The prototypical photoinduced dissociation of Fe(CO)5 in the gas phase is used to test time-resolved x-ray photoelectron spectroscopy for studying photochemical reactions. Upon one-photon excitation at 266 nm, Fe(CO)5 successively dissociates to Fe(CO)4 and Fe(CO)3 along a pathway where both fragments retain the singlet multiplicity of Fe(CO)5. The x-ray free-electron laser FLASH is used to probe the reaction intermediates Fe(CO)4 and Fe(CO)3 with time-resolved valence and core-level photoelectron spectroscopy, and experimental results are interpreted with ab initio quantum chemical calculations. Changes in the valence photoelectron spectra are shown to reflect changes in the valence-orbital interactions upon Fe-CO dissociation, thereby validating fundamental theoretical concepts in Fe-CO bonding. Chemical shifts of CO 3σ inner-valence and Fe 3p core-level binding energies are shown to correlate with changes in the coordination number of the Fe center. We interpret this with coordination-dependent charge localization and core-hole screening based on calculated changes in electron densities upon core-hole creation in the final ionic states. This extends the established capabilities of steady-state electron spectroscopy for chemical analysis to time-resolved investigations. It could also serve as a benchmark for how charge and spin density changes in molecular dissociation and excited-state dynamics are expressed in valence and core-level photoelectron spectroscopy.

We analyze the effects of covalent interactions in Ni 2p3d resonant inelastic X-ray scattering (RIXS) spectra from aqueous Ni2+ ions and find that the relative RIXS intensities of ligand-to-metal charge-transfer final states with respect to the ligand-field final states reflect the covalent mixing between Ni 3d and water orbitals. Specifically, the experimental intensity ratio at the Ni L3-edge allows to determine that the Ni 3d orbitals have on average 5.5% of water character. We propose that 2p3d RIXS at the Ni L3-edge can be utilized to quantify covalency in Ni complexes without the use of external references or simulations.

We prove the hitherto hypothesized sequential dissociation of Fe(CO)5 in the gas phase upon photoexcitation at 266 nm via a singlet pathway with time-resolved valence and core-level photoelectron spectroscopy with an x-ray free-electron laser. Valence photoelectron spectra are used to identify free CO molecules and to determine the time constants of stepwise dissociation to Fe(CO)4 within the temporal resolution of the experiment and further to Fe(CO)3 within 3 ps. Fe 3p core-level photoelectron spectra directly reflect the singlet spin state of the Fe center in Fe(CO)5, Fe(CO)4, and Fe(CO)3 showing that the dissociation exclusively occurs along a singlet pathway without triplet-state contribution. Our results are important for assessing intra- and intermolecular relaxation processes in the photodissociation dynamics of the prototypical Fe(CO)5 complex in the gas phase and in solution, and they establish time-resolved core-level photoelectron spectroscopy as a powerful tool for determining the multiplicity of transition metals in photochemical reactions of coordination complexes.

We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)5 in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)4 which are observed following a charge transfer photoexcitation of Fe(CO)5 as reported in our previous study [Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the 1A1 state of Fe(CO)4. A sub-picosecond time constant of the spin crossover from 1B2 to 3B2 is rationalized by the proposed 1B2 → 1A1 → 3B2 mechanism. Ultrafast ligation of the 1B2 Fe(CO)4 state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the 3B2 Fe(CO)4 ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via 1B2 → 1A1 → 1A′ Fe(CO)4EtOH pathway and the time scale of the 1A1 Fe(CO)4 state ligation is governed by the solute-solvent collision frequency. Our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution.

Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion1,2. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site3,4,5,6,7,8,9,10,11 that need to be controlled to optimize complexes for photocatalytic hydrogen production8 and selective carbon–hydrogen bond activation9,10,11. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst12,13 with an electron deficiency at the Fe centre14,15, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16,17,18,19 and 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes. Mapping the frontier-orbital interactions with atom specificity using X-ray laser-based femtosecond-resolution spectroscopy reveals that spin crossover and ligation determine the sub-picosecond excited-state dynamics of a transition-metal complex in solution. Transition-metal complexes catalyse many reactions of fundamental and practical importance. Their performance is coupled to charge and spin density changes at the metal site caused by electronic excitation, ligand loss from the metal centre or a combination of both. Philippe Wernet et al. show that femtosecond X-ray spectroscopy and quantum chemical theory can deliver unprecedented molecular-level insight into the dynamics of the benchmark transition-metal complex Fe(CO)5, revealing that light-induced dissociation creates a previously unreported excited singlet species and its subsequent reactions. These insights are enabled by the ability of femtosecond X-ray spectroscopy to probe, with atom specificity, frontier-orbital interactions that lie at the heart of chemical transformation. The method, expected to be widely applicable, complements approaches that probe the structural changes accompanying ultrafast chemical reactions.

Time and energy resolved photoelectron distributions of photo-excited NaI are presented. A splitting in the photo-excited state suggested by calculations of the intramolecular potential energy surfaces could be confirmed experimentally for the first time.

We present a table top setup for time- and angle-resolved photoelectron spectroscopy to investigate band structure dynamics of correlated materials driven far from equilibrium by femtosecond laser pulse excitation. With the electron-phonon equilibration time being in the order of 1-2 ps it is necessary to achieve sub-picosecond time resolution. Few techniques provide both the necessary time and energy resolution to map non-equilibrium states of the band structure. Laser-driven high-order harmonic generation is such a technique. In our experiment, a grating monochromator delivers tunable photon energies up to 40 eV. A photon energy bandwidth of 150 meV and a pulse duration of 100 fs FWHM allow us to cover the k-space necessary to map valence bands at different kz and detect outer core states.

We determine the pathways in the photo-dissociation reactions of Fe(CO)5 both in the gas phase and in solution by mapping the valence electronic structure of the reaction intermediates with femtosecond X-ray laser spectroscopy.

At BESSY II a confocal plane grating spectrometer for resonant inelastic X-ray scattering (RIXS) is currently under commissioning. The new endstation operates with a source size of 4 x 1 mu m(2) provided by its dedicated beamline. The RIXS-spectrometer covers an energy range from 50 eV to 1000 eV, providing a resolving power E/Delta E of 5000-15,000. The beamline allows full polarization control and gives a photon flux of up to 7 x 10(14) photons/s/0.1 A/0.1%bandwidth by offering a resolving power E/Delta E of 4000-12,000. (C) 2012 Elsevier B.V. All rights reserved.

Dynamics in materials typically involve different degrees of freedom, like charge, lattice, orbital and spin in a complex interplay. Time-resolved resonant inelastic X-ray scattering (RIXS) as a highly selective tool can provide unique insight and follow the details of dynamical processes while resolving symmetries, chemical and charge states, momenta, spin configurations, etc. In this paper, we review examples where the intrinsic scattering duration time is used to study femtosecond phenomena. Free-electron lasers access timescales starting in the sub-ps range through pump-probe methods and synchrotrons study the time scales longer than tens of ps. In these examples, time-resolved resonant inelastic X-ray scattering is applied to solids as well as molecular systems. (C) 2013 Elsevier B.V. All rights reserved.

The absolute flux of a femtosecond vacuum-ultraviolet (VUV) photon source based on the high-order harmonic generation of a femtosecond Ti:sapphire laser and monochromatized with a grating monochromator is determined both on a shot-to-shot basis and averaged over seconds by a calibrated gas monitor detector. The average flux is compared with the average flux as determined with a calibrated GaAsP semiconductor photodiode. We found that the photodiode is a reliable and easy-to-use tool for estimating the order of magnitude of the average photon flux but that, due to saturation losses, it underestimates the average flux by up to −15%.

Here we present a review of X-ray absorption spectroscopy and X-ray Raman scattering with the perspective to understand the spectra of water including changes with temperature, mass of the water molecule and presence of monovalent ions. The different detection schemes are discussed and it is concluded that transmission X-ray absorption measurements, using a small area where the thickness is uniform, and X-ray Raman scattering give the most reliable spectra. Different model systems are discussed such as the surface and bulk of ice and various adsorbed monolayer structures on metal surfaces.

