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Conspectus: Photoelectron 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.

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.

High-resolution photoelectron (PE) spectra of liquid methanol and ethanol were measured using a liquid microjet and He IIα radiation (40.813 eV). The vertical ionization energy and the ionization threshold were determined as 9.70 ± 0.07 and 8.69 ± 0.07 eV for methanol and 9.52 ± 0.07 and 8.52 ± 0.07 eV for ethanol, respectively. Individual photoemission bands observed for the liquids are well correlated with those in PE spectra of the gaseous samples also measured in the present study, except that the liquid band positions were shifted on average by −1.23 eV for methanol and −1.10 eV for ethanol as compared to the gas. The 5a′ and 7a′ bands of liquid methanol exhibit specifically larger broadening than other bands, for which we attempted spectral fitting with two components, similarly with the case of the 3a1 band of liquid water. PE spectra of both liquid and gaseous ethanol are congested partly due to the presence of the trans and gauche isomers; however, the overall band positions are generally in good agreement with predictions based on quantum chemical calculations. Comparison of the measured PE spectra with experimental and simulated X-ray emission spectra indicate that spectral differences in the lowest ionization band of both methanol and ethanol originate from involvement of nuclear dynamics in the X-ray emission process.

Liquid-jet photoelectron spectroscopy was applied to determine the first acid dissociation constant (pKa) of aqueous-phase glucose while simultaneously identifying the spectroscopic signature of the respective deprotonation site. Valence spectra from solutions at pH values below and above the first pKa reveal a change in glucose’s lowest ionization energy upon the deprotonation of neutral glucose and the subsequent emergence of its anionic counterpart. Site-specific insights into the solution-pH-dependent molecular structure changes are also shown to be accessible via C 1s photoelectron spectroscopy. The spectra reveal a considerably lower C 1s binding energy of the carbon site associated with the deprotonated hydroxyl group. The occurrence of photoelectron spectral fingerprints of cyclic and linear glucose prior to and upon deprotonation are also discussed. The experimental data are interpreted with the aid of electronic structure calculations. Our findings highlight the potential of liquid-jet photoelectron spectroscopy to act as a site-selective probe of the molecular structures that underpin the acid–base chemistry of polyprotic systems with relevance to environmental chemistry and biochemistry.

Photoelectron angular distributions PADs from aqueous solution surfaces reveal details of the spatial arrangement of solute molecules at the solution gas phase interface. This is demonstrated here for mixed equimolar aqueous solutions of dimethyl sulfoxide dimethyl sulfone CH3 2SO CH3 2SO2 and dimethyl sulfoxide dimethyl sulfite CH3 2SO CH3 2SO3 , all molecules having a propensity to reside near the solution surface. Although the surface active molecules coexist at the surface, CH3 2SO2 yields a more intense sulfur 2p surface photoelectron signal than CH3 2SO, and for CH3 2SO3, the effect is even larger. To understand this behavior, we have for one of the solutions mixtures, CH3 2SO CH3 2SO2, performed PAD measurements. Surprisingly, both molecules exhibit almost identical PADs, implying that the emitted photoelectrons have experienced a similar limited amount of scattering interactions. Hence, the molecules reside at the same distance with respect to the solution vacuum interface rather than CH3 2SO2 being closer to the surface than CH3 2SO, as one may have assumed based on the relative photoelectron signal intensities. Instead, the relative surface and bulk concentrations of the two compounds differ. We also report S 2p photoelectron spectra from single component dimethyl sulfide, CH3 2S, aqueous solutions measured at a single detection angle. The exceptionally large surface propensity of CH3 2S is recognized by a narrow, gas phase like photoelectron spectrum, revealing that CH3 2S experiences very few hydration interactions. Experimentally observed trends in surface activity for the different molecules, which are complemented here by molecular dynamics simulations, agree with findings obtained with other surface sensitive techniques. New information on the surface structure of mixed solutions is uniquely obtained from the anisotropic angular distributions of the photoelectrons

We report XRD and IR measurements of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI–BF4) adsorbed in activated carbons, molecular sieving carbon, and single wall carbon nanohorn, where we specifically chose a wide range of pore sizes from 0.5 nm to 2.5 nm. Electron radial distribution function analysis reveals denser packing upon adsorption in two steps, for pore widths larger and comparable to the ion size. Average ion-distance was decreased by 0.05 nm in the latter case. With support of DFT calculations we identify a suppression of specific vibrational modes, which are interpreted as constrainment by the pore walls. Possible consequences for supercapacitor application are discussed.

