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Physical chemistry, chemical physics 24(38), 23119 - 23127 (2022). doi:10.1039/D2CP02383K special issue: "Ions, electrons, coincidences and dynamics: Festschrift for John H.D. Eland", We report the X-ray absorption of isolated H3O+ cations at the O 1s edge. The molecular ions were prepared in a flowing afterglow ion source which was designed for the production of small water clusters, protonated water clusters, and hydrated ions. Isolated H2O+ cations have been analyzed for comparison. The spectra show significant differences in resonance energies and widths compared to neutral H2O with resonances shifting to higher energies by as much as 10 eV and resonance widths increasing by as much as a factor of 5. The experimental results are supported by time-dependent density functional theory calculations performed for both molecular cations, showing a good agreement with the experimental data. The spectra reported here could enable the identification of the individual molecules in charged small water clusters or liquid water using X-ray absorption spectroscopy., Published by RSC Publ., Cambridge

Millisecond plasma pulses are used to convert vaporised melamine into pure TGCN with a large surface area for efficient water splitting.

The F 1s core level photoionization of the ionic molecular radical HF+ has been studied using the photon-ion merged-beams technique at a synchrotron radiation source. Upon analyzing kinetic energy release (KER) dependent photoion yield spectra, complex ultrafast dissociation dynamics of the F 1s core hole excited σ* state can be revealed. By means of configuration-interaction electronic structure calculations of the excited molecular potential energy curves, this complex process can be attributed to a spin-dependent dissociation of the excited σ* biradical state.

We present here the results of low-temperature magnetization and X-ray magnetic circular dichroism studies on the single crystals of BaFe12O19 which reveal for the first time the emergence of a spin glass phase, in coexistence with a long–range ordered ferrimagnetic phase, due to the freezing of the basal plane spin component.

Physical review / A 104(5), 053107 (2021). doi:10.1103/PhysRevA.104.053107, Experimental cross sections for $m$-fold photodetachment ($m=3-6$) of silicon anions via $K$-shell excitation and ionization were measured in the photon-energy range of 1830-1900~eV using the photon-ion merged-beams technique at a synchrotron light source. All cross sections exhibit a threshold behavior that is masked by pre-threshold resonances associated with the excitation of a $1s$ electron to higher, either partly occupied or unoccupied atomic subshells. Results from multi-configuration Dirac-Fock (MCDF) calculations agree with the experimentally derived cross sections for photo-absorption if small energy shifts are applied to the calculated resonance positions and detachment thresholds. Moreover, a systematic approach is applied for modeling the deexcitation cascades that set in after the initial creation of a $K$-shell hole. The resulting product charge-state distributions compare well with the measured ones for direct $K$-shell detachment but less well for resonant $K$-shell excitation. The present results are potentially useful for identifying silicon anions in cold plasmas such as interstellar gas clouds., Published by Inst., Woodbury, NY

Multiple ionization of the ${\mathrm{Ar}}^{+}(3{s}^{2}3{p}^{5})$ ion by a single photon has been investigated in the photon-energy range 250--1800 eV employing the photon-ion merged-beams technique. Absolute partial cross sections were measured for all ${\mathrm{Ar}}^{(1+m)+}$ product-ion channels with $1\ensuremath{\le}m\ensuremath{\le}6$ covering a size range from several tens of Mb down to a few b. Narrow $2p$-subshell excitation resonances were observed in all channels up to quadruple ionization at a photon-energy bandwidth of 52 meV. Double excitations involving a $2p$ and a $3s$ or $3p$ electron were also studied at high resolution and the measurements of the broad $2s$ excitation resonances directly showed their natural widths. Contributions of direct photo double ionization (PDI) to the production of the highest final Ar ion charge states are revealed, with PDI of the $2s$ subshell being mainly responsible for the production of ${\mathrm{Ar}}^{7+}$. The experiment made use of the PIPE setup installed at beamline P04 of the PETRA III synchrotron light source of DESY in Hamburg. The measurements were supported by theoretical calculations to identify the main contributions to the observed cross sections. Comparisons of theory and experiment show remarkable agreement but also hint to additional ionization mechanisms that are not considered in the theoretical models such as core ionization accompanied by excitations with subsequent Auger decays leading to net $m$-fold ionization with $m\ensuremath{\ge}4$.

