frontier-orbital interactions with atom specificity. We anticipate that the method will be broadly applicable in the chemical sciences, and complement approaches that probe structural dynamics in ultrafast processes.In our experimental set-up (Figure 1a), the valence electronic structure of Fe(CO) 5 is probed with femtosecond-resolution resonant inelastic x-ray scattering (RIXS) at the Fe L 3 -edge (Fe experiments. This triplet arises from a singlet state with a time constant of 300 fs, consolidating the notion 6 that sub-ps intersystem crossing appears to be common in the excited-state dynamics of transition-metal complexes 7,[22][23][24] . The persistence of the triplet Fe(CO) 4 ( 3 B 2 ) up to our maximum time delay of 3 ps is consistent with it undergoing a slow, spin-forbidden reaction with intersystem crossing to a solvent-complexed singlet state on the 50-100 ps time scale 4,5, 25 . However, the observed branching on a sub-ps time scale into the competing and simultaneous reaction channels of spin crossover and ligation to form coordinatively saturated species introduces an efficient pathway circumventing this spin barrier. It also supports the idea that the high density of electronic excited states and the relatively large amount of excess energy available in the system determine the course of the excited-state dynamics, rather than spin selection rules alone 5,6 . Fast ligation could be facilitated along the singlet pathway, confirming the general notion that solvent-stabilized metal centers form fast 3, 4, 11 and consistent with the observation of unsaturated carbonyl Cr(CO) 5 forming a solvent complex in alcohol solution within 1.6 ps 26 . An alternative proposal 20 for Fe(CO) 5 involves concerted exchange of CO and EtOH on the time scale of ligand dissociation of 100-150 fs. This would also proceed along a singlet pathway and in agreement with our results, as the temporal resolution of our measurements is not sufficient to distinguish between this concerted and the alternative sequential process. Revealing in detail 8 the influence of solvent-solute interactions will have to be the subject of future studies, which could also explore whether the structure of the solute prior to dissociation 20 influences the excited-state branching ratio between the different pathways.We find that the ligation capability of Fe(CO) 4 is mostly determined by its d σ * LUMO, which receives σ donation from occupied CO or ethanol ligand orbitals. Population of the antibonding d σ * orbital in excited singlet ( 1 B 2 ) and triplet ( 3 B 2 ) Fe(CO) 4 impedes σ donation from ligands (see sketches in Figure 3), explaining the inertness of these species against ligation; this problem is absent in the ligation channel that produces coordinately saturated species. Establishing this correlation of orbital symmetry with spin multiplicity and reactivity 27 is enabled by the atom specificity with which x-ray laser based femtosecondresolution spectroscopy can explore frontier-orbital interactions. This ability gives unique access t...

We present a flexible and compact experimental setup that combines an in vacuum liquid jet with a x-ray emission spectrometer to enable static and femtosecond time-resolved resonant inelastic soft x-ray scattering (RIXS) measurements from liquids at free electron laser (FEL) light sources. We demostrate the feasibility of this type of experiments with the measurments performed at the Linac Coherent Light Source FEL facility. At the FEL we observed changes in the RIXS spectra at high peak fluences which currently sets a limit to maximum attainable count rate at FELs. The setup presented here opens up new possibilities to study the structure and dynamics in liquids.

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.

Figure S1: Calculated bromine (2p) molecular-frame photoelectron angular distributions (MFPADs) for 1,4-dibromobenzene molecules at 20 eV (top) and 35 eV (bottom) photoelectron kinetic energy obtained from the DFT calculations. The polarization axis of the X-rays is along the y-axis (at 90°/270°) and the Xray beam propagation direction along the x-axis (at 0°/180°). In the left panels, the MFPADs are shown for perfectly 3-D aligned molecules with the Br-Br axis along the y-axis and the molecular plane in the yzplane (dashed lines), perfectly 1-D aligned molecules with the Br-Br axis along the y-axis and the molecular plane freely rotating around the Br-Br axis (dotted lines), and for 1-D aligned molecules averaged over a Br-Br axis distribution corresponding to a two-dimensional Gaussian with a FWHM of 50° in order to take into account the experimentally achieved degree of alignment (red solid line). For comparison, the calculated photoelectron angular distributions for randomly oriented molecules are also shown (blue solid lines). In the right panels, the MFPADs for the six molecular orbitals that contribute to the bromine (2p) photoelectron line, weighted by their relative contribution according to the DFT calculations, are shown for perfectly 1-D aligned molecules along with their sum, which corresponds to the dotted lines in the left panels.

