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We report on the experimental observation of the direct decay of a core vacancy in van der Waals clusters by emission of a fast electron from a neighboring atom. The process can be regarded as an interatomic Coulombic decay of core holes (core-level ICD). We identify it unambiguously by electron-electron and electron-electron-photon coincidence spectroscopy of the decay of 2p vacancies in Ar clusters. While several earlier works reported the absence of this channel, we find core-level ICD to be of considerable significance and quantify the branching ratio of this nonlocal electron emission to conventional local Auger decay as (0.8±0.2)%. Our results are supported by calculations on smaller clusters and show a reasonable agreement. This report on a successfully performed electron-electron-photon coincidence experiment provides a perspective for explorations of matter exposed to ionizing radiation. The observed core-level ICD is proposed to be of general importance for studies on charge redistribution after core-level photoionization where van der Waals clusters are often used as prototype systems.

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.

Formamide, a simple model bio-molecule (HCONH2), is irradiated with high intensity, ultrashort pulses from an x-ray free electron laser. Ionic fragments resulting from photoionization and subsequent decay processes are recorded, as well as the electronic signature of the different inner shell ionization events that can take place during the x-ray pulses. The formation of double-core-hole states, where a second inner shell electron is removed before the first core hole has been refilled is observed in the electron spectra, recorded at all three sites (C, N, O) of the molecule. The individual ionization potentials are compared with results of ab initio calculations at different level of theory. Based on our results, future opportunities for advanced studies of inner-shell-induced electronic and nuclear dynamics are explored.

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.

International audience; 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.

With the advent of X-ray free electron lasers and the development of elaborate multi-coincidence methods combined with conventional synchrotron radiation, the interest in double core-hole spectroscopy revived. To describe the measured data and provide the guideline for yet coming experiments, theoretical studies are required. In this paper we review previous theoretical works on double core-hole states and discuss new theoretical results for the XH m –YH n (X, Y = C, N, O, or F; m , n = 0–3) molecules and their fluorine substituted compounds. We compute the single and double core-hole binding energies of these systems using different methods and show that the DFT results agree well with the results of other ab initio methods. In agreement with previous theoretical works, our study demonstrates that the changes in the ionization potential resulting from fluorine substitutions mainly arise due to changes in the ground state and not due to electron relaxation. Special emphasis is given to interatomic relaxation originating from creation of two core holes on different atomic sites. We demonstrate that there exists clear correlation between this quantity, the number of hydrogen atoms and the order of the bond between the heavy atoms in the systems studied.

We demonstrate that K-shell photoelectron spectroscopy is sensitive to binding motifs of cationic cores with excess electrons in solvated metals. Hereby, the satellites in core-level spectra and not the main line provide the important insight into ion-pairing effects. We found that contact ion-pairs in Li − ( NH 3 ) 1 – 3 and a solvent-separated ion-pair in Li − ( NH 3 ) 4 result in remarkably different Li1 s spectra. The former spectra have numerous intense satellites of a charge-transfer character while that of Li − ( NH 3 ) 4 contains only one strong satellite describing excitation of a solvated electron. We also explored K-shell photodetachment of the bare Li − and found that three-electron processes are essential here.

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.

X-ray photoelectron spectroscopy (XPS) and KLL Auger spectra of aqueous KCl solution were measured for the K$^+$ and Cl$^-$ edges. While the XPS spectra of potassium and chloride have similar structures, both exhibiting only weak satellite structures near the main line, the Auger spectra of these isoelectronic ions differ dramatically. A very strong satellite peak was found in the K$^+$ KLL Auger spectrum at the low kinetic energy side of the $^1$D state. Using equivalent core models and ab initio calculations this spectral structure was assigned to electron transfer processes from solvent water molecules to the solvated K$^+$ cation. Contrary to the potassium case, no extra peak was found in the KLL Auger spectrum of solvated Cl$^-$ indicating on a strong dependence of the underlying processes on ionic charge. The observed charge transfer processes are suggested to play an important role in charge redistribution following single and multiple core-hole creation in atomic and molecular systems placed into an environment.

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.

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)].

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.

Here, we report the observation of electron transfer mediated decay (ETMD) involving Mg clusters embedded in helium nanodroplets which is initiated by the ionization of helium followed by removal of two electrons from the Mg clusters of which one is transferred to the He environment neutralizing it while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. The photoelectron spectrum reveals a low energy ETMD peak. For Mg clusters larger than 5 atoms we observe stable doubly-ionized clusters. We argue that ETMD provides a new pathway to the formation of doubly-ionized cold species., 5 pages submitted to Phys. Rev. Lett

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.

