Quantum dynamics of photoinduced processes in extended molecular systems

Thursday, January 15, 2009 - 2:40pm - 3:10pm
EE/CS 3-180
Irene Burghardt (École Normale Supérieure)
The photophysics of extended systems like conjugated polymers or molecular
aggregates is characterized on the one hand by the properties of the
molecular building blocks and on the other hand by the delocalized nature of
the electronic excitations, i.e., the formation of excitonic states. The
dynamical phenomena induced by photoexcitation therefore involve an interplay
of site-site interactions entailing excitation energy transfer, and vibronic
(electron-phonon) coupling which typically leads to ultrafast internal
conversion processes. We propose here a molecular-level, quantum-dynamical
approach as exemplified by our recent study of exciton dissociation at
interfaces of semiconducting polymer phases (so-called heterojunctions) [1].
This study combines a vibronic coupling model parametrized for the three
most relevant electronic states and 20-30 phonon modes, with accurate
multiconfigurational quantum dynamics calculations using the MCTDH method and
a Gaussian-based variant thereof (G-MCTDH) [2]. In addition, we employ
recently developed transformation techniques [1,3] by which a relevant set
of effective modes is constructed which account for the short-time dynamics
in high-dimensional systems involving conical intersection topologies. For
the semiconducting polymer systems under study, which typically involve high-
vs. low-frequency phonon bands, this analysis leads to a mechanistic picture
showing that the dynamical interplay between the two types of phonon modes
is crucial for the ultrafast dissociation of the photogenerated exciton
state. A perspective is given on the effect of averaging over ensembles of
interface structures, on the role of coherence, and on the extension of the
analysis to finite temperatures.

[1] H. Tamura, J. G. S. Ramon, E. R. Bittner, and I. Burghardt,
Phys. Rev. Lett. 100, 107402 (2008), J. Phys. Chem. B, 112, 495 (2008).

[2] G. A. Worth, H.-D. Meyer, H. Koeppel, L. S. Cederbaum, and I. Burghardt,
Int. Rev. Phys. Chem., 27, 569 (2008); I. Burghardt, K. Giri, and G. A. Worth,
J. Chem. Phys., 129, 174104 (2008).

[3] L. S. Cederbaum, E. Gindensperger and I. Burghardt, Phys. Rev. Lett.,
94, 113003 (2005).
MSC Code: