Chenghao Zhang

Chenghao Zhang

Phd student in Department of Physics, University of Illinois at Urbana Champaign

Talks and presentations

Surface crossing and energy flow in many dimensional quantum system

June 21, 2023

Oral presentation, 76th International Symposium for Molecular spectroscopy 2023, Urbana, IL, Urbana, IL

Vibrational energy flow in molecules, like the dynamics of other many dimensional finite systems, involves quantum transport across a dense network of near resonant states. For molecules in their electronic ground state, the network is ordinarily provided by anharmonic vibrational Fermi resonances. Surface crossing between different electronic states provides another route to chaotic motion and energy redistribution. We show that nonadiabatic coupling between electronic energy surfaces facilitates vibrational energy flow, and conversely, anharmonic vibrational couplings facilitate nonadiabatic electronic state mixing. A generalization of the LoganWolynes theory of quantum energy flow in many-dimensional Fermi resonance systems to the two-surface case gives a phase diagram describing the boundary between localized quantum dynamics and global energy flow. We explore these predictions and test them using a model inspired by the problem of electronic excitation energy transfer in the photosynthetic reaction center. Using an explicit numerical solution of the time dependent Schrodinger equation for this ten-dimensional ¨ model, we find quite good agreement with the expectations from the approximate analytical theory.

A phase diagram for energy flow limited reactivity

June 20, 2022

Oral presentation, 76th International Symposium for Molecular spectroscopy 2022, Urbana, IL

Intramolecular vibrational redistributionis often assumed in Rice–Ramsperger–Kassel–Marcus and other rate calculations. In contrast, experimental spectroscopy, computational results, and models based on Anderson localization have shown that ergodicity is achieved rather slowly during molecular energy flow and the statistical assumption might easily fail due to quantum localization.

Quantum Scrambling in Molecules

March 18, 2022

Oral presentation, American Physical Society March Meeting 2022, Chicago, IL

In quantum systems, out of time order correlators (OTOCs) can be used to probe the sensitivity of the dynamics to perturbing the Hamiltonian or changing the initial conditions ordinarily associated with classical chaos or its quantum analog. The vibrations of polyatomic molecules are known to undergo a transition from regular dynamics at low energy to facile energy flow at sufficiently high energy. Molecules therefore represent ideal quantum systems to study the transition to chaos in many-body systems of moderate size (here 6 to 36 degrees of freedom). By computing quantum OTOCs and their classical counterparts we quantify how information becomes ‘scrambled’ quantum mechanically in molecular systems.