Shaul Mukamel

Monitoring Elementary Molecular Events via Vibronic Coherences in Ultrafast X-Ray Spectroscopies

Conical intersections (CoIns) are ubiquitous features on the excited state landscapes of molecular systems. When wavepackets approach and pass a CoIn, a vibronic coherence emerges. The quantum behavior of this process determines the outcome of virtually all photochemistry and -physics. For example, the efficiency of energy transport processes versus ultrafast internal conversion rates depends on the wavepacket branching at CoIns. The direct spectroscopic detection of the passage through CoIns remains an open challenge. 
The simulation and design of novel spectroscopies revealing detailed background-free information about conical intersections will be presented. Recently developed x-ray sources offer novel ways to probe electronic and nuclear dynamics in molecules with high temporal resolution. A stimulated X-ray Raman technique is demonstrated to be sensitive only to coherences emerging at the conical intersection. Fundamental properties like the transient energy distribution of the involved vibronic states are encoded in the signal. We demonstrate its capabilities on the RNA-nucleobase Uracil. Time-resolved X-ray diffraction, records images of electron transition densities associated with the conical intersection, thereby providing a spatial movie of the non-adiabatic passage in azobenzene photoisomerization. 

A major obstacle in both techniques is the intrinsic weakness of coherence-based signatures stemming from the conical intersection. Using a unique combination of spectroscopy and quantum optimal control, strategies are discussed to enhance these contributions.

D. Keefer, J. R. Rouxel, F. Aleotti, F. Segatta, M. Garavelli, S. Mukamel, “Diffractive Imaging of Conical Intersections Amplified by Resonant Infrared Fields”, Journal of the American Chemical Society, jacs.1c06068 (2021).

S. M. Cavaletto, D. Keefer, S. Mukamel, “High Temporal and Spectral Resolution of Stimulated X-Ray Raman Signals with Stochastic Free-Electron-Laser Pulses”, Physical Review X 11, 011029 (2021).

D. Keefer, F. Aleotti, J. R. Rouxel, F. Segatta, B. Gu, A. Nenov, M. Garavelli, S. Mukamel, “Imaging conical intersection dynamics during azobenzene photoisomerization by ultrafast X-ray diffraction”, Proceedings of the National Academy of Sciences 118, e2022037118 (2021).