Mechanism by which a single water molecule affects primary charge separation kinetics in a bacterial photosynthetic reaction center of Rhodobacter sphaeroides

Using quantum-chemical methods, we have studied the role played by water molecules W-A and W-B that are bound by hydrogen bonds to accessory bacteriochlorophyll molecules B _A and B _B in the process of primary charge separation in the reaction center of Rhodobacter Sphaeroides. We have found that the occurrence of a rotational mode of the W-A molecule at 32 cm^?1 and/or its harmonics in stimulated emission of an electron donor P* and the dynamics of population of the states P⁺B _A ^? and P⁺H _A ^? may be related to the structural heterogeneity of the reaction center and the existence of a conformation in which the W-A molecule is predominantly involved in one hydrogen bond (with BA). Based on the calculated redox potentials B _A and P, it has been shown that the appearance of the W-A molecule in the reaction center reduces the energy of the P⁺B _A ^? state by ??600 cm^?1. This is somewhat smaller than the influence of the amino-acid residue Tyr^M210 (??870 cm^?1) and correlates well with a substantial decrease in the electron transfer rate in mutant forms of reaction centers GM203L (which do not contain W-A molecules) and YM210F (in which Tyr^M210 is replaced with Phe). The data obtained allow us to suggest that rotation of the water molecule with a fixed position of its H atom that is involved in a hydrogen bond with the keto carbonyl group of B _A is initiated due to the charge separation between the halves of special pair P and the formation of the state P _A ⁺ P _B ^? . The large effect of this rotation on the kinetics of population of the states P⁺B _A ^? and P⁺H _A ^? after the excitation of P is quite consistent with its influence on the energy of the state P⁺B _A ^? .     

Anionische Silberzentren („B-Zentren“) in Alkalihalogeniden

It is proposed that negatively charged silver ions on anion sites are responsible for theB bands found in silver doped alkalihalides. Experimental investigations confirming this model of theB center are presented. Optical absorption and emission ofB centers were measured in seven alkalihalides in the temperature range from 450 to 4 °K. Configuration coordinate diagrams were obtained for KCl, KBr, and KI. A chemical method was used to determine the charge of theB center. Thermal or optical excitation causes theB centers to dissociate into neutral silver atoms andF centers. The Ag⁰ centers are bound to interstitial positions at low temperature. At high temperature they collect together forming colloidal centers. TetragonalB _A centers were formed during optical bleaching of theB band in mixed crystals of the type KCl+ΔNaCl. Their absorption and emission spectra, optical orientation, and polarized emission were investigated. These properties are shown to be similar to those ofF _A centers.B _A fluorescence polarized almost completely in the [100] direction was observed. A Jahn-Teller splitting of theB absorption band was resolved in RbCl at low temperature. The splitting confirms the configuration5s² for the Ag^? ion. The results are compared with those for the isoelectronic centers In⁺ and Sn⁺⁺. Thin films of alkalihalides containing small amounts of silver, copper, or thallium were condensed simultaneously with alkali vapor. New bands in the UV region were found, possibly due to Cu^? and Tl^? centers.     

Scattering of keV H+ from Au(110)—comparison between experiment and simulation

Abstract

Angle resolved energy spectra for H⁺ ₂ scattering from Au(110) have been compared to simulations using MARLOWE and SABRE binary collision codes. The results of both simulations agree with each other but only qualitatively with the experiment. The simulations indicate that (i) contributions to the surface peak in the energy spectra come from both single and double scattering at the surface (ii) the amount of inelastic loss suffered by the scattered ion is sensitive to the loss model and (iii) azimuthal scans around the double alignment direction can be simulated quite well if an increased experimental background is included.     

Autocatalytic reaction dynamics in systems crowded by catalytic obstacles

Reaction and diffusion dynamics in systems crowded by catalytic obstacles are investigated using a particle-based mesoscopic simulation method. The focus of the work is on effects of correlations induced by the presence of the catalytic obstacles and solvent collective modes. As an example, a system is considered where the reaction A+C→B+C takes place on the surfaces of the C catalytic obstacles, while the autocatalytic reaction A+B→2A occurs in the bulk of the solution. It is shown that mean-field, mass-action rate laws break down and fail to describe the reaction dynamics for large volume fractions of obstacles. The influence of hydrodynamics on the reaction and diffusion dynamics is also studied.     

