Development of a flat jet delivery system for soft X-ray spectroscopy at MAX IV.

One of the most challenging aspects of X-ray research is the delivery of liquid sample flows into the soft X-ray beam. Currently, cylindrical microjets are the most commonly used sample injection systems for soft X-ray liquid spectroscopy. However, they suffer from several drawbacks, such as complicated geometry due to their curved surface.

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).

FlexPES: a versatile soft X-ray beamline at MAX IV Laboratory.

FlexPES is a soft X-ray beamline on the 1.5 GeV storage ring at MAX IV Laboratory, Sweden, providing horizontally polarized radiation in the 40-1500 eV photon energy range and specializing in high-resolution photoelectron spectroscopy, fast X-ray absorption spectroscopy and electron-ion/ion-ion coincidence techniques. The beamline is split into two branches currently serving three endstations, with a possibility of adding a fourth station at a free port.

Superficial Tale of Two Functional Groups: On the Surface Propensity of Aqueous Carboxylic Acids, Alkyl Amines, and Amino Acids.

The gas-liquid interface of water is environmentally relevant due to the abundance of aqueous aerosol particles in the atmosphere. Aqueous aerosols often contain a significant fraction of organics. As aerosol particles are small, surface effects are substantial but not yet well understood.

An atomistic explanation of the ethanol-water azeotrope.

Ethanol and water form an azeotropic mixture at an ethanol molecular percentage of ∼91% (∼96% by volume), which prohibits ethanol from being further purified distillation. Aqueous solutions at different concentrations in ethanol have been studied both experimentally and theoretically. We performed cylindrical micro-jet photoelectron spectroscopy, excited by synchrotron radiation, 70 eV above C1s ionization threshold, providing optimal atomic-scale surface-probing.

Ethanol in Aqueous Solution Studied by Microjet Photoelectron Spectroscopy and Theory.

By combining results and analysis from cylindrical microjet photoelectron spectroscopy (cMJ-PES) and theoretical simulations, we unravel the microscopic properties of ethanol-water solutions with respect to structure and intermolecular bonding patterns following the full concentration scale from 0 to 100% ethanol content. In particular, we highlight the salient differences between bulk and surface. Like for the pure water and alcohol constituents, alcohol-water mixtures have attracted much interest in applications of X-ray spectroscopies owing to their potential of combining electronic and geometric structure probing.

Probing aqueous ions with non-local Auger relaxation.

Non-local analogues of Auger decay are increasingly recognized as important relaxation processes in the condensed phase. Here, we explore non-local autoionization, specifically Intermolecular Coulombic Decay (ICD), of a series of aqueous-phase isoelectronic cations following 1s core-level ionization. In particular, we focus on Na, Mg, and Al ions.

Strong enrichment of atmospherically relevant organic ions at the aqueous interface: the role of ion pairing and cooperative effects.

Surface affinity, orientation and ion pairing are investigated in mixed and single solute systems of aqueous sodium hexanoate and hexylammonium chloride. The surface sensitive X-ray photoelectron spectroscopy technique has been used to acquire the experimental results, while the computational data have been calculated using molecular dynamics simulations. By comparing the single solute solutions with the mixed one, we observe a non-linear surface enrichment and reorientation of the organic ions with their alkyl chains pointing out of the aqueous surface.

Shifted equilibria of organic acids and bases in the aqueous surface region.

Acid-base equilibria of carboxylic acids and alkyl amines in the aqueous surface region were studied using surface-sensitive X-ray photoelectron spectroscopy and molecular dynamics simulations. Solutions of these organic compounds were examined as a function of pH, concentration and chain length to investigate the distribution of acid and base form in the surface region as compared to the aqueous bulk. Results from these experiments show that the neutral forms of the studied acid-base pairs are strongly enriched in the aqueous surface region.

The All-Seeing Eye of Resonant Auger Electron Spectroscopy: A Study on Aqueous Solution Using Tender X-rays.

