Search

The role of orbital polarization for ultrafast charge transfer between an atomic adsorbate and a substrate is explored. Core hole clock spectroscopy with linearly polarized x-ray radiation allows to selectively excite adsorbate resonance states with defined spatial orientation relative to the substrate surface. For c(4×2)S/Ru(0001) the charge transfer times between the sulfur 2s-13p*+1 antibonding resonance and the ruthenium substrate have been studied, with the 2s electron excited into the 3p⊥* state along the surface normal and the 3p∥* state in the surface plane. The charge transfer times are determined as 0.18±0.07 and 0.84±0.23 fs, respectively. This variation is the direct consequence of the different adsorbate-substrate orbital overlap. [ABSTRACT FROM AUTHOR]

Using high resolution S 2p and O 1s x-ray photoelectron spectroscopies, the adsorption of SO2 and its surface bound reaction products on Ru(0001) have been investigated simultaneously while dosing SO2 and while heating the adsorbed species. SO2 is found to adsorb on Ru(0001) at 100 K molecularly in two variants as well as dissociatively and to react to SO3, SO4, SO, and S with increasing coverage. After the monolayer has been saturated, SO2 adsorbs molecularly in multilayers. When heating adsorbed SO2 from 100 K, SO, SO2, and SO4 decompose in a wide temperature range up to 305 K. In contrast SO3 is found to be stable bound to Ru(0001) up to 300 K and to disappear from the surface to below 325 K. At 550 K the surface remains with a saturated atomic sulfur and oxygen layer and some sulfur species in a second layer. Our quantitative analysis of the sulfur amount bound to the surface supports a simple desorption process only for SO4. All other species mainly or partly decompose on the surface. [ABSTRACT FROM AUTHOR]

Potential energy surfaces are a central tool to rationalize, in a multidimensional system, how distortions along defined degrees of freedom link conformation and structural modifications to energy. The local and global mimima define stable conformations and the ground state. Crossings between the ground- and electronically excited-state potential energy surfaces often define rate-limiting steps in chemistry. Here, optically excited or spin-excited states live on different potential energy surfaces. How a molecular wavepacket propagating on a potential energy surface bifurcates at, e.g., a saddle point creates selectivity in a reaction pathway. As a fully quantum mechanical description, wavepacket interference plays an important role. In chemistry, active atomic sites are at the core of excitations, bond creation, and breaking, and thus detailed local information on the potential energy surface surrounding these active atomic centers holds the key to rate and selectivity. The electronic ground state is experimentally elusive to measure, since any measurement determines, at best, modification of the ground state. However, resonant inelastic X-ray scattering (RIXS) has next to electronic and vibronic excitations spectral losses of purely structural excitations [1]. Due to the transient strong distortion in the core excited state, vibrational and rotational excitations up to the dissociation limit within the ground-state potential surface become thus observable at selected atomic sites [1–5]. The ensuing vibrational progressions then allow us to reconstruct the underlying local ground-state potential energy surface [6]. Subnatural linewidth RIXS also allows selective gating of vibrational excitations, which can be efficiently converted into directional excitation within a molecular frame [7]. With this approach of subnatural linewidth RIXS, direct experimental access to potential energy surfaces becomes a reality; in particular, at the active centers and for distortions far from equilibrium that are close to transition and bifurcation points in reactions. Applying this approach of sub-natural line width RIXS to electronically or spin-excitated states will lead to Anti-Stokes spectral signatures [8], which additionally give insight into the potential energy surfaces of excited states. Here, sub-natural line width RIXS with transform-limited pulses will open a completely new approach to excited-state dynamics and reaction pathways. The Heisenberg RIXS apparatus at the European XFEL exemplifies this science field. [ABSTRACT FROM AUTHOR]

As the use of free electron laser (FEL) sources increases, so do the findings mentioning non-linear phenomena occurring at these experiments, such as saturable absorption, induced transparency and scattering breakdowns. These are well known among the laser community, but are still rarely understood and expected among the X-ray community and to date lack tools and theories to accurately predict the respective experimental parameters and results. We present a simple theoretical framework to access short X-ray pulse induced light–matter interactions which occur at intense short X-ray pulses as available at FEL sources. Our approach allows to investigate effects such as saturable absorption, induced transparency and scattering suppression, stimulated emission, and transmission spectra, while including the density of state influence relevant to soft X-ray spectroscopy in, for example, transition metal complexes or functional materials. This computationally efficient rate model based approach is intuitively adaptable to most solid state sample systems in the soft X-ray spectrum with the potential to be extended for liquid and gas sample systems as well. The feasibility of the model to estimate the named effects and the influence of the density of state is demonstrated using the example of CoPd transition metal systems at the Co edge. We believe this work is an important contribution for the preparation, performance, and understanding of FEL based high intensity and short pulse experiments, especially on functional materials in the soft X-ray spectrum. • Tractable model approach based on tabulated atomic cross sections and properties. • Estimate of stimulated scattering for solid state FEL experiments. • Signatures of stimulated scattering e.g. CoPd multilayers. • Pulse induced transparency and scattering breakdown e.g. CoPd multilayers. [ABSTRACT FROM AUTHOR]