The astrophysical journal 908(1), 52 - (2021). doi:10.3847/1538-4357/abcc64, Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for single and up to five-fold photoionization of Fe$^{2+}$ ions in the energy range 690--920~eV. This range contains thresholds and resonances associated with ionization and excitation of $2p$ and $2s$ electrons. Calculations were performed to simulate the total absorption spectra. The theoretical results show very good agreement with the experimental data, if overall energy shifts of up to 2.5~eV are applied to the calculated resonance positions and assumptions are made about the initial experimental population of the various levels of the Fe$^{2+}$([Ar]$3d^6$) ground configuration. Furthermore, we performed extensive calculations of the Auger cascades that result when an electron is removed from the $2p$ subshell of Fe$^{2+}$. These computations lead to a better agreement with the measured product-charge-state distributions as compared to earlier work. We conclude that the $L$-shell absorption features of low-charged iron ions are useful for identifying gas-phase iron in the interstellar medium and for discriminating against the various forms of condensed-phase iron bound to composite interstellar dust grains., Published by Univ.11032, Chicago, Ill. [u.a.]

Scientific reports 11(1), 2996 (2021). doi:10.1038/s41598-021-82518-4, Mass-selected V and Fe monomers, as well as the heterodimer Fe1V1, were deposited on a Cu(001) surface. Their electronic and magnetic properties were investigated via X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. Anisotropies in the magnetic moments of the deposited species could be examined by means of angle resolving XMCD, i.e. changing the X-ray angle of incidence. A weak adatom-substrate-coupling was found for both elements and, using group theoretical arguments, the ground state symmetries of the adatoms were determined. For the dimer, a switching from antiparallel to parallel orientation of the respective magnetic moments was observed. We show that this is due to the existence of a noncollinear spin-flop phase in the deposited dimers, which could be observed for the first time in such a small system. Making use of the two magnetic sublattices model, we were able to find the relative orientations for the dimer magnetic moments for different incidence angles., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

Chem 8, 1-14 (2021). doi:10.1016/j.chempr.2021.11.021, Magnetism in organic materials is very intriguing: the realization of long-range magnetic order in completely metal-free systems com- bines magnetic moments with useful properties of organic mate- rials. An essential step for applications is the knowledge of the magnetic behavior that, so far, was investigated only in the prox- imity of light atoms to heavy metals, impurities, or vacancies, and not as an intrinsic property of thin films that are purely organic. Here, we unravel X-ray magnetic circular dichroism at the carbon and nitrogen K-edges in purely organic radical thin films. Our results show that, controlling the preparation, the magnetic behavior is different than in single crystals. Atomistic simulations indicate that the reason for this is the molecular arrangement in the films when compared with that in the single crystal. The tuning of the film mag- netic properties by the molecular arrangement is an exciting perspective toward revealing new properties and applications., Published by Elsevier, Philadelphia, PA

We present here the results of low-temperature magnetization and X-ray magnetic circular dichroism studies on the single crystals of BaFe12O19 which reveal for the first time the emergence of a spin glass phase, in coexistence with a long-range ordered ferrimagnetic phase, due to the freezing of the basal plane spin component.

Physical review / X 10(4), 041056 (2020). doi:10.1103/PhysRevX.10.041056, The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry., Published by APS, College Park, Md.

Journal of physics / B Atomic, molecular and optical physics 53(19), 1-8 (2020). doi:10.1088/1361-6455/aba719 special issue: "Special issue on Frontiers of AMO Science with FELs and Synchrotron Radiation", We review the recent experimental and theoretical progress in $K$-shell detachment studies of atomic anions. On the experimental side, this field has largely benefitted from technical advances at 3rd generation synchrotron radiation sources. For multiple detachment of C$^-$, O$^-$, and F$^-$ ions, recent results were obtained at the photon-ion merged-beams setup PIPE which is a permanent end station at beamline P04 of the PETRA\,III synchrotron light source in Hamburg. In addition to a much increased photon flux as compared to what was available previously, the PIPE setup has an extraordinary detection sensitivity for heavy charged reaction products that allows one to study detachment processes with extremely low cross sections in the kilobarn range, e.g., for processes involving the simultaneous creation of two core-holes by a single photon as observed in the net triple detachment of F$^-$ and the net five-fold detachment of C$^-$. Moreover, hitherto disregarded photodetachment resonances have been discovered, which exhibit a variety of line-shapes. For O$^-$ the core-hole lifetime could be determined precisely from a high-resolution measurement of a photodetachment resonance. These experimental findings pose new challenges for state-of-the-art atomic theory and require calculations combining photoexcitation (ionization) with decay cascade processes that follow after initial core-hole production., Published by IOP Publ., Bristol

