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Liquid water, with a vapor pressure of 6.1 mbar at freezing point, is rapidly evaporating in high vacuum, rapidly cooling off by the evaporative cooling, and is freezing to ice almost instantly. Nevertheless, liquid water free vacuum surfaces can be prepared for short instances when injecting very small, fast flowing, liquid jets into high vacuum. They provide perfectly suited targets for molecular beams analysis of molecular evaporation of monomers and dimers from liquids. Also, the microjet technology allows ultrahigh vacuum studies of atomic scale liquid surface composition and electronic structures, as will be demonstrated by using highly focused Synchrotron radiation for EUV/XUV-photoelectron spectrocopy on a wide range of chemical solutions.

We report photoelectron spectroscopy measurements from binary acetonitrile–water solutions, for a wide range of acetonitrile mole fractions (xCH3CN = 0.011–0.90) using a liquid microjet. By detecting the nitrogen and carbon 1s photoelectron signal of CH3CN from aqueous surface and bulk solution, we quantify CH3CN’s larger propensity for the solution surface as compared to bulk solution. Quantification of the strong surface adsorption is through determination of the surface mole fraction as a function of bulk solution, xCH3CN, from which we estimate the adsorption free energy using the Langmuir adsorption isotherm model. We also discuss alternative approaches to determine the CH3CN surface concentration, based on analysis of the relative amount of gas- versus liquid-phase CH3CN, obtained from the respective photoelectron signal intensities. Another approach is based on the core-level binding energy shifts between liquid- and gas-phase CH3CN, which is sensitive to the change in solution surface potential an...

By a combination of ab initio calculations and photoelectron spectroscopy, we demonstrate that the DNA surrounding has only a negligible effect on ionization energies of nucleobases in the native a...

In the work described here, the electronic structure of sulfuric acid in water is explored by liquid-jet photoelectron spectroscopy. From the S2p photoelectron spectra of H2SO4 (aq), measured over a large concentration range and aided by previously reported HSO4–/SO42- and HSO4–/H2SO4 concentration ratios in the bulk solution, we obtain detailed electronic structure information of each species. Comparing our results with previous studies on the dissociation of nitric acid, we argue that the solvation structure of H2SO4 (aq) changes around 5–7 M concentration, at which point a dramatic change in both the HSO4– photoelectron peak width and binding energy occurs.

Recent nanomedical applications have again highlighted the significance of silica surface chemistry in solution. Here we report in situ electronic structure measurements at the silica aqueous interface as a function of pH for nanoparticles (NPs) of 7 12 and 22 nm using a liquid microjet in combination with synchrotron radiation. The Si K edge X ray absorption near edge spectroscopy (XANES) spectra reveal a change in shape of the Si 1s ? t 2 (Si 2p 3s) absorption brought about by changes in the silanol protonation state at the interface of the NPs as a result of changes in solution pH. Our results are consistent with the number of silanol groups changing the protonation state being inversely correlated with the SiO 2 NP size. The importance of in situ studies is also demonstrated by comparing the XANES spectra of aqueous 7 nm SiO 2 with the same dehydrated sample in vacuum. © 2012 American Chemical Society.

Photoelectron (PE) spectroscopy measurements from liquid water and from a 4m NaI aqueous solution are performed using a liquid microjet in combination with soft X-ray synchrotron radiation. From th ...

Photoelectron spectroscopy and density functional molecular dynamics simulations are combined to quantify and characterize the redox properties of Ru(bpy)32+/3+ and Ru(H2O)62+/3+ in aqueous solution. We report the energy-resolved photoelectron spectrum of aqueous Ru(bpy)32+ at 200 eV photon energy. From the peak position of the highest molecular orbital at 6.81 eV, an experimental value for the single-ion reorganization free energy of Ru(bpy)33+ is determined to be 1.21 +/- 0.04 eV. Density functional molecular dynamics calculations give a value of 0.84-1.20 eV for Ru(bpy)33+ and 1.92-2.42 eV for Ru(H2O)63+ depending on the method used to extrapolate the results to the infinite dilution limit. Since linear response is an excellent approximation for these systems, we report the same reorganization free energies for the divalent ions. The relatively small reorganization free energy of Ru(bpy)33+ is a consequence of the small changes in the Ru-N bond lengths upon reduction (0.04 eV inner sphere contribution) and of the large hydrophobic cavity formed by the bulky bipyridine ligands, which effectively reduces the dipolar response of the solvent in qualitative agreement with continuum theory. The large difference in redox potential between Ru(bpy)32+/3+ and Ru(H2O)62+/3+ (1 eV) is mainly associated with the difference in reorganization free energy rather than vertical ionization energy. Finally, the measured photoelectron spectrum of Ru(bpy)32+ is compared with the Kohn-Sham density of states for interpretation of occupied as well as computed virtual energy levels. This computational approach, in conjunction with first-ever photoelectron spectroscopy measurements of an aqueous transition metal ion, provides a quantitative benchmark for understanding the effect of water on metal redox potential and lays the groundwork for future studies of redox properties.

