42 research outputs found
Accuracy of approximate methods for the calculation of absorption-type linear spectra with a complex system-bath coupling
The accuracy of approximate methods for calculating linear optical spectra
depends on many variables. In this study, we fix most of these parameters to
typical values found in photosynthetic light-harvesting complexes of plants and
determine the accuracy of approximate spectra with respect to exact calculation
as a function of the energy gap and interpigment coupling in a pigment dimer.
We use a spectral density with the first eight intramolecular modes of
chlorophyll a and include inhomogeneous disorder for the calculation of
spectra. We compare the accuracy of absorption, linear dichroism, and circular
dichroism spectra calculated using the Full Cumulant Expansion (FCE), coherent
time-dependent Redfield (ctR), and time-independent Redfield and modified
Redfield methods. As a reference we use spectra calculated with the Exact
Stochastic Path Integral Evaluation method. We find the FCE method to be the
most accurate for the calculation of all spectra. The ctR method performs well
for the qualitative calculation of absorption and linear dichroism spectra when
pigments are moderately coupled (), but ctR spectra may
differ significantly from exact spectra when strong interpigment coupling
() is present. The dependence of the quality of
Redfield and modified Redfield spectra on molecular parameters is similar, and
these methods almost always perform worse than ctR, especially when the
interpigment coupling is strong or the excitonic energy gap is small (for a
given coupling). The accuracy of approximate spectra is not affected by
resonance between the excitonic energy gap and intramolecular modes when
realistic inhomogeneous disorder is included.Comment: This article has been submitted to the Journal of Chemical Physics
(JCP
Excitation Dynamics and Relaxation in a Molecular Heterodimer
The exciton dynamics in a molecular heterodimer is studied as a function of
differences in excitation and reorganization energies, asymmetry in transition
dipole moments and excited state lifetimes. The heterodimer is composed of two
molecules modeled as two-level systems coupled by the resonance interaction.
The system-bath coupling is taken into account as a modulating factor of the
energy gap of the molecular excitation, while the relaxation to the ground
state is treated phenomenologically. Comparison of the description of the
excitation dynamics modeled using either the Redfield equations (secular and
full forms) or the Hierarchical quantum master equation (HQME) is demonstrated
and discussed. Possible role of the dimer as an excitation quenching center in
photosynthesis self-regulation is discussed. It is concluded that the
system-bath interaction rather than the excitonic effect determines the
excitation quenching ability of such a dimer
Electronic Coherence Dephasing in Excitonic Molecular Complexes: Role of Markov and Secular Approximations
We compare four different types of equations of motion for reduced density
matrix of a system of molecular excitons interacting with thermodynamic bath.
All four equations are of second order in the linear system-bath interaction
Hamiltonian, with different approximations applied in their derivation. In
particular we compare time-nonlocal equations obtained from so-called
Nakajima-Zwanzig identity and the time-local equations resulting from the
partial ordering prescription of the cummulant expansion. In each of these
equations we alternatively apply secular approximation to decouple population
and coherence dynamics from each other. We focus on the dynamics of intraband
electronic coherences of the excitonic system which can be traced by coherent
two-dimensional spectroscopy. We discuss the applicability of the four
relaxation theories to simulations of population and coherence dynamics, and
identify features of the two-dimensional coherent spectrum that allow us to
distinguish time-nonlocal effects.Comment: 14 pages, 8 figure
Polarization-controlled optimal scatter suppression in transient absorption spectroscopy
Ultrafast transient absorption spectroscopy is a powerful technique to study fast photo-induced processes, such as electron, proton and energy transfer, isomerization and molecular dynamics, in a diverse range of samples, including solid state materials and proteins. Many such experiments suffer from signal distortion by scattered excitation light, in particular close to the excitation (pump) frequency. Scattered light can be effectively suppressed by a polarizer oriented perpendicular to the excitation polarization and positioned behind the sample in the optical path of the probe beam. However, this introduces anisotropic polarization contributions into the recorded signal. We present an approach based on setting specific polarizations of the pump and probe pulses, combined with a polarizer behind the sample. Together, this controls the signal-to-scatter ratio (SSR), while maintaining isotropic signal. We present SSR for the full range of polarizations and analytically derive the optimal configuration at angles of 40.5° between probe and pump and of 66.9° between polarizer and pump polarizations. This improves SSR by 33 52 ≈. (or 3 compared to polarizer parallel to probe). The calculations are validated by transient absorption experiments on the common fluorescent dye Rhodamine B. This approach provides a simple method to considerably improve the SSR in transient absorption spectroscopy
Ultrafast Photo-Induced Charge Transfer Unveiled by Two-Dimensional Electronic Spectroscopy
The interaction of exciton and charge transfer (CT) states plays a central
role in photo-induced CT processes in chemistry, biology and physics. In this
work, we use a combination of two-dimensional electronic spectroscopy (2D-ES),
pump-probe measurements and quantum chemistry to investigate the ultrafast CT
dynamics in a lutetium bisphthalocyanine dimer in different oxidation states.
