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Experimental Detection of Branching at a Conical Intersection
Daniel B. Turner
Department of Chemistry, New York University, 100 Washington Square E, New York NY 10003
Scientists envision molecules and chemical reactions through potential energy surfaces, representations of how atomic motions modulate the energy of molecular orbitals . Potential energy surfaces result from invoking the Born–Oppenheimer approximation, which is usually adequate for chemistry restricted to ground states . However, this approximation is insufficient for describing photochemical transformations because it ignores nonadiabatic couplings. These couplings are enormous near conical intersections, features once considered esoteric but now theorized to be ubiquitous in chemical reactions involving excited states. Conical intersections are molecular configurations at which adiabatic potential-energy surfaces touch. Despite their importance and predicted ubiquity, conical intersections have been experimentally detected in only a few molecules because of technical challenges in the condensed phase. Indeed, condensed-phase experiments have focused on the few systems with clear spectroscopic signatures of negligible fluorescence, high photoactivity, or femtosecond electronic kinetics . Although rare, these signatures have become diagnostic for conical intersections. Here we detect a coherent surface-crossing event nearly two picoseconds after optical excitation in a highly fluorescent molecule that has no photoactivity and nanosecond electronic kinetics. Time−frequency analysis of high-sensitivity measurements acquired using sub-6 fs pulses  reveals phase shifts of the signal due to branching of the wavepacket through a conical intersection . The time−frequency analysis methodology demonstrated here on a model compound will enable studies of conical intersections in molecules that do not exhibit their diagnostic signatures, and represents a step toward experimentally mapping the topography of excited states in the condensed phase. Improving the ability to detect and map conical intersections will enrich the understanding of their mechanistic role in molecular photochemistry .
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