Electron spectroscopic study of nanoplasma formation triggered by intense soft x-ray pulses.

Using electron spectroscopy, we investigated the nanoplasma formation process generated in xenon clusters by intense soft x-ray free electron laser (FEL) pulses. We found clear FEL intensity dependence of electron spectra. Multistep ionization and subsequent ionization frustration features are evident for the low FEL-intensity region, and the thermal electron emission emerges at the high FEL intensity. The present FEL intensity dependence of the electron spectra is well addressed by the frustration parameter introduced by Arbeiter and Fennel [New J. Phys. 13, 053022 (2011)].

Damaging Intermolecular Energy and Proton Transfer Processes in Alpha-Particle-Irradiated Hydrogen-Bonded Systems.

Although the biological hazard of alpha-particle radiation is well-recognized, the molecular mechanisms of biodamage are still far from being understood. Irreparable lesions in biomolecules may not only have mechanical origin but also appear due to various electronic and nuclear relaxation processes of ionized states produced by an alpha-particle impact. Two such processes were identified in the present study by considering an acetylene dimer, a biologically relevant system possessing an intermolecular hydrogen bond. The first process is the already well-established intermolecular Coulombic decay of inner-valence-ionized states. The other is a novel relaxation mechanism of dicationic states involving intermolecular proton transfer. Both processes are very fast and trigger Coulomb explosion of the dimer due to creation of charge-separated states. These processes are general and predicted to occur also in alpha-particle-irradiated nucleobase pairs in DNA molecules.

Core Ionization Initiates Subfemtosecond Charge Migration in the Valence Shell of Molecules.

After the ionization of a valence electron, the created hole can migrate ultrafast from one end of the molecule to another. Because of the advent of attosecond pulse techniques, the measuring and understanding of charge migration has become a central topic in attosecond science. Here, we pose the hitherto unconsidered question whether ionizing a core electron will also lead to charge migration. It is found that the created hole in the core stays put, but in response to this hole interesting electron dynamics takes place which can lead to intense charge migration in the valence shell. This migration is typically faster than that after the ionization of a valence electron and transpires on a shorter time scale than the natural decay of the core hole by the Auger process, making the subject very challenging to attosecond science.

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.

Proton-transfer mediated enhancement of nonlocal electronic relaxation processes in X-ray irradiated liquid water.

We have simulated the oxygen 1s Auger-electron spectra of normal and heavy liquid water using ab initio and quantum dynamical methods. The computed spectra are analyzed and compared to recently reported experimental data. The electronic relaxation in liquid water exposed to ionizing X-ray radiation is shown to be far more diverse and complex than anticipated and extremely different than for an isolated water molecule. A core-level ionized water molecule in the liquid phase, in addition to a local Auger process, relaxes through nonlocal energy and charge transfer, such as intermolecular Coulombic decay and electron-transfer mediated decay (ETMD). We evaluate the relative efficiencies for these three classes of relaxation processes. The quantitative estimates for the relative efficiencies of different electronic decay modes help determine yields of various reactive species produced by ionizing X-rays. The ETMD processes which are considered here for the first time in the core-level regime are found to have a surprisingly high efficiency. Importantly, we find that all nonlocal electronic relaxation processes are significantly enhanced by ultrafast proton transfer between the core-ionized water and neighboring molecules.

Communication: electron transfer mediated decay enabled by spin-orbit interaction in small krypton/xenon clusters.

In this work we study the influence of relativistic effects, in particular spin-orbit coupling, on electronic decay processes in KrXe2 clusters of various geometries. For the first time it is shown that inclusion of spin-orbit coupling has decisive influence on the accessibility of a specific decay pathway in these clusters. The radiationless relaxation process is initiated by a Kr 4s ionization followed by an electron transfer from xenon to krypton and a final second ionization of the system. We demonstrate the existence of competing electronic decay pathways depending in a subtle way on the geometry and level of theory. For our calculations a fully relativistic framework was employed where omission of spin-orbit coupling leads to closing of two decay pathways. These findings stress the relevance of an adequate relativistic description for clusters with heavy elements and their fragmentation dynamics.

Probing the interface of doped isotopically mixed helium droplets by the directional anisotropy of interatomic Coulombic decay.

