The electronic structure of free water clusters probed by Auger electron spectroscopy

Interplay of protection and damage through intermolecular processes in the decay of electronic core holes in microsolvated organic molecules

Soft X-ray irradiation of molecules causes electronic core-level vacancies through photoelectron emission. In light elements, such as C, N, or O, which are abundant in the biosphere, these vacancies predominantly decay by Auger emission, leading inevitably to dissociative multiply charged states. It was recently demonstrated that an environment can prevent fragmentation of core-level-ionised small organic molecules through immediate non-local decay of the core hole, dissipating charge and energy to the environment. Here, we present an extended photoelectron-photoion-photoion coincidence (PEPIPICO) study of the biorelevant pyrimidine molecule embedded in a water cluster. It is observed and supported by theoretical calculations that the supposed protective effect of the environment is partially reversed if the vacancy is originally located at a water molecule. In this scenario, intermolecular energy or charge transfer from the core-ionised water environment to the pyrimidine molecule leads to ionisation of the latter, however, presumably in non-dissociative cationic states. Our results contribute to a more comprehensive understanding of the complex interplay of protective and harmful effects of an environment in the photochemistry of microsolvated molecules exposed to X-rays.

Core-ionization spectrum of liquid water

We present state-of-the-art calculations of the core-ionization spectrum of water. Despite significant progress in procedures developed to mitigate various experimental complications and uncertainties, the experimental determination of ionization energies of solvated species involves several non-trivial steps such as assessing the effect of the surface potential, electrolytes, and finite escape depths of photoelectrons. This provides a motivation to obtain robust theoretical values of the intrinsic bulk ionization energy and the corresponding solvent-induced shift. Here we develop theoretical protocols based on coupled-cluster theory and electrostatic embedding. Our value of the intrinsic solvent-induced shift of the 1sO ionization energy of water is -1.79 eV. The computed absolute position and the width of the 1sO peak in photoelectron spectrum of water are 538.47 eV and 1.44 eV, respectively, agreeing well with the best experimental values.

A study of the valence photoelectron spectrum of uracil and mixed water-uracil clusters

The valence ionization of uracil and mixed water-uracil clusters has been studied experimentally and by ab initio calculations. In both measurements, the spectrum onset shows a red shift with respect to the uracil molecule, with the mixed cluster characterized by peculiar features unexplained by the sum of independent contributions of the water or uracil aggregation. To interpret and assign all the contributions, we performed a series of multi-level calculations, starting from an exploration of several cluster structures using automated conformer-search algorithms based on a tight-binding approach. Ionization energies have been assessed on smaller clusters via a comparison between accurate wavefunction-based approaches and cost-effective DFT-based simulations, the latter of which were applied to clusters up to 12 uracil and 36 water molecules. The results confirm that (i) the bottom-up approach based on a multilevel method [Mattioli et al. Phys. Chem. Chem. Phys. 23, 1859 (2021)] to the structure of neutral clusters of unknown experimental composition converges to precise structure-property relationships and (ii) the coexistence of pure and mixed clusters in the water-uracil samples. A natural bond orbital (NBO) analysis performed on a subset of clusters highlighted the special role of H-bonds in the formation of the aggregates. The NBO analysis yields second-order perturbative energy between the H-bond donor and acceptor orbitals correlated with the calculated ionization energies. This sheds light on the role of the oxygen lone-pairs of the uracil CO group in the formation of strong H-bonds, with a stronger directionality in mixed clusters, giving a quantitative explanation for the formation of core-shell structures.