Physical review letters 124, 083203 (2020). doi:10.1103/PhysRevLett.124.083203, We report on new measurements of m-fold photodetachment (m = 2 − 5) of carbon anions via K-shellexcitation and ionization. The experiments were carried out employing the photon-ion merged-beamstechnique at a synchrotron light source. While previous measurements were restricted to doubledetachment (m = 2) and to just the lowest-energy K-shell resonance at about 282 eV, our absoluteexperimental m-fold detachment cross sections at photon energies of up to 1000 eV exhibit a wealth of newthresholds and resonances. We tentatively identify these features with the aid of detailed atomic-structurecalculations. In particular, we find unambiguous evidence for fivefold detachment via double K-holeproduction., Published by APS, College Park, Md.

Journal of physics / Conference series Conference series [...] 1412, 152024 - (2020). doi:10.1088/1742-6596/1412/15/152024, Single and multiple photoionization of Si1+, Si2+, and Si3+ ions have been investigated near thesilicon K-edge using the PIPE setup at beamline P04 of the synchrotron light source PETRA III operated byDESY in Hamburg, Germany. Pronounced resonance structures are observed for all ions which are associatedwith excitation or ionization of a K-shell electron. The experimental cross sections are compared with resultsfrom theoretical calculations., Published by IOP Publ.20382, Bristol

Scientific reports 8(1), 12371 (2018). doi:10.1038/s41598-018-30750-w, Ultra-small clusters containing few atoms are of high interest in both fundamental research and applications due to their specific functional, magnetic or chemical properties which depend on size and composition. The experimental results of the morphology of the size-selected clusters, consisting of few atoms can be an ideal benchmark for sophisticated theoretical models. With this motivation we have investigated the geometrical structure of mass-selected Au$_9$ clusters deposited on a silicon substrate prepared by soft-landing conditions. We present results obtained experimentally by Grazing-Incidence Small-Angle X-ray Scattering (GISAXS). Considering the ultra-small size of the clusters and small quantities of material on the surface, we combined advanced techniques which allowed us to investigate the surface structure of the sample. The resulting structural sizes are in concordance with cluster theory. Using a model-based approach, the advanced X-ray techniques allow for understanding how to resolve the possible cluster structure, identify optimal experimental conditions and obtain the probable morphological information which is challenging to be obtained otherwise., Published by Springer Nature, [London]

Yields of atomic iodine I

Highly conductive silver-nanowire (Ag-NW) networks are used in composite materials as conductive channels. Their resistance tuning can be accomplished by changing the Ag-NW concentration, and, therefore, changing the network structure. In this study, an alternative pathway to resistance engineering of conductive Ag-NW networks by local atmospheric plasma treatment is employed. The corresponding changes in nanowire network morphology and crystallinity as a function of plasma etching time are investigated by time-resolved grazing-incidence X-ray scattering, field-effect scanning electron microscopy, and X-ray photoelectron spectroscopy. Three characteristic etching phases are identified. The first two phases enable the controlled engineering of the electrical properties with different rates of resistance change, which results from changes in nanowire shape, network morphology, and different oxidation rates. Phase III is characterized by pronounced fragmentation and destruction of the Ag-NW networks. These results show the feasibility of atmospheric plasma treatments to tune the local electrical properties of conductive Ag-NW networks. Furthermore, we present a physical Monte Carlo model explaining the electrical network properties as a function of plasma etching time based on the network connectivity and a constant plasma etching rate of 570 ng s−1 cm−2.