Interatomic Coulombic decay (ICD) represents an efficient electronic relaxation mechanism of an ionized or an excited system embedded in an environment. The type of this environment and its size have a great impact on the ICD performance. It is stressed that ICD is sensitive to the arrangement of neighboring atoms when the initially created vacancy has a polarization direction. This is demonstrated in the present paper for the case of a 3p-ionized Ca surrounded by He atoms. Useful explicit expressions are derived for the ICD widths which show that the neighbors located along the polarization direction of the ionized orbital have the largest contribution to the ICD rate. By comparison with ab initio results for small clusters, we also show that in a helium environment, the pairwise approximation represents a reliable approach for computing ICD widths. Using this approximation and the density distribution of the helium atoms obtained within density functional theory, we explore ICD in large isotopically mixed helium droplets doped with Ca. A special emphasis is given to the difference between the ICD widths for the Ca3p orbitals directed perpendicular and parallel to the droplet surface. Depending on the size and isotopic composition of the droplet, Ca resides in the interfacial layer between the (4)He core and the (3)He outer shell. Hence, ICD studies in these droplets may provide valuable information on the properties of this interface.

Interatomic relaxation effects in double core ionization of chain molecules.

Core vacancies created on opposite sides of a molecule operate against each other in polarizing the environment between them. Consequently, the relaxation energy associated with the simultaneous creation of these two core holes turns out to be smaller than the sum of the relaxation energies associated with each individual single core vacancy created independently. The corresponding residual, termed interatomic relaxation energy, is sensitive to the environment. In the present paper we explore how the interatomic relaxation energy depends on the length and type of carbon chains bridging two core ionized nitrile groups (-C≡N). We have uncovered several trends and discuss them with the help of simple electrostatic and quantum mechanical models. Namely, the absolute value of the interatomic relaxation energy depends strongly on the orbital hybridization in carbons being noticeably larger in conjugated chains (sp and sp(2) hybridizations) possessing highly mobile electrons in delocalized π-type orbitals than in saturated chains (sp(3) hybridization) where only σ bonds are available. The interatomic relaxation energy decreases monotonically with increasing chain length. The corresponding descent is determined by the energetics of the molecular bridge, in particular, by the HOMO-LUMO gap. The smallest HOMO-LUMO gap is found in molecules with the sp(2)-hybridized backbone. Here, the interatomic relaxation energy decreases slowest with the chain length.

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+). The comparison of this time constant to those associated with the aqueous K(+), Na(+), Mg(2+), and Al(3+) ions reveals differences of 1 and up to 2 orders of magnitude. Such large variations in the characteristic time scales of the core level ICD processes is qualitatively explained by different internal decay mechanisms in different ions as well as by different ion-solvent distances and interactions.

Inner-shell single and double ionization potentials of aminophenol isomers.

A comprehensive study of single and double core ionization potentials of the aminophenol molecule is reported. The role of relaxation, correlation, relativistic, and basis set effects in these potentials is clarified. Special attention is paid to the isomer dependence of the single and double core ionization potentials. Some of them are also compared with the respective values of the phenol and aniline molecules. It is shown that the core level single ionization potentials of the para-, meta-, and ortho-aminophenol molecules differ only slightly from each other, rendering these structural isomers challenging to distinguish for conventional x-ray photoelectron spectroscopy. In contrast, the energy needed to remove two core electrons from different atoms depends noticeably on the mutual arrangement and even on the relative orientations of the hydroxyl and amine groups. Together with the electrostatic repulsion between the two core holes, relaxation effects accompanying double core ionization play a crucial role here. The pronounced sensitivity of the double ionization potentials, therefore, enables a spectroscopic characterization of the electronic structure of aminophenol isomers by means of x-ray two-photon photoelectron spectroscopy.

Suppression of X-ray-Induced Radiation Damage to Biomolecules in Aqueous Environments by Immediate Intermolecular Decay of Inner-Shell Vacancies.

The predominant reason for the damaging power of high-energy radiation is multiple ionization of a molecule, either direct or via the decay of highly excited intermediates, as, e.g., in the case of X-ray irradiation. Consequently, the molecule is irreparably damaged by the subsequent fragmentation in a Coulomb explosion. In an aqueous environment, however, it has been observed that irradiated molecules may be saved from fragmentation presumably by charge and energy dissipation mechanisms. Here, we show that the protective effect of the environment sets in even earlier than hitherto expected, namely immediately after single inner-shell ionization. By combining coincidence measurements of the fragmentation of X-ray-irradiated microsolvated pyrimidine molecules with theoretical calculations, we identify direct intermolecular electronic decay as the protective mechanism, outrunning the usually dominant Auger decay. Our results demonstrate that such processes play a key role in charge delocalization and have to be considered in investigations and models on high-energy radiation damage in realistic environments.

Interatomic electronic decay driven by nuclear motion.

The interatomic electronic decay after inner-valence ionization of a neon atom by a single photon in a neon-helium dimer is investigated. The excited neon atom relaxes via interatomic Coulombic decay and the excess energy is transferred to the helium atom and ionizes it. We show that the decay process is only possible if the dimer's bond stretches up to 6.2 Å, i.e., to more than twice the equilibrium interatomic distance of the neutral dimer. Thus, it is demonstrated that the electronic decay, taking place at such long distances, is driven by the nuclear motion.