Myelin insulates neuronal axons and enables fast signal transmission, constituting a key component of brain development, aging and disease. Yet, myelin-specific imaging of macroscopic samples remains a challenge. Here, we exploit myelin’s nanostructural periodicity, and use small-angle X-ray scattering tensor tomography (SAXS-TT) to simultaneously quantify myelin levels, nanostructural integrity and axon orientations in nervous tissue. Proof-of-principle is demonstrated in whole mouse brain, mouse spinal cord and human white and gray matter samples. Outcomes are validated by 2D/3D histology and compared to MRI measurements sensitive to myelin and axon orientations. Specificity to nanostructure is exemplified by concomitantly imaging different myelin types with distinct periodicities. Finally, we illustrate the method’s sensitivity towards myelin-related diseases by quantifying myelin alterations in dysmyelinated mouse brain. This non-destructive, stain-free molecular imaging approach enables quantitative studies of myelination within and across samples during development, aging, disease and treatment, and is applicable to other ordered biomolecules or nanostructures.

In the current work, X-ray emission spectra of aqueous solutions of different inorganic salts within the Hofmeister series are presented. The results reflect the direct interaction of the ions with the water molecules and therefore, reveal general properties of the salt−water interactions. Within the experimental precision a significant effect of the ions on the water structure has been observed but no ordering according to the structure maker/structure breaker concept could be mirrored in the results indicating that the Hofmeister effectif existentmay be caused by more complex interactions. ■ INTRODUCTIONAt the end of the 19th century, Franz Hofmeister discovered that common anions and cations could be classified according to their ability to salt in/salt out proteins, in detail to enhance aggregation and precipitation of proteins dissolved in aqueous solution; see Scheme 1. 1 By now, the so-called Hofmeister effect has been observed in a number of systems ranging from atmospheric aerosols up to whole biological entities such as cells, proteins, etc. 2,3 The salting in/out effect even plays an essential role in the metabolism of eukaryotic cells and in the case of mammalians it can lead to critical aggregation effects which were discussed as various prestages of diseases. 4,5 However, the question concerning the effect's origin remains unanswered. Two common models to explain it are (i) water interacting with the hydrated ions and causing precipitation or (ii) a consequence of the direct binding of the salt ions to the biomolecule by electrostatic interactions. 6 The first model leads to the concept of structure makers/ structure breakers (SMB), depending on the anions and cations used. According to this concept, cations and anions stabilize or destabilize the long-range water structure in aqueous solutions, 7 which is conventionally interpreted as strengthening and weakening of the hydrogen bond structure. 8 Theories based on molecular dynamic calculations have been introduced to explain the nature of the effect, some supporting the concept of structure breaking and making, 9−11 some contradicting. 12−15 However, there is a lack of understanding on the molecular level, since SMB is based on experimental observations of macroscopic properties. 16,17 Several experiments have been performed recently regarding the validation of SMB on the molecular level, which reveal objecting results. 8,18−30 Yet, the influence of salt ions on the electronic redistribution of water is unclear. In order to probe the electronic influence of salt ions on the water structure and therefore, if the SMB concept can be pictured on the molecular orbitals of the aqueous systems, corehole X-ray emission spectroscopic (XES) measurements have been performed on the oxygen K-edge of pure water and aqueous salt solutions.

The phytochrome family of light-switchable proteins has long been studied by biochemical, spectroscopic and crystallographic means, while a direct probe for global conformational signal propagation has been lacking. Using solution X-ray scattering, we find that the photosensory cores of several bacterial phytochromes undergo similar large-scale structural changes upon red-light excitation. The data establish that phytochromes with ordinary and inverted photocycles share a structural signaling mechanism and that a particular conserved histidine, previously proposed to be involved in signal propagation, in fact tunes photoresponse.

Polymer conformation in solution is more important for R2R solar cell performance than the crystallinity of the final coated film.