Hydroxide and hydronium, which represent the ionic products of water autolysis, exhibit a peculiar surface behavior. While consensus has been established that the concentration of hydronium cations is enhanced at the surface with respect to the bulk, the affinity of hydroxide anions for the water/vapor interface has been a subject of an ongoing controversy. On the one hand, electrophoretic and titration measurements of air bubbles or oil droplets in water have been interpreted in terms of a dramatic interfacial accumulation of OH−. On the other hand, surface-selective non-linear spectroscopies, surface tension measurements, and molecular simulations show no or at most a weak surface affinity of hydroxide ions. Here, we summarize the current situation and provide new evidence for the lack of appreciable surface enhancement of OH−, based on photoelectron spectroscopy from a liquid jet and on molecular dynamics simulations with polarizable potentials at varying hydroxide concentrations.

Solvation of surface-active tetrabutylammonium iodide (TBAI) in liquid water and in sodium bromide aqueous solution was investigated by VUV photoelectron spectroscopy and by molecular dynamics simulations. The observed signal intensity changes in the photoemission spectra are consistent with the varying propensities of the different ions for the solution interface. While the cations are surface-bound due to hydrophobic interactions, the anions are driven to the vacuum/solution interface by their large polarizability and size. Iodide is more polarizable, and hence more surface-active than bromide, which explains the relatively small decrease of the iodide photoemission signal when TBAI is dissolved in bromide solution.

In the present study we highlight recent applications of liquid beam desorption mass spectrometry for the analysis of biomolecules. The protonated macromolecules are desorbed from a 10 μm thick liquid jet in vacuum with an IR laser pulse tuned in resonance with the OH stretch vibration of water. Cytochrome c, viscotoxin A3, synthetic analogues of DNA and nucleobase substituted β-peptides (PNA like oligomers), bovine serum albumin (BSA), as well as specifically designed pharmaceutical macromolecules have been investigated. The salt and buffer tolerance has been measured for the desorption of cytochrome c. For the PNA oligomers it has been shown that a mixture can be desorbed and that the relative intensity of the mass peaks reflects relative concentrations in solution. With diluted BSA water solutions it has been demonstrated that the desorption technique provides a quantitative measure of BSA in solution. The gas phase signal of singly protonated BSA desorbed from a series of well defined solution concentrations has been found to be linear over at least three orders of magnitude. This feature appears to be promising for quantitative online monitoring applications of this technique. Beyond applications of this technique mechanistic aspects of the poorly characterized desorption mechanism are discussed. With a field-free-drift time-of-flight approach we were able to monitor features of the desorption process by selecting fractions of the broad velocity distribution of desorbed species using an ion optics acting as an ion gate. The observed features were discussed within a “desorption/ionization” model featuring the interplay of an explosive thermal and a shock wave dispersion of the microfilament controlling the ejection of hot nano-droplets and microsolvated molecules as well as their desolvation.

Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.

We report photoelectron measurements and molecular dynamics (MD) simulations with a polarizable force field of surface-active tetrabutylammonium iodide (TBAI) in aqueous solution. Photoemission is studied for a photon energy of 100 eV, using a 6-μm-diameter liquid jet. Surfactant activity of the TBAI salt at the solution surface is proved by a dramatic (×70) increase of the I-(4d) signal, as compared to that of a NaI aqueous solution for identical salt concentrations. Completion of the segregation monolayer is suggested through the growth of the iodide photoelectron emission signal, as a function of the salt concentration. Our experiments reveal identical electron binding energies of iodide in TBAI and NaI aqueous solutions, which are independent of the salt concentration. Zero or very small spectral shifts of any feature, including the low-energy cutoff, suggest that no dipole is formed by TBA+ and I- ion pairs perpendicular to the surface, which is in accord with the simulated ionic density profiles. Bo...