Origin of the Liquid/Gaseous Water Binding Energy Splitting Measured via X-ray Photoelectron Spectroscopy

Ambient-pressure X-ray photoelectron spectroscopy (APXPS) provides an effective way of tackling the challenge of detecting chemical states within complex systems. Here a fundamental understanding of the core-level shift (CLS) of water in the liquid/gas phase observed via APXPS is obtained with computational modeling at the molecular and electronic levels. The CLS value of ∼2 eV derived from experiments is reproduced by modeling in terms of the total shift and photon energy dependence. The contributions of collective electrical effects, including electrostatic potential, orbital deformation, and electronic polarization, to the CLS were further analyzed and discussed. Our results show that the CLS is dominated by the final state effect due to electronic polarization of the surrounding molecules following photoionization, while the peak broadening is mainly determined by the electrostatic potential, which belongs to an initial state effect. The physical insights and computational approaches could be further applied to study more complex molecules or materials.

Electronic state influence on selective bond breaking of core-excited nitrosyl chloride (ClNO)

The potential for selective bond breaking of a small molecule was investigated with electron-spectroscopy and electron-ion coincidence experiments on ClNO. The electron spectra were measured upon direct valence photo-ionization and upon resonant core-excitation at the N 1s- and O 1s-edges followed by emission of resonant Auger (RA) electrons. The RA spectra were analyzed with particular emphasis on the assignment of the participator and spectator states. The latter are of special relevance for investigations of how distinct electronic configurations influence selective bond breaking. The electron-ion coincidence measurements provided branching fractions of the produced ion-fragments as a function of electron binding energy. It explicitly demonstrates the influence of the final electronic states created after the photo-ionization and RA decay, on the fragmentation. In particular, we observe a significantly different branching fraction for spectator states compared with participator states. The bonds broken for the spectator states are also found to correlate with the anti-bonding character of the spectator-electron orbital.

Coupling a magnetic bottle multi-electron spectrometer with a liquid micro-jet device: a comprehensive study of solvated sodium benzoate at the O 1 s threshold

We have developed a magnetic bottle time-of-flight electron-electron coincidence spectrometer to perform measurements on solvated molecules in a liquid micro-jet. We present here the first results obtained after ionization of the oxygen 1s inner-shell of sodium benzoate molecules and show the possibilities to filter out the electron signal arising from the liquid phase from the signal of water molecules in the gas phase. Both photoelectrons and Auger electrons spectra (unfiltered and filtered) are presented.

Water-biomolecule clusters studied by photoemission spectroscopy and multilevel atomistic simulations: hydration or solvation?

The properties of mixed water-uracil nanoaggregates have been probed by core electron-photoemission measurements to investigate supramolecular assembly in the gas phase driven by weak interactions. The interpretation of the measurements has been assisted by multilevel atomistic simulations, based on semi-empirical tight-binding and DFT-based methods. Our protocol established a positive-feedback loop between experimental and computational techniques, which has enabled a sound and detailed atomistic description of such complex heterogeneous molecular aggregates. Among biomolecules, uracil offers interesting and generalized skeletal features; its structure encompasses an alternation of hydrophilic H-bond donor and acceptor sites and hydrophobic moieties, typical in biomolecular systems, that induces a supramolecular core-shell-like organization of the mixed clusters with a water core and an uracil shell. This structure is far from typical models of both solid-state hydration, with water molecules in defined positions, or liquid solvation, where disconnected uracil molecules are completely surrounded by water.

Photoelectron spectroscopy of large water clusters ionized by an XUV comb

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

Temperature Dependence of X-ray-Induced Auger Processes in Liquid Water

Auger spectroscopy has previously been used to study changes in the hydrogen bond network in liquid water, but to the best of our knowledge it has not been used to track such changes as a function of temperature. We show Auger spectroscopy to reflect the weakening of the hydrogen bond network upon heating. This shows that the radiation response of water, i.e., the relative propensity of the different processes occurring after radiation exposure, including femtosecond proton dynamics, depends on the temperature of the system. This proof-of-principle study further demonstrates the suitability of the technique to help elucidate information on the intermolecular structure of liquids such as water, opening the door to further temperature-dependent studies.