Ion yields following X-ray absorption of the cationic series NH

Author(s): Muller, A; Martins, M; Kilcoyne, ALD; Phaneuf, RA; Hellhund, J; Borovik, A; Holste, K; Bari, S; Buhr, T; Klumpp, S; Perry-Sassmannshausen, A; Reinwardt, S; Ricz, S; Schubert, K; Schippers, S | Abstract: Photoprocesses of the endohedral fullerene ions Sc3N@C80+ and Sc3N@C80- in the gas phase have been investigated in the photon energy ranges 30-50 eV and 280-420 eV. Single and double ionization as well as single ionization accompanied by the release of a C2 dimer were observed as a function of the photon energy for the positive parent ion and double detachment was measured for the negative parent ion. The emphasis of the experiments was on the specific effects of the encapsulated trimetallic nitride cluster Sc3N on the observed reactions. Clear evidence of photoexcitation near the Sc L edge is obtained with the dominating contributions visible in the one- and two-electron-removal channels. K-vacancy production in the encapsulated central nitrogen atom is seen in the single ionization of Sc3N@C80+ but is much less pronounced in the photoionization-with-fragmentation channel. Comparison of the cross sections near the carbon K edge with the corresponding channels measured previously in the photoionization of Lu3N@C80+ reveal strong similarities. Previously predicted sharp resonance features in the ionization of Sc3N@C80+ ions below the Sc M edge are not confirmed. The experiments are accompanied by quantum-chemistry calculations in the Hartree-Fock approximation and by model calculations employing density functional theory (DFT).

The astrophysical journal / 1 Part 1 887(2), 189 - (2019). doi:10.3847/1538-4357/ab5118, Relative cross sections for m-fold photoionization (m=1,K,5) of Fe3+ by single-photon absorption weremeasured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source operatedat DESY in Hamburg, Germany. The photon energies used spanned the range of 680–950 eV, covering both thephotoexcitation resonances from the 2p and 2s shells, as well as the direct ionization from both shells.Multiconfiguration Dirac–Hartree–Fock (MCDHF) calculations were performed to simulate the totalphotoexcitation spectra. Good agreement was found with the experimental results. These computations helpedto assign several strong resonance features to specific transitions. We also carried out Hartree–Fock calculationswith relativistic extensions taking into account both photoexcitation and photoionization. Furthermore, weperformed extensive MCDHF calculations of the Auger cascades that result when an electron is removed from the2p and 2s shells of Fe3+. Our theoretically predicted charge-state fractions are in good agreement with theexperimental results, representing a substantial improvement over previous theoretical calculations. The mainreason for the disagreement with the previous calculations is their lack of inclusion of slow Auger decays of severalconfigurations that can only proceed when accompanied by de-excitation of two electrons. In such cases, thisadditional shake-down transition of a (sub)valence electron is required to gain the necessary energy for the releaseof the Auger electron., Published by Institute of Physics Publ., London

Physical chemistry, chemical physics 21(45), 25415-25424 (2019). doi:10.1039/C9CP04640B, Yields of atomic iodine Iq+ (q Z 2) fragments resulting from photoexcitation and photoionisation of thetarget cations CHxI+ (x = 0–3) have been measured in the photon-energy range 610 eV to 670 eV,which comprises the threshold for iodine 3d ionisation. The measured ion-yield spectra show twostrong and broad resonance features due to the excitation of the 3d3/2,5/2 electrons into ef states similarto atomic iodine. In the 3d pre-edge range, electrons are excited into molecular orbitals consisting ofiodine, carbon, and hydrogen atomic orbitals. These transitions have been identified by comparison withliterature data and by simulations using time-dependent density functional theory (TDDFT) with theKMLYP functional. The ion-yield spectrum for CH3I+ resembles the spectrum of IH+ [Klumpp et al., Phys.Rev. A, 2018, 97, 033401] because the highest occupied molecular orbitals (HOMO) of the H and CH3fragments both contain a single vacancy, only. For the molecular cations with higher number ofvacancies in the valence molecular orbitals CHxI+ (x = 0–2), a stronger hybridisation of the molecularorbitals occurs between the organic fragment and the iodine resulting in a change of bonding from asingle s bond in CH3I+ to a triple bond including two p orbitals in CI+. This is reflected in the resonanceenergies of the observed absorption lines below the iodine 3d excitation threshold., Published by RSC Publ., Cambridge

Synopsis Results from the first soft X-ray user experiment at the European XFEL on nonlinear photon-matter interaction will be presented. Angle-resolved electron time-of-flight spectroscopy employed at the AQS (Atom-ic-like Quantum Systems) endstation of the SQS (Small Quantum Systems) instrument reveals insight into the character of resonances in highly transient, core ionized neon ions, i.e. Ne+ 1s12s22p6 → Ne+* 1s02s22p6np, together with their respective relaxation dynamics. Enabled by the unique properties of the European XFEL, novel perspectives on efficient nonlinear spectroscopy will be discussed.

