Tracking coherent vibronic and vibrational motions in ultrafast proton transfer.

Decades of theoretical and experimental work on excited-state intramolecular proton transfer systems indicate that electronic-vibrational (vibronic) coupling plays a significant role in ultrafast proton transfer reactions. However, the participating atomic motions of this chemical reaction have yet to be directly observed. The challenge in observing these rapid proton transfer processes lies in simultaneously probing both the electronic and vibrational degrees of freedom. Addressing these challenges requires advanced spectroscopic techniques that provide multidimensional (electronic and vibrational) structural information with femtosecond time resolution. Here, we use multidimensional electronic-vibrational spectroscopy to directly observe the vibronic motions driving the non-adiabatic excited-state proton transfer process in 10-hydroxybenzo[h]quinoline. We measure the interplay between high and low-frequency vibrations, mapping the proton transfer trajectory on two vibronically coupled electronic states. Our study enables the observation of vibronic coherence transfer, structural rearrangements, and intramolecular vibrational redistribution during and following proton transfer.