Abstract: Soft X-ray spectroscopy is one of the best tools to directly address the electronic structure, the driving force of chemical reactions. It enables selective studies on sample surfaces to single out reaction centers in heterogeneous catalytic reactions. With core-hole clock methods, specific dynamics are related to the femtosecond life time of a core-hole. Typically, this method is used with photoemission spectroscopy, but advancements in soft X-ray emission techniques render more specific studies possible. With the advent of bright femtosecond pulsed soft X-ray sources, highly selective pump-probe X-ray emission studies are enabled with temporal resolutions down to tens of femtoseconds. This finally allows to study dynamics in the electronic structure of adsorbed reaction centers on the whole range of relevant time scales – closing the gap between kinetic soft X-ray studies and the atto- to femtosecond core-hole clock techniques. [Copyright &y& Elsevier]

Abstract: Charge transfer pathways and charge transfer times in molecular films and in adsorbate layers depend both on the details of the electronic structure as well as on the degree of the initial localization of the propagating excited electronic state. For C6F6 molecular adsorbate films on the Cu(111) surface we determined the interplay between excited state localization and charge transfer pathways. In particular we selectively prepared a free-particle-like LUMO band excitation and compared it to a molecularly localized core-excited C1s→ π ⁎ C6F6 LUMO state using time-resolved two-photon photoemission (tr-2PPE) and core–hole–clock (CHC) spectroscopy, respectively. For the molecularly localized core-excited C1s→ π ⁎ C6F6 LUMO state, we separate the intramolecular dynamics from the charge transfer dynamics to the metal substrate by taking the intramolecular dynamics of the free C6F6 molecule into account. Our analysis yields a generally applicable description of charge transfer within molecular adsorbates and to the substrate. [Copyright &y& Elsevier]

Abstract: With modern soft X-ray methods, the whole field of electron–phonon interactions becomes accessible directly in the ultrafast time domain with ultrashort pulsed X-ray sources, as well as in the energy domain through modern highly resolving spectrometers. The well-known core-hole clock approach plays an intermediate role, resolving energetic and temporal features at the same time. In this perspective paper, we review several experiments to illustrate the modern advances in the selective study of electron–phonon interactions as fundamentally determining ingredients for materials properties. We present the different complementary approaches that can be taken with soft X-ray methods to conquer this field beyond the Born–Oppenheimer approximation. [Copyright &y& Elsevier]

We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses. [ABSTRACT FROM AUTHOR]

Photon-induced band bending changes and subsequent relaxation at the buried insulator/semiconductor interface of BaF2/Si(001) have been investigated. This is achieved by creating free charge carriers through absorption of a visible pump laser pulse (λ=532 nm, E=2.33 eV) in the silicon, followed by time resolved photoelectron spectroscopy using vacuum-ultraviolet synchrotron radiation (E=18 eV) as a probe of the BaF2 valence band (VB). Since the excitation takes place in the semiconductor and the probe photoemission signal originates from the BaF2, the relaxation at the interface has been probed. [ABSTRACT FROM AUTHOR]

Abstract: Core-hole-clock spectroscopy is increasingly used to determine femtosecond and attosecond charge transfer processes initiated at an atomically defined starting point. We give proof that the time constant of a charge transfer process determined by the core-hole-clock method is probed, but not governed by the spectroscopic process. To this end, charge transfer in c(4×2)S/Ru(0001) has been probed by two investigations using core level resonances with identical symmetry but excitation photon energies one order of magnitude apart (S2s→3p at 227.5eV, Ss→3p at 2471.1±0.3eV). Despite the large difference in excitation energy, the measured charge transfer times agree. [Copyright &y& Elsevier]

In complex systems, composed from many atomic units, it is of equal importance to determine the time scales of dynamic processes and to have control at which atomic site a dynamic process was initiated. With resonant X-ray spectroscopic tools like autoionization and resonant inelastic X-ray scattering, an atomically localized excited state can be created and its dynamics monitored in comparison to the ultrafast time scale of the transient core-hole resonance. With this core-hole-clock method, charge transfer dynamics of only 320±90 attoseconds could be determined for Sulphur adsorbed on the Ru(0001) surface, extending the core-hole-clock method into the range of attoseconds. Exploiting the symmetry selection rules of resonant inelastic X-ray scattering, ultrafast atomic motion can also be investigated, which changes the molecular symmetry during a transient core-excited state. With this approach the vibrational dynamics of C2H4 on the time scale of a Carbon 1s core-hole resonance has been investigated. [ABSTRACT FROM AUTHOR]

Abstract: The orientation of hexafluorobenzene (C6F6) on the Cu(111) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C6F6 differ significantly in comparison to its hydrocarbon analog C6H6. C6F6 is found to adsorb on Cu(111) with the ring plane parallel to the surface for coverages below 10ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate. [Copyright &y& Elsevier]