Recently, a new family of autoionization processes has been identified in aqueous phases. The processes are initiated by core-electron ionization of a solute molecule and involve proton transfer along the solute-solvent hydrogen bond. As a result, short-lived singly charged cations form with structures sharing a proton between solute and solvent molecules. These molecular transients decay by autoionization, which creates reactive dicationic species with the positive charges delocalized over the entire molecular entity. Here, we investigate the ultrafast electron and nuclear dynamics following the core ionization of hydrated ammonia and glycine. Both molecules serve as models for exploring the possible role of the nonlocal relaxation processes in the chemical reactivity at the interface between, for instance, a protein surface and aqueous solution. The nature of the postionization dynamical processes is revealed by high-accuracy Auger-electron spectroscopy measurements on liquid microjets in vacuum. The proton-transfer-mediated processes are identified by electron signals in the high-energy tail of the Auger spectra with no analogue in the Auger spectra of the corresponding gas-phase molecule. This high-energy tail is suppressed for deuterated molecules. Such an isotope effect is found to be smaller for aqueous ammonia as compared to the hydrated H2O molecule, wherein hydrogen bonds are strong. An even weaker hydrogen bonding for the hydrated amino groups in glycine results in a negligibly small proton transfer. The dynamical processes and species formed upon the nitrogen-1s core-level ionization are interpreted using methods of quantum chemistry and molecular dynamics. With the assistance of such calculations, we discuss the conditions for the proton-transfer-mediated relaxation processes to occur. We also consider the solvent librational dynamics as an alternative intermolecular ultrafast relaxation pathway. In addition, we provide experimental evidence for the umbrella-type motion in aqueous ammonia upon core ionization. This intramolecular channel proceeds in parallel with intermolecular relaxation processes in the solution.

The capacity and kinetics of CO2 capture of K2CO3 were studied to determine the mechanism for CO2 sequestration under ambient conditions. Bicarbonate formation of K2CO3 was examined by thermogravimetric analysis under various CO2 concentrations in the presence of water vapor, and the accompanying structural changes of K2CO3 were demonstrated by X-ray diffraction (XRD). Morphological variations were observed during the reaction in the presence of different CO2 concentrations through scanning electron microscopy (SEM). Structural changes and morphological variations, which occurred during the course of the reaction, were then connected to the kinetic and exothermic properties of the CO2 capture process from XRD and SEM measurements. The XRD results showed that the bicarbonate formation process of K2CO3 could be divided into three reactions, such as the formation of K2CO3·1.5H2O from K2CO3, the subsequent formation of K4H2(CO3)3·1.5H2O from K2CO3·1.5H2O, and the slow formation of KHCO3 from K4H2(CO3)3·1.5H2O...

Nanostructured silicon (N-Si) was prepared by anodic electroetching of p-type silicon wafers. The obtained magnetic particles were separated by a permanent neodymium magnet as a magnetic nanostructured silicon (mN-Si). The N-Si and mN-Si exhibited different magnetic properties: the N-Si exhibited ferromagnetic-like behaviour, whereas the mN-Si exhibited superparamagnetic-like behaviour.

The charge-carrier dynamics of anatase TiO2 nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of generated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300–400 fs, which we assign to the structural distortion dynamics near the surface.

