Ultrafast Photochemistry and Electron Diffraction for Cyclobutanone in the S2 State: Surface Hopping with Time-Dependent Density Functional Theory

We simulate the photodynamics of gas-phase cyclobutanone excited to the S₂ state using fewest switches surface hopping (FSSH) dynamics powered by time-dependent density functional theory (TDDFT). We predict a total C3+C2 photoproduct yield of 9%, with a C3:C2 product ratio of 1:8. Two primary S₂_1$ conical intersections are identified: $β$ stretch and CCH bend, with the higher energy $β$ stretch being associated with sub-picosecond S$_2$ decay. Excited state lifetimes computed with respect to electronic state populations were found to be 7.0 ps (S$_2→S₁) and 550 fs (S₁_0$). We also generate time-resolved difference pair distribution functions ($Δ$PDFs) from our TDDFT-FSSH dynamics results in order to generate direct comparisons to ultrafast electron diffraction experiment observables. Global and target analysis of time-resolved $Δ$PDFs produced a distinct set of lifetimes: i) a 0.462 ps decay, and ii) a 16.8 ps decay that both resemble the S$_2$ minimum, as well as iii) a long ($>$ nanosecond) decay that resembles the S$_1$ minimum geometry and the fully separated C3/C2 products. Finally, we contextualize our results by considering the impact of the most likely sources of significant errors.