Abstract: The influence of the surface projected electronic structure on resonant charge transfer is investigated on the Cu(111) and Cu(100) surfaces using core hole clock spectroscopy and wave packet propagation computations. The charge transfer time of core excited adsorbed in a dense Ar monolayer is 5.6±0.1fs and 3.5±0.1fs for the Cu(111) and Cu(100) surfaces. A two times longer residence time on Cu(111) than on Cu(100) is also obtained from theory and is attributed to the differences in the projected electronic band structure of the two metal surfaces, especially to the different energies of the surface projected band gap. [Copyright &y& Elsevier]

Abstract: The general aspects of CO bonding in carbonyls and on metal surfaces are discussed in terms of a molecular orbital description, based on calculations and direct experimental measurements. In particular, the excellent agreement between experiment and theory for CO adsorbed on Ni(100) is the starting point to derive in the same theoretical approach the orbital-based electronic structure of the carbonyls Ni(CO)4, Fe(CO)5, Cr(CO)6 and CO coordinated to the heme group. A frontier orbital interaction scheme involving only the 2π* and the 5σ orbitals is shown to be inadequate for the description of the electronic structure; all orbitals change due to the interaction. Instead, we demonstrate how the formation of an allylic configuration in the π-system, involving both the 1π and 2π* orbitals, gives a more complete description of the π electronic structure. The rearrangement of the σ orbitals involves mainly mixing of occupied CO and metal orbitals resulting in Pauli repulsion. The latter is minimized by polarization of charge away from the CO–Me bond. The bonding picture for CO in organometallic systems is very similar to the surface adsorption case; the main idea of repulsive σ and bonding π interactions seems generally valid when CO interacts with metals. [Copyright &y& Elsevier]

Dynamical processes are commonly investigated using laser pump–probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10-18 s) is a promising recent development. These ultrafast pulses have been fully characterized, and used to directly measure light waves and electronic relaxation in free atoms. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10-15 s). Here we monitor the dynamics of ultrafast electron transfer—a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices—on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as. [ABSTRACT FROM AUTHOR]

The bonding of ethylene to Cu(1 1 0) and Ni(1 1 0) is analyzed in detail using symmetry-resolved X-ray absorption (XAS) and emission (XES) spectroscopies in conjunction with density functional theory (DFT) calculations of geometric structure and spectra. XES, which probes the occupied valence states, reveals the formation of bonding and non-bonding orbitals of π-3d as well as π*-3d character. Additional mixing of σ and π states indicates rehybridization upon adsorption. The anti-bonding π-3d and π*-3d combinations are unoccupied and seen in XAS. A lower intensity of the π* transition for Ni is evidence of larger π* occupancy upon bonding. The position of the σ* shape-resonance indicates a 0.02 Å longer C–C bond on Ni than on Cu, in good agreement with the DFT structure optimizations. The XE spectra are well-reproduced both by specific spectrum calculations based on cluster models and by the carbon p-density of states calculated using periodic boundary conditions. The contribution of both π and π* levels to the new, surface-induced occupied states close to the Fermi-level lends support to the traditional Dewar–Chatt–Duncanson picture of the bonding. Theoretical charge-density difference plots support an alternative view of ethylene bonding in terms of the specific involvement of the excited molecular triplet state. Based on the variation in XE intensities the main difference between ethylene bonding to Cu and Ni is found to be an about two times larger occupancy of the π* orbital upon chemisorption on the transition metal, which comes along with C–C bond elongation and stronger σ–π rehybridization. [Copyright &y& Elsevier]

We have investigated the resonant charge transfer from an adsorbed Ar atom in an electronically excited 2p3/254s1 state into different metal substrates, i.e., Ag(1 1 1), Ni(1 1 1) and Cu(1 1 1). Using the so-called core hole clock method we have been able to determine the resonant charge transfer times for the three systems. They differ roughly by a factor of two with the fastest transfer time being observed for Ni(1 1 1) and the slowest for Ag(1 1 1). Furthermore we observe strong energy dependences of the charge transfer times across the resonances which are opposite to the expectations from a simple tunneling model, or from a detuning induced shortening of the interaction time. Possible reasons for the observed behavior are discussed. [Copyright &y& Elsevier]

We present a polarization resolved near edge X-ray absorption fine structure (NEXAFS) investigation of ethylene (C2H4) adsorbed on the oriented single-domain Si(0 0 1)-(2 × 1) surface.From the detected resonances and their polarization dependences C2H4 is found to be strongly bound to the silicon dimers with the carbon atoms in a sp3-hybridized state. The molecular axis is rotated around the surface normal with respect to the dimer axis. [Copyright &y& Elsevier]

Investigates the solvolysis reactions of thiatricycle. Desulfurization of thionesters; Synthesis of bornane derivatives; Formation of thiolactone.

