Webinar series on

Light-Matter Interaction

This series of webinars aims at discussing exciting new concepts and developments related to light-matter interaction. They also offer an occasion to interact with early-career researchers and discuss stimulating new ideas in the field of photophysics and photochemistry – compensating somehow for the discussions one may have during a workshop. 

If you are interested in joining our webinars, please contact Basile directly.

You will find below the full list of confirmed webinars:

24 September 2020 – 4.30pm (BST)

Mixed Quantum-Classical Dynamics in Cavity Quantum Electrodynamics

by Dr. Norah Hoffmann (Columbia University)

Chemical reactions under the influence of light play a paramount role in everyday life, from fundamental biological processes such as photosynthesis, vision, and DNA radiation-damage, to sustainable energy applications such as solar cells. Thus, controlling or optimizing the underlying chemical processes is of great interest. Recently, experimental developments in cavity Quantum Electrodynamics (QED) have opened up new possibilities of modifying and controlling chemical reactions by taking advantage of the strong light-matter interaction that arises when matter is confined in cavities. In such experiments the quantized nature of light becomes important and can profoundly change the chemical landscape. However, investigating these new developments from the theoretical point of view presents a challenging and especially high-dimensional problem, i.e. many molecules (electrons and nuclei) interacting with quantized light fields (many photon modes). Therefore, the theoretical simulation and prediction of realistic cavity QED experiments is by far not a trivial task and requires accurate and computationally efficient approximations.
To this end, we investigated and benchmarked extensions of mixed-quantum classical trajectory methods and time-dependent potential energy surfaces, both traditionally introduced for electron-nuclear problems, to the photonic degrees of freedom. In this talk I will discuss, how Wigner-sampling schemes for photons enable us to introduce quantized light-fields and multi-photon-mode treatments to quantum dynamics simulations in a computational efficient way, by properly accounting for the quantum statistics of the vacuum field while using classical/semi-classical trajectories to describe the time-evolution. Additionally, I will give an overview of our latest work on extending the exact-factorization approach to the photonic degrees of freedom, in order to set a starting point for the development of new mixed-quantum classical approaches within strongly coupled light-matter systems.

A schematic of a mixed quantum-classical approach for photons, the cavity Multi-Trajectory Ehrenfest (cMTE) algorithm (left). A molecule coupled to many photon modes (right) and the corresponding nuclear dynamics when coupling to a single photon mode (a) vs. the effect of coupling to many cavity photon modes, where spBO denotes the self-polarization modified Born-Oppenheimer-surfaces.

15 October 2020 – 2pm (BST)

Developing and applying efficient DD-vMCG method for nonadiabatic simulations

by Georgia Christopoulou (UCL)

Direct Dynamics is the branch of molecular dynamics simulations that solves the time- dependent Schrödinger equation (TDSE) by allowing the calculation of potential energy surfaces on- the-fly [1]. One of the major advantage of this method is that it is feasible to treat any system available to quantum chemistry as simply as using a modern quantum chemistry computer program. Also, since a quantum dynamics method is used to propagate the nuclei, the analysis of the influence of the quan- tum effects on reactivity is enabled. In the Direct Dynamics variational multi-configuration Gaussian wavepacket (DD-vMCG) method, a fully variational solution of the TDSE is made for the nuclei. The nuclear wavefunction is described as a superposition of Gaussian Wavepackets and the potential surfaces are provided on-the-fly [2]. During every timestep in a DD-vMCG propagation the calculated energies, gradients and hessian matrix are stored in a database. One important challenge of this method is the time needed to continually reread, sort and analyze this database which makes the calculation of a large system very expensive. To this end, the aim is to improve the existing method to be more efficient so we can treat complex chemical systems. In this talk our recent methodological updates to the DD-vMCG implementation in the Quantics sets of programs along with benchmark calculations on the allene radical cation will be discussed in detail.

[1] G. A. Worth, M. A. Robb, and B. L. Lasorne. Mol. Phys., 106:2077–2091, (2008).
[2] B. Lasorne, M.A. Robb, and G.A. Worth. PCCP, 9:3210 – 3227, (2007).

Schematic representation of DD-vMCG method for excited-state dynamics.

12 November 2020 – 2pm (BST)

Intramolecular Singlet Fission Mechanism in Donor-Acceptor Copolymers: Nonadiabatic Quantum Dynamics

by Dr. Maria Fumanal (EPFL)

Singlet Fission (SF) is a photophysical phenomenon that promises to overcome the limit of photo-conversion efficiency in organic photovoltaics by converting one singlet into two triplet excitons following S11TT→2T1. Since many years, SF has been reported for molecular crystals as well as isolated dimers through intermolecular and intramolecular mechanism, respectively. The latter is particularly attractive because the SF efficiency is not dependent on the crystal packing but an intrinsic property much easy to tune via molecular design. In this context, the donor-acceptor (D-A) modular strategy of copolymers has shown great potential for SF given that it can incorporate low-lying charge-transfer (CT) and multi-excitonic states in a single polymer chain [1-2]. Few D-A copolymers have remarkably succeed in displaying intramolecular SF (iSF), however the reasons behind the extraordinary efficiency found in some of these systems remains unknown. SF is an intrinsically dynamic process that can be hardly understood from the static picture considering that several (de)activation channels may compete with one another. To this end, we investigated how singlet splitting S11TT occurs in a prototypical iSF D-A copolymer by means of nonadiabatic quantum dynamics simulations. In this talk, I will discuss how to comprehensively build up a model Hamiltonian that serves as the basis for wave-packet propagations performed in a set of coupled diabatic potentials. This strategy allows to identify which excited states, vibrations and electronic coupling drive the ultrafast excited state decay in these systems and reveal the interplay between a direct, a mediated and a potentially delayed mechanism.

[1] Busby et al. Nat. Mater. 2015, 14, 426
[2] Blaskovits et al. Chem. Mat. 2020, 32, 6515