X-ray absorption and Auger electron spectroscopies are demonstrated to be powerful tools to unravel the electronic structure of solvated ions. In this work for the first time, we use a combination of these methods in the tender X-ray regime. This allowed us to address electronic transitions from deep core levels, to probe environmental effects, specifically in the bulk of the solution since the created energetic Auger electrons possess large mean free paths, and moreover, to obtain dynamical information about the ultrafast delocalization of the core-excited electron.

Anomalous surface behavior of hydrated guanidinium ions due to ion pairing.

Surface affinity of aqueous guanidinium chloride (GdmCl) is compared to that of aqueous tetrapropylammonium chloride (TPACl) upon addition of sodium chloride (NaCl) or disodium sulfate (NaSO). The experimental results have been acquired using the surface sensitive technique X-ray photoelectron spectroscopy on a liquid jet. Molecular dynamics simulations have been used to produce radial distribution functions and surface density plots.

Ethanol Solvation in Water Studied on a Molecular Scale by Photoelectron Spectroscopy.

Because of the amphiphilic properties of alcohols, hydrophobic hydration is important in the alcohol-water system. In the present paper we employ X-ray photoelectron spectroscopy (XPS) to investigate the bulk and surface molecular structure of ethanol-water mixtures from 0.2 to 95 mol %.

Surface Partitioning in Organic-Inorganic Mixtures Contributes to the Size-Dependence of the Phase-State of Atmospheric Nanoparticles.

Atmospheric particulate matter is one of the main factors governing the Earth's radiative budget, but its exact effects on the global climate are still uncertain. Knowledge on the molecular-scale surface phenomena as well as interactions between atmospheric organic and inorganic compounds is necessary for understanding the role of airborne nanoparticles in the Earth system. In this work, surface composition of aqueous model systems containing succinic acid and sodium chloride or ammonium sulfate is determined using a novel approach combining X-ray photoelectron spectroscopy, surface tension measurements and thermodynamic modeling.

Acid-base speciation of carboxylate ions in the surface region of aqueous solutions in the presence of ammonium and aminium ions.

The acid-base speciation of surface-active carboxylate ions in the surface region of aqueous solutions was studied with synchrotron-radiation-based photoelectron spectroscopy. The protonated form was found at an extraordinarily large fraction compared to that expected from the bulk pH. When adding salts containing the weak acid NH4(+) to the solution, the fraction of the acidic form at the surface increases, and to a much greater extent than expected from the bulk pH of the solution.

Succinic acid in aqueous solution: connecting microscopic surface composition and macroscopic surface tension.

The water-vapor interface of aqueous solutions of succinic acid, where pH values and bulk concentrations were varied, has been studied using surface sensitive X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations. It was found that succinic acid has a considerably higher propensity to reside in the aqueous surface region than its deprotonated form, which is effectively depleted from the surface due to the two strongly hydrated carboxylate groups. From both XPS experiments and MD simulations a strongly increased concentration of the acid form in the surface region compared to the bulk concentration was found and quantified.

Surface behavior of hydrated guanidinium and ammonium ions: a comparative study by photoelectron spectroscopy and molecular dynamics.

Through the combination of surface sensitive photoelectron spectroscopy and molecular dynamics simulation, the relative surface propensities of guanidinium and ammonium ions in aqueous solution are characterized. The fact that the N 1s binding energies differ between these two species was exploited to monitor their relative surface concentration through their respective photoemission intensities. Aqueous solutions of ammonium and guanidinium chloride, and mixtures of these salts, have been studied in a wide concentration range, and it is found that the guanidinium ion has a greater propensity to reside at the aqueous surface than the ammonium ion.

Solvent dependence of the electronic structure of I(-) and I3(-).

We present synchrotron-based I4d photoelectron spectroscopy experiments of solutions from LiI and LiI3 in water, ethanol, and acetonitrile. The experimentally determined solvent-induced binding energy shifts (SIBES) for the monatomic I(–) anion are compared to predictions from simple Born theory, PCM calculations, as well as multiconfigurational quantum chemical spectral calculations from geometries obtained through molecular dynamics of solvated clusters. We show that the SIBES for I(–) explicitly depend on the details of the hydrogen bonding configurations of the solvent to the I(–) and that static continuum models such as the Born model cannot capture the trends in the SIBES observed both in experiments and in higher-level calculations.

