198 research outputs found
A probability current analysis of energy transport in open quantum systems
We introduce a probability current analysis of excitation energy transfer
between states of an open quantum system. Expressing the energy transfer
through currents of excitation probability between the states in a site
representation enables us to gain key insights into the energy transfer
dynamics. It allows to, i) identify the pathways of energy transport in large
networks of sites and to quantify their relative weights, ii) quantify the
respective contributions of unitary dynamics, dephasing, and
relaxation/dissipation processes to the energy transfer, and iii) quantify the
contribution of coherence to the energy transfer. Our analysis is general and
can be applied to a broad range of open quantum system descriptions (with
coupling to non-Markovian environments) in a straightforward manner
Vibronic Lineshapes of PTCDA Oligomers in Helium Nanodroplets
Oligomers of the organic semiconductor PTCDA are studied by means of helium
nanodroplet isolation (HENDI) 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 quasi continuous spectra, resembling the broad oligomer spectra
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
Lichtabsorption und Energietransfer in molekularen Aggregaten
Aggregate aus Molekülen, in denen die Moleküle über ihre elektronischen Übergangsdipole miteinander wechselwirken, finden wegen ihrer besonderen optischen und Energietransfer-Eigenschaften vielfach Anwendung in Natur, Technik, Biologie und Medizin. Beispiele sind die wechselwirkenden Farbstoffmoleküle, die in den Lichtsammelkomplexen Photosynthese betreibender Lebewesen Sonnenlicht absorbieren und die Energie als elektronische Anregung hocheffizient zu Reaktionszentren weiterleiten, oder Aggregate aus tausenden von organischen Farbstoffmolekülen in einem flüssigen Lösungsmittel. Die Wechselwirkung der Moleküle (Monomere) führt zu über mehrere Moleküle delokalisierten angeregten elektronischen Zuständen, die die Energietransfer-Dynamik und die Absorptionsspektren der Aggregate prägen.
Die Lichtabsorption und der Energietransfer in molekularen Aggregaten werden oft stark von Vibrationen beeinflusst, sowohl von internen Vibrationsfreiheitsgraden der Monomere als auch von Vibrationen der Umgebung (z. B. das Proteingerüst in Lichtsammelkomplexen oder eine Flüssigkeitsumgebung), an die die elektronische Anregung koppelt.
Da es schwierig ist, diese Vibrationen in die theoretische Beschreibung des Transfers und der Spektren einzubeziehen, ist ihr genauer Einfluss noch nicht gut verstanden. Um dieses Verständnis zu verbessern, entwickeln wir in dieser Arbeit neue Berechnungsmethoden und untersuchen damit die Auswirkungen der Vibrationen.
Zuerst betrachten wir die diskreten internen Vibrationsfreiheitsgrade der Monomere. Dazu haben wir eine effiziente numerische Methode entwickelt, die es uns erlaubt, mehrere Freiheitsgrade pro Monomer explizit einzubeziehen und die volle Schrödinger-Gleichung zu lösen. Mit den Modellrechnungen können wir experimentelle Aggregat-Spektren der Helium-Nanotröpfchen-Isolation-Spektroskopie, mit der man die einzelnen Vibrationslinien der Monomere auflösen kann, zum ersten Mal quantitativ reproduzieren.
In früheren theoretischen Behandlungen wurde oft nur ein einziger Vibrationsfreiheitsgrad pro Monomer berücksichtigt – nun zeigen wir, dass die Einbeziehung möglichst vieler Freiheitsgrade für eine realistische Beschreibung von Aggregat-Spektren wichtig ist.
Um neben den internen Vibrationen auch den Einfluss der Umgebung beschreiben zu können, nutzen wir den Zugang offener Quantensysteme und nehmen an, dass die elektronische Anregung an ein strukturiertes Kontinuum von Vibrationsfreiheitsgraden koppelt. Erstmals wenden wir die sogenannte nicht-markovsche Quanten-Zustands-Diffusion auf die molekularen Aggregate an, wodurch wir mit Hilfe einer Näherung Spektren und Transfer mit einer sehr effizienten stochastischen Schrödinger-Gleichung berechnen können. So
können wir Merkmale gemessener Aggregat-Spektren, wie das schmale J-Band und das breite strukturierte H-Band, in Abhängigkeit der Anzahl der Monomere und der Wechselwirkungsstärke zwischen den Monomeren beschreiben. Auch können wir den Übergang von kohärentem zu inkohärentem Transfer erfassen.