A method for the fast and accurate solution of the radiative transfer equation in plane-parallel media with coherent isotropic scattering is presented. This largely analytical approach uses the formalism of meromorphic functions in order to obtain the total solution, i.e. the angle and depth variation of the radiation field. A discretization of space and angle coordinates is not required. For the further application in the accretion disks a particular case of a finite slab whose the midplane is the symmetry plane is considered., 20 pages, 4 figures, submitted to Reports on Mathematical Physics

Core ionization in systems with several equivalent atoms gives rise to a set of near-degenerate core-hole states each associated with the removal of an electron from one of the delocalized orbitals. The energy splitting between the core-hole states is the intrinsic feature of core ionized systems and should not be neglected. The conventional equivalent core model (ECM) predicts strictly degenerate core-hole states because the core-hole is thought of as completely localized on one of the equivalent centers. This failure as well as several others inherent to the ECM are successfully removed in the framework of the corrected ECM presented in this paper. Two approaches are available in accordance with the two representations of the core-hole, either delocalized or localized. The Z+1 approximation is an excellent starting point in the localized representation. It must be modified, however, when the delocalized picture is used. To this end we introduce a (Z+Q) system where Q is the magnitude of point charges ad...

The ability of the recently developed corrected equivalent core model (cECM) to predict properties of core hole states is examined for the CO molecule. It is shown that systematic corrections derived in this approach significantly improve the results of the conventional equivalent core model (ECM). This opens new possibilities to apply the equivalent core formalism to calculate the energy of core hole states which cannot be usefully done by the ECM itself. On the self-consistent-field level the predictions of geometry changes upon core ionization made by the cECM and those of direct core hole calculations are found to be very similar to each other. There exists, however, an appreciable difference between the total energies of the core hole states obtained in both approaches. A new procedure enabling us to improve the results of the cECM, in particular, to reduce this energy difference is proposed. In contrast to the cECM, where the corrections to the ECM energy are found by deriving the Hamiltonian HZ of ...

The correlated nature of electronic states in atoms and molecules is manifested in the simultaneous emission of two electrons after absorption of a single photon close to the respective threshold. Numerous observations in atoms and small molecules demonstrate that the double ionization efficiency close to threshold is rather small. In this Letter we show that this efficiency can be dramatically enhanced in the environment. To be specific, we concentrate on the case where the species in question has one or several He atoms as neighbors. The enhancement is achieved by an indirect process, where a He atom of the environment absorbs a photon and the resulting He(+) cation is neutralized fast by a process known as electron transfer mediated decay, producing thereby doubly ionized species. The enhancement of the double ionization is demonstrated in detail for the example of the Mg · He cluster. We show that the double ionization cross section of Mg becomes 3 orders of magnitude larger than the respective cross section of the isolated Mg atom. The impact of more neighbors is discussed and the extension to other species and environments is addressed.

The analytical solution of the plane-parallel radiative transfer equation is obtained in the two-stream approximation for a large class of continuous distributions of the de-excitation coecient " (constant, linear, parabolic, with spikes etc.). We present also the method of the discrete space theory for obtaining solutions of the transfer equation in the media with strong density inhomogeneities. These sets of the analytical solutions can be used for the solution of the inverse problem. The deduction of the internal distribution of" from observational data is facilitated in the case of isothermal media, since the characteristic behavior of the solution refers to the certain behavior of ". As an example, we nd the corresponding parameters of the constant and linear distributions of " precisely.

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.

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.

In this work, the four-component two-particle propagator technique is employed for the calculation of double ionization spectra of the noble gas atoms Ne through Rn. For a correct assignment of the individual final states, inclusion of spin-orbit coupling and electron correlation is mandatory and is accounted for in the framework of the relativistic propagator. It was observed that the $n{s}^{2}n{p}^{4}$(${}^{3}\phantom{\rule{-0.16em}{0ex}}{P}_{2,1,0}$, ${}^{1}\phantom{\rule{-0.16em}{0ex}}{D}_{2}$, ${}^{1}\phantom{\rule{-0.16em}{0ex}}{S}_{0}$) manifolds of all investigated noble gas dications exhibit a clear main-state character with only small admixture from other configurations. This also refers to the $2{s}^{1}2\phantom{\rule{-0.16em}{0ex}}{p}^{5}$(${}^{3}\phantom{\rule{-0.16em}{0ex}}{P}_{2,1,0}^{o}$, ${}^{1}\phantom{\rule{-0.16em}{0ex}}{P}_{1}^{o}$) states of Ne${}^{2+}$. In the argon, krypton, and xenon dications, the $n{s}^{1}n{p}^{5}$(${}^{3}\phantom{\rule{-0.16em}{0ex}}{P}_{2,1,0}^{o}$) states, and especially the $n{s}^{1}\phantom{\rule{-0.16em}{0ex}}n{p}^{5}$(${}^{1}\phantom{\rule{-0.16em}{0ex}}{P}_{1}^{o}$) ones, lose intensity due to pronounced configuration interaction. These states experience strong mixings with ground-state shake-up satellites, which occupy the same energy region. The composition of the $5{s}^{1}5{p}^{5}$(${}^{1}\phantom{\rule{-0.16em}{0ex}}{P}_{1}^{o}$) singlet state of Xe${}^{2+}$ is studied in detail by analyzing the corresponding eigenvector. As long as a $LS$ coupling picture can be approximately maintained, the amount of singlet-triplet splitting decreases in the sequence from neon to xenon. In the $6{s}^{1}6{p}^{5}$ manifold of Rn${}^{2+}$, a complete disappearance of well-defined main states takes place leading to a dense and complicated spectrum governed by very strong multiconfiguration effects. Relativistic corrections to the Coulomb interaction are accounted for by inclusion of the Gaunt (magnetic) term.

