electron kinetic energies from 10 to 300 eV, we find that the IMFP is noticeably larger than the EAL. The EMFP is longer than that of gas-phase water and the IMFP is longer compared to latest theoretical estimations, but both EMFP and IMFP are much shorter than suggested by experimental measurements of integral cross sections for amorphous ice.

Low-Energy Electron Escape from Liquid Interfaces: Charge and Quantum Effects

The high surface sensitivity and controlled surface charge state of submicron sized droplets is exploited to study low-energy electron transport through liquid interfaces using photoelectron imaging. Already a few charges on a droplet are found to modify the photoelectron images significantly. For narrow escape barriers, the comparison with an electron scattering model reveals pronounced quantum effects in the form of above-barrier reflections at electron kinetic energies below about 1 eV. The observed susceptibility to the characteristics of the electron escape barrier might provide access to these properties for liquid interfaces, which are generally difficult to investigate.

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

Synchrotron radiation provides insight into spectroscopy and dynamics in clusters and nanoparticles.

Relaxation Dynamics and Genuine Properties of the Solvated Electron in Neutral Water Clusters

We have investigated the solvation dynamics and the genuine binding energy and photoemission anisotropy of the solvated electron in neutral water clusters with a combination of time-resolved photoelectron velocity map imaging and electron scattering simulations. The dynamics was probed with a UV probe pulse following above-band-gap excitation by an EUV pump pulse. The solvation dynamics is completed within about 2 ps. Only a single band is observed in the spectra, with no indication for isomers with distinct binding energies. Data analysis with an electron scattering model reveals a genuine binding energy in the range of 3.55-3.85 eV and a genuine anisotropy parameter in the range of 0.51-0.66 for the ground-state hydrated electron. All of these observations coincide with those for liquid bulk, which is rather unexpected for an average cluster size of 300 molecules.

Absolute cross sections for chemoradiation therapy: Damages to cisplatin-DNA complexes induced by 10 eV electrons

In chemoradiation therapy, the synergy between the radiation and the chemotherapeutic agent (CA) can result in a super-additive treatment. A priori, this increased effectiveness could be estimated from model calculations, if absolute cross sections (ACSs) involved in cellular damage are substantially higher, when the CA binds to DNA. We measure ACSs for damages induced by 10 eV electrons, when DNA binds to the CA cisplatin as in chemotherapy. At this energy, DNA is damaged essentially by the decay of core-excited transient anions into bond-breaking channels. Films of cisplatin-DNA complexes of ratio 5:1 with thicknesses 10, 15, and 20 nm were irradiated in vacuum during 5-30 s. Conformation changes were quantified by electrophoresis and yields extrapolated from exposure-response curves. Base damages (BDs) were revealed and quantified by enzymatic treatment. The ACSs were generated from these yields by two mathematical models. For 3197 base-pair plasmid DNA, ACS for single strand breaks, double strand breaks (DSBs), crosslinks, non-DSB cluster damages, and total BDs is 71 ± 2, 9.3 ± 0.4, 10.1 ± 0.3, 8.2 ± 0.3, and 115 ± 2 ×10^-15 cm², respectively. These ACSs are higher than those of nonmodified DNA by factors of 1.6 ± 0.1, 2.2 ± 0.1, 1.3 ± 0.1, 1.3 ± 0.3, and 2.1 ± 0.4, respectively. Since LEEs are produced in large quantities by radiolysis and strongly interact with biomolecules, we expect such enhancements to produce substantial additional damages in the DNA of the nucleus of cancer cells during concomitant chemoradiation therapy. The increase damage appears sufficiently large to justify more elaborate simulations, which could provide a quantitative evaluation of molecular sensitization by Pt-CAs.

