Excitation Trapping Research Articles

Excitation trapping on two-dimensional percolating clusters

The trapping problem in disordered condensed phase systems is addressed by calculating the survival probability of a random walker on two-dimensional percolating clusters containing a random distribution of trap sites. The observable is directly determined by numerically solving the Pauli master equation via the Lanczos algorithm and the recursive residue generation method. From the results, we delimit the time and density regimes over which approximations developed for the corresponding statistic on regular lattices and deterministic fractals may be extended. In addition, recent predictions regarding the survival probability's long-time behavior are confirmed for moderately high trap concentrations; it is also shown that on this time scale, the fluctuations in the survival probability become enormous.

Directed picosecond excitation transport in purple photosynthetic bacteria

Picosecond spectrally resolved fluorescent measurements together with the coupled kinetic rate equation model simulations of the data have been employed to determine the rates and pathways of heterogeneous excitation transport in the membranes of photosynthetic bacteria Rhodobacter sphaeroides and Chromatium minutissimum. Excitation transport from bacteriochlorophyll molecules B800 to B850, belonging to the same pigment-pigment-protein complex B800–850, proceeds in 1–2 ps at room temperature and at 77 K. The intercomplex excitation transport from B850 to B875 takes about 10 ps for most excitations. In the minor part of B800–850 complexes, the excitation transport to B875 complex takes much longer, about 50 ps. The macroscopic rate constant of excitation trapping by open reaction centres is shown to be the same for all the bacteria studied, although the number of antenna molecules per reaction centre differs significantly. This seems to be an indication of the intrinsic homogeneity of the long-wavelength bacteriochlorophyll band, which facilitates an additional localization of excitations in the vicinity of the reaction centre and, due to the shortening of the trapping time, increases the overall quantum yield of photosynthesis.

Nonlinear spectroscopy, scattering and excitation trapping of exciton-polaritons in naphthalene and doped naphthalene crystals

Phase-matched two-photon excited (TPE) fluorescence and second-harmonic generation (SHG) are used to determine the transvere polariton dispersion curves and longitudinal exciton associated with the 31475 cm − a -exciton of naphthalene. Crystal angle tuning is shown to provide polariton group velocities in the range 10 6 gVp ≲ν p≲5×10 8 cm s −. Scattering of these polaritons with phonons is completely characterized and compared with the same for the longitudinal exciton and the relatively slow polaritons (ν p ≈ 2×10 5 cm s −) produced by one-photon excitation. Narrowing of the exciton-phonon interaction results from fast polariton motion. As a result, the semi-classical theory of nonlinear susceptibilities is shown to break down. The temperature-dependent branching ratio for SHG and two-photon absorption yield a low T residual scattering frequency of γ 0≈4×l0 9 s − for the polaritons in pure strain-free crystals. Macroscopic polariton coherence lengths ( l p) are achieved at low T. A novel methodology for the study of energy transfer in doped molecular crystals is described which employs wavevector- and velocity-selected polaritons. For naphthalene doped with anthracene (trap) the donor-trap “reaction complex” radius ( a) is ≈ 50 Å with an associated scattering frequency of p≈ 10 12 s −. It is necessary to consider scattering of the polariton (exciton) off of the trap ( p sc) as well as actual trapping or capture ( p tr). The ratio p sc/ p tr≈7 and p= p sc+ p tr. Analysis of the data with a simple theoretical model shows that energy transfer via the phase-matched polaritons utilized is trap limited (i.e. not motion limited) with concomitant (and “tunable”) high probabilities for transmission of the polariton through the trap region. On the other hand, the energy transfer via the relatively slow polaritons (ν p ≈10 5 cm s −) or even slower unclothed excitons is shown to be motionlimited. In this limit, the transmission probability is zero and the quasiparticle dephasing frequency (sum of the rate constants for capture and scattering) is given by the trap encounter frequency, R enc. For the intermediate regime it is necessary to write R enc p/( p+ t enc −1) for the dephasing frequency, where t enc −1) is the quasiparticle escape frequency (from the trap region).

Fractal Reaction Kinetics

Classical reaction kinetics has been found to be unsatisfactory when the reactants are spatially constrained on the microscopic level by either walls, phase boundaries, or force fields. Recently discovered theories of heterogeneous reaction kinetics have dramatic consequences, such as fractal orders for elementary reactions, self-ordering and self-unmixing of reactants, and rate coefficients with temporal "memories." The new theories were needed to explain the results of experiments and supercomputer simulations of reactions that were confined to low dimensions or fractal dimensions or both. Among the practical examples of "fractal-like kinetics" are chemical reactions in pores of membranes, excitation trapping in molecular aggregates, exciton fusion in composite materials, and charge recombination in colloids and clouds.

