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Photophysics of extended molecular systems: Electronic structure and dynamics

We aim to study elementary energy transfer (ET) and charge transfer (CT) processes induced by photoexcitation in materials and biological systems (e.g., organic semiconducting polymers, donor-acceptor materials, chromophore-carbon nanotube (CNT) assemblies, DNA, and various related systems). The processes in question are often ultrafast (fs to ps scale) and involve coherent transfer mechanisms and coupled electronic-nuclear (vibronic) dynamics. Examples include the primary events of organic photovoltaics, i.e., exciton dissociation at polymer heterojunctions (collaboration with Bittner (UH Texas) and Tamura (U Tohoku) [Tamura, Bittner, Burghardt, Phys. Rev. Lett. 100, 107402 (2008), Tamura, Burghardt, Tsukada J. Phys. Chem. C 115, 10205 (2011)]. Current projects include the investigation of exciton migration in poly-phenylene-vinylene (PPV) type systems and ET/CT in novel donor-acceptor-donor systems which are developed and spectroscopically investigated at Strasbourg University (collaboration with S. Haacke).

Quantum dynamics in extended systems: effective-mode approach

Quantum dynamics and quantum dissipation in extended systems often necessitate an explicit treatment of a large number of degrees of freedom. We employ recently developed effective-mode techniques [ Martinazzo, Hughes, Burghardt, Phys. Rev. E (Rapid Comm.) 84, 030102(R) (2011), Martinazzo, Hughes, Vacchini, Burghardt, J. Chem. Phys. 134, 011101 (2011), Cederbaum, Gindensperger, Burghardt, Phys. Rev. Lett. 94, 113003 (2005)], which allow for meaningful reduced-dimensional representations that are typically combined with multiconfigurational quantum dynamics simulations (see below). This set-up is ideal to describe, e.g., the role of coherent effects vs. fluctuations and disorder in ultrafast ET. At a formal level, these studies connect to the formulation of the non-Markovian dynamics problem in terms of reduced-dimensional effective-mode picture.

Photoswitches and RNA folding

This topic connects directly to SFB 902 on the topic "Molecular principles of RNA regulation''. Our particular interest concerns ligand-RNA binding involving photoswitches of the azobenzene and spiropyrane family, which have been found to trigger RNA folding dynamics. The aim is to (i) characterize the photoswitches as such, (ii) their interaction with RNA, e.g., by intercalation into an RNA helix, and (iii) the RNA folding dynamics that is triggered by the interaction with the photoswitch. These steps cover a wide range of time scales, from the ultrafast (fs to ps) regime to long times (ms to s).

Multiconfigurational hybrid approaches

Building upon the Multiconfiguration Time-Dependent Hartree (MCTDH) method, we have developed the G-MCTDH variant that employs time-dependent Gaussian basis sets [Burghardt, Giri, Worth, J. Chem. Phys., 129, 174104 (2008); Worth, Meyer, Koeppel, Cederbaum, Burghardt, Int. Rev. Phys. Chem., 27, 569 (2008)]. This method can be employed as an on-the-fly approach, see recent work by Graham Worth (Birmingham). Novel developments include, in particular, a two-layer variant of the G-MCTDH approach and the classical limit of the method.

On-the-fly QM/MM surface-hopping approaches: the NEWTON-X method

The NEWTON-X method, (A package for Newtonian dynamics close to the crossing seam) performs surface-hopping (SH) nonadiabatic dynamics using the COLUMBUS, TURBOMOLE, and GAUSSIAN program packages. One of the developers of this method, Matthias Ruckenbauer, has recently joined our team.

Mixed quantum-classical approach with a mesoscopic environment

We have developed a quantum-classical hybrid method that is tailored to describe the evolution of a quantum subsystem coupled to a non-equilibrium environment (e.g., solvent) described in a mesoscopic setting [Burghardt, Bagchi, Chem. Phys. 134, 343 (2006), Bousquet, Hughes, Micha, Burghardt, J. Chem. Phys. 134, 064116 (2011)]. The approach is suitable to describe, e.g., ultrafast solvation dynamics coupled with charge transfer, or quantum-classical transport phenomena. We have formulated a rigorous theoretical basis of this approach (see the above references), along with a first numerical demonstration for the case of translational solvation dynamics [Hughes, Bousquet, Ramanathan, Burghardt, submitted]. The approach can be derived from the so-called quantum-classical Liouville Equation (QCLE), and one of its limits is an extended hydrodynamic or DDFT (Dynamical Density Functional Theory) description.

Non-linear time series analysis

This area is the central research interest of Rainer Hegger. Non-linear time series analysis employs information provided by measurements and reconstruct the original phase space from the "scalar" information of the time series. For applications, see the TISEAN package