Physical Chemistry Seminar: Thomas Allison |Stony Brook University

Title: Imaging Exciton and Charge-Transfer Dynamics at Surfaces with Time-resolved Momentum Microscopy

Abstract: Many of our current technological ambitions hinge on our ability to engineer and control the light-driven dynamics of energy, charge, and spin at the nanoscale. Prominent examples occur in the development of photovoltaics and photocatalysts, for the conversion of abundant sunlight into electrical power or chemical fuels, and the development of optoelectronics for information technology. The coherent manipulation of excited states also lies at the heart of emerging quantum information technology, in the context of quantum memories and quantum repeaters which work with photons as mobile qubits.

The excited states created by optical excitation of matter are inherently complex, with many strongly-coupled degrees of freedom and dynamics occurring over a wide range of time and length scales. Ultrafast spectroscopy can in principle be used to dissect these complex interactions. However, most ultrafast spectroscopy measurements involve drastic averaging over many quantum states, which reduces the information content of the data and obscures its meaning. Optical spectroscopy methods are also blind to “dark states” that cannot be probed due to selection rules. So while the time resolution of ultrafast spectroscopy is often sufficient to resolve dynamics in complex systems, the interpretation of the observables remains a major challenge.

I will present recent results on advancing the sensitivity of time- and angle-resolved photoemission (tr-ARPES) from surfaces and applying these methods to exciton and charge-transfer dynamics in quantum materials and molecule-surface interfaces. The unique combination of an XUV light source based on cavity-enhanced high-order harmonic generation at 60 MHz repetition rate and time-of-flight momentum microscopy enable experiments to be conducted under low excitation fluence—which is critical for studying the intrinsic dynamics of the system—while still varying many parameters.

Speaker: T. K. Allison, Department of Chemistry and Department of Physics, Stony Brook University, Stony Brook, NY.

References:
[1] T. K. Allison, A. Kunin, G. Schönhense, APL Photonics 10, 010906 (2025).
[2] C. Corder, P. Zhao, J. Bakalis, X. Li, M. D. Kershis, A. R. Muraca, M. G. White, T. K. Allison, Structural Dynamics 5, 054301 (2018).
[3] K. Medjanik, O. Fedchenko, S. Chernov, D. Kutnyakhov, M. Ellguth, A. Oelsner, B. Schönhense, T. R. F. Peixoto, P. Lutz, C.-H. Min, F. Reinert, S. Daster, Y. Acremann, J. Viefhaus, W. Wurth, H. J. Elmers, G. Schönhense, Nature Materials 16, 615–621 (2017).