We present a time-dependent theory for non-resonant x-ray emission spectrum (XES) and normal Auger spectrum (NAS) calculation, based on a fully quantum description of nuclear dynamics using the vibrational wave packet concept. We compare two formulations of the time-dependent theory, either employing a two-time propagation scheme or using spectral integration over the electron energy continuum. We find that the latter formulation is more efficient for numerical simulations, providing a reasonable accuracy when the integration step is shorter than the lifetime broadening of the core-ionized state. We demonstrate our approach using the example of non-resonant x-ray emission from a water molecule, considering the lowest core-ionized K−1 and first core-ionized shake-up K−1V−1V1 intermediate states. These channels exemplify the developed theory on bound–bound, bound–continuum, continuum–bound, and continuum–continuum transitions. Our results suggest that the time-dependent approach is efficient for simulating XES involving dissociative states, whereas the time-independent approach, based on Franck–Condon factors, is more efficient for bound–bound transitions expressed as discrete frequency dependence in the energy domain. The methods and discussion have general applicability, including both NAS and more complex systems, such as liquid water. [ABSTRACT FROM AUTHOR]

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for the quantum nuclear wave packet simulations, showing the important role it plays in RIXS. [ABSTRACT FROM AUTHOR]

The so-called vacuum space-charge acceleration of emitted electrons triggered by absorption of soft X-rays (a few 100 eV) under intense near-infrared laser excitation is studied experimentally and theoretically. The influence of high excitation densities on the properties of Auger and photo electrons liberated by a probe X-ray beam is investigated for grazing-incidence photons interacting with an atomically clean Cu (111) surface as a model system. Transient electron spectra have been taken in a pump–probe setup at the BESSY II storage ring using a newly developed compact electrostatic retarding Bessel-box spectrometer. Strong electron-energy shifts have been found and assigned to space-charge acceleration. Model calculations based on experimental input parameters are in good agreement with experimentally obtained energy-gain values indicating ultimate limits of ultrafast X-ray experiments with photo or Auger electrons as a probe. [ABSTRACT FROM AUTHOR]

The nitrogen and sulfur K-edge X-ray absorption spectra of aqueous 2-thiopyridone, a model system for excited-state proton transfer in several recent time-resolved measurements, have been simulated from ab initio molecular dynamics. Spectral signatures of the local intra- and inter-molecular structure are identified and rationalized, which facilitates experimental interpretation and optimization. In particular, comparison of aqueous and gas phase spectrum simulations assesses the previously unquantified solvatization effects, where hydrogen bonding is found to yield solvatochromatic shifts up to nearly 1 eV of the main peak positions. Thereby, while each K-edge can still decisively determine the local protonation of its core-excited site, only their combined, complementary fingerprints allow separating all of the three relevant molecular forms, giving a complete picture of the proton transfer. [ABSTRACT FROM AUTHOR]

We report on a combined theoretical and experimental study of core-excitation spectra of gas and liquid phase methanol as obtained with the use of X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). The electronic transitions are studied with computational methods that include strict and extended second-order algebraic diagrammatic construction [ADC(2) and ADC(2)-x], restricted active space second-order perturbation theory, and time-dependent density functional theory—providing a complete assignment of the near oxygen K-edge XAS. We show that multimode nuclear dynamics is of crucial importance for explaining the available experimental XAS and RIXS spectra. The multimode nuclear motion was considered in a recently developed "mixed representation" where dissociative states and highly excited vibrational modes are accurately treated with a time-dependent wave packet technique, while the remaining active vibrational modes are described using Franck–Condon amplitudes. Particular attention is paid to the polarization dependence of RIXS and the effects of the isotopic substitution on the RIXS profile in the case of dissociative core-excited states. Our approach predicts the splitting of the 2a″ RIXS peak to be due to an interplay between molecular and pseudo-atomic features arising in the course of transitions between dissociative core- and valence-excited states. The dynamical nature of the splitting of the 2a″ peak in RIXS of liquid methanol near pre-edge core excitation is shown. The theoretical results are in good agreement with our liquid phase measurements and gas phase experimental data available from the literature. [ABSTRACT FROM AUTHOR]

Electron spectroscopy with the unprecedented transmission of angle-resolved time-of-flight detection, in combination with pulsed X-ray sources, brings new impetus to functional materials science. We showcase recent developments towards chemical sensitivity from electron spectroscopy for chemical analysis and structural information from photoelectron diffraction using the phase transition properties of 1T-TaS2. Our development platform is the SurfaceDynamics instrument located at the Femtoslicing facility at BESSY II, where femtosecond and picosecond X-ray pulses can be generated and extracted. The scientific potential is put into perspective to the current rapidly developing pulsed X-ray source capabilities from Lasers and Free-Electron Lasers. [ABSTRACT FROM AUTHOR]