Proton dynamics in molecular solvent clusters as an indicator for hydrogen bond network strength in confined geometries

Hydrogen bonding leads to the formation of strong, extended intermolecular networks in molecular liquids such as water. However, it is less well-known how robust the network is to environments in which surface formation or confinement effects become prominent, such as in clusters or droplets. Such systems provide a useful way to probe the robustness of the network, since the degree of confinement can be tuned by altering the cluster size, changing both the surface-to-volume ratio and the radius of curvature. To explore the formation of hydrogen bond networks in confined geometries, here we present O 1s Auger spectra of small and large clusters of water, methanol, and dimethyl ether, as well as their deuterated equivalents. The Auger spectra of the clusters and the corresponding macroscopic liquids are compared and evaluated for an isotope effect, which is due to proton dynamics within the lifetime of the core hole (proton-transfer-mediated charge-separation, PTM-CS), and can be linked to the formation of a hydrogen bond network in the system. An isotope effect is observed in water and methanol but not for dimethyl ether, which cannot donate a hydrogen bond at its oxygen site. The isotope effect, and therefore the strength of the hydrogen bond network, is more pronounced in water than in methanol. Its value depends on the average size of the cluster, indicating that confinement effects change proton dynamics in the core ionised excited state.

Effective one-particle energies from generalized Kohn-Sham random phase approximation: A direct approach for computing and analyzing core ionization energies

Generalized-Kohn-Sham (GKS) orbital energies obtained self-consistently from the random phase approximation energy functional with a semicanonical projection (spRPA) were recently shown to rival the accuracy of GW quasiparticle energies for valence ionization potentials. Here, we extend the scope of GKS-spRPA correlated one-particle energies from frontier-orbital ionization to core orbital ionization energies, which are notoriously difficult for GW and other response methods due to strong orbital relaxation effects. For a benchmark consisting of 23 1s core electron binding energies (CEBEs) of second-row elements, chemical shifts estimated from GKS-spRPA one-particle energies yield mean absolute deviations from experiment of 0.2 eV, which are significantly more accurate than the standard GW and comparable to Δ self-consistent field theory without semiempirical adjustment of the energy functional. For small ammonia clusters and cytosine tautomers, GKS-spRPA based chemical shifts capture subtle variations in covalent and noncovalent bonding environments; GKS-spRPA 1s CEBEs for these systems agree with equation-of-motion coupled cluster singles and doubles and ADC(4) results within 0.2-0.3 eV. Two perturbative approximations to GKS-spRPA orbital energies, which reduce the scaling from O(N⁶) to O(N⁵) and O(N⁴), are introduced and tested. We illustrate the application of GKS-spRPA orbital energies to larger systems by using oxygen 1s CEBEs to probe solvation and packing effects in condensed phases of water. GKS-spRPA predicts a lowering of the oxygen 1s CEBE of approximately 1.6-1.7 eV in solid and liquid phases, consistent with liquid-jet X-ray photoelectron spectroscopy and gas phase cluster experiments. The results are rationalized by partitioning GKS-spRPA electron binding energies into static, relaxation, and correlation parts.

Site-specific X-ray induced dynamics in liquid methanol

Complex chemical and biochemical systems are susceptible to damage from ionising radiation. However, questions remain over the extent to which such damage is influenced by the nature of the surrounding chemical environment, which can consist of both hydrophobic and hydrophilic domains. To gain fundamental insight into the first crucial mechanistic steps of radiation damage in such systems, we need to understand the initial radiation response, i.e. dynamics occurring on the same timescale as electronic relaxation, which occur in these different environments. Amphiphilic molecules contain both hydrophobic and hydrophilic domains, but the propensity for charge delocalisation and proton dynamics to occur in these different domains has been largely unexplored so far. Here, we present carbon and oxygen 1s Auger spectra for liquid methanol, one of the simplest amphiphilic molecules, as well as its fully deuterated equivalent d4-methanol, in order to explore X-ray induced charge delocalisation and proton dynamics occurring on the few femtosecond timescale. Unexpectedly, we find a similar propensity for proton dynamics to occur at both the carbon and oxygen site within the lifetime of the core hole. Our results could serve as a model for decay processes that are likely to occur in other more complex amphiphilic systems.

