Phonon-assisted excitation energy transfer in photosynthetic systems

The phonon-assisted process of energy transfer aiming at exploring the newly emerging frontier between biology and physics is an issue of central interest.This article shows the important role of the intramolecular vibrational modes for excitation energy transfer in the photosynthetic systems.Based on a dimer system consisting of a donor and an acceptor modeled by two two-level systems,in which one of them is coupled to a high-energy vibrational mode,we derive an effective Hamiltonian describing the vibration-assisted coherent energy transfer process in the polaron frame.The effective Hamiltonian reveals in the case that the vibrational mode dynamically matches the energy detuning between the donor and the acceptor,the original detuned energy transfer becomes resonant energy transfer.In addition,the population dynamics and coherence dynamics of the dimer system with and without vibration-assistance are investigated numerically.It is found that,the energy transfer efficiency and the transfer time depend heavily on the interaction strength of the donor and the high-energy vibrational mode,as well as the vibrational frequency.The numerical results also indicate that the initial state and dissipation rate of the vibrational mode have little influence on the dynamics of the dimer system.Results obtained in this article are not only helpful to understand the natural photosynthesis,but also offer an optimal design principle for artificial photosynthesis.     

Migration transfer of electronic excitation energy in activated solids

An analytic expression for the kinetics of quenching of donor luminescence in the presence of acceptors with allowance for migration of energy over the donor subsystem is derived. An extended criterion for the applicability of the hopping migration model is obtained.     

Light absorption and excitation energy transfer calculations in primitive photosynthetic bacteria

In photosynthetic organisms, light energy is converted into chemical energy through the light absorption and excitation energy transfer (EET) processes. These processes start in light-harvesting complexes, which contain special photosynthetic pigments. The exploration of unique mechanisms in light-harvesting complexes is directly related to studies, such as artificial photosynthesis or biosignatures in astrobiology. We examined, through ab initio calculations, the light absorption and EET processes using cluster models of light-harvesting complexes in purple bacteria (LH2). We evaluated absorption spectra and energy transfer rates using the LH2 monomer and dimer models to reproduce experimental results. After the calibration tests, a LH2 aggregation model, composed of 7 or 19 LH2s aligned in triangle lattice, was examined. We found that the light absorption is red shifted and the energy transfer becomes faster as the system size increases. We also found that EET is accelerated by exchanging the central pigments to lower energy excited pigments. As an astrobiological application, we calculated light absorptions efficiencies of the LH2 in different photoenvironments.     

Effect of correlations on the relation between excitation energy and temperature

Formulae for the excitation energy of a nucleus as a function of its temperature are derived in the framework of the random phase approximation (RPA). Two methods are investigated. We first calculate the energy as the derivative of the grand potential obtained in random phase approximation. We also calculate the energy as the thermal average of the Hamiltonian operator using RPA Green functions at finite temperature. The two methods are compared by analyzing their content in terms of diagrams. We also discuss the effect of correlations on the relation between chemical potential and particle number.     

Extracting fluorescence signal due to direct excitation of the energy acceptor from quantum dot-based FRET

Journal of Nanoparticle Research - An “in situ” strategy for extracting the fluorescence signal of dye acceptors due to direct excitation from Qdot-based FRET systems has been reported....     

Migration of electronic excitation energy in heterogeneous media under conditions of inhomogeneous broadening of energy levels

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 919–922, June, 1991.     

Atomic state reduction and energy balance relation in spectrally resolved resonance fluorescence

The paper is devoted to a study of properties of the reduced atomic state directly after detection of a resonance fluorescence photon that passed through a spectral (Fabry–Perot) filter. We establish the energy balance relation for the reduced state and discuss its observable consequences.     