A comprehensive investigation of the emission characteristics for electrons induced by X-rays of a few hundred eV at grazing-incidence angles on an atomically clean Cu(111) sample during laser excitation is presented. Electron energy spectra due to intense infrared laser irradiation are investigated at the BESSY II slicing facility. Furthermore, the influence of the corresponding high degree of target excitation (high peak current of photoemission) on the properties of Auger and photoelectrons liberated by a probe X-ray beam is investigated in time-resolved pump and probe measurements. Strong electron energy shifts have been found and assigned to space-charge acceleration. The variation of the shift with laser power and electron energy is investigated and discussed on the basis of experimental as well as new theoretical results. [ABSTRACT FROM AUTHOR]

In heterogeneous catalysis the surface-adsorbate bond strength is critical for the function of the system. Here we study a series consisting of multilayer, bilayer and monolayer N 2 O on Ni(111) and employ Auger-photoelectron coincidence spectroscopy (APECS) to study the interaction between the molecule and the substrate directly. We observe intensity in the nitrogen Auger spectra that arise from the interaction between molecule and surface (not observed in free molecules) whereas the oxygen spectra are thickness-independent. Since the two nitrogen atoms of N 2 O are chemically inequivalent we can assign the intensity present in the bilayer and monolayer cases to orbitals centered on the terminal nitrogen which is closest to the Ni(111) surface. Using ab initio , molecular dynamics and solid-state density functional theory calculations we infer a Blyholder model of the surface bond as consisting of donation from the terminal nitrogen lone-pair valence orbital with back-donation from the metal into the unoccupied orbitals on that nitrogen. This coincidence technique can readily be used to study substrate–adsorbate interactions directly with chemical and orbital specificity — this opens up prospects to study fundamental steps of molecular adsorption and heterogeneous catalysis with unprecedented detail. [Display omitted] • Direct observation of surface–molecular electron sharing. • Coincidence spectroscopy reveals hidden spectral features. • Blyholder backdonation between N 2 O and Ni(111). [ABSTRACT FROM AUTHOR]

The unprecedented beam properties of free-electron laser based X-ray sources enable novel resonant inelastic X-ray scattering (RIXS) experiments. Femtosecond time-resolved RIXS can be used to follow charge, spin and structural dynamics of dilute solute molecules in solution. Ultrashort X-ray pulses allow probing of highly radiation sensitive states of matter such as the metastable phase of supercooled liquid water. Nonlinear X-ray probes like amplified spontaneous emission and stimulated resonant X-ray scattering provide an enhanced selectivity and sensitivity as well as a path to control radiation damage and increase the photon yields in RIXS experiments. [ABSTRACT FROM AUTHOR]

The inherent stability of methylated formamides is traced to a stabilization of the deep-lying σ-framework by resonant inelastic X-ray scattering at the nitrogen K-edge. Charge transfer from the amide nitrogen to the methyl groups underlie this stabilization mechanism that leaves the aldehyde group essentially unaltered and explains the stability of secondary and tertiary amides. [ABSTRACT FROM AUTHOR]

The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H2O molecular moieties. This is mirrored in a manifold of the H2O molecular potentials. Subnatural line width resonant inelastic X-ray scattering allowed us to quantify the manifold of molecular potential energy surfaces along the H2O symmetric normal mode and the local asymmetric O–H bond coordinate up to 1 and 1.5 Å, respectively. The comparison of the single H2O molecular potentials and spectroscopic signatures with the ambient conditions liquid phase H2O molecular potentials is done on various levels. In the gas phase, first principles, Morse potentials, and stepwise harmonic potential reconstruction have been employed and benchmarked. In the liquid phase the determination of the potential energy manifold along the local asymmetric O–H bond coordinate from resonant inelastic X-ray scattering via the bound state oxygen 1s to 4a1 resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1s to 2b2 resonance is based on stepwise harmonic reconstruction. We find in liquid water at ambient conditions H2O molecular potentials ranging from the weak interaction limit to strongly distorted potentials which are put into perspective to established parameters, i.e., intermolecular O–H, H–H, and O–O correlation lengths from neutron scattering. [ABSTRACT FROM AUTHOR]

We present calculations on the quantum electrodynamics (QED) effects in 1s and 2s single and double ionization potentials of noble gases from Ne to Rn as perturbations on relativistic fourcomponent Dirac-Fock wavefunctions. The most dominant effect originates from the self-energy of the core-electron that yields corrections of similar order as the transverse interaction. For 1s ionization potentials, a match within few eV against the known experimental values is obtained, and our work reveals considerable QED effects in the photoelectron binding energies across the periodic table-most strikingly even for Ne. We perform power-law fits for the corrections as a function of Z and interpolate the QED correction of ~-0.55 eV for S1s. Due to this, the K-edge electron spectra of the third row and below need QED for a match in the absolute energy when using state-of-the-art instrumentation. [ABSTRACT FROM AUTHOR]

