Pseudorotation in photoionized benzene captured by time-resolved Coulomb explosion imaging

Vibronic coupling and coherence are crucial in the charge and energy transfer of photoexcited molecules. Here we investigate the coupled electron-nuclear dynamics of the photoionized benzene molecule using the time-resolved Coulomb-explosion imaging method. An unusual long-period oscillation is experimentally observed in the ion yields of the C₆H₆²⁺ channel, as well as the C⁺+C⁺, and the C⁺+C⁺+C⁺ Coulomb explosion channels. Quantum dynamics simulations reveal that this ~600 fs oscillation, which notably exceeds the period of any vibrational modes, originates from pseudorotation of the benzene cation. This motion arises from quantum beating between two coherent vibronic states of the benzene molecule coupled via the Jahn-Teller effect around the conical intersection. The structural evolution of the benzene cation via pseudorotation is visualized by the time-resolved momentum imaging in the C⁺+C⁺+C⁺ three-body Coulomb explosion channel. Our work offers a comprehensive characterization of coherent vibronic dynamics of the benzene cation and demonstrates the power of the time-resolved Coulomb-explosion imaging for unraveling coupled electronic and nuclear motions in aromatic molecules.