51 research outputs found
Theory of energy and charge transport in organic molecular systems.
No full textIn this thesis we investigate energy and charge transport in organic molecular systems. The main focus of this work is development and application of the stochastic model of the dynamics of open quantum systems. First of all, we derive the stochastic Schrödinger equation describing the evolution of a bosonic system linearly interacting with its environment. We demonstrate the performance of this equation on a small model system. Further we apply the stochastic Schrödinger equation to model the energy excitation transfer dynamics in the photosynthetic Fenna-Matthews-Olson (FMO) complex. The obtained results allow us to estimate the energy transfer pathways in this aggregate and their dependence on environmental factors. The stochastic Schrödinger equation is also used to model charge separation dynamics in organic Solar cells. Comparing theoretical results with experimental data we determine that in the femtosecond – picosecond timescale the motion of electron can be divided into three stages: coherent, transient and incoherent. We also analyze the influence of morphology of active layer in organic Solar cells on the process of charge separation
Energijos ir krūvio pernašos teorija organinėse molekulinėse sistemose.
No full textIn this thesis we investigate energy and charge transport in organic molecular systems. The main focus of this work is development and application of the stochastic model of the dynamics of open quantum systems. First of all, we derive the stochastic Schrödinger equation describing the evolution of a bosonic system linearly interacting with its environment. We demonstrate the performance of this equation on a small model system. Further we apply the stochastic Schrödinger equation to model the energy excitation transfer dynamics in the photosynthetic Fenna-Matthews-Olson (FMO) complex. The obtained results allow us to estimate the energy transfer pathways in this aggregate and their dependence on environmental factors. The stochastic Schrödinger equation is also used to model charge separation dynamics in organic Solar cells. Comparing theoretical results with experimental data we determine that in the femtosecond – picosecond timescale the motion of electron can be divided into three stages: coherent, transient and incoherent. We also analyze the influence of morphology of active layer in organic Solar cells on the process of charge separation
Description of coupling between molecular system vibrational degrees of freedom using time-dependent variational approach.
No full textCharacteristics and efficiency of majority of modern organic photoactive devices such as organic light-emitting diodes, light sensitive detectors, photovoltaic cells and others directly depend on different organic materials generating exciton pairs, carrying out energy transfer and performing charge separation reactions. Better understanding of finely tuned photosynthetic systems found in nature and ability to accurately simulate them, could help produce cheaper and more efficient photoactive devices. When trying to mathematically describe organic molecular aggregates, one must include a few different interaction types. Firstly, Coulomb forces between separated molecular charge densities allow for coherent energy transfer between different molecules to occur. Secondly, molecular vibrations must be coupled to electronic degrees of freedom, which perturb its energetic landscape. These vibrations can be distinguished by their origin and characteristics: low frequency environment over-damped phonon modes and high frequency under-damped molecular vibrations. Thirdly, vibrations of different origin should also be intercoupled for molecular vibrations to become damped. Most widely used theoretical approaches are pertubative and, due to strong coupling, are unable to accurately describe electron-vibration interactions. Also, phonon-vibration interactions are not straightforward to implement and often are neglected, resulting in unquenched molecular vibrations. The aim of this work is to derive general set of differential equations, using time-dependent variational approach, which explicitly include coupling between different vibrational modes, and can be used to describe excitation evolution in arbitrary molecular system. Using these equations, we simulated a model molecular system consisting of a single chlorophyll A molecule with one intra-molecular vibrational mode, which was coupled to phonon modes. Environment vibration frequency characteristics were described by experimentally measured spectral density function. Due to coupling, molecular vibration mode exhibited amplitude decay and frequency spectral line broadening. After analyzing changes to molecular and environment vibrational mode frequency power spectra, we deduced that intra-molecular mode was acting as a medium for phonon modes to redistribute vibrational energy. To better understand this effect, we constructed an effective spectral density function, which allowed to measure changes to electron-phonon coupling strength, induced by phonon-vibration coupling. In conclusion, (1) the derived equations for describing excitation and bath dynamics in molecular systems, with couplings between all system degrees of freedom, are sufficient for bath relaxation to occur, (2) and the effective spectral density function exhibited changes to the coupling of environment vibrations with electronic subsystem
Influence of environment spectral density on the absorption spectrum of two state system.
No full textInfluence of Environment Spectral Density on the Absorption Spectrum of Two State System
Modeling of nonlinear optical spectra of molecular complexes using exciton scattering approach.