When materials are exposed to X-ray pulses with sufficiently high intensity, various nonlinear effects can occur. The most fundamental one consists of stimulated electronic decays after resonant absorption of X-rays. Such stimulated decays enhance the number of emitted photons and the emission direction is confined to that of the stimulating incident photons which clone themselves in the process. Here we report the observation of stimulated resonant elastic (REXS) and inelastic (RIXS) X-ray scattering near the cobalt L3 edge in solid Co/Pd multilayer samples. We observe an enhancement of order 106 of the stimulated over the conventional spontaneous RIXS signal into the small acceptance angle of the RIXS spectrometer. We also find that in solids both stimulated REXS and RIXS spectra contain contributions from inelastic electron scattering processes, even for ultrashort 5 fs pulses. Our results reveal the potential and caveats of the development of stimulated RIXS in condensed matter. X-ray free electron lasers provide brilliant light with sufficient intensity to strongly drive and probe nonlinear X-ray-matter interactions. Here, spectroscopy of Co/Pd multilayers reveals a nearly one-million-fold enhancement of stimulated resonant inelastic X-ray scattering versus conventional, spontaneous scattering. [ABSTRACT FROM AUTHOR]

Abstract: The lithium aluminium hydride reduction of 2,2,4,4-tetrachloro-8-oxabicyclo[3.2.1]oct-6-en-3-one (8) was reinvestigated. In contrast to most halogeno-substituted oxabicyclic ketones, which give predominantly the corresponding endo alcohols, the expected (3endo)-2,2,4,4-tetrachloro-8-oxabicyclo[3.2.1]oct-6-en-3-ol (9n) is formed in a minute proportion. An X-ray structure analysis of the dominating product gave proof of the exo-alcohol, i.e., (3exo)-2,2,4,4-tetrachloro-8-oxabicyclo[3.2.1]oct-6-en-3-ol (9x). On the other hand, reduction of trichloroketone 11, 2,2,endo-4-trichloro-8-oxabicyclo[3.2.1]oct-6-en-3-one, and the methoxy-substituted chloroketones 13 and 14 provided the corresponding endo alcohols (12 and 15). [Copyright &y& Elsevier]

The adsorption of acetylene on single-domain Si(0 0 1)-(2 × 1) surfaces has been investigated at room temperature using fully polarization resolved near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Two coexisting adsorption species have been observed, which we ascribe to the dimerized and end-bridge structures. In both cases, the Si dimers remain intact and the acetylene carbon atoms rehybridize to sp2. [Copyright &y& Elsevier]

The electron–phonon scattering is one of the main microscopic mechanisms responsible for the spin-flip in the transient state of ultrafast demagnetization. Here, we present an experimental determination of the temperature-dependent electron–phonon scattering rate in Gd. Using a static x-ray emission spectroscopy method, where the reduction of the decay peak intensities when increasing the temperature is quantified, we measure independently the electron-phonon scattering rate for the 5d and the 4f electrons. We deduce the temperature dependence of scattering for the 5d electrons, while no effect on the phonon population is observed for the 4f electrons. Our results suggest that the ultrafast magnetization dynamics in Gd is triggered by the spin-flip in the 5d electrons. We also evidence the existence of a temperature threshold, above which spin-flip scattering of the 5d electrons takes place. We deduce that during the transient state of ultrafast demagnetization, the exchange energy between 5d electrons has to be overcome before the microscopic electron-phonon scattering process can occur. [ABSTRACT FROM AUTHOR]

The chemical shift of core level binding energies makes electron spectroscopy for chemical analysis (ESCA) a workhorse analytical tool for science and industry. For some elements, close lying and overlapping spectral features within the natural life time broadening restrict applications. We establish how the core level binding energy chemical shift can be picked up experimentally by the additional selectivity through Auger electron photoelectron coincidence spectroscopy (APECS). Coincident measurement of Ni 3p photoemission with different MVV Auger regions from specific decay channels, narrows the 3p core-levels to a width of 1.2 eV, resolves the spin–orbit splitting of 1.6 eV and determines the chemical shift of Ni 3p levels of a Ni(111) single crystal and its oxidized surface layer to 0.6 eV. [ABSTRACT FROM AUTHOR]

The layered dichalcogenide MoS 2 is relevant for electrochemical Li adsorption/intercalation, in the course of which the material undergoes a concomitant structural phase transition from semiconducting 2H-MoS 2 to metallic 1T-Li x MoS 2 . With the core hole clock approach at the S L 1 X-ray absorption edge we quantify the ultrafast directional charge transfer of excited S3p electrons in-plane (‖ ) and out-of-plane (⊥ ) for 2H-MoS 2 as τ 2 H , ‖ = 0.38 ± 0.08 fs and τ 2 H , ⊥ = 0.33 ± 0.06 fs and for 1T-Li x MoS 2 as τ 1 T , ‖ = 0.32 ± 0.12 fs and τ 1 T , ⊥ = 0.09 ± 0.07 fs. The isotropic charge delocalization of S3p electrons in the semiconducting 2H phase within the S-Mo-S sheets is assigned to the specific symmetry of the Mo-S bonding arrangement. Formation of 1T-Li x MoS 2 by lithiation accelerates the in-plane charge transfer by a factor of ∼ 1.2 due to electron injection to the Mo-S covalent bonds and concomitant structural repositioning of S atoms within the S-Mo-S sheets. For excitation into out-of-plane orbitals, an accelerated charge transfer by a factor of ∼ 3.7 upon lithiation occurs due to S-Li coupling. [ABSTRACT FROM AUTHOR]