Disorder in Aqueous Solutions and Peak Broadening in X-ray Photoelectron Spectroscopy

The microscopic structure and photoelectron spectra of an aqueous solution are investigated with ab initio molecular dynamics simulations and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). The simulation results show that the structural fluctuations in an aqueous solution can lead to remarkable peak broadening (∼1 eV) of ionic species, which is in good agreement with the results from AP-XPS experiments. We find that this broadening of the XPS peaks can be directly correlated with the local structural fluctuations in the aqueous solution, such as the evolution of solvation shells. This work demonstrates that the rich dynamics of solvation shells in aqueous solutions can be revealed by combining advanced simulations with in situ AP-XPS, and may stimulate new developments in the in situ XPS characterization of complex electrochemical reactions.

Extreme-ultraviolet high-harmonic generation in liquids

High–harmonic generation (HHG) in gases has been the main enabling technology of attosecond science since its discovery. Recently, HHG from solids has been demonstrated, opening a lively area of research. In contrast, harmonic generation from liquids has so far remained restricted to low harmonics in the visible regime. Here, we report the observation and detailed characterization of extreme ultraviolet HHG from liquid water and several alcohols extending beyond 20 eV. This advance was enabled by the implementation of the recent liquid flat–microjet technology, which we show to facilitate the spatial separation of HHG from the bulk liquid and the surrounding gas phase. We observe striking differences between the HHG spectra of water and several alcohols. A comparison with a strongly–driven few–band model establishes the sensitivity of HHG to the electronic structure of liquids. Our results suggest liquid–phase high–harmonic spectroscopy as a new method for studying the electronic structure and ultrafast scattering processes in liquids.

While high–harmonic generation from gases, and more recently also from solids, has been extensively studied, there is little data on HHG from liquids. Here, Luu et al. experimentally demonstrate and study HHG up to 27^th order from the bulk of liquid water and different alcohols.

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

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

Microhydration of LiOH: Insight from electronic decays of core-ionized states

We compute and compare the autoionization spectra of a core-ionized LiOH molecule both in its isolated and microhydrated states. Stepwise microhydration of LiOH leads to gradual elongation of the Li-OH bond length and finally to molecular dissociation. The accompanying changes in the local environment of the OH(-) and Li(+) counterions are reflected in the computed O 1s and Li 1s spectra. The role of solvent water molecules and the counterion in the spectral shape formation is assessed. Electronic decays of the microhydrated LiOH are found to be mostly intermolecular since the majority of the populated final states have at least one outer-valence vacancy outside the initially core-ionized ion, mainly on a neighboring water molecule. The charge delocalization occurs through the intermolecular Coulombic and electron transfer mediated decays. Both mechanisms are highly efficient that is partly attributed to hybridization of molecular orbitals. The computed spectral shapes are sensitive to the counterion separation as well as to the number and arrangement of solvent molecules. These sensitivities can be used for studying the local hydration structure of solvated ions in aqueous solutions.

X-ray and Electron Spectroscopy of Water

Here we present an overview of recent developments of X-ray and electron spectroscopy to probe water at different temperatures. Photon-induced ionization followed by detection of electrons from either the O 1s level or the valence band is the basis of photoelectron spectroscopy. Excitation between the O 1s and the unoccupied states or occupied states is utilized in X-ray absorption and X-ray emission spectroscopies. These techniques probe the electronic structure of the liquid phase and show sensitivity to the local hydrogen-bonding structure. Both experimental aspects related to the measurements and theoretical simulations to assist in the interpretation are discussed in detail. Different model systems are presented such as the different bulk phases of ice and various adsorbed monolayer structures on metal surfaces.