No full textOptical two-dimensional (2D) coherent spectroscopy has been developed as a new method over the last 20 years. It probes the structure and dynamics of materials by exciting them with a sequence of phase-coherent pulses and recording their response as two or more delays are varied. It excels at determining if resonances are coupled, overcoming the effects of inhomogeneous broadening and disentangling congested resonances by spreading them in two dimensions. Objective of this work is to try the optimization of the exciton scattering method with the exciton overlap parameter. Likewise, attempt using the exciton scattering approach to model 2D spectra of the heliobacterial (hBc) reaction center. We have implemented the quasiparticle representation of the optical response as a computer package. The quasiparticle picture naturally emerges out of equations of motion for exciton variables, the nonlinear-exciton equations (NEE), that were derived and gradually developed at different levels. The nonlinear response is then attributed to exciton-exciton scattering. The scattering process let’s us simplify the NEE’s that are being solved and saves us tremendous computational time when applying to the 2D spectroscopy. In this work we used the simple J-aggregate system in order to test additional optimisations. We introduced the exciton overlap parameter. With it we could cut off some of the calculation cycles when excitonic overlaps are too low. When comparing the computational time between setting this parameter as 0 and setting it as 0.01 we saved considerable amount of time without affecting the spectres themselves. However, by increasing the parameter furthermore, the amount of time we win becomes negligible. And taking this parameter too big will start to affect the resulting 2D spectra. The exciton scattering approach was applied to model 2D spectra of a realistic system - the heliobacterial reaction center. The system chosen was, the hBc reaction center. The structure was characterized by Gisriel and coworkers. We used the excitonic model of Kimura Itoh for our calculations. While absorbtion spectrum coincided nicely with experimental data, the cross-peaks in the 2D spectrum do not compare to experiments. The homogenous broadening of the main peaks was too large and the cross-peaks in our calculations was hardly to be seen. In conclusion, usage of the exciton overlap parameter can optimize 2D spectra calculations for many pigment excitonic systems, while not loosing the precision of the results. Furthermore, our modelled diagonal peaks agreed nicely with the experiment. However, diagonal peak background covers the cross-peak amplitudes. Thus, Kimura and Itoh model is not viable to model cross-peaks of the 2D spectra
Variacinė elektroninių sužadinimų pernašos ir relaksacijos teorija.
No full textIn this thesis we investigate electronic energy transfer and relaxation in molecular aggregates. The main focus of this work is the development and application of the stochastic time-dependent variational approach to open quantum systems. First of all, we derive the equations of motion for an electronic system interacting with vibrational degrees of freedom. Then we analyze the dynamics and regimes of excitonic polaron formation in a model system. The accuracy of the approach is benchmarked, comparing it to a widely used theoretical method. The stochastic time-dependent variational approach is then applied to model the dynamics of optical excitations and the ultrafast time resolved fluorescence spectra in the Light Harvesting Complex 2. Finally, the effect of nonlinear coupling between the molecular system and the vibrational modes on the energy transfer and optical spectra is demonstrated
Modeling of multiphotonic spectra of molecular complexes.
No full textThe description of the nature by quantum mechanics is conceptually and computationally challenging. There is an unfulfillment in quantum technologies - it's hard to simulate and control quantum systems. With classical computers it is impossible to simulate quantum systems. In 1982 it was suggested by Richard Feynman and other people, that quantum systems should be simulated by quantum computers. Nevertheless, till this day we have a little progress to achieve this goal. There are different approaches how to build quantum computer, but still there is no practical use of it. The purpose of this work is to investigate water-soluble chlorophyll binding protein (WSCP) as a model for qubit system. WSCP consists of four pigments, which are described by Frenklel excitons. We interpret these excitons as four qubits. The main tool for this work is multiphotonic spectroscopic techniques with help of them we can draw two-dimensional correlation spectra of molecular complexes. We can use a laser to excite two, three or four excitons and observe from spectra their correlations. From broadening of peaks in the spectra we can find out the times of coherence between qubits. We found out that there is coherence between two qubits, which can be manipulated by femtosecond laser. In addition we can equate our system of four excitons (qubits) which is described by Frenkel Hamiltonian to four spin system described by Ising Hamiltonian
Wavepacket dynamics of coupled molecules interacting with anharmonic vibrations.
