Simulating nonadiabatic dynamics in benzophenone: Tracing internal conversion through photoelectron spectra

Benzophenone serves as a prototype chromophore for studying the photochemistry of aromatic ketones, with applications ranging from biochemistry to organic light-emitting diodes. In particular, its intersystem crossing from the first singlet excited state to triplet states has been extensively studied, but experimental or theoretical studies on the preceding internal conversion within the singlet manifold are very rare. This relaxation mechanism is particularly important because direct population transfer of the first singlet excited state from the ground state is inefficient due to its low oscillator strength. In this work, our aim is to fill this gap by employing mixed quantum-classical and full quantum dynamics simulations and time-resolved photoelectron spectroscopy for gas-phase benzophenone and meta-methyl benzophenone. Our results show that nonadiabatic relaxation via conical intersections leads to an increase in the population of the first singlet excited state, which appears linear within the simulation time of 500 fs. This population transfer due to conical intersections can be directly detected by a bifurcation of the photoelectron signal. In addition, we discuss to clarify the role of the third singlet excited state degenerate to the second excited state-a topic that remains largely unexplored in the existing literature on benzophenone.