Science Drivers

For the LJE Endstation

Understanding the fundamental processes of photo-chemistry is essential for directed design of photo-catalytic systems for chemical transformation and solar energy conversion that are efficient, chemically selective, robust, and based on earth-abundant elements. The central events of excited state chemistry critically influence the performance of photo-catalysts since stable charge separation, transport, and localization are mediated by strong interaction between electronic and nuclear structure.

New tools that enable direct observation of these central events, on fundamental time scales, and with chemical specificity, will qualitatively advance our understanding of chemical dynamics in photo-catalytic systems, and advance the development of design principles for directing molecular and materials synthesis. The high repetition rate of LCLS-II combined with advanced instrumentation for time-resolved resonant inelastic X-ray scattering, RIXS (NEH 2.2) will map the energy distribution and evolution of occupied and unoccupied molecular orbitals of model complexes and functional photo-catalysts in operating (liquid) environments.

First Experiments for LJE Will Include:

One important example from nature that illustrates the potential for this new class of science at LCLS-II is the site for water oxidation (Mn4CaO5 cluster) that acts as a charge storage device in the reaction center of photosystem II (PS-II). Tremendous progress has been made in the structural characterization of this catalytic center in its resting state, with record atomic resolution and damage-free room-temperature results provided by XFELs. However, a major knowledge gap is the transient electronic configurations of the metal sites and the flow of charge within the catalytic center during all four steps of the reaction cycle, and particularly the electronic structure of the metastable S4 state, where O-O bond formation occurs. Time-resolved RIXS experiments at LCLS-II will provide the first mapping of the transient electronic structure of the Mn4CaO5 cluster under operating conditions.

For the RIXS Endstation

Strong coupling between charge, spin, orbitals, and lattice motion in quantum materials gives rise to collective modes that determine the macroscopic material properties of profound interest such as high-temperature superconductivity, colossal magnetoresistivity, and topologically protected phases. Momentum-resolved resonant inelastic X-ray scattering (q-RIXS) has emerged as a powerful tool to characterize collective excitations for comparison with fundamental theoretical models based on the Kramers-Heisenberg approach. Because the ground states of quantum materials arise from a subtle balance among competing interactions, the relevant emergent collective modes appear at modest energies, typically up to a few hundreds of meVs but with a number of excitations lying below 100 meV, where the required combination of photon flux and energy resolution press the limits of modern X-ray sources and spectrometers.

The high repetition rate of LCLS-II will offer transformative capabilities—for both characterizing collective modes and excited states (energy and momentum dependence across the Brillouin zone), and for following their response to tailored external stimuli to disentangle coupled phenomena in the time domain. For example recent studies have shown that broadband THz pulses can selectively couple to electronic order, and thereby transiently decouple charge and lattice modes. Such approaches can also trigger phase transitions and create new phases that are inaccessible in thermal equilibrium. Tailored ultrafast vibrational excitation has been shown to drive insulator-to-metal phase transitions in colossal magnetoresistant (CMR) manganites, and enhanced superconductivity is claimed to result from transiently-driven nonlinear lattice dynamics. These novel photo-induced phenomena are ultimately related to the emergent properties in equilibrium and are a key step towards active control, yet a clear interpretation and characterization of the collective modes in the transient regime is still lacking.

First Experiments for RIXS Will Include:

First experiments at LCLS-II will provide crucial pieces of information by time- and momentum- resolved RIXS (q-RIXS instrument, NEH 2.2). In cuprates, Cu L-edge RIXS will map the evolution of magnetic excitations and phonons in time, energy, and momentum to provide a more complete microscopic picture about the transient superconducting phase. The time-evolution of charge-stripe order, a co-existing state in superconducting cuprates, and its associated excitations can be simultaneously monitored. This will provide new insights into the much-debated issue of the role of charge order in high-TC superconductivity, as well as provided quantitative assessment of the strength of spin fluctuations and electron-phonon coupling as candidates for a superconducting pairing mechanism. This approach is applicable to many other outstanding problems in quantum materials, such as the relation of recently discovered collective modes near the zone center and the role of magnetic fluctuations in the electron-doped cuprates, as well as more generally to excitations in multi-ferroics, topological spin liquids, and understanding battery cathodes.

See document on first experiments for NEH 2.2