Synopsis Photoprocesses of the endohedral fullerene ions Sc 3 N @ C 80 + and Sc 3 N @ C 80 − in the gas phase have been experimentally investigated with emphasis on the specific effects of the encapsulated trimetallic nitride cluster Sc3N on the observed reaction yields. Clear evidence of photoexcitation near the Sc L edge is obtained with the dominating contributions visible in the one- and two-electron-removal channels. The experiments are accompanied by model calculations employing density functional theory (DFT).

Author(s): Muller, A; Lindroth, E; Bari, S; Borovik, A; Hillenbrand, PM; Holste, K; Indelicato, P; Kilcoyne, ALD; Klumpp, S; Martins, M; Viefhaus, J; Wilhelm, P; Schippers, S | Abstract: In a joint experimental and theoretical endeavor, photoionization of metastable C4+(1s2sS13) ions via intermediate levels with hollow, double-K-vacancy configurations 2s2p, 2s3p, 2p3s, 2p3d, 2s4p, 2p4s, and 2p4d has been investigated. High-resolution photon-ion merged-beams measurements were carried out with the resolving power reaching up to 25 800 which is sufficient to separate the leading fine-structure components of the 2s2pP3 term. Many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. Their uncertainties are estimated to be below ±1 meV. Detailed balance confirms the present photoionization cross-section results by comparison with previous dielectronic-recombination measurements. The high accuracy of the theoretical transition energies together with the present experimental results qualify photoabsorption resonances in heliumlike ions as new, greatly improved energy-reference standards at synchrotron radiation facilities.

Review of scientific instruments 89(3), 036103 - (2018). doi:10.1063/1.5018396, A transmission polarizer for producing elliptically polarized soft X-ray radiation from linearly polarized light is presented. The setup is intended for use at synchrotron and free-electron laser beamlines that do not directly offer circularly polarized light for, e.g., X-ray magnetic circular dichroism (XMCD) measurements or holographic imaging. Here, we investigate the degree of ellipticity upon transmission of linearly polarized radiation through a cobalt thin film. The experiment was performed at a photon energy resonant to the Co L3-edge, i.e., 778 eV, and the polarization of the transmitted radiation was determined using a polarization analyzer that measures the directional dependence of photo electrons emitted from a gas target. Elliptically polarized radiation can be created at any absorption edge showing the XMCD effect by using the respective magnetic element., Published by American Institute of Physics, [S.l.]

Double and triple detachment of the ${\mathrm{F}}^{\ensuremath{-}}(1{s}^{2}2{s}^{2}2{p}^{6})$ negative ion by a single photon have been investigated in the photon energy range 660 to 1000 eV. The experimental data provide unambiguous evidence for the dominant role of direct photodouble detachment with a subsequent single-Auger process in the reaction channel leading to ${\mathrm{F}}^{2+}$ product ions. Absolute cross sections were determined for the direct removal of a $(1s+2p)$ pair of electrons from ${\mathrm{F}}^{\ensuremath{-}}$ by the absorption of a single photon.

Author(s): Muller, A; Borovik, A; Buhr, T; Hellhund, J; Holste, K; Kilcoyne, ALD; Klumpp, S; Martins, M; Ricz, S; Viefhaus, J; Schippers, S | Abstract: Single, double, and triple ionization of the C+ ion by a single photon have been investigated in the energy range 286 to 326 eV around the K-shell single-ionization threshold at an unprecedented level of detail. At energy resolutions as low as 12 meV, corresponding to a resolving power of 24 000, natural linewidths of the most prominent resonances could be determined. From the measurement of absolute cross sections, oscillator strengths, Einstein coefficients, multielectron Auger decay rates, and other transition parameters of the main K-shell excitation and decay processes are derived. The cross sections are compared to results of previous theoretical calculations. Mixed levels of agreement are found despite the relatively simple atomic structure of the C+ ion with only five electrons. This paper is a followup to a previous Letter [A. Muller, Phys. Rev. Lett. 114, 013002 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.013002].

Clusters are structures in the nano- or sub-nanometer regime ranging from a few atoms up to several thousand atoms per cluster. Supported metal clusters and adatoms are interesting systems for magnetic studies as their magnetic properties can strongly depend on the size, composition and the substrate due to quantum size effects. This offers rich possibilities to tailor systems to specific applications by choosing the proper size and composition of the cluster. In this article the magnetic properties of small 3d metal and alloy clusters in the few atom limit measured by X-ray magnetic circular dichroism and how their spin and orbital moments depend on size and composition are discussed. Special emphasis is put on the non-collinear magnetic coupling in the clusters resulting in complex spin structures and the influence of oxidation on the magnetic properties of the clusters.