Expansion dynamics of supercritical water probed by picosecond time-resolved photoelectron spectroscopy

Vibrational excitation of liquid water with femtosecond laser pulses can create extreme states of water. Yet, the dynamics directly after initial sub-picosecond delocalization of molecular vibrations remain largely unclear. We study the ultrafast expansion dynamics of an accordingly prepared supercritical water phase with a picosecond time resolution. Our experimental setup combines vacuum-compatible liquid micro-jet technology and a table top High Harmonic light source driven by a femtosecond laser system. An ultrashort laser pulse centered at a wavelength of 2900 nm excites the OH-stretch vibration of water molecules in the liquid. The deposited energy corresponds to a supercritical phase with a temperature of about 1000 K and a pressure of more than 1 GPa. We use a time-delayed extreme ultraviolet pulse centered at 38.6 eV, and obtained via High Harmonic generation (HHG), to record valence band photoelectron spectra of the expanding water sample. The series of photoelectron spectra is analyzed with noise-corrected target transform fitting (cTTF), a specifically developed multivariate method. Together with a simple fluid dynamics simulation, the following picture emerges: when a supercritical phase of water expands into vacuum, temperature and density of the first few nanometers of the expanding phase drop below the critical values within a few picoseconds. This results in a supersaturated phase, in which condensation seeds form and grow from small clusters to large clusters on a 100 picosecond timescale.

Hydrogen bond dynamics of superheated water and methanol by ultrafast IR-pump and EUV-photoelectron probe spectroscopy

Snapshot of superheated water 40 ps after fs-IR laser excitation; representative aggregates formed during the simulation (close-up) compared to one obtained from superheated methanol phase (inset).

Angle-resolved valence shell photoelectron spectroscopy of neutral nanosized molecular aggregates

Angle-resolved photoelectron spectroscopy opens a new avenue to probe the orbital character of solutes and solvents from the nanoscale to the bulk.

A multi purpose source chamber at the PLEIADES beamline at SOLEIL for spectroscopic studies of isolated species: cold molecules, clusters, and nanoparticles

This paper describes the philosophy and design goals regarding the construction of a versatile sample environment: a source capable of producing beams of atoms, molecules, clusters, and nanoparticles in view of studying their interaction with short wavelength (vacuum ultraviolet and x-ray) synchrotron radiation. In the design, specific care has been taken of (a) the use standard components, (b) ensuring modularity, i.e., that swiftly switching between different experimental configurations was possible. To demonstrate the efficiency of the design, proof-of-principle experiments have been conducted by recording x-ray absorption and photoelectron spectra from isolated nanoparticles (SiO2) and free mixed clusters (Ar/Xe). The results from those experiments are showcased and briefly discussed.

Exploring Protonation and Deprotonation Effects with Auger Electron Spectroscopy

Auger electron spectroscopy is demonstrated to be a very efficient tool to probe alterations in local chemical environment due to changes in protonation states. We show that electronic and geometric structure changes induced by protonation or deprotonation are well reflected in Auger spectra through characteristic chemical shifts and spectral shape variations. We also present evidence that Auger spectra are sensitive to relative concentrations of compounds in different protonation states. Special attention is paid to the high kinetic energy spectral regions that exhibit remarkable features resulting from core ICD-like transitions in normal species and Auger transitions in deprotonated fragments. The latter contribution was so far ignored when explaining Auger spectra of species embedded in the environment. This contribution should be reconsidered, taking into account the recently discovered possibility of ultrafast dissociation of core-ionized hydrogen-bonded systems in media.

Accurate predictions of water cluster formation, (H₂O)(n=2-10)

An efficient mixed molecular dynamics/quantum mechanics model has been applied to the water cluster system. The use of the MP2 method and correlation consistent basis sets, with appropriate correction for BSSE, allows for the accurate calculation of electronic and free energies for the formation of clusters of 2-10 water molecules. This approach reveals new low energy conformers for (H(2)O)(n=7,9,10). The water heptamer conformers comprise five different structural motifs ranging from a three-dimensional prism to a quasi-planar book structure. A prism-like structure is favored energetically at low temperatures, but a chair-like structure is the global Gibbs free energy minimum past 200 K. The water nonamers exhibit less complexity with all the low energy structures shaped like a prism. The decamer has 30 conformers that are within 2 kcal/mol of the Gibbs free energy minimum structure at 298 K. These structures are categorized into four conformer classes, and a pentagonal prism is the most stable structure from 0 to 320 K. Results can be used as benchmark values for empirical water models and density functionals, and the method can be applied to larger water clusters.