No full textWavepacket Dynamics of Coupled Molecules Interacting with Anharmonic Vibrations Various approaches exist when describing the nature of light induced excitations in the photosynthetic complexes. There is still the discussion how the electronic excitations appear and behave in such environments. However, it is concluded that molecular vibrations and thermal environment plays the crucial role in the behavior of excitation dynamics. In cases of strong interaction between the exciton and molecular vibrations the polaronic effects may be detected as the lattice of the molecular structure becomes strongly deformed and exciton becomes self-trapped in formed potential well. It is also well known that molecular vibrations are anharmonic in natural systems and effects of such vibrations are detectable in the low-temperature spectroscopy experiments of excited photosynthetic complexes. Multitude of theoretical methods have been developed to describe complex nature of polaronic excitation dynamics in photosynthetic molecular aggregates. In this work we apply the time dependent variational method, which allows us to describe the coorelated evolution of the vibrational modes and excitonic amplitudes. Variational method has been widely studied using different trial functions and Hamiltonians, however there were a lack of applications analyzing dynamics of anharmonic vibrations coupled with electronic excitation. In this work we propose the modified Davydov Ansatz, which includes squeezed coherent states and thus allows for more accurate description of wavepackets oscillations in phase space. As the Morse potential is well known anharmonic potential and widely used in the spectroscopy, we apply it to model the environmental oscillators of the system. We conclude that our equations reveal the shift of the lowest energy state of Morse oscillators in comparison with harmonic oscillators. The modified Davydov Ansatz leads to much more stable formation of excitonic polarons. If the squeezed coherent states are taken into account the vibrational bound and unbound states of Morse oscillators can be observed in the absorption spectra
Kvantinių sistemų relaksacijos teorija su grįžtamuoju ryšiu: nuo laiko priklausančio variacinio principo metodų vystymas.
No full textThe function of many biological molecular systems is closely tied to the process of energy relaxation. Understanding the pathways and rates of energy relaxation induced by photoexcitation is relevant across a range of molecular spatial scales. This includes the smallest molecules, intermediate-sized molecular aggregates, and the largest photosynthetic complexes found in nature, which may involve physical processes such as molecular energy relaxation, charge transfer, and spatial energy transfer across structures composed of tens or hundreds of molecules. The work explores the use of the family of Davydov’s trial wavefunctions, which expand the model’s vibrational eigenstates in terms of time-dependent coherent states. During time evolution, these wavefunctions continuously adjust to align with the most relevant eigenstates at any given moment. As excitation energy relaxation occurs in the model, the temperature of the model increases. The standard approaches lack a direct energy exchange mechanism between vibrational degrees of freedom, violating the assumption of constant temperature and introduces errors. The theoretical problem of vibrational heating mirrors a natural process, during which a large amount of thermal energy to its immediate surroundings. Subsequently, a cooling process – thermalization, occurs where the excess heat dissipates away from the molecules. This work proposes a theoretical formulation, implementation and investigation of thermalization to be used with Davydov’s trial wavefunctions
Simulation of charge carrier diffusion in algan epitaxial layers.
No full textDiffusion in disordered systems, such as amorphous or disordered semiconductors, does not follow classical laws which describe particle transport in ordered crystalline media, and this leads to anomalous subdiffusive transport properties. While in classical diffusion the mean square displacement of a particle is proportional to time to the first power, in subdiffusion the mean square displacement is proportional to time to some real power α < 1. Subdiffusion is described by fractional diffusion equation whose solution gives the evolution of particle concentration. Subdiffusive process can be simulated in computer assuming that particles move in energetically irregular lattice. Two one-dimensional lattice models are presented, each with different hopping mechanisms form one node to another. Statistically generated distributions of particle concentration are compared to the solutions of fractional diffusion equation. After analysing characteristics of subdiffusion in one dimension, the lattice model is upgraded to three dimensions and is used for simulating charge carrier dynamics in real materials. Particularly, in this work the lattice model is used for trying to explain the decrease of charge carrier lifetimes at lower energies in AlGaN epitaxial layers which was observed by using photoluminescence spectroscopy. We conclude that in order for a system to undergo subdiffusive process described by fractional diffusion equation, the energies of localized states (which in our models are the energies of the nodes of the lattice) have to be distributed according to the exponential law. Since both models produce same results, we also conclude that a particular mechanism of random walk has no influence to overall evolution of particle concentration. By simulating charge carrier dynamics and recombination in the upgraded three-dimensional lattice we can achieve the desired decrease of charge carrier lifetimes at lower energies with a particular set of parameters. However, the model cannot explain all experimentally observed characteristics and we conclude that there are more complicated mechanisms of charge carrier transport and recombination in AlGaN epitaxial layers
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