Computational Spectroscopy of Ultrafast Molecular Dynamics

Conical intersections (CIs) are key features of molecular potential energy surfaces that govern how molecules behave after absorbing light. Acting as “doorways” between electronic states, they play a central role in determining chemical outcomes on ultrafast timescales. Yet, directly observing CIs remains a major challenge in spectroscopy.We use quantum dynamics simulations to uncover how CI-driven processes leave distinct fingerprints in spectroscopic signals. By modeling molecules like furan and its derivatives, we predict how different reaction pathways—such as ring puckering vs. ring opening—can be distinguished using time-resolved photoelectron spectroscopy, X-ray spectroscopy, and ultrafast electron diffraction.In a second example, we study the photodissociation of methyl iodide, a benchmark system in ultrafast science. We discover that quantum coherences formed at the CI surprisingly survive long after the molecule splits, showing up in the atomic products. To probe these hidden dynamics, we propose a novel approach using heterodyne-detected four-wave mixing, which not only captures CI-induced coherence but also enables a form of quantum state tomography.