The operation of a liquid polymer jet laser-plasma target and the characterization of the absolute x-ray emission in the extreme ultraviolet wavelength window from 9–19 nm is reported. The target is a liquid polymer (perfluoro-polyether) that is exposed to pulsed and focused laser light at 532 nm in the form of a thin, liquid microjet (d=40 to 160 μm) in vacuum. The spectral brightness of the source in the 13 nm range is relatively high because a large fraction of radiative energy is emitted in one single line only, which is assigned to be the 2p–3d FVII doublet at 12.8 nm, with a laser energy conversion efficiency of 0.45% (2π sr, 2% bandwidth) in our initial experiment. A further increase of the relative emission has been found in the wavelength range between 7 and 17 nm when the jet diameter was increased from 40 to 160 μm. The two-dimensional spatial profile of the source plasma (d=40 to 50 μm) has been analyzed with a pinhole camera.

The valence band photoemission of aqueous alkali-metal halide solutions is studied for photon energies from 90 to 110 eV. A 6 μm diameter liquid microjet provides a free vacuum surface, allowing water molecules to evaporate without collisions, and hence enables the direct detection of photoelectrons originating from the liquid. The experiments were performed at the MBI undulator beamline of the synchrotron radiation facility BESSY. Here, we focus on the determination of electron binding energies of solvated anions and cations. The effect of different countercations (Li+, Na+, K+, and Cs+), and salt concentrations is systematically investigated. Electron binding energies of the solvated ions are found to differ considerably from those in the gas phase; contrary to intuition, the energies do not depend on the salt concentration. Measured binding energies can be surprisingly well explained within a simple dielectric cavity model. For a NaI aqueous solution, negative surface excess is inferred from the evolut...

The valence band photoelectron spectra of liquid water (H2O and D2O) are studied in the photon energy range from hν = 60 to 120 eV. The experiments use a 6 μm diameter liquid-jet free vacuum surface at the MBI undulator beamline of the synchrotron radiation facility BESSY. Photoelectron emission from all four valence molecular orbitals (MOs) is observed. In comparison to those of the gas phase, the peaks are significantly broadened and shifted to lower binding energies by about 1.5 eV. This is attributed primarily to the electronic polarization of the solvent molecules around an ionized water molecule. Energy shifts, peak broadening, and relative peak intensities for the four MOs differ because of their specific participation in the hydrogen bonding in liquid water. Relative photoionization cross sections for MOs were measured for hν = 60, 80, and 100 eV. The main difference for liquid water, as compared to the gas phase, is the relative intensity decrease of the 1b2 and 3a1 orbitals, reflecting changes o...

Oxidative damage to DNA and hole transport between nucleobases in oxidized DNA are important processes in lesion formation for which surprisingly poor thermodynamic data exist, the relative ease of oxidizing the four nucleobases being one such example. Theoretical simulations of radiation damage and charge transport in DNA depend on accurate values for vertical ionization energies (VIEs), reorganization energies, and standard reduction potentials. Liquid-jet photoelectron spectroscopy can be used to directly study the oxidation half-reaction. The VIEs of nucleic acid building blocks are measured in their native buffered aqueous environment. The experimental investigation of purine and pyrimidine nucleotides, nucleosides, pentose sugars, and inorganic phosphate demonstrates that photoelectron spectra of nucleotides arise as a spectral sum over their individual chemical components; that is, the electronic interactions between each component are effectively screened from one another by water. Electronic structure theory affords the assignment of the lowest energy photoelectron band in all investigated nucleosides and nucleotides to a single ionizing transition centered solely on the nucleobase. Thus, combining the measured VIEs with theoretically determined reorganization energies allows for the spectroscopic determination of the one-electron redox potentials that have been difficult to establish via electrochemistry.