Auger-electron spectroscopy is applied to hydrogen peroxide aqueous solution to identify ultrafast electronic relaxation processes, specifically those involving a proton transfer between core-ionized hydrogen peroxide and solvating water molecules (proton transfer mediated-charge separation, PTM-CS). Such processes yield dications where the two positive charges resulting from the Auger decay are delocalized over the two molecules. These species contribute to the high-energy tail of the Auger-electron spectrum as do also species resulting from charge delocalization in the ground-state geometry. However, the immediate and secondary transient species are different for ground-state and proton-transferred structures. Here we show that it is possible to experimentally distinguish the species by studying the H2O2/D2O2 isotope effect on the Auger spectra. To interpret the measured Auger-electron spectra, we complement the experiment with ab initio based dynamical calculations.

Autoionization electron spectroscopy is applied to study nonradiative relaxation processes of hydrogen peroxide aqueous solution irradiated by soft X-rays. The high kinetic energy part of the oxygen 1s H2O2(aq) Auger-electron spectrum reveals dicationic final states with considerably lower energy than for neat liquid water. Assisted by quantum chemical calculations, it is argued that such lower-energy states arise from two fundamentally different relaxation processes. One is (local) Auger decay, yielding H2O22+(aq) species, and here the low final-state energy arises from charge delocalization across the molecular O–O bond. Alternatively, nonlocal dicationic states can form, corresponding to a charge-separated complex comprising H2O2 and a neighboring water molecule. Different charge-separation mechanisms, depending on whether or not proton dynamics of the core-level excited or ionized H2O2 molecule is involved, are discussed. We also present for the first time the partial electron yield X-ray absorption s...

To understand the yield and patterns of damage in aqueous condensed matter, including biological systems, it is essential to identify the initial products subsequent to the interaction of high-energy radiation with liquid water. Until now, the observation of several fast reactions induced by energetic particles in water was not possible on their characteristic timescales. Therefore, some of the reaction intermediates involved, particularly those that require nuclear motion, were not considered when describing radiation chemistry. Here, through a combined experimental and theoretical study, we elucidate the ultrafast proton dynamics in the first few femtoseconds after X-ray core-level ionization of liquid water. We show through isotope analysis of the Auger spectra that proton-transfer dynamics occur on the same timescale as electron autoionization. Proton transfer leads to the formation of a Zundel-type intermediate [HO*···H···H2O]+, which further ionizes to form a so-far unnoticed type of dicationic charge-separated species with high internal energy. We call the process proton-transfer mediated charge separation. Previously unobserved types of reactive species formed on the core ionization of liquid water have been identified using a combination of liquid microjet photoemission spectroscopy and ab initio calculations. The charge-separated di-cationic species are formed within a few femtoseconds, through proton-transfer-mediated processes followed by autoionization.

Photoionization is at the heart of X-ray photoelectron spectroscopy (XPS), which gives access to important information on a sample's local chemical environment. Local and non-local electronic decay after photoionization-in which the refilling of core holes results in electron emission from either the initially ionized species or a neighbour, respectively-have been well studied. However, electron-transfer-mediated decay (ETMD), which involves the refilling of a core hole by an electron from a neighbouring species, has not yet been observed in condensed phase. Here we report the experimental observation of ETMD in an aqueous LiCl solution by detecting characteristic secondary low-energy electrons using liquid-microjet soft XPS. Experimental results are interpreted using molecular dynamics and high-level ab initio calculations. We show that both solvent molecules and counterions participate in the ETMD processes, and different ion associations have distinctive spectral fingerprints. Furthermore, ETMD spectra are sensitive to coordination numbers, ion-solvent distances and solvent arrangement.

The electronic structures of formic acid (HCOOH) and formate (HCOO(-)) have been determined in aqueous solutions over a pH range of 1.88-8.87 using a combination of X-ray photoelectron spectroscopy (XPS), partial electron-yield X-ray absorption spectroscopy (PEY XAS), and density functional theory (DFT). The carbon 1s XPS measurements reveal a binding energy shift of -1.3 eV for deprotonated HCOO(-) compared with neutral HCOOH. Such distinction between neutral HCOOH and deprotonated HCOO(-) cannot be made based solely on the respective carbon K-edge PEY XA spectra. Independent of pH, the C1s → π* state excitations occur at 288.0 eV and may lead to the incorrect conclusion that the energy levels of the π* state are the same for both species. The DFT calculations are consistent with the experimental observations and show a shift to higher energy for both the occupied C1s (lower binding energy) and unoccupied π* orbitals of deprotonated HCOO(-) compared to neutral HCOOH in aqueous solutions.

