Jan Roden

Influence of complex exciton-phonon coupling on optical absorption and energy transfer of quantum aggregates.

We present a theory that efficiently describes the quantum dynamics of an electronic excitation that is coupled to a continuous, highly structured phonon environment. Based on a stochastic approach to non-Markovian open quantum systems, we develop a dynamical framework that allows us to handle realistic systems where a fully quantum treatment is desired yet the usual approximation schemes fail. The capability of the method is demonstrated by calculating spectra and energy transfer dynamics of mesoscopic molecular aggregates, elucidating the transition from fully coherent to incoherent transfer.

An efficient method to calculate excitation energy transfer in light-harvesting systems: application to the Fenna-Matthews-Olson complex

A master equation derived from non-Markovian quantum state diffusion is used to calculate the excitation energy transfer in the photosynthetic Fenna–Matthews–Olson pigment–protein complex at various temperatures. This approach allows us to treat spectral densities that explicitly contain the coupling to internal vibrational modes of the chromophores. Moreover, the method is very efficient and as a result the transfer dynamics can be calculated within about 1 min on a standard PC, making systematic investigations w.r.t. parameter variations tractable. After demonstrating that our approach is able to reproduce the results of the numerically exact hierarchical equations of motion approach, we show how the inclusion of vibrational modes influences the transfer.

Vibronic line shapes of PTCDA oligomers in helium nanodroplets

Oligomers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic-dianhydride, C(24)H(8)O(6) (PTCDA) are studied by means of helium nanodroplet isolation spectroscopy. In contrast to the monomer absorption spectrum, which exhibits clearly separated, very sharp absorption lines, it is found that the oligomer spectrum consists of three main peaks having an apparent width orders of magnitude larger than the width of the monomer lines. Using a simple theoretical model for the oligomer, in which a Frenkel exciton couples to internal vibrational modes of the monomers, these experimental findings are nicely reproduced. The three peaks present in the oligomer spectrum can already be obtained taking only one effective vibrational mode of the PTCDA molecule into account. The inclusion of more vibrational modes leads to quasicontinuous spectra, resembling the broad oligomer spectra.

Non-Markovian quantum state diffusion for absorption spectra of molecular aggregates

In many molecular systems one encounters the situation where electronic excitations couple to a quasi-continuum of phonon modes. The interaction to that often structured continuum may be highly frequency dependent, e.g. due to some weakly damped high frequency modes. To handle such a situation, an approach combining the non-markovian quantum state diffusion description of open quantum systems with an efficient but abstract approximation was recently applied to calculate energy transfer and absorption spectra of molecular aggregates [J. Roden, A. Eisfeld, W. Wolff, W. T. Strunz, Phys. Rev. Lett. 103, 058301 (2009)]. To explore the validity of the used approximation for such complicated systems, in the present work we compare the calculated (approximative) absorption spectra with exact results. These are obtained from the method of pseudomodes, which we show to be capable of determining the exact spectra for small aggregates and a few pseudomodes. It turns out that in the cases considered, the results of the two approaches mostly agree quite well. The advantages and disadvantages of the two approaches are discussed.

Electronic energy transfer on a vibronically coupled quantum aggregate

We examine the transfer of electronic excitation (an exciton) along a chain of electronically coupled monomers possessing internal vibronic structure and which also interact with degrees of freedom of the surrounding environment. Using a combination of analytical and numerical methods, we calculate the time evolution operator or time-dependent Greens function of the system and thereby isolate the physical parameters influencing the electronic excitation transport. Quite generally, we show that coupling to vibrations slows down and inhibits migration of electronic excitation due to dephasing effects on the coherent transfer present without vibrations. In particular, coupling to a continuous spectrum of environment states leads to a complete halting of transfer, i.e., a trapping of the exciton.

The J- and H-bands of dye aggregate spectra: Analysis of the coherent exciton scattering (CES) approximation

The validity of the CES approximation is investigated by comparison with direct diagonalisation of a model vibronic Hamiltonian of N identical monomers interacting electronically. Even for quite short aggregates ðN J 6Þ the CES approximation is shown to give results in agreement with direct diagonalisation, for all coupling strengths, except that of intermediate positive coupling (the H-band region). However, previously excellent agreement of CES calculations and measured spectra in the H-band region was obtained [A. Eisfeld, J.S. Briggs, Chem. Phys. 324 (2006) 376]. This is shown to arise from use of the measured monomer spectrum which includes implicitly dissipative effects not present in the model calculation.

SPECTRAL PROPERTIES OF MOLECULAR OLIGOMERS: A NON-MARKOVIAN QUANTUM STATE DIFFUSION APPROACH

Absorption spectra of small molecular aggregates (oligomers) are considered. The dipole-dipole interaction between the monomers leads to shifts of the oligomer spectra with respect to the monomer absorption. The line-shapes of monomer as well as oligomer absorption depend strongly on the coupling to vibrational modes. Using a recently developed approach [Roden et al., PRL 103, 058301] we investigate the length dependence of spectra of one-dimensional aggregates for various values of the interaction strength between the monomers. It is demonstrated, that the present approach is well suited to describe the occurrence of the J- and H-bands.

Anomalous strong exchange narrowing in excitonic systems

We investigate theoretically the phenomenon of exchange narrowing in the absorption spectrum of a chain of monomers, which are coupled via resonant dipole-dipole interaction. The individual (uncoupled) monomers exhibit a broad absorption line shape due to the coupling to an environment consisting of a continuum of vibrational modes. Upon increasing the interaction between the monomers, the absorption spectrum of the chain narrows. For a non-Markovian environment with a Lorentzian spectral density, we find a narrowing of the peak width [full width at half maximum (FWHM)] by a factor 1∕N, where N is the number of monomers. This is much stronger than the usual 1/√N narrowing. Furthermore, it turns out that for a Markovian environment no exchange narrowing at all occurs. The relation of different measures of the width (FWHM, standard deviation) is discussed.