Eine für den Transfer relevante Größe ist die Anzahl der kohärent gekoppelten Monomere im Aggregat. Diese schätzt man häufig aus der Verschmälerung des Aggregat-Spektrums ab. Wir finden jedoch für verschiedene Spektraldichten des Vibrationskontinuums sehr unterschiedliche Verschmälerungen des Aggregat-Spektrums, die wir analytisch erklären. So zeigen wir, dass die bisherige einfache Abschätzung der Anzahl der kohärent gekoppelten Monomere nicht gerechtfertigt ist, da die Verschmälerung stark vom angenommenen Modell abhängt
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. That continuum may be
highly structured e.g. due to some weakly damped high frequency modes. To
handle such a situation, an approach combining the non-Markovian quantum state
diffusion (NMQSD) description of open quantum systems with an efficient but
abstract approximation was recently applied to calculate energy transfer and
absorption spectra of molecular aggregates [Roden, Eisfeld, Wolff, Strunz, PRL
103 (2009) 058301]. 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
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Absence of Quantum Oscillations and Dependence on Site Energies in Electronic Excitation Transfer in the Fenna–Matthews–Olson Trimer
Energy transfer in the photosynthetic Fenna–Matthews–Olson (FMO) complex of green sulfur bacteria is studied numerically taking all three subunits (monomers) of the FMO trimer and the recently found eighth bacteriochlorophyll (BChl) molecule into account. The coupling to the non-Markovian environment is treated with a master equation derived from non-Markovian quantum state diffusion. When the excited-state dynamics is initialized at site eight, which is believed to play an important role in receiving excitation from the main light harvesting antenna, we see a slow exponential-like decay of the excitation. This is in contrast to the oscillations and a relatively fast transfer that usually occurs when initialization at sites 1 or 6 is considered. We show that different sets of electronic transition energies can lead to large differences in the transfer dynamics and may cause additional suppression or enhancement of oscillations.Chemistry and Chemical Biolog
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
Long-range energy transport in photosystem II.
We simulate the long-range inter-complex electronic energy transfer in photosystem II-from the antenna complex, via a core complex, to the reaction center-using a non-Markovian (ZOFE) quantum master equation description that allows the electronic coherence involved in the energy transfer to be explicitly included at all length scales. This allows us to identify all locations where coherence is manifested and to further identify the pathways of the energy transfer in the full network of coupled chromophores using a description based on excitation probability currents. We investigate how the energy transfer depends on the initial excitation-localized, coherent initial excitation versus delocalized, incoherent initial excitation-and find that the overall energy transfer is remarkably robust with respect to such strong variations of the initial condition. To explore the importance of vibrationally enhanced transfer and to address the question of optimization in the system parameters, we systematically vary the strength of the coupling between the electronic and the vibrational degrees of freedom. We find that the natural parameters lie in a (broad) region that enables optimal transfer efficiency and that the overall long-range energy transfer on a ns time scale appears to be very robust with respect to variations in the vibronic coupling of up to an order of magnitude. Nevertheless, vibrationally enhanced transfer appears to be crucial to obtain a high transfer efficiency, with the latter falling sharply for couplings outside the optimal range. Comparison of our full quantum simulations to results obtained with a "classical" rate equation based on a modified-Redfield/generalized-Förster description previously used to simulate energy transfer dynamics in the entire photosystem II complex shows good agreement for the overall time scales of excitation energy transport
Suppression of quantum oscillations and the dependence on site energies in electronic excitation transfer in the Fenna-Matthews-Olson trimer
Energy transfer in the photosynthetic complex of the Green Sulfur Bacteria
known as the Fenna-Matthews-Olson (FMO) complex is studied theoretically taking
all three subunits (monomers) of the FMO trimer and the recently found eighth
bacteriochlorophyll (BChl) molecule into account. We find that in all
considered cases there is very little transfer between the monomers. Since it
is believed that the eighth BChl is located near the main light harvesting
antenna we look at the differences in transfer between the situation when BChl
8 is initially excited and the usually considered case when BChl 1 or 6 is
initially excited. We find strong differences in the transfer dynamics, both
qualitatively and quantitatively. When the excited state dynamics is
initialized at site eight of the FMO complex, we see a slow exponential-like
decay of the excitation. This is in contrast to the oscillations and a
relatively fast transfer that occurs when only seven sites or initialization at
sites 1 and 6 is considered. Additionally we show that differences in the
values of the electronic transition energies found in the literature lead to a
large difference in the transfer dynamics
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