Aqueous solutions of ferrous and ferric iron (Fe(2+/3+)) and of the iron-hexacyano complexes [Fe(CN)(6)](4-/3-) are studied by photoelectron spectroscopy using a liquid microjet in conjunction with synchrotron soft X-rays for ionization. For Fe(2+)(aq) we observe two well-resolved peaks at 7.09 and 9.16 eV electron binding energy (BE) that can be assigned to the iron-hexaaquo complex. For Fe(3+)(aq) we observe only one peak above the highest valence band of liquid water, at 10.08 eV BE. Interpreting the spectra in terms of the one-electron levels of Kohn-Sham density functional theory, we find that the two peaks for Fe(2+)(aq) originate from the energy splitting between the highest occupied β (= minority) spin level (Fe d(t(2g))) and the five highest occupied α (= majority) spin levels (Fe d(t(2g)) and d(e(g))). The peak for Fe(3+)(aq) arises from d-levels that are strongly mixed with the solvent. The spectra of the aqueous hexacyano complexes show a single strong peak at 6.11 and 7.52 eV BE for [Fe(CN)(6)](4-) and [Fe(CN)(6)](3-), respectively, originating from the highest occupied Fe d(t(2g)) levels, and two further peaks at higher BE originating from the cyano ligands. The PE spectra of the reduced aquo and cyano ions are then used to obtain-solely on experimental grounds-values for the reorganization free energy of the oxidized ions. DFT/continuum calculations of this important parameter in the Marcus theory of oxidation reactions are in fairly good agreement with experiment.

Characterization of the structure and properties of matter would be incomplete without the detailed knowledge of electronic structure, and yet, for aqueous solutions, not even the binding energies of the valence electrons are generally known. Thus, fundamental interactions between solute electronic structure and water, essentially the key to chemical reactivity, have remained poorly understood. This work describes how, by the development of the vacuum liquid microjet technique for X-ray photoelectron spectroscopy, electronic structure measurements from aqueous solutions have advanced to date. Direct and resonant second-order electron emission processes are discussed in light of the specific electron structure information accessible from aqueous solutions. Several examples of solutes in their natural aqueous environment will be presented along with future research directions and prevailing challenges in the field.

The electronic structure of a Ti(3+) aqueous solution is studied by liquid-jet soft X-ray photoelectron (PE) spectroscopy. Measured valence and Ti 2p core-level binding energies, together with the Ti 2p resonant photoelectron (RPE) spectra and the derived partial electron-yield L-edge X-ray absorption (PEY-XA) spectra, reveal mixing between metal 3d and water orbitals. Specifically, ligand states with metal character are identified through the enhancement of signal intensities in the RPE spectra. An observed satellite 3d peak structure is assigned to several different metal-ligand states. Experimental energies and the delocalized nature of the respective orbitals are supported by ground-state electronic structure calculations. We also show that by choice of the detected Auger-electron-decay channel, from which different PEY-XA spectra are obtained, the experimental sensitivity to the interactions of the metal 3d electrons with the solvent can be varied. The effect of such a state-dependent electronic relaxation on the shape of the PEY-XA spectra is discussed in terms of different degrees of electron delocalization.

The charge-carrier dynamics of anatase TiO2 nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of generated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300–400 fs, which we assign to the structural distortion dynamics near the surface., 光触媒ナノ粒子における光照射後10兆分の1秒での電子の動きをX線自由電子レーザーで観測. 京都大学プレスリリース. 2017-07-06.

The nonradiative dark channels in the L-edge fluorescence spectra from transition-metal aqueous solution identify the ultrafast charge-transfer processes playing an important role in many biological and chemical systems. Yet, the exact origin of such spectral dips with respect to the X-ray transmission spectrum has remained unclear. In the present study we explore the nature of the underlying decay mechanism of 2p core-excited Co(2+) in water by probing the nonradiative Auger-type electron emission channel using photoelectron spectroscopy from a liquid microjet. Our measurements demonstrate unequivocally that metal-to-water charge transfer quenches fluorescence and will inevitably lead to a dip in the total-fluorescence-yield X-ray absorption spectrum. This is directly revealed from the resonant enhancement of valence signal intensity arising from the interference of two identical final states created by a direct and Auger-electron emission, respectively.