Charge-Transfer-to-Solvent Reaction in a Hydrophobic Tetrabutylammonium Iodide Molecular Layer in Aqueous Solution

We present ultrafast photoelectron spectroscopy of the charge-transfer-to-solvent reaction in a segregated TBAI (tetrabutylammonium iodide) molecular layer in aqueous solution. The reaction times and electron binding energies of transient species vary with TBAI concentration from a very low value of 1 × 10^-3 mol L^-1, which is in contrast to NaI solution exhibiting no concentration (0.01-1.0 mol L^-1) dependence. The result from soft X-ray N(1s) spectroscopy indicates that the photoelectron intensity in TBAI aqueous solution is about 70 times enhanced as compared to that in NH₄Cl aqueous solution for an identical salt concentration, and TBA⁺ drags I^- to the surface region. At high TBAI concentrations, electrons released from I^- are trapped and held in the TBAI molecular layer owing to electrostatic attraction by TBA⁺.

Simulated XUV photoelectron spectra of THz-pumped liquid water

Highly intense, sub-picosecond terahertz (THz) pulses can be used to induce ultrafast temperature jumps (T-jumps) in liquid water. A supercritical state of gas-like water with liquid density is established, and the accompanying structural changes are expected to give rise to time-dependent chemical shifts. We investigate the possibility of using extreme ultraviolet photoelectron spectroscopy as a probe for ultrafast dynamics induced by sub-picosecond THz pulses of varying intensities and frequencies. To this end, we use ab initio methods to calculate photoionization cross sections and photoelectron energies of (H₂O)₂₀ clusters embedded in an aqueous environment represented by point charges. The cluster geometries are sampled from ab initio molecular dynamics simulations modeling the THz-water interactions. We find that the peaks in the valence photoelectron spectrum are shifted by up to 0.4 eV after the pump pulse and that they are broadened with respect to unheated water. The shifts can be connected to structural changes caused by the heating, but due to saturation effects they are not sensitive enough to serve as a thermometer for T-jumped water.

Do water's electrons care about electrolytes?

Ions have a profound effect on the geometrical structure of liquid water and an aqueous environment is known to change the electronic structure of ions. Here we combine photoelectron spectroscopy measurements from liquid microjets with molecular dynamical and quantum chemical calculations to address the reverse question, to what extent do ions affect the electronic structure of liquid water? We study aqueous solutions of sodium iodide (NaI) over a wide concentration range, from nearly pure water to 8 M solutions, recording spectra in the 5 to 60 eV binding energy range to include all water valence and the solute Na+ 2p, I- 4d, and I- 5p orbital ionization peaks. We observe that the electron binding energies of the solute ions change only slightly as a function of electrolyte concentration, less than 150 ± 60 meV over an ∼8 M range. Furthermore, the photoelectron spectrum of liquid water is surprisingly mildly affected as we transform the sample from a dilute aqueous salt solution to a viscous, crystalline-like phase. The most noticeable spectral changes are a negative binding energy shift of the water 1b2 ionizing transition (up to -370 ± 60 meV) and a narrowing of the flat-top shape water 3a1 ionization feature (up to 450 ± 90 meV). A novel computationally efficient technique is introduced to calculate liquid-state photoemission spectra using small clusters from molecular dynamics (MD) simulations embedded in dielectric continuum. This theoretical treatment captured the characteristic positions and structures of the aqueous photoemission peaks, reproducing the experimentally observed narrowing of the water 3a1 feature and weak sensitivity of the water binding energies to electrolyte concentration. The calculations allowed us to attribute the small binding energy shifts to ion-induced disruptions of intermolecular electronic interactions. Furthermore, they demonstrate the importance of considering concentration-dependent screening lengths for a correct description of the electronic structure of solvated systems. Accounting for electronic screening, the calculations highlight the minimal effect of electrolyte concentration on the 1b1 binding energy reference, in accord with the experiments. This leads us to a key finding that the isolated, lowest-binding-energy, 1b1, photoemission feature of liquid water is a robust energetic reference for aqueous liquid microjet photoemission studies.

Absolute cross section for DNA damage induced by low-energy (10 eV) electrons: Experimental refinements and sample characterization by AFM

This work describes multiple experimental improvements for measuring absolute cross sections of DNA damage induced by low-energy electrons in nanometer-thick films in vacuum. Measurements of such cross sections are particularly sensitive to film thickness and uniformity. Using atomic force microscopy in 70% ethanol, we present a novel and effective method to determine plasmid DNA film thickness and uniformity that combines height histograms and force-distance curves. We also investigate film deposition with DNA intercalated with 1,3-diaminopropane (Dap) on tantalum-coated substrates as a convenient and cost-effective alternative to the previously-used graphite substrate. The tantalum substrate permits deposition of films very similar to those formed on graphite. Using these refinements and further optimizations of the experimental procedure, we measure an absolute cross section of (7.4 ± 2.3) × 10^-18 cm² per nucleotide for conformational damage to a 3197 base-pair plasmid, induced by 10 eV electrons, which we believe should be considered as a reference value.

