# The Linac Coherent Light Source: Recent Developments and Future Plans
## 3 LCLS-II Science Opportunities
### Energy and Charge Dynamics in Atoms and Molecules  
The fundamental processes of charge migration, redistribution and localization are at the heart of complex processes such as photosynthesis, catalysis, and bond formation/dissolution that govern all chemical reactions. Such charge dynamics are closely coupled with atomic motion, and substantial evidence points to the importance of the concurrent evolution of electronic and nuclear wave functions (i.e., beyond the Born-Oppenheimer approximation [28]) in many molecular systems. Recent evidence also suggests that quantum coherence in chemical and biological complexes may play a more important role than previously appreciated [29]. Our understanding of these processes at the quantum level is limited, even for simple molecules. We have not been able to directly observe these processes to date, and they are beyond the description of conventional chemistry models.  
Ultrafast soft X-rays at a high repetition rate from advanced XFELs will enable new experimental methods that will directly map valence charge distributions and reaction dynamics in the molecular frame. One powerful approach that will be qualitatively advanced by the capabilities of LCLS-II  
Figure 8: Projected LCLS-II X-ray pulse energies for the soft X-ray undulator (SXU) (red) and the hard X-ray undulator (HXU) (blue) undulators for 100 pC per bunch (from ref. [4]). The X-ray pulse energy is expected to be constant up to \(\sim\)300 kHz, and will scale inversely with repetition rate (i.e., constant average X-ray power) for repetition rates above \(\sim\)300 kHz.  
is the dynamic molecular reaction microscope. As illustrated in Figure 9, the molecular reaction microscope [30; 31; 32] employs sophisticated coincidence measurements of photoelectrons (scattered from a molecular structure at the moment of photo absorption) and ion fragments (of the dissociating molecule) to enable a reconstruction of the molecule at a fixed orientation in space. The unique combination of ultrafast X-rays and high repetition rate at LCLS-II will advance these techniques to the time domain to follow molecular dynamics in the excited-state on fundamental time scales. Here specific molecular dynamics are initiated via tailored transient excitations, such as charge transfer, vibrational excitation, the creation of a valence hole via ionization, or the creation of non-equilibrium Rydberg wavepackets.  
Recent experiments highlight the promising opportunities for dynamic reaction microscope studies at high repetition rate XFELs [34; 35; 36; 37]. For example, LCLS experiments at 120 Hz by [PERSON] et al. on charge-transfer processes in gas-phase iodomethane [33] identified three dissociative channels based on the time-dependent kinetic energy distributions of the charged fragments (Figure 9 right). Measurements at high repetition rate will enable the complete spatial reconstruction of the excited-state charge transfer and subsequent dissociation of iodomethane at each time step for a particular molecular orientation. The more than 1000-fold increase in coincidence rates (and information content) from LCLS-II will transform this into a powerful approach for visualizing a broad range of ultrafast molecular dynamics from dissociation of simple diatomic molecules, to charge-transfer processes, to isomerization and ring-opening reactions [5], to non-Born-Oppenheimer relaxation processes [38], to quantum symmetry breaking events from which chirality emerges [39].