We have used analytical laser induced liquid beam desorption in combination with high resolution mass spectrometry ( m/Δm≥ 1000) for the study of protonated amino acids (ornithine, citrulline, lysine, arginine) and their non-covalently bound complexes in the gas phase desorbed from water solutions. We report studies in which the desorption mechanism has been investigated. The results imply that biomolecule desorption at our conditions is a single step process involving laser heating of the solvent above its supercritical temperature, a rapid expansion, ion recombination and finally isolation and desorption of only a small fraction of preformed ions and charged aggregates. In addition, we report an investigation of the aqueous solution concentration and pH-dependence of the laser induced desorption of protonated species (monomers and dimers). The experimental findings suggest that the desorption process depends critically upon the proton affinity of the molecules, the concentration of other ions, and of the pH value of the solution. Therefore the ion concentrations measured in the gas phase very likely reflect solution properties (equilibrium concentrations). Arginine self-assembles large non-covalent singly protonated multimers (n = 1...8) when sampled by IR laser induced water beam desorption mass spectrometry. The structures of these aggregates may resemble those of the solid state and may be preformed in solution prior to desorption. A desorption of mixtures of amino acids in water solution enabled us to study (mixed) protonated dimers, one of the various applications of the present technique. Reasons for preferred dimerization - leading to simple cases of molecular recognition - as well as less preferred binding is discussed in terms of the number of specific H-bonds that can be established in the clusters.

In this reply to comments to our Frontiers Article (B. Winter, M. Faubel, R. Vacha, P. Jungwirth Chem. Phys. Lett. 474 (2009) 241) we reiterate in detail on spectroscopic and computational evidence arguing against strong adsorption of hydroxide ions at or near the water/vapor interface. In particular, we stress that the putative strongly enhanced OH- layer within one or several nm from the surface would have been observable in our previously reported photoelectron experiments.

The free surfaces of liquid water, formamide and ethylene glycol are prepared as a microjet in a high vacuum where they are studied by UV photoelectron spectroscopy with He I ( hν =21.22 eV) photons. The low vapour pressure liquid formamide UPS spectrum, which has already been reported, serves as an independent test of the accuracy of the present method. In agreement with a detailed investigation of experimental offset potentials, a measurement of the electrical current transported by the jet provides a highly accurate absolute calibration of the photoelectron spectra with respect to the vacuum zero energy level. The corrected spectra show several ionization bands which are shifted by approximately 1 eV to lower ionization energies with respect to the related gas phase molecular orbitals as a result of the electronic relaxation energy contribution to the molecular solvation forces.

The F+D 2 elastic scattering as well as the D 2 ( j i =0 → j f =2), D 2 ( j i =1 → j f =3) and, for the first time, D 2 ( j i =0 → j f =4) inelastic rotational transitions have been measured in a crossed molecular beam experiment at a center-of-mass collision energy of 112±2 meV. Differential cross-sections for the measured rotational transitions have been calculated on three different potential energy surfaces (Takayanagi–Sato, DMBE-6SEC of Truhlar and coworkers, and Stark–Werner) using different quasiclassical and semiclassical techniques. Although an overall qualitative agreement is found between the experimental and computed differential cross-sections, significant discrepancies are also apparent. In particular, the dynamical calculations lead to a systematic underestimation of the scattering yield into the non-reactive scattering channels, which can be attributed to shortcomings in the topography of all the three surfaces probed. Replacing the free deuterium molecule with a rigid or a vibrating rotor (both of these models exclude reaction) in the dynamical treatment considerably reduces the disagreement between the experimental and the calculated differential cross-sections for the different inelastic transitions.

Absolute differential and integral cross-sections for the DF(vf=0−4) vibrational products of the F + D2(vi=0,ji=0−2) → DF(vf) + F reaction have been evaluated from the time-of-flight spectra measured in high-resolution crossed molecular beam scattering experiments at five collision energies within the range 90−240 meV. The observed rise of the total reactive cross-section with increasing collision energy can be fitted with an empirical, modified line-of-centers model, yielding an activation energy of 1.8 kcal·mol-1. Remarkably pronounced angular structures are observed in the vibrationally resolved differential reactive cross-sections, especially for the largest accessible vibrations, vf = 3 and vf = 4. The experimental absolute cross-sections are compared with the results of previously reported quantum mechanical and quasiclassical trajectory calculations on the most recent potential energy surfaces for the F + D2 system. An overall excellent agreement is found within the experimental uncertainty. Howeve...