We report the first time-resolved x-ray absorption study of liquid water. Structural changes in the hydrogen-bond network as induced by resonant femtosecond-infrared excitation arc monitored via transient x-ray absorption at the oxygen K-edge.

Time-resolved valence band photoelectron spectroscopy with a temporal resolution of 135 fs is used to map the entire occupied valence electronic structure of photoexcited gas-phase Br2 molecules during dissociation. The observed shifting and mixing of valence energy levels defines a transition period where the system appears to be intermediate between atoms and molecules. The surprisingly short bond breaking or dissociation time is determined by monitoring in real time how the photoelectron multiplet structure of the free atom arises from the valence states of the photoexcited molecule.

Laser and X-ray spectroscopic methods are both frequently used to elucidate structure and bonding in matter. We aim at developing and applying methods where we use a laser pulse to excite atoms and molecules in the gas phase, in liquids and in solids and where we interrogate the evolution of the system with ultrafast X-ray spectroscopic methods. Two examples will be discussed. First, our results on the ultrafast dissociation of Br2 molecules in the gas phase with femtosecond photoelectron spectroscopy will be presented. We use high-order harmonic generation of an ultrafast Ti:Sa laser (800 nm) to produce our femtosecond VUV photon pulses with a photon energy of around 20 eV. Second, a novel infrared pump-X-ray probe technique is used to unravel structural dynamics in liquid water. A femtosecond Ti:Sa laser system is used to drive an optical parametric amplifier to generate intense infrared pulses and the X-ray probe pulses are generated in a soft X-ray undulator beamline at the synchrotron radiation source BESSYII.

We report the use of zone plate optics as a monochromator for the spectral selection of a single high-order harmonic of a femtosecond laser generated in a rare gas medium. We show that this is a convenient way of monochromatizing the vacuum ultraviolet (VUV) pulses in the photon energy range from 30 up to 70 eV while keeping the pulse duration in the femtosecond range. We measure the emission spectra with a typical energy resolution of E/ΔE=60. Comparably high transmission and diffraction efficiencies allow the record of images of different harmonics using a camera within a few seconds of acquisition time. This is our first step toward a VUV microscope with a submicrometer spatial resolution and femtosecond time resolution. Possible applications of this setup are also discussed.

We report the first infrared pump-x-ray probe study of the structural dynamics of liquid water. Femtosecond infrared excitation via the O–H stretching band induces an ultrafast temperature jump that gives rise to changes in the hydrogen bond network. Such changes are probed via the transient x-ray absorption at the oxygen K edge using 70 ps x-ray pulses from a storage ring source. We measure spectra and time evolution of the transient x-ray absorption and calibrate the absolute change of temperature. Our work paves the way for future studies with femtosecond x-ray probe pulses.

The 4d photoelectron spectrum of free thulium atoms is measured and calculated in the single-configuration intermediate-coupling approximation considering multiplet splitting and lifetime broadening of the spectral components due to radiationless transitions following the 4d ionization. The individual natural widths of the 4d(9)4f(13) multiplet components are found to differ considerably, and the experimental profile of the spectrum can be reproduced theoretically only if this effect is taken into account.

The isotope effect on the local structure of liquid water at room temperature is studied by x-ray Raman spectroscopy. The difference between the room-temperature spectra of liquid H2O and D2O is compared to the difference spectrum between liquid H2O at 22 and 2 degrees C. The spectral changes between H2O and D2O can partly be attributed to structural changes similar to a temperature change, in agreement with diffraction data. Additionally, we find that isotope substitution affects the local asymmetry in the hydrogen-bonded network: hydrogen-bonding configurations are more asymmetric on the donor side for H2O than for D2O. A cluster model is used to computationally illustrate the spectral changes that arise due to the increased asymmetry, capturing all essential features in the difference spectrum. We infer from our study that quantum effects contribute to the formation of asymmetrical species in the liquid.