Quenching of vibrational excitations in resonant inelastic X-ray scattering (RIXS) spectra of liquid acetic acid is observed. At the oxygen core resonance associated with localized excitations at the O–H bond, the spectra lack the typical progression of vibrational excitations observed in RIXS spectra of comparable systems. We interpret this phenomenon as due to strong rehybridization of the unoccupied molecular orbitals as a result of hydrogen bonding, which however cannot be observed in x-ray absorption but only by means of RIXS. This allows us to address the molecular structure of the liquid, and to determine a lower limit for the average molecular chain length. [ABSTRACT FROM AUTHOR]

How different microscopic mechanisms of ultrafast spin dynamics coexist and interplay is not only relevant for the development of spintronics but also for the thorough description of physical systems out-of-equilibrium. In pure crystalline ferromagnets, one of the main microscopic mechanism of spin relaxation is the electron-phonon (el-ph) driven spin-flip, or Elliott-Yafet, scattering. Unexpectedly, recent experiments with ferro- and ferrimagnetic alloys have shown different dynamics for the different sublattices. These distinct sublattice dynamics are contradictory to the Elliott-Yafet scenario. In order to rationalize this discrepancy, it has been proposed that the intra- and intersublattice exchange interaction energies must be considered in the microscopic demagnetization mechanism, too. Here, using a temperature-dependent x-ray emission spectroscopy (XES) method, we address experimentally the element specific el-ph angular momentum transfer rates, responsible for the spin-flips in the respective (sub)lattices of Fe 20 Ni 80 , Fe 50 Ni 50 and pure nickel single crystals. We establish how the deduced rate evolution with the temperature is linked to the exchange coupling constants reported for different alloy stoichiometries and how sublattice exchange energies threshold the related el-ph spin-flip channels. Thus, these results evidence that the Elliott-Yafet spin-flip scattering, thresholded by sublattice exchange energies, is the relevant microscopic process to describe sublattice dynamics in alloys and elemental magnetic systems. [ABSTRACT FROM AUTHOR]

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface. [ABSTRACT FROM AUTHOR]

The evolution of the geometric and electronic structures within the entire series of lanthanide orthophosphate nanoparticles (∼2–∼5 nm) has been determined experimentally with X-ray diffraction and near edge X-ray absorption fine structure spectroscopy. In particular, the interplay between electronic structure, crystal morphology, and crystal phase has been systematically studied. A missing local order in the crystal structure accompanied by multiple ion sites in the nanoparticles was revealed to be due to the small crystal size and large surface contribution. All lanthanide ions were found to be in “3+” configuration and accommodated in three different crystallization states: the larger lanthanide ions (La, Ce, Pr, Nd, Sm) in the monoclinic phase, the smaller ones (Er, Tm, Yb, Lu) in the tetragonal phase, and the intermediate lanthanide ions (Eu, Gd, Tb, Dy, Ho) in a “mixed phase” between monoclinic and tetragonal phases. [ABSTRACT FROM AUTHOR]

Es wird gezeigt, dass Kovalenz die Ladungstrennung nach Photooxidation des Metallzentrums in einem Metall‐Ligand‐Ladungstransferzustand eines Eisensensibilisators ausgleicht. Die Fähigkeit der Liganden zur σ‐Bindung kompensiert den Verlust einer 3d‐Elementarladung am Eisenzentrum, sodass die ursprüngliche Ladungsdichte und lokale Edelgaskonfiguration am Metall erhalten bleibt. Diese Erkenntnisse werden durch elementspezifische und orbitalselektive zeitaufgelöste Röntgenabsorptionsspektroskopie an der Eisen‐L‐Kante ermöglicht. Die Population von Valenzorbitalen um das Metallzentrum wird so direkt zugänglich. Mit Hilfe von Dichtefunktionaltheorie stellen wir fest, dass das Bild einer lokalisierten Ladungstrennung inadäquat ist. Die ungepaarte Spindichte bietet jedoch eine geeignete Beschreibung des mit dem Elektronentransferprozess assoziierten Elektron‐Loch‐Paares. [ABSTRACT FROM AUTHOR]

Covalency is found to even out charge separation after photo‐oxidation of the metal center in the metal‐to‐ligand charge‐transfer state of an iron photosensitizer. The σ‐donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble‐gas configuration. These findings are enabled through element‐specific and orbital‐selective time‐resolved X‐ray absorption spectroscopy at the iron L‐edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge‐separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron‐transfer process. [ABSTRACT FROM AUTHOR]