The recently developed technique of accessing volatile liquids in a high vacuum environment by using a very thin liquid jet is implemented to carry out the first measurements of photoelectron spectra of pure liquid water, methanol, ethanol, 1-propanol, 1-butanol, and benzyl alcohol as well as of liquid n-nonane. The apparatus, which consists of a commercial hemispherical (10 cm mean radius) electron analyzer and a hollow cathode discharge He I light source is described in detail and the problems of the sampling of the photoelectrons in such an environment are discussed. For water and most of the alcohols up to six different electronic bands could be resolved. The spectra of 1-butanol and n-nonane show two weakly discernable peaks from which the threshold ionization potential could be determined. A deconvolution of the photoelectron spectra is used to extract ionization potentials of individual molecular bands of molecules near the surface of the liquid and shifts of the order of 1 eV compared to the gas phase are observed. A molecular orientation for water molecules at the surface of liquid water is inferred from a comparison of the relative band strengths with the gas phase. Similar effects are also observed for some of the alcohols. The results are discussed in terms of a simple “Born-solvation” model.

The free surfaces of two binary liquid mixture systems over the whole range of bulk molar composition have been studied using HeI photoelectron spectroscopy in a thin, fast flowing jet arrangement. The spectra of the first group of mixtures, benzyl alcohol and formamide, are found to be deconvolutable by a weighted sum of the pure component spectra. Slight deviations in the procedure allow an estimate to be made of the surface orientation of single benzyl alcohol molecules on top of the formamide solvent. The relative surface composition in the top layer is extracted by the deconvolution procedure. Time relaxation of the surface composition is observed with time constants of the order of several tens of mus. The results of the second group of mixtures, water and formamide, show more complicated features, with one component rapidly evaporating on the time scale of the experiment. The relative surface composition is estimated by a constrained deconvolution involving mainly the first two ionization bands of ...

The important properties of thermal ovens for producing intense beams of F atoms by thermal dissociation of are analysed. After reviewing previous constructions a new source made of a single crystal of is described and characterized. This source has been tested for more than 3000 h at temperatures up to with a mixture of 10% in Ar at pressures up to 12 bar. A degree of dissociation of 50% and a F atom beam velocity spread of about 7% were achieved.

Quasi-classical trajectory calculations (QCT) have been carried out for the F+D 2 reaction at the collision energies and initial rotational states necessary to simulate the molecular beam results presented in the preceding paper of this issue by Faubel et al. Although the general trends are well accounted for by the QCT calculations, there are significant differences between experiment and theoretical results. The vibrational resolved differential cross section are in an overall good agreement; however, the QCT calculations clearly underestimate both backward and forward scattering. The comparison between the product state distributions indicates that the QCT ones are somewhat broader than the experimental ones for most of the vibrational states. The limitations of the theoretical results become more clear when the laboratory frame (LAB) angular distributions (AD) and time-of-flight (TOF) spectra are simulated using the calculated DCS resolved into the final rovibrational states, v f , j f . The theoretical findings and, especially, the roles of translational energy and initial rotational momentum on the dynamics of this reaction are discussed in some detail.

Photoelectron angular distributions (PADs) from the liquid-water surface and from bulk liquid water are reported for water oxygen-$1s$ ionization. Although less so than for the gas phase, the measured PADs from the liquid are remarkably anisotropic, even at electron kinetic energies lower than 100 eV, when elastic scattering cross sections for the outgoing electrons with other water molecules are large. The PADs reveal that theoretical estimates of the inelastic mean free path are likely too long at low kinetic energies, and hence the electron probing depth in water, near threshold ionization, appears to be considerably smaller than so far assumed.

The F + D2(ji=0, 1, 2) → DF(νf, jf) + D reaction has been studied in a high-resolution crossed molecular beam experiment at a collision energy of 3.2 kcal/mol (140.0 meV) with an energy resolution of the time-of-flight spectra of typically 25 meV. The new results for the vibrationally resolved differential cross sections for the DF(νf = 1, 2, 3, 4) products show the same general trends as an earlier experiment of Neumark et al., but some significant discrepancies are found. New evidence is presented that the vibrational product state distributions depend strongly on the initial rotational state of the D2 reagents. This effect reconciles some of the apparent discrepancies between our experiment and the earlier one.

New measurements of the elastic and rotationally inelastic differential cross sections for the Ne–D2, Ne–H2 system are compared with exact and approximate quantum calculations. The three most recent high quality, semiempirical interaction potentials available in the literature for the Ne–H2 system yield consistent theoretical scattering cross sections for Ne–H2 and for Ne–D2. They also agree with previous and with present inelastic cross section measurements for D2. However, the theory underestimates by 30% the newly measured rotational excitation in Ne–H2 collisions discussed here. We therefore propose a new potential with a modified repulsive barrier that succeeds in describing both Ne–D2 and Ne–H2 rotationally inelastic scattering experiments for j=0→j’=2 within an accuracy of a few percent.

