Molecular Physics seminar

Molecular dynamics of photochemical processes

by Morgane Vacher (Uppsala University and Imperial College London)

FA 31 ()

FA 31

A photochemical process is a chemical reaction that is caused by the absorption of light and involves several electronic states. In this presentation, we will focus on two types of photochemical processes. In the first part, we will discuss coupled electron and nuclear dynamics following photoionisation. The hope to observe electronic motion on its own time scale arose a few years ago in the field of attoscience. Several experiments have aimed to observe electron dynamics following ionisation of molecules.1 Theoretical studies of pure electron dynamics in molecules have predicted oscillatory charge migration,2 but often at just a single fixed nuclear geometry, i.e. neglecting both nuclear motion and the natural nuclear distribution within the vibrational ground state wavepacket.3,4 Using our implementation of the Ehrenfest method,5 we simulate pure electron dynamics in a substituted bismethylene-adamantane cation, and its subsequent decoherence driven by nuclear motion and the natural zero point distribution in geometries.6 In the second part of the presentation, we will discuss a “reversed photochemical process”, called chemiluminescence, where the emission of light results from a chemical reaction. The basic understanding of today is that a thermally activated molecule decomposes and by doing so, it undergoes a non-adiabatic transition to an electronic excited state of the product (which then releases the excess energy in the form of light).7 To understand and rationalise experimental observations, time and efforts were until now devoted to theoretically investigate the detailed nature of 1,2-dioxetane molecule (the simplest light emitting species) and reaction mechanisms by computing cuts of potential energy surfaces and identifying critical points and pathways.8,9 We aim to provide more insights of the chemiluminescence mechanism by simulating the actual dynamics of the system. 1. F. Calegari et al, Science, 346, 336, 2014. 2. A. I. Kuleff and L. S. Cederbaum, J. Phys. B, 47, 124002, 2014. 3. M. Vacher, D. Mendive-Tapia, M. J. Bearpark and M. A. Robb, J. Chem. Phys., 142, 094105, 2015. 4. M. Vacher, L. Steinberg, A. J. Jenkins, M. J. Bearpark and M. A. Robb, Phys. Rev. A, 92, 040502(R), 2015. 5. M. Vacher, D. Mendive-Tapia, M. J. Bearpark and M. A. Robb, Theo. Chem. Acc., 113 1505, 2014. 6. M. Vacher, F. E. Albertani, A. J. Jenkins et al, Faraday Discuss., 10.1039/C6FD00067C, 2016. 7. I. Navizet, Y.-J. Liu, N. Ferré, D. Roca-Sanjuán and R. Lindh Chem. Phys. Chem., 12, P3064-3076, 2011. 8. L. De Vico, Y.-J. Liu, J. W. Krogh and R. Lindh, J. Phys. Chem. A, 111, 8013-8019, 2007. 9. P. Farahani, D. Roca-Sanjuán, F. Zapata and R. Lindh, J. Chem. Theory Comput., 9, 5404-5411, 2013