The phase diagram of water harbors controversial views on underlying structural properties of its constituting molecular moieties, its fluctuating hydrogen-bonding network, as well as pair-correlation functions. In this work, long energy-range detection of the X-ray absorption allows us to unambiguously calibrate the spectra for water gas, liquid, and ice by the experimental atomic ionization cross-section. In liquid water, we extract the mean value of 1.74 ± 2.1% donated and accepted hydrogen bonds per molecule, pointing to a continuousdistribution model. In addition, resonant inelastic X-ray scattering with unprecedented energy resolution also supports continuous distribution of molecular neighborhoods within liquid water, as do X-ray emission spectra once the femtosecond scattering duration and proton dynamics in resonant X-ray-matter interaction are taken into account. Thus, X-ray spectra of liquid water in ambient conditions can be understood without a two-structure model, whereas the occurrence of nanoscalelength correlations within the continuous distribution remains open. [ABSTRACT FROM AUTHOR]

Excited‐state proton transfer (ESPT) is a fundamental process in biomolecular photochemistry, but its underlying mediators often evade direct observation. We identify a distinct pathway for ESPT in aqueous 2‐thiopyridone, by employing transient N 1s X‐ray absorption spectroscopy and multi‐configurational spectrum simulations. Photoexcitations to the singlet S2 and S4 states both relax promptly through intersystem crossing to the triplet T1 state. The T1 state, through its rapid population and near nanosecond lifetime, mediates nitrogen site deprotonation by ESPT in a secondary intersystem crossing to the S0 potential energy surface. This conclusively establishes a dominant ESPT pathway for the system in aqueous solution, which is also compatible with previous measurements in acetonitrile. Thereby, the hitherto open questions of the pathway for ESPT in the compound, including its possible dependence on excitation wavelength and choice of solvent, are resolved. Intersystem crossing: Efficient intersystem‐crossing (ISC) drives proton‐transfer in 2‐thiopyridone (2‐TP, aq). Transient N 1s X‐ray absorption spectroscopy reveals prompt ISC upon photoexcitation of 2‐TP. The dominant population of the T1 state mediates excited‐state proton‐transfer on sub‐nanosecond timescales. Comparison to existing data indicates invariance of the pathway to excitation wavelength and solvent environment (see scheme). [ABSTRACT FROM AUTHOR]

The simultaneous detection of energy, momentum and temporal information in electron spectroscopy is the key aspect to enhance the detection efficiency in order to broaden the range of scientific applications. Employing a novel 60° wide angle acceptance lens system, based on an additional accelerating electron optical element, leads to a significant enhancement in transmission over the previously employed 30° electron lenses. Due to the performance gain, optimized capabilities for time resolved electron spectroscopy and other high transmission applications with pulsed ionizing radiation have been obtained. The energy resolution and transmission have been determined experimentally utilizing BESSY II as a photon source. Four different and complementary lens modes have been characterized. [ABSTRACT FROM AUTHOR]

The implementation of a high-transmission, angular-resolved time-of-flight electron spectrometer with a 1.25 MHz pulse selector at the PM4 soft X-ray dipole beamline of the synchrotron BESSY II creates unique capabilities to inquire electronic structure via photoelectron spectroscopy with a minimum of radiation dose. Solid-state samples can be prepared and characterized with standard UHV techniques and rapidly transferred from various preparation chambers to a 4-axis temperature-controlled measurement stage. A synchronized MHz laser system enables excited-state characterization and dynamical studies starting from the picosecond timescale. This article introduces the principal characteristics of the PM4 beamline and LowDosePES end-station. Recent results from graphene, an organic hole transport material for solar cells and the transition metal dichalcogenide MoS 2 are presented to demonstrate the instrument performances. [ABSTRACT FROM AUTHOR]

Die Femtosekundendynamik nach resonanten Photoanregungen mit optischen und Röntgenpulsen ermöglicht eine selektive Verformung von chemischen N ‐ H ‐ und N ‐ C ‐ Bindungen in 2 ‐ Thiopyridon in wässriger Lösung. Die Untersuchung der orbitalspezifischen elektronischen Struktur und ihrer Dynamik auf ultrakurzen Zeitskalen mit resonanter inelastischer Röntgenstreuung an der N1s ‐ Resonanz am Synchrotron und dem Freie ‐ Elektronen ‐ Laser LCLS in Kombination mit quantenchemischen Multikonfigurationsberechnungen erbringen den direkten Nachweis dieser kontrollierten photoinduzierten Molekülverformungen und ihrer ultrakurzen Zeitskala. Die photoinduzierte Dynamik in 2 ‐ Thiopyridon (2 ‐ TP) wurde mit zeitaufgelöster resonanter inelastischer Röntgenstreuung untersucht. Die spektralen Fingerabdrücke für die N ‐ Deprotonierung geben Einblick in die ersten Schritte eines Protonentransferprozesses im valenzangeregten Zustand. Röntgenanregung von 2 ‐ TP verursacht eine Schwächung der C ‐ N ‐ Bindung, wodurch eine selektive Bindungsverformung durch Anpassung der Photonenenergie möglich wird. [ABSTRACT FROM AUTHOR]