Photovoltaic instabilities

Abstract A review of recently discovered instability phenomena related to the photovoltaic current is presented. It includes the photo-electric domain instability observed in ruby crystals and the instability causing excitation trap recharging waves in photorefractive crystals. The photoelectric instability leading to formation of stationary domain structure is due to an absolute negative photoconductivity of the crystal in zero electric field. It is shown that such instability should occur in some ferroelectrics e.g. in KDP crystals. The spontaneous excitation of trap recharging waves in uniformly illuminated noncentrosymmetric crystals is induced by the photovoltaic current which becomes inhomogeneous in the presence of such waves. Effects related to trap recharging waves in photorefractive crystals, namely beam coupling, space charge stratification and stimulated light scattering are discussed. These effects are important for dynamic holography.

Nonexponential picosecond trapping in poly-(N-vinylcarbazole). A theoretical study

A theoretical model for excitation trapping in poly-(N-vinylcarbazole) in the presence of excimer dissociation is presented. We assume a time-dependent trapping coefficient in the form k(t)=b+ct−1/2, which reflects both the diagonal disorder and quasi-one-dimensionality of carbazole hopping sites [Kauffmann et al., J. Chem. Phys. 85, 3566 (1986)]. The equations obtained for monomer and excimer fluorescence decays are given in terms of Laplace transforms. Respective equations for fluorescence decays have been obtained also in the case when excimer formation is time dependent. Equations for monomer and excimer quantum yields are also given.

Multiatom squeezed states: a new class of collective atomic states

We introduce states of correlated pairs of atoms with the property that the collective atomic dipole operators exhibit squeezing. These multiatom squeezed states are constructed by analogy with the multimode squeezed states and thermofield states of the radiation field. Atoms prepared in a multiatom squeezed state can radiate strongly squeezed light. The production of multiatom squeezing might be possible by using coherent excitation and population trapping techniques.

Antenna size dependence of fluorescence decay in the core antenna of photosystem I: estimates of charge separation and energy transfer rates.

We have examined the photophysics of energy migration and trapping in photosystem I by investigating the spectral and temporal properties of the fluorescence from the core antenna chlorophylls as a function of the antenna size. Time-correlated single photon counting was used to determine the fluorescence lifetimes in the isolated P700 chlorophyll a-protein complex and in a mutant of Chlamydomonas reinhardtii that lacks the photosystem II reaction center complex. The fluorescence decay in both types of sample is dominated by a fast (15-45 psec) component that is attributed to the lifetime of excitations in the photosystem I core antenna. These excitations decay primarily by an efficient photochemical quenching on P700. The measured lifetimes show a linear relationship to the core antenna size. A linear dependence of the excitation lifetime on antenna size was predicted previously in a lattice model for excitation migration and trapping in arrays of photosynthetic pigments [Pearlstein, R.M. (1982) Photochem. Photobiol. 35, 835-844]. Based on this model, our data predict a time constant for photochemical charge separation in the photosystem I reaction center of 2.8 +/- 0.7 or 3.4 +/- 0.7 psec, assuming monomeric or dimeric P700, respectively. The predicted average single-step transfer time for excitation transfer between core antenna pigments is 0.21 +/- 0.04 psec. Under these conditions, excitation migration in photosystem I is near the diffusion limit, with each excitation making an average of 2.4 visits to the reaction center before photoconversion.

Excitation transport in solution: A quantitative comparison between GAF theory and time-resolved fluorescence profiles

Time-correlated single photon counting was used to monitor fluorescence concentration depolarization for DODCI in glycerol. For DODCI concentrations below ∼10−3 M, the present experiments have minimized self-absorption and excitation trapping artifacts to the extent where they contribute negligibly to the observed differences between experimental fluorescence profiles and profiles computed from the Gochanour–Andersen–Fayer three-body theory for excitation transport in solution. The three-body theory accurately describes fluorescence depolarization at the lower dye concentrations. At higher concentrations, the measured decay in Gs(t), the probability that the excitation resides on the laser-excited molecule, is perceptibly slower than predicted by the three-body theory. This deviation may arise from nonrandom dye distributions in solution, rather than from errors in the three-body theory. The experimental decay is equally well described at all concentrations by an earlier analytic theory which was developed by Huber, Hamilton, and Barnett.