High resolution time of flight spectra of DF products have been measured for 12 different center‐of‐mass angles in the range θc.m.=114° to 180° for the reaction F+D2→DF+D at a center‐of‐mass collision energy of Ec.m.=82.5±2.6 meV. The resolution is sufficient to clearly resolve the different final product vibrational states and to extract rotational product distributions for each of the vibrational states. Absolute reactive cross sections for the final vibrational states vf=1, 2, 3, and 4 were determined from a careful calibration of the beam source intensities and detector sensitivity. For all final vibrational states, nearly the same large rotational surprisal values of ΘR=5.3 were found. From the rotational distributions, it has also been possible to estimate opacity functions for these final vf states via the method of Elsum and Gordon [J. Chem. Phys. 76, 3009 (1982)]. The angular distributions for different vf states are compared to recent infinite order sudden approximation (IOSA) and classical tra...

Since the pioneering work of Kai Siegbahn, electron spectroscopy for chemical analysis (ESCA) has been developed into an indispensable analytical technique for surface science. The value of this powerful method of photoelectron spectroscopy (PES, also termed photoemission spectroscopy) and Siegbahn's contributions were recognized in the 1981 Nobel Prize in Physics. The need for high vacuum, however, originally prohibited PES of volatile liquids, and only allowed for investigation of low-vapor-pressure molecules attached to a surface (or close to a surface) or liquid films of low volatility. Only with the invention of liquid beams of volatile liquids compatible with high-vacuum conditions was PES from liquid surfaces under vacuum made feasible. Because of the ubiquity of water interfaces in nature, the liquid water-vacuum interface became a most attractive research topic, particularly over the past 10 years. PES studies of these important aqueous interfaces remained significantly challenging because of the need to develop high-pressure PES methods. For decades, ESCA or PES (termed XPS, for X-ray photoelectron spectroscopy, in the case of soft X-ray photons) was restricted to conventional laboratory X-ray sources or beamlines in synchrotron facilities. This approach enabled frequency domain measurements, but with poor time resolution. Indirect access to time-resolved processes in the condensed phase was only achieved if line-widths could be analyzed or if processes could be related to a fast clock, that is, reference processes that are fast enough and are also well understood in the condensed phase. Just recently, the emergence of high harmonic light sources, providing short-wavelength radiation in ultrashort light pulses, added the dimension of time to the classical ESCA or XPS technique and opened the door to (soft) X-ray photoelectron spectroscopy with ultrahigh time resolution. The combination of high harmonic light sources (providing radiation with laserlike beam qualities) and liquid microjet technology recently enabled the first liquid interface PES experiments in the IR/UV-pump and extreme ultraviolet-probe (EUV-probe) configuration. In this Account, we highlight features of the technology and a number of recent applications, including extreme states of matter and the discovery and detection of short-lived transients of the solvated electron in water. Properties of the EUV radiation, such as its controllable polarization and features of the liquid microjet, will enable unique experiments in the near future. PES measures electron binding energies and angular distributions of photoelectrons, which comprise unique information about electron orbitals and their involvement in chemical bonding. One of the future goals is to use this information to trace molecular orbitals, over time, in chemical reactions or biological transformations.

The local electronic structure of glycine in neutral, basic, and acidic aqueous solution is studied experimentally by X-ray photoelectron spectroscopy and theoretically by molecular dynamics simulations accompanied by first-principle electronic structure and spectrum calculations. Measured and computed nitrogen and carbon 1s binding energies are assigned to different local atomic environments, which are shown to be sensitive to the protonation/deprotonation of the amino and carboxyl functional groups at different pH values. We report the first accurate computation of core-level chemical shifts of an aqueous solute in various protonation states and explicitly show how the distributions of photoelectron binding energies (core-level peak widths) are related to the details of the hydrogen bond configurations, i.e. the geometries of the water solvation shell and the associated electronic screening. The comparison between the experiments and calculations further enables the separation of protonation-induced (covalent) and solvent-induced (electrostatic) screening contributions to the chemical shifts in the aqueous phase. The present core-level line shape analysis facilitates an accurate interpretation of photoelectron spectra from larger biomolecular solutes than glycine.

