Satellite TRIXS 29th RAU

Title: Principles of RIXS and applications on molecular systems

Abracts: What governs rate and selectivity in chemical processes down to the atomic level is the bearing point of high resolution resonant inelastic X-ray scattering on molecular systems. Here both active moieties and the interacting fluctuating network of the solvent, most notably liquid water, are the constituting entities. RIXS derives excited state dynamics at active sites and determine the ground state potential energy surface in directional cuts spanning from equilibrium geometries to strong distortions along bond coordinates, mapping out the parameter space on which thermal chemistry takes place. Thus, the multitude of potential energy surfaces of hydrogen bridge bonded water molecules in liquid water can be experimentally determined.

Diverging views exist on liquid water, dating back to Wilhelm Conrad Röntgen, postulating distinct phases to coexist in liquid water even under ambient conditions – competing with a single-phase liquid in a fluctuating hydrogen bonding network – the continuous distribution model. Over time, X-ray spectroscopic methods have repeatedly been interpreted in support of Röntgen’s postulate. However, quantitative X-ray spectroscopic analyses show that this is not the case. At room temperature and normal pressure, the X-ray spectroscopic observables can be fully and consistently described with continuous distribution models of near-tetrahedral liquid water at ambient conditions: The water molecules form a fluctuating network with an average of 1.74 ±2.1% donor and acceptor hydrogen bridge bonds per molecule each, allowing tetrahedral coordination to neighbours. In addition, across the full phase diagram, correlations to e.g. second shell coordination is established and the influence of ultrafast dynamics from X-ray matter interaction is separated and quantified.

Can these X-ray spectroscopic conclusions on water at ambient conditions now also resolve the heavily debated question of the existence of a second critical point in the so-called “no man’s land” of supercooled water? This postulated second critical point is conceptually based on the extension of the established low- and high-density amorphous ice phases into purported low- and high-density liquid phases along a Widom line where the second critical point is found as the extrapolated divergence of stable and supercooled water‘s thermodynamic response functions around -45°C at atmospheric pressure. From the physics of critical fluctuations, it is known, that well above a critical point one should view the state of matter as homogeneous. Incipient and large fluctuations are allowed as one approaches closely the phase boundary and the critical point: How close one has to approach it in energy and on what time scale to sense the divergence is not fully answered, but expectations from observations in solid state physics are that you have to be close to realize the 2-phase effects. Even if the purported second critical point at -45°C and ambient pressure existed, the ambient conditions of liquid water in equilibrium would be by any means far away in temperature. Thus, the fluctuating continuous distribution model of near-tetrahedral liquid water at ambient conditions holds true independent of whether the second critical point of water in the supercooled region exists or not.

(1) Intramolecular soft modes and intermolecular interactions in liquid acetone, Y.-P. Sun et al., Phys. Rev. B 84 132202 (2011)
(2) Internal symmetry and selection rules in resonant inelastic soft x-ray scattering, Y.-P.Sun et al., J PHYSICS B-AMOP 44 161002 (2011)
(3) Snapshots of the Fluctuating Hydrogen Bond Network in Liquid Water on the Sub-Femtosecond Timescale with Vibrational Resonant Inelastic x-ray Scattering, A Pietzsch et al. Phys. Rev. Lett. 114 088302 (2015)
(4) Ground state potential energy surfaces around selected atoms from resonant inelastic x-ray scattering S Schreck et al. Nat. Sci. Rep. 7 20054 (2016)
(5) A study of the water molecule using frequency control over nuclear dynamics in resonant X-ray scattering, V. Vaz da Cruz et al. PCCP 19 19573 (2017)
(6) Selective gating to vibrational modes through resonant X-ray scattering R. Carvalho Couto et al. Nat. Com. 8 14165 (2017)
(7) Local Maps of Potential Energy Surfaces and Chemical Pathways, A Pietzsch, A Föhlisch, Synchrotron Radiation News 30 8 (2017)
(8) One-dimensional cuts through multidimensional potential-energy surfaces by tunable x rays, S Eckert et al. Phys. Rev. A 97 053410 (2018)
(9) Compatibility of quantitative X-ray spectroscopy with continuous distribution models of water at ambient conditions, J Niskanen, et al. PNAS 116 4058 (2019)
(10) Probing hydrogen bond strength in liquid water by resonant inelastic X-ray scattering, V. Vaz da Cruz, et al. Nat. Com. 10 1013 (2019)

Short CV: Marco Moretti obtained his PhD in Physics at Politecnico di Milano (Milano, Italy) in 2011 under the supervision of Prof. Lucio Braicovich and prof. Giacomo Ghiringhelli. Marco then moved to ESRF – The European Synchrotron (Grenoble, France), where he started as post-doc on ID16. He participated in the design, construction and commissioning of ID20, the inelastic hard-x-ray scattering beamline , where he became scientist and, eventually, beamline responsible. Finally, in 2017 he became professor at Politecnico di Milano (Italy). His main research interest is the study of strongly correlated electron systems by means of synchrotron-based experimental techniques.

Title: Resonant inelastic x-ray scattering of correlated electron systems

Abstract: Correlated systems show a variety of interactions involving charge, spin, lattice and orbital degrees of freedom of the electrons, which give rise to a number of fascinating properties, the most prominent example being high-Tc superconductivity in layered cuprates. Resonant inelastic x-ray scattering (RIXS) proved very effective as an experimental technique to investigate the electronic structure of correlated electron systems; indeed, RIXS probes various type of excitations over a broad energy range, thus providing quantitative information on the relevant electronic interactions. In this talk, I will introduce the technique for the study of electronic excitations in solids, including a detailed discussion on the mechanism giving rise to vibrational, magnetic and orbital excitations in L2,3 edge RIXS of transition metals. Key experimental results are finally reviewed, with emphasis to the case of antiferromagnetic insulating cuprates and iridates.