Resolving Photochemical Reaction Pathways Using Field-Induced Surface Hopping Simulation of fs-EUV-TRPES: The Case of Furan

Recent advances in femtosecond extreme ultraviolet photoelectron spectroscopy have enabled insight into photochemistry in gases, condensed phases, and at interfaces. Assigning spectral features to reaction pathways requires theoretical predictions connecting nonadiabatic dynamics to measured observables. Here, we present a general approach for resolving photochemical reaction pathways by combining field-induced surface hopping with explicit pulse excitation, RMS-CASPT2, and simulations of time-resolved photoelectron spectra. The pump field prepares the excited-state ensemble under experimental conditions, while the multireference treatment describes excited states, conical intersections, and bond-breaking coordinates. Mapping trajectories onto photoelectron signals, supported by a systematic analysis of photoionization approximations, enables comparison with experiment. We apply this approach to furan excited by a 200 nm, 10 fs pulse. Among trajectories returning to the ground state, approximately 87 % hop near the ring-puckering conical intersection and 13 % near the ring-opening intersection. Subsequent ground-state vibrational energy redistribution nevertheless populates ring-opening structures. Five ground-state products are identified. Simulated spectra reproduce time-dependent electron-binding-energy shifts, characteristic kinks, and bands with predicted photoproduct contributions. Compared with Dyson-norm-based intensities, plane- and Coulomb-wave continuum functions improve the early-time shape of the lowest binding energy signal and show that photoelectron anisotropy is more sensitive to the continuum approximation than the angle-integrated spectrum. More generally, these findings demonstrate how explicit-pulse nonadiabatic dynamics and dynamically correlated multireference electronic structure can resolve competing photochemical reaction pathways through simulated fs-EUV-TRPES.