Physics Colloquium with Doran Raccah

Abstract: How do excitations and charges thread their way through soft, vibrating molecular materials to power new technologies?  At the 10nm – 1µm scale—where molecular aggregates become materials—photoexcited states and charge carriers can delocalize over many molecules while coupling to a dense spectrum of local and non-local vibrations. Recent work in spatially resolved non-linear spectroscopies have revealed the importance of dynamic and static heterogeneities in controlling key material functions, from singlet fission to charge separation, and highlighted deficiencies in our mechanistic models of how excitations/charges migrate and drive chemical processes. 

In this talk I will describe recent progress from my group toward that goal: a library of formally exact, open-quantum-system algorithms with O(1) scaling that make it practical to simulate excitation and charge transport—and the corresponding spectroscopic signals—in realistic molecular materials.  These methods retain a quantum description of both the excitation and molecular vibrations while sidestepping the exponential scaling that has historically limited simulations to small molecular aggregates.  We have released the core developments as the open-source package MesoHOPS and used this code to simulate excited-state processes in materials containing over a thousand molecules, accessing the mesoscopic regime that links molecular design to material function.

Bio: Doran Raccah earned his Ph. D. from the University of California at Berkeley in 2013. After a two-year adventure at the Dow Chemical Company, he took a post-doctoral fellowship at Harvard University with Prof. Alán Aspuru-Guzik. He was an assistant professor of chemistry at Southern Methodist University in Dallas from 2019-2023. He joined the Department of Chemistry at The University of Texas at Austin in 2023. Doran leads the MesoScience Lab where members work on developing new algorithms for scalable quantum dynamics simulations and integrative models of the bioenergetic membranes involved in photosynthesis.