Electronic excitation migration and trapping among cationic porphyrins adsorbed on anionic vesicle surface. Fractal-like behaviors

Picosecond time-correlated single-photon-counting studies have been made on the electronic excitation migration among positively charged porphyrins, TAPP (5,10,15,20-tetrakis(4-trimethylammonio)phenyl)porphyrin, tosylate) and ZnTMPyP (zinc 5,10,15,20-tetrakis(1-methyl-4-pyridinyl)porphyrin, chloride), adsorbed on a negatively charged surface of DHP (dihexadecyl phosphate) vesicles and its trapping at porphyrin aggregates (dimer or higher associates). The observed fluorescence decay curves cannot be reproduced by decay functions derived assuming single-step transfer from porphyrin monomer to nonfluorescent aggregate uniformly distributed in two dimensions or three dimensions. The decay function for the case of single-step transfer derived on the basis of the fractal theory is also unable to reproduce the results. It has been confirmed that the fluorescence decay function derived for multistep excitation migration on fractal and fast trapping at the first trap encounter can reproduce satisfactorily the observed results, and the fracton dimension relevant to porphyrin monomer distribution on the vesicle has been estimated to be 1.65.

Transient grating spectroscopy of LaP<inf>5</inf>O<inf>14</inf>: Nd<sup>3+</sup>

Degenerate four-wave mixing spectroscopy is used to measure the diffusion of electronic excitation in the system Nd x La (1-x) P 5 O 14 . For samples with x = 0.2 , 0.5, and 1.0, diffusion lengths of 165, 242, and 253 nm were obtained. When the period of the grating is less than 1.3μm and greater than 2.2μm, the observed scattered beam decay exhibits unusual features which are discussed in terms of excitation trapping and the influence of other types of laser-induced grating phenomena.

Excitation transport and fluorescence anisotropy of Rhodamine 3B on amorphous quartz

Time-resolved fluorescence depolarization has been measured by time-correlated photon counting for submonolayers of rhodamine 3B adsorbed onto optically flat quartz. The two-dimensional excitation transport theory developed by Loring and Fayer yields fluorescence profiles which agree well with the experimental profiles for reduced surface coverages up to approx.0.4. At higher coverages, excitation trapping by rhodamine 3B aggregates truncates the depolarization process, yielding apparent reduced coverages which are appreciably lower than the true coverages. The authors present dynamic evidence that the dye transition moments are not adsorbed at a uniform angle from the macroscopic surface normal; this and other effects influencing comparisons between depolarization experiments and excitation transport theory are discussed.

Electro-induced changes of chlorophyll fluorescence in individual intact chloroplasts

The intensity of chlorophyll fluorescence in isolated chloroplasts of Peperomia metallica and in situ chloroplasts of Anthoceros sp. was found to change by 5–10% on passing the electric current through the microelectrode inserted in the intrathylakoid space. The dependence of fluorescence deflections on the direction of a current flow and the absence of electro-induced fluorescence changes in chloroplasts made ion-permeable with gramicidin indicate that the emission was affected by the membrane potential created during the passage of current. A hyperpolarizing displacement of the membrane potential, positive on the inside, produced an increase in the emission in the absence of inhibitors, but has no effect on the fluorescence in chloroplasts treated with DCMU. A stimulation of a variable fluorescence by a positive membrane potential was interpreted as due to slowing down of charge separations and a respective decrease of excitation trapping in Photosystem II. A displacement of the membrane potential to the negative direction resulted in a decrease of chlorophyll fluorescence in both the absence and presence of DCMU. The fluorescence emission seems to be sensitive to variations of the membrane potential of about 100 mV or even less. Possible effects of the light-induced membrane potential on charge separations and the fluorescence emission in Photosystem II are discussed.

Fluorescence depolarization of Rhodamine 6G in glycerol: a photon-counting test of three-dimensional excitation transport theory

Time-correlated photon counting has been used to measure fluorescence concentration depolarization for rhodamine 6G in glycerol. The excitation transport theory developed by Gochanour, Andersen, and Fayer yields good approximations to the experimental decay profiles over the concentration range 1.7 x 10/sup -4/ to 2.4 x 10/sup -3/ M. Although the differences between optimized theoretical and experimental profiles are fractionally small, they are readily characterized under present counting statistics. They prove to be dominated by experimental artifacts, arising from excitation trapping by rhodamine 6G aggregates and from self-absorption in solution cells thicker than approx. 10 ..mu..m.

