Unraveling the Photochemical Ring-Opening Dynamics of 1,3-cyclohexadiene with Ultrafast Electron Diffraction and \textit{Ab initio} Multiple Spawning*

While the photoinduced ring-opening of 1,3-cyclohexadiene (CHD) has been the topic of numerous experimental and theoretical studies, none have achieved the temporal and spatial resolution needed to unambiguously observe its ring-opening dynamics. Here, we directly observe the photoinduced ring-opening of CHD on the femtosecond and sub-{\AA}ngstr\"{o}m scales using MeV Ultrafast Electron Diffraction (UED) and Ab Initio Multiple Spawning (AIMS). We show that CHD experiences a substantial acceleration of the ring-opening motion upon relaxation to the ground state via a conical intersection, which is transformed into rotation of the terminal ethylene groups in 1,3,5-hexatriene (HT). Additionally, we observe a coherent oscillation between the cZc, tZc/cZt, and tZt rotamers of HT upon relaxation to the ground state.