The Photon-Ion Spectrometer at PETRA III - in short, PIPE - is a permanently installed user facility at the "Variable Polarization XUV Beamline" P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The careful design of the PIPE ion-optics in combination with the record-high photon flux at P04 has lead to a breakthrough in experimental studies of photon interactions with ionized small quantum systems. This short review provides an overview over the published scientific results from photon-ion merged-beams experiments at PIPE that were obtained since the start of P04 operations in 2013. The topics covered comprise photoionization of ions of astrophysical relevance, quantitative studies of multi-electron processes upon inner-shell photoexcitation and photoionization of negative and positive atomic ions, precision spectroscopy of photoionization resonances, photoionization and photofragmentation of molecular ions and of endohedral fullerene ions., Comment: 10 pages, 9 figures

The fragmentation pattern of the iodine-containing molecules CH2I2 and CH3I following a strong multiphoton excitation in the vicinity of the iodine 4d giant resonance regime is studied using soft X-ray free electron laser radiation. A strong difference of the charge distribution and the kinetic energy release (KER) for the two molecules is found. The effects can be attributed to charge rearrangement processes induced by the photoexcitation. The difference in the observed distribution for higher charge states of iodine and carbon fragments is consistent with an over-the-barrier model for the charge rearrangement in the dissociating molecules. The KER for singly ionised carbon fragments indicates an ultrafast charge rearrangement before the dissociation starts.

The astrophysical journal 836(2), 166 (2017). doi:10.3847/1538-4357/836/2/166, Single, double, and triple photoionization of Ne$^{+}$ ions by single photons have been investigated at the synchrotron radiation source PETRA III in Hamburg, Germany. Absolute cross-sections were measured by employing the photon–ion merged-beams technique. Photon energies were between about 840 and 930 eV, covering the range from the lowest-energy resonances associated with the excitation of one single K-shell electron up to double excitations involving one K- and one L-shell electron, well beyond the K-shell ionization threshold. Also, photoionization of neutral Ne was investigated just below the K edge. The chosen photon energy bandwidths were between 32 and 500 meV, facilitating the determination of natural line widths. The uncertainty of the energy scale is estimated to be 0.2 eV. For comparison with existing theoretical calculations, astrophysically relevant photoabsorption cross-sections were inferred by summing the measured partial ionization channels. Discussion of the observed resonances in the different final ionization channels reveals the presence of complex Auger-decay mechanisms. The ejection of three electrons from the lowest K-shell-excited Ne$^{+}$$(1s 2s^{2}$ $2p^{6}$ $^{2}$$S_{1/2})$ level, for example, requires cooperative interaction of at least four electrons.

XXX International Conference on Photonic, Electronic, and Atomic Collisions, Cairns, Australia, 25 Jul 2017 - 1 Aug 2017 ; Journal of physics / Conference Series 875, 032034(2017). doi:10.1088/1742-6596/875/4/032034, Cross sections for photoionization and photofragmentation of singly, doubly and triply charged Lu3N@C80 ions have been measured using the photon-ion merged-beams technique at the PETRA III synchrotron light source. The measured spectra exhibit prominent resonance features at energies around the carbon K edge. In contrast to that, no signatures of the nitogen K and lutetium M edges have been found in the experimentally investigated reaction channels., Published by IOP Publ., Bristol

Absolute cross sections for m-fold photoionization (m=1,...,6) of Fe+ by a single photon were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680-920 eV which covers the photoionization resonances associated with 2p and 2s excitation to higher atomic shells as well as the thresholds for 2p and 2s ionization. The corresponding resonance positions were measured with an uncertainty of +- 0.2 eV. The cross section for Fe+ photoabsorption is derived as the sum of the individually measured cross-sections for m-fold ionization. Calculations of the Fe+ absorption cross sections have been carried out using two different theoretical approaches, Hartree-Fock including relativistic extensions and fully relativistic Multi-Configuration Dirac Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross sections are in good agreement with each other and with the experimental results. In addition, the complex deexcitation cascades after the creation of inner-shell holes in the Fe+ ion have been tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium., Comment: 18 pages, 10 figures, 4 tables, accepted for publication in The Astrophysical Journal

Clusters are small systems ranging from a few atoms up to several thousand atoms. They are of high interest in basic research, but also for applications due to their specific electronic, magnetic or chemical properties depending on size and composition. For small clusters, quantum size effects play an important role and specific material properties might be tailored by choosing a special size or composition of the cluster. Here, we review the magnetic properties of adatoms and supported small mass-selected transition-metal clusters in the few-atom limit investigated by x-ray magnetic circular dichroism spectroscopy in the soft x-ray regime. The influence of cluster size, composition, the cluster-surface and intra-cluster interaction on the spin and orbital magnetic moments will be discussed.