We present a first experimental study of inelastic, rotational excitation cross-sections in a reactive collision. By preliminary model calculations we show that these measurements provide an ideal method for probing the anisotropy in the rate-determining entrance-channel valley of the potential-energy surface for the F–H2 reaction. A comparison with the non-reactive Ne–D2, H2 scattring system reveals the importance of a surprisingly large fourth-order Legendre term in the anisotropic expansion of the F–H2 potential. This particular shape feature is related to the incipient chemical reaction and is absent in the more ellipsoidal shape of non-reactive potentials for rare gas–H2 systems.

Energy-resolved photoemission spectroscopy and density functional molecular dynamics simulations are combined to construct an energy level diagram for the Mn(2+/3+) redox reaction in aqueous solution. Two peaks centered at 8.88 and 10.26 eV electron binding energies can be assigned to the Mn2+ hexa-aquo complex with a peak area ratio of 2:2.83. Using the notation of crystal field theory, the peak at lower energies can be interpreted as arising from ionization from the e(g) levels (highest occupied molecular orbital, HOMO), and the peak at higher energies are from ionization of the t(2g) levels. The difference corresponds to the average crystal field splitting, 1.38 eV. From the position of the HOMO level and the absolute redox potential, an experimental value for the reorganization free energy of the aqueous Mn3+ hexa-aquo complex is estimated to be 2.98 eV. Density functional molecular dynamics simulations can reproduce the experimental vertical ionization energy, redox free energy, and reorganization free energies fairly well, provided that the absolute potential shift in periodic boundary conditions, finite size effects, and inaccuracies of the exchange correlation functional are taken into account. Most strikingly, in the simulations, we observe spontaneous and reversible deprotonation of the aqueous Mn3+ hexa-aquo complex to form MnOH(H2O)5(2+) + H+, in line with the low experimental pKa value of this ion. The interconversion between protonation states leads to interesting redox phenomena for aqueous Mn3+, culminating in a bimodal thermal distribution of the electron affinity.

Solvated electrons in liquid water are one of the seemingly simplest, but most important, transients in chemistry and biology, but they have resisted disclosing important information about their energetics, binding motifs and dynamics. Here we report the first ultrafast liquid-jet photoelectron spectroscopy measurements of solvated electrons in liquid water. The results prove unequivocally the existence of solvated electrons bound at the water surface and of solvated electrons in the bulk solution, with vertical binding energies of 1.6 eV and 3.3 eV, respectively, and with lifetimes longer than 100 ps. The unexpectedly long lifetime of solvated electrons bound at the water surface is attributed to a free-energy barrier that separates surface and interior states. Beyond constituting important energetic and kinetic benchmark and reference data, the results also help to understand the mechanisms of a number of very efficient electron-transfer processes in nature.

Depth-resolved ion spatial distributions of nitrate and nitrite anions in aqueous solution have been quantitatively measured using X-ray photoemission spectroscopy on a 15 microm aqueous liquid jet containing 3 M NaNO(3), 3 M NaNO(2), or an equimolar mixture of the two. The surface region, which extends to photoelectron kinetic energies of 400-500 eV, is partially depleted in anions relative to the bulk 3 M concentration. The nitrate and nitrite solutions exhibit similar depth-dependent anion profiles. The results presented here are compared with recent molecular dynamics simulations of a NaNO(3) solution and are found to agree at high photoelectron kinetic energies. At shallower probe depths, the experiment measured a surface anion concentration less than that predicted by theory. Possible origins of the discrepancy are discussed in terms of the confined size of the simulation box and uncertainties that remain in regard to the inelastic mean free path of photoelectrons in aqueous media. The importance of our findings is discussed in relation to the observed increase in photochemical activity of nitrate-containing aerosols in the atmosphere.

Vertical ionization energies of the nucleosides cytidine and deoxythymidine in water, the lowest ones amounting in both cases to 8.3 eV, are obtained from photoelectron spectroscopy measurements in aqueous microjets. Ab initio calculations employing a nonequilibrium polarizable continuum model quantitatively reproduce the experimental spectra and provide molecular interpretation of the individual peaks of the photoelectron spectrum, showing also that lowest ionization originates from the base. Comparison of calculated vertical ionization potentials of pyrimidine bases, nucleosides, and nucleotides in water and in the gas phase underlines the dramatic effect of bulk hydration on the electronic structure. In the gas phase, the presence of sugar and, in particular, of phosphate has a strong effect on the energetics of ionization of the base. Upon bulk hydration, the ionization potential of the base in contrast becomes rather insensitive to the presence of the sugar and phosphate, which indicates a remarkable screening ability of the aqueous solvent. Accurate aqueous-phase vertical ionization potentials provide a significant improvement to the corrected gas-phase values used in the literature and represent important information in assessing the threshold energies for photooxidation and oxidation free energies of solvent-exposed DNA components. Likewise, such energetic data should allow improved assessment of delocalization and charge-hopping mechanisms in DNA ionized by radiation.