Auger electron spectroscopy as a probe of the solution of aqueous ions

Aqueous potassium chloride has been studied by synchrotron-radiation excited core-level photoelectron and Auger electron spectroscopy. In the Auger spectrum of the potassium ion, the main feature comprises the final states where two outer valence holes are localized on potassium. This spectrum exhibits also another feature at a higher kinetic energy which is related to final states where outer valence holes reside on different subunits. Through ab initio calculations for microsolvated clusters, these subunits have been assigned as potassium ions and the surrounding water molecules. The situation is more complicated in the Auger spectrum of the chloride anion. One-center and multicenter final states are present here as well but overlap energetically.

Auger processes in the 21st century

PURPOSE

The extreme radiotoxicity of Auger electrons and their exquisite capacity to irradiate specific molecular sites has prompted scientists to extensively investigate their radiobiological effects. Their efforts have been punctuated by quadrennial international symposia that have focused on biophysical aspects of Auger processes. The latest meeting, the 6th International Symposium on Physical, Molecular, Cellular, and Medical Aspects of Auger Processes, was held 5-6 July 2007 at Harvard Medical School in Boston, Massachusetts, USA. This article provides a review of the research in this field that was published during the years 2004-2007, the period that has elapsed since the previous meeting.

CONCLUSION

The field has advanced considerably. A glimpse of the potential of this unique form of ionizing radiation to contribute to future progress in a variety of fields of study is proffered.

Charge delocalization dynamics of ammonia in different hydrogen bonding environments: free clusters and in liquid water solution

Valence and core level photoelectron spectra and Auger electron spectra of ammonia in pure clusters have been measured. The Auger electron spectra of gas-phase ammonia, pure ammonia clusters and ammonia in aqueous solution are compared and interpreted via ab initio calculations of the Auger spectrum of the ammonia monomer and dimer. The calculations reveal that the final two-hole valence states can be delocalized over both ammonia molecules. Features at energies pertaining to delocalized states involving one, or more, hydrogen bonding orbitals can be found in both the ammonia cluster Auger electron spectrum and in that of the liquid solvated molecule. The lower Coulombic repulsion between two delocalized valence final state holes gives higher kinetic energy of the Auger electrons which is also observed in the spectra. This decay path--specific to the condensed phase--is responsible for more than 5% of the total cluster Auger intensity. Moreover, this interpretation is also applicable to the solid phase since the same features have been observed, but not assigned, in the Auger spectrum of solid ammonia.

Angular distribution of Auger electrons from fixed-in-space and rotating C 1s-->2pi photoexcited CO: theory

The one-center approach for molecular Auger decay is applied to predict the angular distribution of Auger electrons from rotating and fixed-in-space molecules. For that purpose, phase shifts between the Auger decay amplitudes have been incorporated in the atomic model. The approach is applied to the resonant Auger decay of the photoexcited C 1s-->2pi resonance in carbon monoxide. It is discussed how the symmetry of the final ionic state is related to features in the angular distributions and a parametrization for the molecular frame Auger electron angular distribution is suggested. The angular distribution of Auger electrons after partial orientation of the molecule by the sigma-->pi-excitation process is also calculated and compared to available experimental and theoretical data. The results of the one-center approach are at least of the same quality as the available theoretical data even though the latter stem from a much more sophisticated method. As the one-center approximation can be applied with low computational demand even to extended systems, the present approach opens a way to describe the angular distribution of Auger electrons in a wide variety of applications.