Linnaeus Physics Colloquium: Time-resolved adsorbate-surface and donor-acceptor dynamics

Lecturer: Claudio Verdozzi, Department of Mathematical Physics and European Theoretical Spectroscopy Facility, Lund University, Sweden
Title: Time-resolved adsorbate-surface and donor-acceptor dynamics:
scope and challenges for non-equilibrium green's function methods

Place: Kalmar – room N304, Norrgård. Växjö – through link, room D0073, building D. Live – through Adobe Connect, https://connect.sunet.se/cmp-kalmar.

Coffee and buns at 13.45 at Norrgård, room N304.

Abstract

Non-Equilibrium Green's Functions (NEGF) provide a general theoretical framework for a description of non-equilibrium phenomena. In this talk, we illustrate their scope to address the real-time dynamics of adsorbate-surface and donor-acceptor complexes. Specifically, after a brief review of NEGF and some approximations commonly involved in their practical implementations,

1. We consider a model approach to atomic desorption which is exactly solvable for finite systems. With more realistic systems in mind, we then introduce the Ehrenfest approximation for nuclei, treat electronic interactions within NEGF based on many-body perturbation theory, and include model semi-infinite substrates via embedding self-energy schemes. The progressive introduction of such approximations allows us to scrutiny each of them separately, and critically assess different level of NEGF treatments against exact benchmarks.
2. We then investigate the non-adiabatic charge transfer dynamics in a donor-acceptor complex, where the donor consists of HOMO and LUMO molecular levels, and the acceptor is a C60 molecule, modelled within a tight-binding description and augmented by parameterised electronic interactions. In this case, the electronic dynamics is performed within the Generalized Kadanoff-Baym ansatz for NEGF (tested agains density-matrix renormalization group, tDMRG) and also at the mean-field (Hartree-Fock) level, with the Ehrenfest dynamics used for nuclei. Our results for C60-based donor-acceptor complexes confirm that, in some parameter regimes, electron-phonon interactions can be expected to be the main provider of the charge transfer mechanism, as previously pointed out in the literature. However, the results also depict alternative scenarios, namely a large number of situations where donor-acceptor electronic correlations (i.e. effects beyond mean-field treatments) are primarily responsible for the donor-substrate charge transfer following to the photo-excitation.

While preliminary in character and based on rather simple systems, our results convey the potential of NEGF to study the non equilibrium dynamics of adsorbates, donor-acceptor complexes and, more in general, clean and covered material surfaces subject to ultrafast laser fields. At the same time, they also give a perspective on some challenges to be met by NEGF in future developments for time-resolved spectroscopies.