On the biphoton excitation of the fluorescence of the bacteriochlorophyll molecules of purple photosynthetic bacteria by powerful near IR femto-picosecond pulses

The authors of a number of experimental works detected nonresonance biphoton excitation of bacteriochlorophyll molecules, which represent the main pigment in the light-absorbing natural “antenna” complexes of photosynthesizing purple bacteria, by femtosecond IR pulses (1250–1500 nm). They believe that IR quanta excite hypothetic forbidden levels of the pigments of these bacteria in the double frequency range 625–750 nm. We propose and ground an alternative triplet mechanism to describe this phenomenon. According to our hypothesis, the mechanism of biphoton excitation of molecules by IR quanta can manifest itself specifically, through high triplet levels of molecules in the high fields induced by femtosecond-picosecond laser pulses.     

Migration of excitation in transfer of spectral-line radiation

A simple mathematical model is developed for the transfer of energy through a gas by the combined effect of radiative transfer and migration of excitation. The “excitation” is carried through the gas by a succession of atoms which experience resonant excitation exchange; it thus appears to random walk through the medium. The theory developed here is valid when the distance traveled by an atom while excited is much larger than the typical distance at which two atoms can exchange excitation (roughly 10^-6cm). The model is expressed in terms of a pair of coupled transport equations for the intensity of radiation and the density of excited atoms, which are solved by means of a generalized discrete-ordinate technique. Extensive numerical results are obtained and discussed in terms of characteristics lengths for the various transfer processes. Substantial effects of migration are seen in both the distribution of excited atoms near the cell windows and the line profile of the emergent radiation for typical laboratory conditions.     

Spectral changes in the fluorescence of chlorophyll during photosynthesis induction

With the help of a light-emitting diode with a radiation maximum at 407 nm and an S-2000 UV-VIS spectrometer connected with a computer, the spectral changes in the fluorescence of chlorophyll from acacia leaves (Acacia sp.) preliminarily subjected to a dark adaptation are studied. It is found that, in the slow induction phase, the Kautsky effect manifests itself in the “compression” of the intensity of the chlorophyll fluorescence spectrum, with this spectrum exponentially approaching the steady-state shape with a time constant of 10–20 s. Once the steady-state fluorescence spectrum of chlorophyll was established, the amount of energy delivered to the upper singlet level is one and a half times greater than at 735 nm. The ratio between the energies spent for photochemical processes of photosynthesis and for fluorescence depends on the wavelength and the instant of time of the induction period. In the steady-state state of the chlorophyll fluorescence, the values of this ratio at 685 and 735 nm are equal to 5.9 and 3.4, respectively.     

Possible dynamical limitations to excitation energy storage in nuclei

The dependence of the relative populations of particle unbound states on the associated charged particle multiplicity and on the total kinetic energy of the two decay products was investigated for ⁴⁰Ar induced reactions on ¹⁹⁷Au at E/A = 60 MeV. The measurements indicate that the relative populations exhibit little sensitivity to the violence of the collision and to the time of emission. Implications of these results on the dynamics of the reaction are discussed.     

Directed energy transfer due to orientational broadening of energy levels in photosynthetic pigment solutions

The directed non-radiative energy transfer through monomeric molecules of chlorophyll “a” and pheophytin “a” at high concentrations (c~10^-2 M) in a rigid matrix of polyvinylbutyral has been found by using the nanosecond laser spectrofluorimeter. The phenomenon is caused by orientational broadening of pigment molecular spectra owing to its interaction with a solvent. The observed temporal shift of the luminescence spectrum to the red region in a nanosecond time scale as well as the red shift of the time integrated spectrum at a high concentration of pigment molecules and the monotonic growth of the luminescence lifetime with a shift to the red region of the spectrum served as indications of the directed energy transfer in the sample. The non-radiative energy transfer from monomeric molecules towards aggregates is also directly demonstrated by the deformation of instantaneous luminescence spectra in the long-wavelength range (λ>700 nm). The role and the possibility of the directed energy transfer between molecules with orientationally broadened spectra in the biological systens are discussed.     