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.

X-ray photoelectron and Auger spectroscopies are well-suited for exploring the chemical state of a selected system. Chemical shifts of electronic transitions and line broadening in the respective spectra contain a wealth of information on the interaction of the core ionized system with its local environment. The presence of neighbors in the vicinity of the core ionized system is responsible for a number of other remarkable effects such as charge-transfer satellites in core ionization spectra and intermolecular electronic transitions in Auger spectra. In addition, due to the environment, some electronic states resulting from Auger decay may further decay electronically via the intermolecular Coulombic decay mechanism. This decay by emission of an electron would be impossible if the core ionized system were isolated. All of the above phenomena happen in the small ammonia clusters whose core ionization and Auger spectra were computed from first principles and are discussed in the present paper.

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.

After simultaneous ionization and excitation of one helium atom within the giant weakly bound helium dimer, the excited ion can relax via interatomic Coulombic decay (ICD) and the excess energy is transferred to ionize the neighboring helium atom. We showed [Sisourat et al. Nature Phys. 6, 508 (2010)] that the distributions of the kinetic energy released by the two ions reflect the nodal structures of the ICD-involved vibrational wave functions. We also demonstrated that energy transfer via ICD between the two helium atoms can take place over more than 14 \AA{}. We report here a more detailed analysis of the ICD process and of the impact of the nuclear dynamics on the electronic decay. Nonadiabatic effects during the ICD process and the accuracy of the potential energy curve of helium dimer and of the computed decay rates are also investigated.

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 A, 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.

The energy gained by either of the two helium atoms in a helium dimer through simultaneous ionization and excitation can be efficiently transferred to the other helium atom, which then ionizes. The respective relaxation process called interatomic Coulombic decay (ICD) is the subject of the present paper. Specifically, we are interested in ICD of the lowest of the ionized excited states, namely, the He{sup +}(n=2)He states, for which we calculated the relevant potential-energy curves and the interatomic decay widths. The full-configuration interaction method was used to obtain the potential-energy curves. The decay widths were computed by utilizing the Fano ansatz, Green's-function methods, and the Stieltjes imaging technique. The behavior of the decay widths with the interatomic distance is examined and is elucidated, whereby special emphasis is given to the asymptotically large interatomic separations. Our calculations show that the electronic ICD processes dominate over the radiative decay mechanisms over a wide range of interatomic distances. The ICD in the helium dimer has recently been measured by Havermeier et al. [Phys. Rev. Lett. 104, 133401 (2010)]. The impact of nuclear dynamics on the ICD process is extremely important and is discussed by Sisourat et al. [Nat. Phys. 6, 508 (2010)] based onmore » the ab initio data computed in the present paper.« less

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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3462246]

We explore the potential of double core hole electron spectroscopy for chemical analysis in terms of x-ray two-photon photoelectron spectroscopy (XTPPS). The creation of deep single and double core vacancies induces significant reorganization of valence electrons. The corresponding relaxation energies and the interatomic relaxation energies are evaluated by CASSCF calculations. We propose a method how to experimentally extract these quantities by the measurement of single and double core-hole ionization potentials (IPs and DIPs). The influence of the chemical environment on these DIPs is also discussed for states with two holes at the same atomic site and states with two holes at two different atomic sites. Electron density difference between the ground and double core-hole states clearly shows the relaxations accompanying the double core-hole ionization. The effect is also compared with the sensitivity of single core hole ionization potentials (IPs) arising in single core hole electron spectroscopy. We have demonstrated the method for a representative set of small molecules LiF, BeO, BF, CO, N2, C2H2, C2H4, C2H6, CO2 and N2O. The scalar relativistic effect on IPs and on DIPs are briefly addressed., 35 pages, 6 figures. To appear in J. Chem. Phys.

Physical review letters 103, 013002 (2009). doi:10.1103/PhysRevLett.103.013002, Published by APS, College Park, Md.

