The role of quantum mechanical coherences or coherent superposition states in excited state processes has received considerable attention in the last two decades owing much to advances in ultrafast laser spectroscopy. The existence of coherence effects shows promise for enhancing the efficiency and robustness of functionally relevant processes, even when confronted with strong energy disorder and environmental fluctuations. Therefore, an in-depth understanding of coherence propels us to push the frontier to the grand challenge of using optical control of coherence to improve functions or create new ones in molecular and material systems. In this frontier, the role of electronic and vibrational interplay, or more specifically the role of vibrations in directing electronic dynamics, has emerged as the leading principle, where two energetically disparate quantum degrees of freedom work in-sync to dictate the trajectory of an excited state reaction. Moreover, with the vibrational degree being directly related to the structural composition of molecular or material systems, new molecular designs could be inspired by tailoring certain structural elements.

In this talk, I envision acquainting the quantum world of molecules to physicists. I will summarize essential aspects concerning the interplay of electronic and vibrational dynamics afforded from state-of-the-art ultrafast laser spectroscopy in three exemplary processes: photo-induced electron transfer, singlet-triplet intersystem crossing, and intramolecular vibrational energy-driven energy flow in molecular systems. More precisely, I will showcase crucial experimental signatures that offer deeper insights into the complex electronic-vibrational trajectories encompassing excited states. I will discuss rapid decoherence—loss of phase and amplitude correlations—of vibrational coherences along promoter vibrations in comparison to spectator coherences in a near-ballistic (~30-femtosecond time constant) electron transfer reaction in the Marcus-inverted region and during a sub-picosecond intersystem crossing dynamics in a series of binuclear platinum complexes. The rapid decoherence depicts the vibration-driven reactive pathways from Franck-Condon state to the curve crossing region. I will also discuss the generation of new vibrational coherences induced by impulsive reaction dynamics—not by the laser pulse—in these systems which informs on specific energy dissipation pathways and thereby on the progression of the reaction trajectory in the vicinity of the curve crossing on the product side. While the decoherence or impulsive generation of vibrational wavepackets has emerged as a prominent indicator of the interplay of electronic dynamics and vibrations, the other property of vibrational coherences, amplitude, also relays how energy can flow from one vibration to another vibration in the electronic excited state. 

From these studies, we learn that superposition states along vibrations that are part of the complex reactive trajectories are extremely sensitive to electronic dynamics. Thus, vibrational superposition states act as quantum mechanical windows for visualizing the interplay of electronic and vibrational dynamics. This frontier could change the outlook on how vibrations might soon become a control element in the hands of a chemist, influencing the outcome of a reaction.