Collective hydrogen-bond dynamics dictates the electronic structure of aqueous I3(-).

The molecular and electronic structures of aqueous I3(-) and I(-) ions have been investigated through ab initio molecular dynamics (MD) simulations and photoelectron (PE) spectroscopy of the iodine 4d core levels. Against the background of the theoretical simulations, data from our I4d PE measurements are shown to contain evidence of coupled solute-solvent dynamics. The MD simulations reveal large amplitude fluctuations in the I-I distances, which couple to the collective rearrangement of the hydrogen bonding network around the I3(-) ion.

Holding onto electrons in alkali metal halide clusters: decreasing polarizability with increasing coordination.

The connection between the electronic polarizability and the decrease of the system size from macroscopic solid to nanoscale clusters has been addressed in a combined experimental and model-calculation study. A beam of free neutral potassium chloride clusters has been probed using synchrotron-radiation-based photoelectron spectroscopy. The introduction of "effective" polarizability for chlorine, lower than that in molecules and dimers and decreasing with increasing coordination, has allowed us to significantly improve the agreement between the experimental electron binding energies and the electrostatic model predictions.

Molecular sinkers: X-ray photoemission and atomistic simulations of benzoic acid and benzoate at the aqueous solution/vapor interface.

In this work, we provide a detailed microscopic picture of the behavior of benzoic acid at the aqueous solution/vapor interface in its neutral as well as in its dissociated form (benzoate). This is achieved through a combination of highly surface-sensitive X-ray photoelectron spectroscopy experiments and fully atomistic molecular simulations. We show that significant changes occur in the interface behavior of the neutral acid upon release of the proton.

The new ambient-pressure X-ray photoelectron spectroscopy instrument at MAX-lab.

The new instrument for near-ambient-pressure X-ray photoelectron spectroscopy which has been installed at the MAX II ring of the Swedish synchrotron radiation facility MAX IV Laboratory in Lund is presented. The new instrument, which is based on a SPECS PHOIBOS 150 NAP analyser, is the first to feature the use of retractable and exchangeable high-pressure cells. This implies that clean vacuum conditions are retained in the instrument's analysis chamber and that it is possible to swiftly change between near-ambient and ultrahigh-vacuum conditions.

Ionic-charge dependence of the intermolecular coulombic decay time scale for aqueous ions probed by the core-hole clock.

Auger electron spectroscopy combined with theoretical calculations has been applied to investigate the decay of the Ca 2p core hole of aqueous Ca(2+). Beyond the localized two-hole final states on the calcium ion, originating from a normal Auger process, we have further identified the final states delocalized between the calcium ion and its water surroundings and produced by core level intermolecular Coulombic decay (ICD) processes. By applying the core-hole clock method, the time scale of the core level ICD was determined to be 33 ± 1 fs for the 2p core hole of the aqueous Ca(2+).

Cations strongly reduce electron-hopping rates in aqueous solutions.

We study how the ultrafast intermolecular hopping of electrons excited from the water O1s core level into unoccupied orbitals depends on the local molecular environment in liquid water. Our probe is the resonant Auger decay of the water O1s core hole (lifetime ∼3.6 fs), by which we show that the electron-hopping rate can be significantly reduced when a first-shell water molecule is replaced by an atomic ion.

The protonation state of small carboxylic acids at the water surface from photoelectron spectroscopy.

We report highly surface sensitive core-level photoelectron spectra of small carboxylic acids (formic, acetic and butyric acid) and their respective carboxylate conjugate base forms (formate, acetate and butyrate) in aqueous solution. The relative surface propensity of the carboxylic acids and carboxylates is obtained by monitoring their respective C1s signal intensities from a solution in which their bulk concentrations are equal. All the acids are found to be enriched at the surface relative to the corresponding carboxylates.