Possible electronic decay channels in the ionization spectra of small clusters composed of Ar and Xe: A four-component relativistic treatment.

Electronic decay of the inner-valence Ar 3s(-1) vacancy is energetically forbidden in an isolated argon atom and in all rare gas dimers where argon is present. However, if an argon atom has at least two suitable rare gas atoms in its neighborhood, the Ar 3s(-1) vacancy may decay electronically via an electron transfer mediated decay (ETMD) mechanism. An ArXe(2) cluster is considered in the present paper as an example of such systems. The single and double ionization spectra of different ArXe(2) isomers as well as of homonuclear Ar(2) and Xe(2) and heteronuclear ArXe clusters have been calculated by means of propagator methods to reveal possible electronic decay channels. A four-component version of the one-particle propagator utilizing the Dirac-Coulomb Hamiltonian was employed to obtain the single ionization potentials of the clusters studied. Hereby electron correlation, scalar relativistic effects, and spin-orbit couplings are described in a consistent manner. A two-particle propagator in its one-component form, in conjunction with effective core potentials to account consistently for correlation and scalar relativistic effects, was used to calculate the double ionization potentials. ETMD is shown to be the only possible electronic decay process of the Ar 3s(-1) vacancy in the ArXe(2) cluster. In clusters with more Xe atoms, alternative electronic decay mechanisms may appear.

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.

Core ionization of Na+ microsolvated in water and ammonia.

Microsolvation of ions is a subject of active explorations pursuing the objectives of understanding microscopic details of ion solvation in bulk solutions. In the present work core ionization of positively charged microsolvated clusters is addressed. This research area did not receive much attention so far. The Na 1s(-1) core ionization spectra of Na(+)(H(2)O)(n) and Na(+)(NH(3))(n) clusters have been calculated by means of an accurate ab initio Green's function method. Various dependencies of the core ionization energy on the number of solvent molecules, their arrangement, and chemical type are investigated and explained. Special emphasis is also given to the low-energy parts of the calculated core ionization spectra that exhibit numerous shake-up satellites mostly originating from the transfer of electrons from the solvent molecules to the Na(+) ion induced by core ionization. Spectral characteristics of selected charge-transfer satellites are discussed in detail.

Possible electronic decay channels in the ionization spectra of small clusters composed of Ar and Kr: a four-component relativistic treatment.

In this work single and double ionization spectra of the homo- and heteronuclear argon/krypton dimers and trimers are calculated by means of propagator methods where a four-component implementation was employed for the single ionizations. Scalar relativistic effects play only a minor role for the outer valence spectral structure, whereas spin-orbit coupling and electron correlation have to be treated adequately in order to reproduce the features correctly. Nonradiative decay mechanisms of subvalence vacancies in the argon and krypton dimers and trimers are discussed both for the interatomic Coulombic decay and the electron transfer mediated decay (ETMD). In the heteronuclear triatomic system which serves as a model for larger clusters, a possible ETMD process of the Ar 3s vacancy is found for the linear arrangement of the atoms. In the bent configuration the ETMD channel is closed.

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. In the considered exemplary aqueous KCl solution, the solvated isoelectronic K+ and Cl- ions exhibit notably different Auger electron spectra as a function of the photon energy. Differences appear due to dipole-forbidden transitions in aqueous K+ whose occurrence, according to the performed ab initio calculations, becomes possible only in the presence of solvent water molecules.

Observation of electron-transfer-mediated decay in aqueous solution.

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

Relaxation Processes in Aqueous Systems upon X-ray Ionization: Entanglement of Electronic and Nuclear Dynamics.

The knowledge of primary processes following the interaction of high-energy radiation with molecules in liquid phase is rather limited. In the present Perspective, we report on a newly discovered type of relaxation process involving simultaneous autoionization and proton transfer between adjacent molecules, so-called proton transfer mediated charge separation (PTM-CS) process. Within PTM-CS, transients with a half-transferred proton are formed within a few femtoseconds after the core-level ionization event. Subsequent nonradiative decay of the highly nonequilibrium transients leads to a series of reactive species, which have not been considered in any high-energy radiation process in water. Nonlocal electronic decay processes are surprisingly accelerated upon proton dynamics. Such strong coupling of electronic and nuclear dynamics is a general phenomenon for hydrogen-bonded systems, however, its probability correlates strongly with hydration geometry. We suggest that the newly observed processes will impact future high-energy radiation-chemistry-relevant modeling, and we envision application of autoionization spectroscopy for identification of solution structure details.