Pavel Heřman

Exciton scattering in light-harvesting systems of purple bacteria

Abstract Using the reduced density matrix formalism the exciton scattering in light-harvesting systems of purple bacteria is calculated. Static disorder (fluctuations of the site energies) as well as dynamic disorder (dissipation) is taken into account in this work. Circular aggregates within 18 pigments are studied to model the B850 ring of bacteriochlorophylls within LH2 complexes. It can be shown that the influence of dissipation may not be neglected in the simulation of the time-dependent anisotropy of fluorescence. Also an elliptical deformation of the ring could be essential.

Influence of static and dynamic disorder on the anisotropy of emission in the ring antenna subunits of purple bacteria photosynthetic systems

Using the reduced density matrix formalism the time dependence of the exciton scattering in light-harvesting ring systems of purple bacteria is calculated. In contrast to the work of Kumble and Hochstrasser [J. Chem. Phys. 109 (1998) 855] static disorder (fluctuations of the site energies) as well as dynamic disorder (dissipation) is taken into account. For the description of dissipation we use Redfield theory in exciton eigenstates without secular approximation. This is shown to be equivalent to the Markovian limit of Capeks theory in local states. Circular aggregates with 18 pigments are studied to model the B850 ring of bacteriochlorophyls within LH2 complexes. It can be demonstrated that the dissipation is important for the time-dependent anisotropy of the fluorescence. Smaller values of static disorder are sufficient to produce the same decay rates in the anisotropy in comparison with the results by Kumble and Hochstrasser.

Towards proper parametrization in the exciton transfer and relaxation problem. II. Trimer

Abstract We extend our previous critical comparison of several parametrization schemes which have been used for transfer and relaxation of Frenkel exciton interacting with a responsive bath to the case of homogeneous cyclic trimer. Standard projection formalism yields dynamic equations for the single-exciton density matrix ρ complementing the usual single-exciton Liouville equation by adding terms describing the bath influence. The latter terms must, on the other hand, be properly parametrized. We found, mostly on the ground of analytical calculations which were allowed due to the simple structure of trimer, that the Redfield secular approximation could distort true time dependence of physical quantities calculated by means of exciton density matrix ρ .

Non-Markovian Effects in the Anisotropy of Emission in the Ring Antenna Subunits of Purple Bacteria Photosynthetic Systems

Using the reduced density matrix formalism the time dependence of the anisotropy of the fluorescence in light-harvesting ring systems LH2 of purple bacteria is calculated. In contrast to the work of Kumble and Hochstrasser [J. Chem. Phys. 109 (1998) 855] static disorder (fluctuations of the site energies) as well as dynamic disorder (dissipation) is taken into account. For the description of dissipation we use the Non-Markovian version of Čápeks theory in local states. It can be demonstrated that the dissipation is important for the time-dependent anisotropy of the fluorescence. Smaller values of static disorder are sufficient to produce the same decay rates in the anisotropy in comparison with the results by Kumble and Hochstrasser. The influence of the non-Markovian terms is thoroughly discussed.

Computer Simulation of the Anisotropy of Fluorescence in Ring Molecular Systems: Tangential vs. Radial Dipole Arrangement

Time dependence of the anisotropy of fluorescence in recently discovered cyclic antenna units of the BChl photosystem is modeled. Interaction with a bath and a static disorder here modeled as uncorrelated Gaussian disorder in the transfer integrals is taken into account. Parallel computer enviroment is used because one is forced to recalculate every physical quantity for several thousands of different realizations of disorder. Results for the ring LH4 with radial optical transition dipole arrangement are compared with those for the ring LH2 with the tangential one. The difference between LH2 and LH4 results for the static disorder in transfer integrals has an opposite sign in comparison with that one for the static disorder in local energies. Equivalent differences are shifted to a smaller times for the stronger interaction with a bath.

Computer Simulation of Emission and Absorption Spectra for LH2 Ring

Computer simulation of absorption and steady state fluorescence spectra for molecular system is presented. We focus on the B850 ring from peripheral cyclic antenna unit LH2 of the bacterial photosystem from purple bacteria. Uncorrelated static disorder in radial positions of molecules on the ring is taking into account in our simulations. We consider also influence of dynamic disorder, interaction with phonon bath, in Markovian approximation. Spectral responses are calculated by the cumulant-expansion method of Mukamel et al. Procedure in Fortran was created for calculation of single ring spectra within full Hamiltonian model. These new results are compared with our previous ones (within the nearest neighbour approximation model) that were obtained by software package Mathematica.

Coherent memory functions for finite systems: Hexagonal photosynthetic unit

Coherent memory functions entering the Generalized Master Equation are presented for a hexagonal model of a photosynthetic unit. Influence of an energy heterogeneity and possible superstructure on an exciton transfer in an antenna system as well as to a reaction center is investigated.

Computer simulation of the anisotropy of fluorescence in ring molecular systems

The time dependence of the anisotropy of fluorescence after an impulsive excitation in the molecular ring (resembling the B850 ring of the purple bacterium Rhodopseudomonas acidophila) is calculated. Fast fluctuations of the environment are simulated by dynamic disorder and slow fluctuations by static disorder. Without dynamic disorder modest degrees of static disorder are sufficient to cause the experimentally found initial drop of the anisotropy on a sub-100 fs time scale. In the present investigation we are comparing results for the time-dependent optical anisotropy of the molecular ring for four models of the static disorder: Gaussian disorder in the local energies (Model A), Gaussian disorder in the transfer integrals (Model B), Gaussian disorder in radial positions of molecules (Model C) and Gaussian disorder in angular positions of molecules (Model D). Both types of disorder – static and dynamic – are taken into account simultaneously.

Femtosecond Dynamics in the Anisotropy of Emission in LH2 Units

Circular aggregates of 18 pigments are studied to model the B850 ring of bacteriochlorophylls within the LH2 complexes of the purple bacterium Rhodopseudomonas acidophila . Previously, the time evolution of the anisotropy of emission after an impulsive excitation has been calculated for varying degrees of static disorder but without dynamic disorder (interaction with a heat bath). In this case, modest degrees of static disorder are sufficient to cause the experimentally found initial drop of the anisotropy on a sub-100 v; fs time scale. We have taken into account the simultaneous influence of static and dynamic disorders and found that larger static disorder is needed to guarantee temperature independent initial drop of the anisotropy. In the present paper we extend our previous calculations and investigate non-Markovian effects in the description of the bath.

Trapping effects in exciton motion in the framework of CTRW

Abstract The influence of a trap (sink) on an exciton transfer in molecular aggregates is investigated. Pausing time distribution functions ψ p m ( t ) and probability densities Q mn ( t ) of the continuous time random walk theory are calculated from memory functions entering the generalized master equations. The presence of the sink changes their analytical form. We used trimer as an example to show that also large trapping rates could not give a guarantee that pausing time distribution functions and probability densities are always nonnegative.