Electron spectroscopy for chemical analysis (ESCA) is a powerful tool for the quantitative analysis of the composition and the chemical environment of molecular systems. Due to the lack of compatibility of liquids and vacuum, liquid-phase ESCA is much less well established. The chemical shift in the static ESCA approach is a particularly powerful observable quantity for probing electron orbital energies in molecules in different molecular environments. Employing high harmonics of 800 nm (40 eV), near-infrared femtosecond pulses, and liquid-water microbeams in vacuum we were able to add the dimension of time to the liquid interface ESCA technique. Tracing time-dependent chemical shifts and energies of valence electrons in liquid interfacial water in time, we have investigated the timescale and molecular signatures of laser-induced liquid-gas phase transitions on a picosecond timescale.

Charge-transfer-to-solvent (CTTS) excited states of aqueous chloride are studied by a novel experimental approach based on resonant inner-shell photoexcitation, Cl(-)aq 2p --e(i), i = 1-4, which denotes a series of excitations to lowest and higher CTTS states. These states are clearly identified through the occurrence of characteristic spectator Auger decays to double Cl 3p valence-hole states, where the CTTS states can be more stabilized as compared to single Cl(-)aq 2p core excitations and optical valence excitations. Furthermore, we have found for the first time that the CTTS electron e(i) bound by a single Cl 2p hole not only behaves as a spectator e(i) --e'(i), bound by a double valence-hole state before relaxation of the excited electron (i) itself, but also shows electron dynamics to the relaxed lowest state, e(i) --e'(1*). This interpretation is supported by ab initio calculations. The key to performing photoelectron and Auger-electron spectroscopy studies from aqueous solutions is the use of a liquid microjet in ultrahigh vacuum in conjunction with synchrotron radiation.

We achieved a continuous, stable, ultrashort pulse hard x-ray point source by focusing 1.8-W, 1-kHz, 50-fs laser pulses onto a novel, 30-microm -diameter, high-velocity, liquid-metal gallium jet. This target geometry avoids most of the debris problems of solid targets and provides nearly 4pi illumination. Photon fluxes of 5x10(8) photons/s are generated in a two-component spectrum consisting of a broad continuum from 4 to 14 keV and strong K(alpha) and K(beta) emission lines at 9.25 and 10.26 keV. This source will find wide use in time-resolved x-ray diffraction studies and other applications.

We demonstrate the applicability of X-ray photoelectron spectroscopy to obtain charge- and site-specific electronic structural information of biomolecules in aqueous solution. Changing the pH of an aqueous solution of lysine from basic to acidic results in nitrogen 1s and carbon 1s chemical shifts to higher binding energies. These shifts are associated with the sequential protonation of the two amino groups, which affects both charge state and hydrogen bonding to the surrounding water molecules. The N1s chemical shift is 2.2 eV, and for carbon atoms directly neighboring a nitrogen the shift for C1s is approximately 0.4 eV. The experimental binding energies agree reasonably with our calculated energies of lysine(aq) for different pH values.

We have measured resonant and off-resonant Auger-electron spectra of liquid water. Continuumlike transitions near and above the O1s vertical ionization energy are identified by the characteristic normal Auger-electron spectra. On the contrary, well-resolved spectator shifts of the main Auger-electron peak are observed at the liquid-water O1s absorption main edge and near the absorption pre-edge. The shifts of 1.4 and 1.9 eV arise from the localized nature of the excitation. Excited-state localization/delocalization is also discussed for the analogous vacuum ultraviolet (VUV) transitions, and we point out the similarities between x-ray and VUV absorption spectra of liquid water.

The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.

The electronic structure of hydrated H3O+ and OH- is probed in a water jet by photoelectron spectroscopy employing 100 eV photons. The first ionization potential for OH- at 9.2 eV and the second ionization potential for H3O+ at 20 eV are resolved, corresponding to the removal of an electron from the 2ppi highest occupied molecular orbital and from the 1e orbital, respectively. These assignments are supported by present computational results based on a combination of molecular dynamics and ab initio calculations.