It is found that in the anionic form, the combination of strong CT-exciton
interaction and electronic asymmetry induced by a counter-ion enables CT
between the two macrocycles of the complex on a 30 fs timescale. Following
optical excitation, a chain of electron and hole transfer steps gives rise to
characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor
how the excited state charge density ultimately localizes on the macrocycle
closest to the counter-ion within 100 fs. A comparison with the dynamics in the
radical species further elucidates how CT states modulate the electronic
structure and tune fs-reaction dynamics. Our experiments demonstrate the unique
capability of 2D-ES in combination with other methods to decipher ultrafast CT
dynamics.Comment: 14 pages, 11 figures, and Supporting informatio
Exciton Dynamics in Photosynthetic Complexes: Excitation by Coherent and Incoherent Light
In this paper we consider dynamics of a molecular system subjected to
external pumping by a light source. Within a completely quantum mechanical
treatment, we derive a general formula, which enables to asses effects of
different light properties on the photo-induced dynamics of a molecular system.
We show that once the properties of light are known in terms of certain
two-point correlation function, the only information needed to reconstruct the
system dynamics is the reduced evolution superoperator. The later quantity is
in principle accessible through ultrafast non-linear spectroscopy. Considering
a direct excitation of a small molecular antenna by incoherent light we find
that excitation of coherences is possible due to overlap of homogeneous line
shapes associated with different excitonic states. In Markov and secular
approximations, the amount of coherence is significant only under fast
relaxation, and both the populations and coherences between exciton states
become static at long time. We also study the case when the excitation of a
photosynthetic complex is mediated by a mesoscopic system. We find that such
case can be treated by the same formalism with a special correlation function
characterizing ultrafast fluctuations of the mesoscopic system. We discuss
bacterial chlorosom as an example of such a mesoscopic mediator and propose
that the properties of energy transferring chromophore-protein complexes might
be specially tuned for the fluctuation properties of their associated antennae.Comment: 12 page
Exciton/Charge-transfer Electronic Couplings in Organic Semiconductors
Charge transfer (CT) states and excitons are important in energy conversion processes that occur in organic light emitting devices (OLEDS) and organic solar cells. An ab initio density functional theory (DFT) method for obtaining CT−exciton electronic couplings between CT states and excitons is presented. This method is applied to two organic heterodimers to obtain their CT−exciton coupling and adiabatic energy surfaces near their CT−exciton diabatic surface crossings. The results show that the new method provides a new window into the role of CT states in exciton−exciton transitions within organic semiconductors.United States. Dept. of Energy (DEFG02- 07ER46474)David & Lucile Packard Foundation (Fellowship
Origin of Long Lived Coherences in Light-Harvesting Complexes
A vibronic exciton model is developed to investigate the origin of long lived
coherences in light-harvesting complexes. Using experimentally determined
parameters and uncorrelated site energy fluctuations, the model predicts
oscillations in the nonlinear spectra of the Fenna-Matthews-Olson (FMO) complex
with a dephasing time of 1.3 ps at 77 K. These oscillations correspond to the
coherent superposition of vibronic exciton states with dominant contributions
from vibrational excitations on the same pigment. Purely electronic coherences
are found to decay on a 200 fs timescale.Comment: 4 pages, 2 figure
Isothermal Maxwell daemon as a molecular rectifier
A standard quantum model of the interacting particle vs.
single-mode phonon system under the influence of a thermodynamic bath is
considered for very specific values of the parameters involved. The resulting
computer-exact solutions show then the isothermal Maxwell daemon property
reported so far for just a few exceptional non-trivial models. Owing to
specific dynamical effects in the particle-phonon system, the resulting
long-time particle site occupation probabilities are incompatible with the
usual equilibrium statistical thermodynamics