Migration of chlorine in the X-ray fluorescence analysis of rocks

The intensity of the analytical ClKα-line continuously increases during the measurement of a rock sample. The main cause of this effect is the migration of chlorine into the irradiated (emitting) part of the sample. The hypothesis was credibly proved through the experiment in which a rock sample was measured first on one side, and then on the other. An increase in the intensity of the analytical line was observed in both measurements. The intensity in the second measurement registers an increase from the same or even the lower level than in the first part. The following chlorine migration model is proposed. Negative chlorine ions arise in the rock sample under the influence of primary X-ray radiation. X-ray photons knock out part of the ions from the sample; therefore, a positive electric field is formed in it at the sample–vacuum interface. It attracts chlorine ions from the depths of the sample, as well as chlorine ions flying outside. Thus, the number of chlorine atoms in the surface layer of the sample increases and consequently, the intensity of the analytical line of chlorine increases. Formation of chlorine ions, their migration, and concentration in the surface layer of the sample occur only under the influence of primary X-ray radiation.     

Fluorescence and fluorescence excitation spectra of jet-cooled carbazole

The fluorescence excitation spectra of jet-cooled carbazole molecules at vibrational temperatures of 55 and 80 K and the fluorescence spectrum of these molecules excited by radiation at the frequency of a pure electronic transition are measured. As the vibrational temperature increases, the excitation spectra exhibit a series of lines of the same symmetry, which are caused by the interaction of the active vibration with a subensemble of optically inactive vibrations. The final symmetry of the totally and nontotally symmetric vibrations is determined from the shape of the rotational contours of the lines of vibronic transitions. The values of a decrease in the frequency of the nontotally symmetric vibrations in the first excited electronic state S ₁ due to their interaction with the electronic state S ₂ are calculated to be up to 100 cm^?1. The frequencies of the pure electronic transitions in the absorption and fluorescence spectra coincide with each other and are equal to 30809 cm^?1, the frequencies of vibrations in the ground state S ₀ exceeding the frequencies of the corresponding vibrations in the excited state S ₁. The degree of polarization of the integral fluorescence is determined for a series of vibronic transitions of the a ₁ and b ₂ final symmetry that are observed in the fluorescence excitation spectra, and the contribution of the intensity with the borrowed polarization θ_⊥ to the integral fluorescence is calculated. It is found that the intensity θ_⊥ is higher for the transitions of the b ₂ symmetry and can reach ≈50%.     

Experimental determination and interpretation of the fluorescence and fluorescence excitation spectra of chrysene cooled in a supersonic jet

The fluorescence and fluorescence excitation spectra of jet-cooled chrysene are measured. The frequencies of in-plane vibrations in the ground and first excited singlet electronic states, as well as the relative intensities of transitions between them, are calculated with the MO/M8ST method. Based on these data, experimental spectra are interpreted. In the fluorescence excitation spectrum, the position of the line of the 0–0 transition (28 195 ± 1 cm^?1), which is the most intense, is determined. In the experimental fluorescence excitation spectrum, 21 lines correspond to fundamental vibrations (altogether, 37 lines are attributed). This supports our assignment and is consistent with the group-theoretical analysis of vibronic interactions. Upon excitation at the frequency of the 0–0 transition, 10 lines corresponding to the excitation of fundamental vibrations are detected, and all 17 lines observed are attributed. In the fluorescence excitation spectrum, the standard deviation between the calculated and measured frequencies of attributed fundamental vibrations is 19 cm^?1, while that in the fluorescence spectrum is 15 cm^?1.     

Liquid-drop description of the nuclear excitation energy

An extended Fermi-gas model is presented to describe volume, surface and asymmetry contributions to the nuclear excitation energy. The potential part of the nuclear energy is treated within Brueckner's energy-density formalism; appropriate corrections due to surface and asymmetry effects are included. An excited nucleus is interpreted as a metastable superheated liquid. The excitation energy as a function of the temperature is given by a sum of volume, surface and asymmetry terms. The corresponding level-density formula is in reasonable agreement with experimental values for a suitable choice of the nuclear incompressibility modulus. It is shown that the surface contribution is appreciable.