Electronic Coherences Built by Ultrashort Pulse Drive the Vibronic Forces at Nonadiabatic Seams

We show that the quantum force that the electrons in a molecule exert on the nuclei remains local even in the presence of strong nonadiabatic interactions. At every geometry, the total force is the sum of a potential gradient term driven by the local population and a nonclassical, nonadiabatic force driven by the local electronic coherence. On the nonadiabatic seams, the nonadiabatic force competes effectively with the potential terms. Following a sudden photoionization, we show how the nonadiabatic component of the local force drives the dynamics of the structural Jahn–Teller rearrangement of the methane cation toward the C_2v geometry using 2 effective nuclear coordinates and 3 coupled electronic states for either CH⁺ ₄ or CD⁺ ₄. Toward the progress in reaching shorter femtosecond resolution in time-resolved x-ray and electron diffraction experiments, our work provides a quantitative understanding and pictorial visualization of the local forces that drive ultrafast molecular rearrangement and chemical reactivity with a special reference to the role of electronic coherences and nonadiabatic couplings.