We report here on the electron binding energies and ultrafast electronic relaxation of the Fe(3+)(aq) complex in FeCl(3) aqueous solution as measured by soft X-ray photoelectron (PE) spectroscopy from a vacuum liquid microjet. Covalent mixing between the 3d valence orbitals of the iron cation and the molecular orbitals of water in the ground-state solution is directly revealed by spectroscopy of the highest partially occupied molecular orbitals. Valence PE spectra, obtained for photon energies near the iron 2p absorption edge, exhibit large resonant enhancements. These resonant PE features identify 3d-O2p transient hybridization between iron and water-derived orbitals and are an indication of charge transfer within the electronically excited Fe(3+)(aq)* complex. Charge transfer from water to iron is also revealed by the 2p core-level PE spectrum, and the asymmetric peak shape additionally identifies the characteristic multiplet interactions in the 2p core-hole state. The electronic structure of water molecules in the first hydration shell is selectively probed by Auger decay from water molecules, at excitation energies well below the O1s absorption edge of neat water. These experiments lay the groundwork for establishing resonant PE spectroscopy for the study of electronic-structure dynamics in the large family of transition metal (aqueous) solutions.

The effect of hydration on the electronic structure of H(2)O(2) is investigated by liquid-jet photoelectron spectroscopy measurements and ab initio calculations. Experimental valence electron binding energies of the H(2)O(2) orbitals in water are, on average, 1.9 eV red-shifted with respect to the gas-phase molecule. A smaller width of the first peak was observed in the photoelectron spectrum from the solution. Our experiment is complemented by simulated photoelectron spectra, calculated at the ab initio level of theory (with EOM-IP-CCSD and DFT methods), and using path-integral sampling of the ground-state density. The observed shift in ionization energy upon solvation is attributed to a combination of nonspecific electrostatic effects (long-range polarization) and of the specific interactions between H(2)O(2) and H(2)O molecules in the first solvation shell. Changes in peak widths are found to result from merging of the two lowest ionized states of H(2)O(2) in water due to conformational changes upon solvation. Hydration effects on H(2)O(2) are stronger than on the H(2)O molecule. In addition to valence spectra, we report oxygen 1s core-level photoelectron spectra from H(2)O(2)(aq), and observed energies and spectral intensities are discussed qualitatively.

X-Ray photoelectron spectroscopy has been extended to colloidal nanoparticles in aqueous solution using a liquid microjet in combination with synchrotron radiation, which allowed for depth-dependent measurements. Two distinct electronic structures are evident in the Si 2p photoelectron spectrum of 7 nm SiO(2)-nanoparticles at pH 10. A core-shell model is proposed where only the outermost layer of SiO(2) nanoparticles, which is mainly composed of deprotonated silanol groups,Si-O(-), interacts with the solution. The core of the nanoparticles is not affected by the solvation process and retains the same electronic structure as measured in vacuum. Future opportunities of this new experiment are also highlighted.

Recent advancement in quantitative liquid-jet photoelectron spectroscopy enables the accurate determination of the absolute-scale electronic energetics of liquids and species in solution. Our major objective is the determination of the absolute lowest-ionization energy of liquid water 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. Considerably different behavior is revealed: In the NaI(aq) solutions, water's 1b1 energy increases by 300 meV upon increasing the concentration near-saturation concentrations, whereas for TBAI the energy decreases by about 0.7 eV upon formation of a surface layer. The solute-induced effects on the solute binding energies are quantified as well, as inferred from concentration-dependent energy shifts of the I- 5p peak 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 this result in NaI(aq) 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) 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 3a1 orbital of liquid water which is a sensitive signature of water-water hydrogen bond interactions., Comment: 28 pages, 12 figures, including Supporting Information