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A setup for resonant inelastic soft x-ray scattering on liquids at free electron laser light sources
(2012)
We present a flexible and compact experimental setup that combines an in vacuum liquid jet with an 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 demonstrate the feasibility of this type of experiments with the measurements 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.
In this combined theoretical and experimental study we report a full analysis of the resonant inelastic X-ray scattering (RIXS) spectra of H2O, D2O and HDO. We demonstrate that electronically-elastic RIXS has an inherent capability to map the potential energy surface and to perform vibrational analysis of the electronic ground state in multimode systems. We show that the control and selection of vibrational excitation can be performed by tuning the X-ray frequency across core-excited molecular bands and that this is clearly reflected in the RIXS spectra. Using high level ab initio electronic structure and quantum nuclear wave packet calculations together with high resolution RIXS measurements, we discuss in detail the mode coupling, mode localization and anharmonicity in the studied systems.
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.
Symmetry and its breaking crucially define the chemical properties of molecules and their functionality. Resonant inelastic X-ray scattering is a local electronic structure probe reporting on molecular symmetry and its dynamical breaking within the femtosecond scattering duration. Here, we study pyrimidine, a system from the C-2v point group, in an aqueous solution environment, using scattering though its 2a(2) resonance. Despite the absence of clean parity selection rules for decay transitions from in-plane orbitals, scattering channels including decay from the 7b(2) and 11a(1) orbitals with nitrogen lone pair character are a direct probe for molecular symmetry. Computed spectra of explicitly solvated molecules sampled from a molecular dynamics simulation are combined with the results of a quantum dynamical description of the X-ray scattering process. We observe dominant signatures of core-excited Jahn-Teller induced symmetry breaking for resonant excitation. Solvent contributions are separable by shortening of the effective scattering duration through excitation energy detuning.
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 continuous-distribution 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 nanoscale-length correlations within the continuous distribution remains open.
Charge-density rearrangements after metal-to-ligand charge-transfer excitation in an iron photosensitizer are investigated by R. M Jay, A. Fohlisch et al. in their Communication (DOI: 10.1002/anie.201904761). By using time-resolved X-ray absorption spectroscopy, surprising covalency-effects are revealed that inhibit charge-separation at the intra-molecular level. Furthermore, the underlying mechanism is proposed to be generally in effect for all commonly used photosensitizers in light-harvesting applications, which challenges the common perception of electronic charge-transfer.
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 angstrom, 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 ls to 4a(1) resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1 s to 2b(2) 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.
Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. pi-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences sigma-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.
Chelating agents are an integral part of transition metal complex chemistry with broad biological and industrial relevance. The hexadentate chelating agent ethylenediaminetetraacetic acid (EDTA) has the capability to bind to metal ions at its two nitrogen and four of its carboxylate oxygen sites. We use resonant inelastic X-ray scattering at the 1s absorption edge of the aforementioned elements in EDTA and the iron(III)-EDTA complex to investigate the impact of the metal-ligand bond formation on the electronic structure of EDTA. Frontier orbital distortions, occupation changes, and energy shifts through metal- ligand bond formation are probed through distinct spectroscopic signatures.
The speciation of 2-Mercaptopyridine in aqueous solution has been investigated with nitrogen 1s Near Edge X-ray Absorption Fine Structure spectroscopy and time dependent Density Functional Theory. The prevalence of distinct species as a function of the solvent basicity is established. No indications of dimerization towards high concentrations are found. The determination of different molecular structures of 2-Mercaptopyridine in aqueous solution is put into the context of proton-transfer in keto-enol and thione-thiol tautomerisms. (C) 2016 The Authors. Published by Elsevier B.V.