Average Charge Separation Research Articles

Ultrafast excitonic dynamics in DNA: Bridging correlated quantum dynamics and sequence dependence.

After photoexcitation of DNA, the excited electron (in the LUMO) and the remaining hole (in the HOMO) localized on the same DNA base form a bound pair, called the Frenkel exciton, due to their mutual Coulomb interaction. In this study, we demonstrate that a tight-binding (TB) approach, using TB parameters for electrons and holes available in the literature, allows us to correlate relaxation properties, average charge separation, and dipole moments to a large ensemble of double-stranded DNA sequences (all 16384 possible sequences with 14 nucleobases). This way, we are able to identify a relatively small subensemble of sequences responsible for long-lived excited states, high average charge separation, and high dipole moment. Further analysis shows that these sequences are particularly T rich. By systematically screening the impact of electron-hole interaction (Coulomb forces), we verify that these correlations are relatively robust against finite-size variations of the interaction parameter, not directly accessible experimentally. This methodology combines simulation methods from quantum physics and physical chemistry with statistical analysis known from genetics and epigenetics, thus representing a powerful bridge to combine information from both fields.

The antenna of far-red absorbing cyanobacteria increases both absorption and quantum efficiency of Photosystem II

Cyanobacteria carry out photosynthetic light-energy conversion using phycobiliproteins for light harvesting and the chlorophyll-rich photosystems for photochemistry. While most cyanobacteria only absorb visible photons, some of them can acclimate to harvest far-red light (FRL, 700–800 nm) by integrating chlorophyll f and d in their photosystems and producing red-shifted allophycocyanin. Chlorophyll f insertion enables the photosystems to use FRL but slows down charge separation, reducing photosynthetic efficiency. Here we demonstrate with time-resolved fluorescence spectroscopy that on average charge separation in chlorophyll-f-containing Photosystem II becomes faster in the presence of red-shifted allophycocyanin antennas. This is different from all known photosynthetic systems, where additional light-harvesting complexes increase the overall absorption cross section but slow down charge separation. This remarkable property can be explained with the available structural and spectroscopic information. The unique design is probably important for these cyanobacteria to efficiently switch between visible and far-red light.

Size dependent charge separation and recombination in CsPbI3 perovskite quantum dots.

CsPbI3 perovskite quantum dots (QDs) have shown great potential in light-harvesting and light-emitting applications, which often involve the transfer of charge carriers in and out of these materials. Here, we studied size-dependent charge separation (CS) and charge recombination (CR) between CsPbI3 QDs and rhodamine B (RhB) molecules, using transient absorption spectroscopy. When the average size decreases from 11.8 nm to 6.5 nm, the average intrinsic CS time constant decreases from 872 ± 52 ps to 40.6 ± 4.3 ps and the corresponding charge recombination time constant decreases from 3829 ± 51 ns to 1384 ± 54 ns. The observed trend of size-dependent CS and CR rates can be well explained by Marcus theory using the theoretically calculated CS and CR driving forces (ΔGCS and ΔGCR), molecular reorganization energy (λRhB), and electronic coupling strength between QD and RhB (HCS and HCR). Unlike the extensively studied more strongly quantum confined Cd chalcogenide QDs, the CsPbI3 QDs are in a weak quantum confinement regime in which size-dependent coupling strength plays a dominant role in the size-dependent charge transfer properties.

Aggregation of Plate-like Colloids Induced by Charged Polymer Chains: Organization at the Nanometer Scale Tuned by Polymer Charge Density.

We study the aggregation of charged plate-like colloids, Na-montmorillonite clays, in the presence of ionenes, oppositely charged polymer chains. The choice of the charged polymer allows tuning its linear charge density to match/mismatch the average charge separation on the clay surfaces. We assess the nanoscale structure of the aggregates formed by small-angle X-ray and neutron scattering. The nanoscale features of the formed clay aggregates are dominated by the presence of a stacking peak, giving clear evidence for the formation of clay tactoids, that is, a face-to-face aggregation geometry of the clay platelets. The chain charge density of ionenes influences not only the stacking repeat distance within the clay tactoids but also the extent of stacking and abundance of the tactoids. We may distinguish two regimes as a function of clay and ionene polymer charge densities (ρc and ρp, respectively). The first regime applies to ρp > ρc and ρp ≈ ρc, that is, for highly and "matching" charged chains. Under these conditions, the intercalated chains lie in a flat conformation within the tactoids, irrespective of the ionic strength (within the range studied, i.e., up to 0.05 M NaBr). For weakly charged chains, ρp < ρc, undulation of the ionene chains within the tactoid is seen. The degree of undulation increases with ionic strength due to the decreasing persistence length of the ionene chains. The extent of stacking (5-10 platelets per tactoid) is a general feature of all the systems, and its origin remains unknown. The system corresponding to the closest match in charge separations on the clay surface and on the polymer chain (ρp ≈ ρc) features the highest abundance of tactoids. This coincides with the highest macroscopic density as deduced from simple visual inspection of sediment volumes. This leads to the open question regarding the link between the density at the nanoscale and the macroscopic density and sedimentation behavior of the aggregate.