Time-delayed mass spectrometry of the low-energy electron impact with a liquid beam surface

We set up an experimental apparatus to investigate the low-energy electron impact with a liquid beam surface, in which a cylindrical liquid beam with a diameter of 25 μm emits as the laminar flow from a microjet and the positively charged ions produced by the electron-impact ionizations are detected with a linear time-of-flight mass spectrometer. We propose a time-delayed mass spectrometry for this apparatus to identify the cationic fragments produced on the liquid surface, in which the application of the ion extracting pulse is delayed with different time intervals after the electron beam pulse. Sensitivity and specificity of the present methodology are demonstrated by the combinational experiments of the gas-phase and liquid ethanol. In comparison with the gas-phase experiments, the ion peaks become much broader in the mass spectra of the liquid beam, primarily due to the molecular evaporation and diffusion. After delaying with about 2 μs, we find that the hydrocarbon ions are ultimately the predominant products in the mass spectra of the liquid ethanol and they are proposed to be produced on the liquid surface. Above observations are in line with the widely accepted picture of the molecular orientation on the liquid surface; namely, the ethanol's CH₃-CH₂- group on the liquid surface prefers to be oriented outside. Therefore, we demonstrate a new mass spectrometry to explore the molecular structures of the liquid surface.

Electron scattering in large water clusters from photoelectron imaging with high harmonic radiation

Low-energy electron scattering in water clusters (H2O)n with average cluster sizes of n < 700 is investigated by angle-resolved photoelectron spectroscopy using high harmonic radiation at photon energies of 14.0, 20.3, and 26.5 eV for ionization from the three outermost valence orbitals. The measurements probe the evolution of the photoelectron anisotropy parameter β as a function of cluster size. A remarkably steep decrease of β with increasing cluster size is observed, which for the largest clusters reaches liquid bulk values. Detailed electron scattering calculations reveal that neither gas nor condensed phase scattering can explain the cluster data. Qualitative agreement between experiment and simulations is obtained with scattering calculations that treat cluster scattering as an intermediate case between gas and condensed phase scattering.

Molecular species forming at the α-Fe2O3 nanoparticle-aqueous solution interface

We report on electronic structure measurements of the interface between hematite nanoparticles (6 nm diameter) and aqueous solutions. Using soft X-ray photoelectron spectroscopy from a liquid microjet we detect valence and core-level photoelectrons as well as Auger electrons from liquid water, from the nanoparticle-water interface, and from the interior of the aqueous-phase nanoparticles. Most noteworthy, the method is shown to be sufficiently sensitive for the detection of adsorbed hydroxyl species, resulting from H2O dissociation at the nanoparticle surface in aqueous solution. We obtain signal from surface OH from resonant, non-resonant, and from so-called partial-electron-yield X-ray absorption (PEY-XA) spectra. In addition, we report resonant photoelectron measurements at the iron 2p excitation. The respective Fe iron 2p3/2 edge (L3-edge) PEY-XA spectra exhibit two main absorption peaks with their energies being sensitive to the chemical environment of the Fe3+ ions at the nanoparticle-solution interface. This manifests in the 10Dq value which is a measure of the ligand-field strength. Furthermore, an observed intensity variation of the pre-peak, when comparing the PEY-XA spectra for different iron Auger-decay channels, can be assigned to different extents of electron delocalization. From the experimental fraction of local versus non-local autoionization signals we then find a very fast, approximately 1 fs, charge transfer time from interfacial Fe3+ into the environment. The present study, which is complementary to ambient-pressure photoemission studies on solid-electrolyte systems, also highlights the multiple aspects of photoemission that need to be explored for a full characterization of the transition-metal-oxide nanoparticle surface in aqueous phase.