Tracking Light-Driven Molecular Transformations Using High-Repetition Rate Coulomb Explosion Imaging

Professor Ruaridh Forbes at the University of California, Davis is developing and applying next-generation ultrafast laser technologies to directly visualize structural rearrangements during prototypical photochemical reactions, including excited-state intramolecular proton transfer (ESIPT) and molecular photoswitching, with femtosecond and atomic-scale resolution. Light-driven molecular transformations underpin processes of fundamental and technological importance, from energy conversion and biological photoreception to molecular switches and logic devices. A central challenge is that the nuclear rearrangements involved unfold on femtosecond timescales across complex potential energy landscapes, and no spectroscopic method directly images evolving atomic positions in real time. Professor Forbes and his students will address this challenge using Coulomb explosion imaging (CEI), in which rapid multiple ionization causes a molecule to fragment along its instantaneous nuclear geometry, allowing direct reconstruction of molecular structure from the momenta of the resulting ions. Their studies will advance fundamental understanding of light-matter interactions and the mechanisms governing proton transfer and isomerization dynamics, with implications for organic electronics, molecular computing, and optically responsive materials. The project will train undergraduate and graduate researchers in interdisciplinary techniques spanning ultrafast laser physics, molecular dynamics, and data analysis, with outreach activities and open-access publishing ensuring broad dissemination to the scientific community and the public. The research program will establish two complementary CEI platforms to study photochemical dynamics. At UC Davis, Professor Forbes and his students will develop high-repetition rate laser sources producing few-cycle ultraviolet pulses, generated via resonant dispersive wave conversion and tunable across the UV, for use as pump pulses in time-resolved CEI experiments. These tabletop measurements will target ESIPT dynamics in prototypical systems including salicylic acid and pyruvic acid, using femtosecond UV excitation followed by intense infrared probe pulses to trigger Coulomb explosion and image hydrogen migration in real time. In parallel, element- and site-selective X-ray-induced CEI will be conducted at next-generation X-ray free-electron laser facilities, including LCLS-II, where inner-shell photoionization provides atomic-site selectivity inaccessible with optical probes. These facility-based experiments will target photoswitching dynamics in polyatomic molecules including hemithioindigo and azobenzene. High-dimensional covariance and cumulant analysis will be applied across both platforms to extract correlated multi-particle fragmentation data and reconstruct evolving molecular geometries. Collaboration with theoretical chemistry groups will provide benchmark comparisons to nonadiabatic dynamics simulations, deepening the mechanistic interpretation of the structural dynamics observed. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria. NSF Award ID: 2534781 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Ruaridh Forbes | Institution: University of California-Davis, DAVIS, CA | Award Amount: $490,000 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2534781 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2534781.html