The Photochemistry of Rydberg Excited Cyclobutanone: Photoinduced Processes and Ground State Dynamics

Owing to ring-strain, cyclic ketones exhibit complex excited-state dynamics with multiple competing photochemical channels active on the ultrafast timescale. While the excited-state dynamics of cyclobutanone after π^← n excitation into the lowest-energy excited singlet state (S₁) has been extensively studied, the dynamics following 3s← n excitation into the higher-lying singlet Rydberg (S₂) state are less well understood. Herein, we couple quantum and excited-state trajectory surface-hopping molecular dynamics simulations to study the relaxation of cyclobutanone following 3s← n excitation and to predict the ultrafast electron diffraction scattering signal that we anticipate to arise from the relaxation dynamics that we observe. Our simulations indicate that relaxation from the initially-populated singlet Rydberg state occurs on the hundreds-of-femtosecond to picosecond timescale consistent with the symmetry-forbidden nature of the state-to-state transition involved. Once cyclobutanone has relaxed non-radiatively to the electronic ground state (S₀), the vibrationally hot molecules have sufficient energy to form multiple fragmentory products on the electronic ground-state surface including C₂H₄ + CH₂CO (C2; 20%), and C₃H₆ + CO (C3; 2.5%). We discuss the limitations of our simulations, how these may influence the outcome of the excited-state dynamics we observe, and -- ultimately -- the predictive power of the simulated experimental observable.