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.

Relaxation processes following inner-valence ionization of a system can be modified dramatically by embedding this system in a suitable environment. Surprisingly, such an environment can be even composed of helium atoms, the most inert species available. As demonstrated by the examples of Ne and Ca atoms embedded in He droplets, a fast relaxation process [Interatomic Coulombic Decay (ICD)] takes place merely due to the presence of the He surroundings. We have computed ICD widths for both 4He_N and 3He_N droplets doped with Ne and Ca and discuss the findings in some detail. In the case of Ne, ICD is by far the dominating relaxation pathway. In Ca, atomic Auger decay is also possible but ICD becomes a competitive relaxation pathway in the droplets., 10 pages, 3 figures

Anion-molecule clusters constitute a very suitable class of systems for studying intermolecular (interatomic) charge-transfer (CT) processes following core ionization. A weakly bound electron of the anion in these clusters can be easily transferred to the core-ionized molecule. The screening effect of this electron may have a dramatic impact on core-level spectra and even account for a breakdown of the quasiparticle picture of core ionization. This is demonstrated here by calculating the O1s(-1) and C1s(-1) core ionization spectra of the Na- CO cluster using an ab initio fourth-order Green's-function method. Interestingly, along with the CT processes in this cluster there exist also very efficient energy-transfer (ET) processes favored by the low excitation energies of Na-. These ET processes constitute an appreciable part of the electronic excitations following core ionization of Na- CO and exert thereby a strong influence on the spectra studied. The spectral features attributed to the ET processes are as pronounced as those attributed to the CT processes. Major differences in the behavior of CT and ET satellites as a function of the anion-molecule separation are found and explained. We compare also the O1s(-1) core ionization spectra of the Na- CO and Na- H2O clusters. Along with a certain similarity, these spectra exhibit substantial differences which are essentially attributed to the distinct cluster geometries.

Ab initio calculations of core-ionization spectra of the anion-molecule Cl-CO cluster are performed. Particular attention is paid to the investigation of charge-transfer screening processes accompanying core ionization of the CO molecule in the cluster. The charge-transfer processes are very efficient and favored by the presence of a low-lying unoccupied pi* orbital in CO capable of accepting an electron from Cl-. The O1s(-1) and C1s(-1) core-ionization spectra are calculated and compared. Both reveal a breakdown of the quasiparticle picture of core ionization caused by the charge-transfer processes. Remarkable differences between these two spectra are found which manifest themselves in distinct intensity distributions in the prominent low-energy spectral bands. The underlying reason for these differences is elucidated and linked with the preference of the pi* orbital to localize mainly on carbon. Core-ionization spectra of anion-molecule clusters are very sensitive to the type of the molecule involved as the comparative analysis of the O1s(-1) core-ionization spectra of the Cl-CO and Cl-H(2)O clusters show.

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.

The interatomic relaxation (IR) effects of two-site double core hole (tsDCH) states in selected molecules with a polarizable unit have been systematically investigated using ab initio calculations. The IR effects are analysed by varying the size of this polarizable unit and its position relative to the DCHs. The systems with the DCHs located on the opposite sides of the polarizable unit show large negative IR energies, while those on the same side of the polarizable unit have smaller negative IR effects. Here, the IR energies can even be positive if the polarizable unit is large enough. The generalized Wagner plots of tsDCH states are used to visualize the trend of the IR effects in the molecules studied.

Intermolecular Coulombic decay (ICD) is an ultrafast relaxation mechanism of a highly excited system embedded in a chemical environment. Our theoretical investigation of ammonia clusters shows that the ICD efficiency can be regulated by protonation or deprotonation. It is suggested that by varying ICD rate through changing the environmental pH value a control over the relaxation dynamics can be achieved.

On the example of the giant helium dimer, we present an efficient electronic decay process for excited atoms or molecules embedded in a chemical environment, called Interatomic (intermolecular) Coulombic decay (ICD). After simultaneous ionization and excitation of a helium atom within a helium dimer, the excited ion relaxes by ICD to He+(1s) and the neighbor neutral helium is ionized to He+(1s) as well and emits a secondary electron. A short review on ab initio methods developed during the last 10 years to accurately describe ICD is reported. Finally, the main striking results on the helium dimer obtained experimentally and theoretically are summarized.

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.

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.

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.

A finite element method for solving the monochromatic radiation transfer equation including scattering in three dimensions is presented. The algorithm employs unstructured grids which are adaptively refined. Adaptivity as well as ordinate parallelization reduce memory requirements and execution time and make it possible to calculate the radiation field across several length scales for objects with strong opacity gradients. An a posteriori error estimate for one particular quantity is obtained by solving the dual problem. The application to a sample of test problems reveals the properties of the implementation.