# The Linac Coherent Light Source: Recent Developments and Future Plans
## 3 LCLS-II Science Opportunities
### Photochemistry and Catalysis  
The directed design of photo-catalytic systems for chemical transformation and solar energy conversion (particularly systems that are efficient, chemically selective, robust, and based on earth-abundant elements) remains a major scientific and technological challenge. Meeting this challenge requires a much deeper understanding of the fundamental processes of photo-chemistry that influence the performance of photo-catalysts; namely, stable charge separation, transport, and localization. In molecular systems, these events are mediated by internal conversion, intersystem crossing, and conformational changes on the ultrafast time scale. Understanding such processes  
Figure 9: (**a**) Illustration of a molecular reaction microscope (also known as COLd Target Recoil Ion Momentum Spectroscopy, COLTRIMS). Only one molecule is in the X-ray beam on each pulse (i.e., less than one ionization event per pulse). The electron and ion momenta are fully characterized in coincidence via position-sensitive time-of-flight detectors (graphic courtesy of [PERSON], Goethe [PERSON]). (**b–d**) Schematic of charge transfer in iodomethane during dissociation. Reprinted with permission from [33]. Copyright AAAS, 2014. Dynamic reaction microscope studies at high repetition rate would enable a full reconstruction of the charge-sharing dynamics in iodomethane and similar complexes in the molecular frame. in molecular systems is hindered by the limited ability of conventional experimental or theoretical approaches to directly observed or calculate these charge dynamics. For example, ultrafast optical spectroscopy can capture charge dynamics (and reveal quantum coherences), but is limited to probing electronic states delocalized over multiple atomic sites (or overlapping vibrational spectra), and thus lack element specificity and struggle to identify nuclear degrees of freedom coupled to excited-state dynamics. Since charge separation, charge transport, and catalysis are local phenomena, the resolving power and element specificity of X-rays can provide insight that is unavailable from non-local probes. Ultrafast X-ray can thus disentangle the coupled motion of electrons and nuclear dynamics, making them uniquely powerful for studying chemical dynamics. High repetition rate X-rays from LCLS-II will enable a powerful suite of spectroscopy tools for understanding the physics and chemistry of photo-catalysts in operating environments. One important example where LCLS-II capabilities will provide a qualitative advance beyond what is presently possible at LCLS is resonant inelastic X-ray scattering (RIXS). As illustrated in Figure 2 RIXS measures the energy distribution of both occupied and unoccupied molecular orbitals, via resonant transitions from a specific atomic core level, thus providing sensitivity to the local chemistry with high resolution. As highlighted in Section 2.2, recent demonstration experiments at LCLS have applied femtosecond time-resolved RIXS to investigate the charge transfer and ligand exchange dynamics of Fe(CO)\({}_{5}\) in solution [7]. By comparing 2D RIXS maps with quantum chemical calculations, the dynamics of the frontier orbitals and their interactions are revealed for the first time with element specificity. These studies demonstrate the potential of time-resolved RIXS to capture short-lived reaction intermediates and correlate the underlying orbital symmetry with spin multiplicity and reactivity. However, the low repetition rate (and corresponding low average spectral flux: ph/s/meV) of LCLS presently limits the application of time-resolved RIXS to studies of gross structural changes in model molecular systems at high concentrations (e.g., \(\sim\)1 M in the case of the Fe(CO)\({}_{5}\) studies [7]). With the more than 1000-fold increase in average brightness provided by LCLS-II, time-resolved RIXS with high spectral resolution will enable complete time-sequenced measurements at high fidelity.