Transport in InGaAs/InP heterostructures grown by MOCVD

Using the persistent photoconductivity effect in InP/InGaAs heterojunctions we present a detailed analysis of the factors controlling the transport in normal and inverted interfaces. We report the highest mobilities yet seen for this system and find no difference between the two configurations. The persistent photoconductivity appears to originate with excitation and hole trapping in the substrate.

Excitation trapping and primary charge stabilization in Rhodopseudomonas viridis cells, measured electrically with picosecond resolution

The transmembrane primary charge separation in the photosynthetic bacterium Rhodopseudomonas viridis was monitored by electric measurements of the light-gradient type [Trissl, H. W. & Kunze, U. (1985) Biochim. Biophys. Acta 806, 136-144]. Excitation of whole cells with 30-ps laser pulses at either 532 nm or 1064 nm gave rise to a biphasic increase of the photovoltage. The fast phase, contributing about 50% of the total, rose with an exponential time constant </=40 ps and was independent of the redox state of the quinone electron acceptor. It is assigned to the migration of the excitation energy in the antenna and its subsequent trapping by the reaction center, monitored by the ultrafast charge separation between the primary electron donor and the bacteriopheophytin intermediary acceptor. The slower phase (125 +/- 50 ps) only occurred when the quinone was oxidized and disappeared when it was reduced (either chemically or photochemically). It is assigned to the forward electron transfer from the bacteriopheophytin to the quinone. The relative amplitudes of these two electrogenic steps demonstrate that the bacteriopheophytin intermediary acceptor is located halfway between the primary donor and the quinone.

Electronic excitation transport in disordered infinite volume systems

Abstract An account of recent theoretical and experimental advances in the field of excitation transport in disordered systems is presented. First the problem of excitation transport among chromophores (donors) randomly distributed in solution is discussed. Picosecond fluorescence mixing experiments demonstrate that the recent nonperturbative theory for this type of system is accurate at all times and concentrations examined. Next we consider systems with two types of chromophores, donors and traps, randomly distributed in solution. Experiments and theory describe the transport and trapping of excitations over the full range of donor and trap concentrations. These ideas and picosecond transient grating experiments are used to explain the mechanism for fluorescence quenching in concentrated dye solutions, which is shown to be due to excitation transport and trapping by dimers of dye molecules. Finally, the problems of excitation transfer among molecules randomly distributed on a lattice (rather than in a s...

Electron transfer in photosystem II

The picture presently emerging from studies on the mechanism of photosystem II electron transport is discussed. The reactions involved in excitation trapping, charge separation and stabilization of the charge pair in the reaction center, followed by the reactions with the substrates, plastoquinone reduction and water oxidation, are described successively. Finally, a brief discussion on photosystem II heterogeneity is presented.

Analysis of structure-function correlations in light-harvesting photosynthetic antenna: Structure optimization parameters

Our previous theoretical analysis of the consistency between the results of experimental studies of two most important stages in the primary conversion of light energy in photosynthesis—the excitation energy transfer within a light-harvesting antenna and the stabilization of this energy in reaction centres—showed that the photosynthetic unit structure is to be strongly optimized in vivo to operate with a 90% quantum yield of primary charge separation in reaction centres, which means that a macroscopic photosynthetic unit is neither uniform nor isotropic. The principles of the structural organization of a light-harvesting antenna of a model photosynthetic unit which allow optimization of excitation energy transfer from antenna molecules to reaction centres are studied here. The time of excitation energy trapping by reaction centres was computed for two-dimensional systems simulating a photosynthetic unit of purple bacteria and the longest-wavelength part of higher plant Photosystem-1 photosynthetic unit. The calculations assume a Förster inductive resonance mechanism for energy transfer within antenna and pairwise dipolar interactions. It was shown that the co-operative effect of several optimizing factors, i.e. (1) anisotropy of intermolecular distances in an antenna; (2) its spectral heterogeneity; (3) mutual orientation of transition moment vectors; (4) availability of the “focusing zone” around reaction centres; (5) definite spatial clustering of reaction centres in macroscopic antenna, can decrease 300-fold the time required for a high probability of excitation trapping to be obtained compared to a similar array of undifferentiated antenna molecules. As the analysis that we offer is of more or less general nature, it is to be hoped that our major conclusions will be valid for any photosynthetic organisms. The basic principles of the structural Organization of an optimal artificial light-converting system are formulated.

Excitation transport and trapping in a two-dimensional disordered system: cresyl violet on quartz

Excitation transport and trapping in a two-dimensional disordered system: cresyl violet on quartz