New journal of physics 21(3), 033017 (2019). doi:10.1088/1367-2630/ab1056, We present studies on intra-molecular charge redistribution in iodine containing molecules upon iodine-4d photoionization. For this, we employed an XUV-pump-XUV-probe scheme based on time-delayed femtosecond pulses delivered by the free-electron laser at DESY in Hamburg (FLASH). The experimental results show delay dependent and molecule-specific iodine charge state distributions that arise upon multiple iodine-4d photoionization. Using the example of CH$_3$I and CH$_2$I$_2$, we compare the delay-dependent yields of I$^{3+}$. We model the involved processes using advanced ab initio electronic structure calculations which include electron correlations combined with a classical model of the nuclear motion. The qualitative agreement of our model with the experimental results allows us to relate the observed, strongly molecule-specific efficiencies of the intra-molecular charge rearrangement not only to molecule-specific fragmentation timescales but also to molecule-specific electronic structure and molecular environment., Published by IOP, [London]

Physical review / B 93(14), 144421 (2016). doi:10.1103/PhysRevB.93.144421, Ultrasmall magnetic clusters and adatoms are of strong current interest because of their possible use in future technological applications. Here, we demonstrate that the magnetic coupling between the adsorbates and the substrate can be significantly changed through oxidation. The magnetic properties of Co adatoms and CoPd dimers deposited on a remanently magnetized Ni/Cu(100) substrate have been investigated by x-ray absorption and x-ray magnetic circular dichroism spectroscopy at the Co L$_{2,3}$ edges. Using spectral differences, pure and oxidized components are distinguished, and their respective magnetic moments are determined. The Co adatoms and the CoPd dimers are coupled ferromagnetically to the substrate, while their oxides, Co-O and CoPd-O, are coupled antiferromagnetically to the substrate. Along with the spin reorientation from the pure to the oxidized state, the magnetic moment of the adatom is highly reduced from Co to Co-O. In contrast, the magnetic moment of the dimer is of similar order for CoPd and CoPd-O., Published by Inst., Woodbury, NY

The photon-ion merged-beams technique was used at a synchrotron light source for measuring absolute cross sections of double and triple photodetachment of O$^{-}$ ions. The experimental photon energy range of 524-543 eV comprised the threshold for K-shell ionization. Using resolving powers of up to 13,000, the position, strength and width of the below-threshold 1s 2s2 2p6 2S resonance as well as the positions of the 1s 2s2 2p5 3P and 1s 2s2 2p5 1P thresholds for K-shell ionization were determined with high precision. In addition, systematically enlarged multi-configuration Dirac-Fock calculations have been performed for the resonant detachment cross sections. Results from these ab-initio computations agree very well with the measurements for the widths and branching fractions for double and triple detachment, if double shake-up (and -down) of the valence electrons and the rearrangement of the electron density is taken into account. For the absolute cross sections, however, a previously found discrepancy between measurements and theory is confirmed., Comment: 5 pages, 2 figures

We investigate the photodouble ionization of H2 molecules with 400 eV photons. We find that the emitted electrons do not show any sign of two-center interference fringes in their angular emission distributions if considered separately. In contrast, the quasiparticle consisting of both electrons (i.e., the "dielectron") does. The work highlights the fact that nonlocal effects are embedded everywhere in nature where many-particle processes are involved, This work was funded by the Deutsche Forschungsgemeinschaft (DFG), the BMBF, the European COST Action Grant No. XLIC CM1204, the European Research Council Advanced XCHEM Grant No. 290853, and the MINECO Project Grant No. FIS2013-42002-R. J. L. thanks the DFG for support. We are grateful to the staff of PETRA III for excellent support during the beam time. Raw data are archived at the Goethe-University Frankfurt am Main and are available on request