A simple relationship of bond dissociation energy and average charge separation to impact sensitivity for nitro explosives

The bond dissociation energy (BDE) of the weakest bonds in 33 explosives were calculated and analyzed using the B3LYP method with the 6-311++G** basis set. A comparison between BDE and the impact sensitivity H50 showed that cleavage of the weakest bond plays an important role in the initiation of detonation. Using the generalized gradient approximation (GGA) with the Perdew?Burke?Ernzerhof (PBE) method and dispersion-corrected density functional theory (DFT-D), the simulation of compressed TNT (2-methyl-1,3,5- -trinitrobenzene) and royal demolition explosive (RDX, hexahydro- -1,3,5-trinitro-1,3,5-triazine) crystals showed that an imbalance of the electrostatic surface potential (ESP) leads to molecular deformation and instability of the explosive under impact pressures. The average charge separation (?) of the molecules was calculated and used to demonstrate the ESP balances. Based on the BDE, ? and the experimental H50 values, simple quantitative structure? sensitivity correlations were established for the nitro heterocycles, nitramines, picryl heterocycles and nitro aromatics, respectively. The fitting relationship is simple yet statistically significant with only two variables. The correlation coefficients, R2, are larger than 0.8 with F&gt;F**(0.05) (95 % confidence intervals).

Chiral kinetic approach to the chiral magnetic effect in isobaric collisions

Based on the chiral kinetic approach using quarks and antiquarks from a multiphase transport model as initial conditions, we study the chiral magnetic effect, i.e., the magnetic field induced separation of charged particles in the transverse plane, in non-central isobaric collisions of Zr$+$Zr and Ru$+$Ru, which have the same atomic number but different proton numbers. For the observable $\gamma^{OS}-\gamma^{SS}$ related to the difference between the correlations of particles of opposite charges and of same charges, we find a difference between the two collision systems if the magnetic field has a long lifetime of 0.6 fm$/c$ and the observable is evaluated using the initial reaction plane. This signal of the chiral magnetic effect becomes smaller and comparable to the background contributions from elliptic flow if the event plane determined from particle emission angles is used. For the other observable given by the $R(\Delta S)$ correlator related to the distribution of average charge separation in a collision, the signal due to the chiral magnetic effect is found to depend less on whether the reaction or event plane is used in the analysis, and their difference between the two isobaric collision systems is thus a more robust observable.

Determination of sin2 θ w eff using jet charge measurements in hadronic Z decays

The electroweak mixing angle is determined with high precision from measurements of the mean difference between forward and backward hemisphere charges in hadronic decays of the Z. A data sample of 2.5 million hadronic Z decays recorded over the period 1990 to 1994 in the ALEPH detector at LEP is used. The mean charge separation between event hemispheres containing the original quark and antiquark is measured for\(b\bar b\) and\(c\bar c\) events in subsamples selected by their long lifetimes or using fastD*’s. The corresponding average charge separation for light quarks is measured in an inclusive sample from the anticorrelation between charges of opposite hemispheres and agrees with predictions of hadronisation models with a precision of 2%. It is shown that differences between light quark charge separations and the measured average can be determined using hadronisation models, with systematic uncertainties constrained by measurements of inclusive production of kaons, protons andΛ’s. The separations are used to measure the electroweak mixing angle precisely as sin2ϑweff=0.2322±0.0008(exp.stat.) ±0.0007(exp.syst.)±0.0008(sep.). The first two errors are due to purely experimental sources whereas the third stems from uncertainties in the quark charge separations.

Action of extracellular pH on Na+ and K+ membrane currents in the giant axon of Loligo vulgaris.

Voltage-clamp currents and resting membrane potential of squid giant axons have been studied at extracellular pH varying between 4 and 10. The membrane currents, analyzed according to the Hodgkin-Huxley equations, showed that sodium permeability, PNa(E), and potassium conductance gK(E), curves were shifted toward positive voltages by different amounts and slightly depressed as the external pH was lowered. Under the same conditions, taum(E) and taun(E) were found to be enhanced and shifted to a larger extent in the same direction. The rate constants alpham and alphan were shifted substantially toward positive voltages, but betam and betan changed hardly at all. The shift of the alpham(E) curve was analyzed in terms of a fixed surface charge model; it indicates that unspecific negative groups with an approximate pKa of 4.5 are located in the vicinity of sodium active sites with an average charge separation of 8 A. A similar figure is obtained for the potassium system from the shift of the alphan(E) curve.

Electrification Associated with Breakup of Drops at Terminal Velocity in Air

Abstract The electrification accompanying the breakup of large drops falling in air in the absence of an electric field has been studied. A closed circuit vertical tunnel has been used, and the chemical composition of the drops has been carefully controlled. The results obtained for the average charge separation between the large and small fragments are non-specific regarding the nature of the ions present (except for surface-active species), but strongly dependent on the electrolytic conductivity. The magnitude of the effect goes from about +10−2 esu per breakup (5 × 10−2 esu cm−3) for pure water to −10−2 esu for concentrated solutions, reversing sign at about 10−5 Ω−1 cm−1 (concentrations ∼10−4 N). It was shown that these results can be interpreted on the basis of two competing processes, one of which is the shelling of the electrical double layer on the water–air interface while the air develops the characteristic liquid film bag or bubble.