\AA ngstrom 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 anisotropy parameter characterizing the PAD scales linearly with the average distance of atoms along the surface normal. 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~\AA~along the surface normal can be clearly distinguished with this method, achieving excellent depth resolution.Comment: Submitted to Phys. Rev. Let

Spectroscopic evidence for a gold-coloured metallic water solution

Insulating materials can in principle be made metallic by applying pressure. In the case of pure water, this is estimated1 to require a pressure of 48 megabar, which is beyond current experimental capabilities and may only exist in the interior of large planets or stars2–4. Indeed, recent estimates and experiments indicate that water at pressures accessible in the laboratory will at best be superionic with high protonic conductivity5, but not metallic with conductive electrons1. Here we show that a metallic water solution can be prepared by massive doping with electrons upon reacting water with alkali metals. Although analogous metallic solutions of liquid ammonia with high concentrations of solvated electrons have long been known and characterized6–9, the explosive interaction between alkali metals and water10,11 has so far only permitted the preparation of aqueous solutions with low, submetallic electron concentrations12–14. We found that the explosive behaviour of the water–alkali metal reaction can be suppressed by adsorbing water vapour at a low pressure of about 10−4 millibar onto liquid sodium–potassium alloy drops ejected into a vacuum chamber. This set-up leads to the formation of a transient gold-coloured layer of a metallic water solution covering the metal alloy drops. The metallic character of this layer, doped with around 5 × 1021 electrons per cubic centimetre, is confirmed using optical reflection and synchrotron X-ray photoelectron spectroscopies

Core Level Photoelectron Angular Distributions at the Liquid Vapor Interface

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 amp; 8764;1 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 interface

High resolution analysis of 32 S18O2 spectra: The ν1 and ν3 interacting bands

Highly accurate, image, ro-vibrational spectrum of S18O2 was recorded with Bruker IFS 120 HR Fourier transform interferometer in the region of 1050–1400 cm−1 where the bands ν1 and ν3 are located. About 1560 and 1840 transitions were assigned in the experimental spectrum with the maximum values of quantum numbers image equal to 65/22 and 58/16 to the bands ν3 and ν1, respectively. The further weighted fit of experimentally assigned transitions was made with the Hamiltonian model which takes into account Coriolis resonance interaction between the vibrational states (100) and (001). To make the ro-vibrational analysis physically more suitable, the initial values of main spectroscopic parameters have been estimated from the values of corresponding parameters of the S16O2 species on the basis of the results of the Isotopic Substitution theory. Finally, the set of 23 spectroscopic parameters obtained from the fit reproduces values of 1292 initial “experimental” ro-vibrational energy levels (about 3400 transitions assigned in the experimental spectrum) with the image. Also, the ground state parameters of the S18O2 molecule were improved

High resolution analysis of 32 S18O2 spectra: The ν1 and ν3 interacting bands

Highly accurate, image, ro-vibrational spectrum of S18O2 was recorded with Bruker IFS 120 HR Fourier transform interferometer in the region of 1050–1400 cm−1 where the bands ν1 and ν3 are located. About 1560 and 1840 transitions were assigned in the experimental spectrum with the maximum values of quantum numbers image equal to 65/22 and 58/16 to the bands ν3 and ν1, respectively. The further weighted fit of experimentally assigned transitions was made with the Hamiltonian model which takes into account Coriolis resonance interaction between the vibrational states (100) and (001). To make the ro-vibrational analysis physically more suitable, the initial values of main spectroscopic parameters have been estimated from the values of corresponding parameters of the S16O2 species on the basis of the results of the Isotopic Substitution theory. Finally, the set of 23 spectroscopic parameters obtained from the fit reproduces values of 1292 initial “experimental” ro-vibrational energy levels (about 3400 transitions assigned in the experimental spectrum) with the image. Also, the ground state parameters of the S18O2 molecule were improved

Photoelectron angular distributions as sensitive probes of surfactant layer structure at the liquid vapor interface

The characterization of liquid vapor interfaces at the molecular level is an important underpinning for a basic understanding of fundamental heterogeneous processes in many areas, such as atmospheric science. Here we use X ray photoelectron spectroscopy to study the adsorption of a model surfactant, octanoic acid, at the water gas interface. In particular, we examine the information contained in photoelectron angular distributions and show that information about the relative depth of molecules and functional groups within molecules can be obtained from these measurements. Focusing on the relative location of carboxylate COO amp; 8722; and carboxylic acid COOH groups at different solution pH, the former is found to be immersed deeper into the liquid vapor interface, which is confirmed by classical molecular dynamics simulations. These results help establish photoelectron angular distributions as a sensitive tool for the characterization of molecules at the liquid vapor interfac

Valence and Core Level X ray Photoelectron Spectroscopy of a Liquid Ammonia Microjet

Photoelectron spectroscopy of microjets expanded into vacuum allows access to orbital energies for solute or solvent molecules in the liquid phase. Microjets of water, acetonitrile and alcohols have previously been studied; however, it has been unclear whether jets of low temperature molecular solvents could be realized. Here we demonstrate a stable 20 μm jet of liquid ammonia (−60 °C) in a vacuum, which we use to record both valence and core-level band photoelectron spectra using soft X-ray synchrotron radiation. Significant shifts from isolated ammonia in the gas-phase are observed, as is the liquid-phase photoelectron angular anisotropy. Comparisons with spectra of ammonia in clusters and the solid phase, as well as spectra for water in various phases potentially reveal how hydrogen bonding is reflected in the condensed phase electronic structure

Spectroscopic evidence for a gold coloured metallic water solution

Insulating materials can in principle be made metallic by applying pressure. In the case of pure water, this is estimated to require a pressure of 48 megabar, which is beyond current experimental capabilities and may only exist in the interior of large planets or stars. Indeed, recent estimates and experiments indicate that water at pressures accessible in the laboratory will at best be superionic with high protonic conductivity, but not metallic with conductive electrons1. Here we show that a metallic water solution can be prepared by massive doping with electrons upon reacting water with alkali metals. Although analogous metallic solutions of liquid ammonia with high concentrations of solvated electrons have long been known and characterized the explosive interaction between alkali metals and water has so far only permitted the preparation of aqueous solutions with low, submetallic electron concentrations. We found that the explosive behaviour of the water alkali metal reaction can be suppressed by adsorbing water vapour at a low pressure of about 10 amp; 8722;4 millibar onto liquid sodium potassium alloy drops ejected into a vacuum chamber. This set up leads to the formation of a transient gold coloured layer of a metallic water solution covering the metal alloy drops. The metallic character of this layer, doped with around 5 1021 electrons per cubic centimetre, is confirmed using optical reflection and synchrotron X ray photoelectron spectroscopie

Following in Emil Fischer s Footsteps A Site Selective Probe of Glucose Acid Base Chemistry

Liquid jet photoelectron spectroscopy was applied to determine the first acid dissociation constant pKa of aqueousphase 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 amp; 8722;base chemistry of polyprotic systems with relevance to environmental chemistry and biochemistr