Enskog’s kinetic theory of dense gases for chemically reacting binary mixtures, II: Light scattering and sound propagation

Enskog’s kinetic theory for a symmetric moderately dense reaction A+A?B+B is used to determine Fick’s and Fourier’s law. The transport coefficients of diffusion, thermal-diffusion rate and thermal conductivity are represented graphically for endothermic and exothermic reactions and are analyzed as a function of the activation energy and of the density of the mixture. The Onsager reciprocity relations are numerically investigated and verified. The problems related to sound propagation and light scattering are investigated for such a mixture and it is shown that the influence of chemical reactions on phase velocity, attenuation coefficient and light scattering spectra is more pronounced for rarefied gases although there is a considerable change in these quantities as the mixture becomes denser.     

One-electron capture in collisions of fast ions with atoms

The properties of one-electron charge transfer between ions and atomsB ^Z++A →B ^(Z?1)+ A ⁺ are studied at relative velocities of the colliding particles higher than target electron velocities. Calculations of partial and total cross sections in collisions of protons and multiply-charged ions with neutral atoms are performed and compared with experimental data. The universal curve for the capture of the targetK- andL-electrons is given. In all cases at sufficiently high collision energies the electron capture from outer shells decreases and the capture of electrons from inner shells of the target atom becomes predominant.     

Dynamic deformation of light nuclei in excited states

An important property—the dynamic deformation of B* light aligned nuclei—is investigated for the nuclear reactions A(x, y)B* → γ + B ₀ by measuring the y-γ correlations. Dynamic deformation is determined from the orientation tensors of multipolar moments. Normalization constants of the contributions from even-rank orientation tensors are determined from the condition of coincidence between dynamic and static deformations of the B* nucleus for θ _y = 0°. Experimental dynamic deformations of ¹²C(2⁺) nuclei caused by the inelastic scattering of α particles and deuterons are determined, along with the ¹⁰Be(2⁺) nuclei formed in reaction ⁹Be(d, p)¹⁰Be(2⁺). It is shown that the dynamic deformation of the aligned nuclei depends on how they are formed and their structure, and evolves substantially when the angle θ _y is varied.     

The reaction mechanism of heavy ion scattering at intermediate energies

The contribution to the real and imaginary nucleus-nucleus (N-N) optical potential from nucleon-nucleon scattering in the medium is calculated in a local density approximation from a two Fermi sphere nuclear matter picture for the N-N collision. This reaction mechanism is shown to be dominant for ¹²C + ¹²C scattering at all considered energies (160 MeV < E_lab < 2250 MeV) giving a weakly energy dependent reaction cross section of about 900 mb. Inclusion of the collective 2⁺, 3^? excitations in a coupled channel calculations gives good agreement for both the measured elastic and inelastic 2⁺ cross section at E_lab = 1016 MeV. This fully microscopic parameter free calculation indicates that the energy dependence of the reaction cross section for this system is mostly due to the decrease of the collective contribution with increasing energy contrary to current theoretical models.     

Time-reversal invariance-like relations for spin-effects in elastic and inelastic reactions; vector-meson photoproduction versus compton scattering from nucleons

For elastic scattering, relations between spin-effects (for example, the well-known asymmetry-polarization equality) follow from time-reversal invariance. We show that if certain amplitude combinations vanish, there are strikingly similar relations between spin-effects for elastic and also inelastic reactions. This vanishing of amplitude combinations (denoted M-purity) corresponds asymptotically to purely natural or purely unnatural parity in the crossed channel. The M-purity relations hold for spin-configurations much more general than do the corresponding time-reversal invariance relations.The experimental evidence for purely natural parity exchanges in high energy vector meson photoproduction from nucleons is shown to be good for all amplitudes involving nonzero meson helicity, but less conclusive for the zero helicity ones. Using time-reversal invariance and a vector meson-dominance argument, this implies no unnatural parity contributions in high energy Compton scattering from nucleons.Because of this empirical evidence for M-purity in these two processes, a detailed application to spin-effects in Compton scattering and in vector meson photoproduction is made. Some time-reversal invariance relations in Compton scattering resemble the corresponding M-purity relations though the applicability of the two is different, and there are examples where only one of the two exists. Out of our illustrations, the only M-purity relations which change in form due to the extra amplitudes present in the inelastic reaction are the M-purity analogue and extensions of the asymmetry-polarization equality (of Compton scattering) referring to the photon; the change is the appearance of the elements ?⁰⁰ of the vector meson density-matrix ?. Our other examples of M-purity relations do not change in form in going over from the elastic reaction (Compton scattering) to the inelastic reaction (vector meson photoproduction).     

Spectral properties of chlorines and electron transfer with their participation in the photosynthetic reaction center of photosystem II

Structural factors that provide localization of excited states and determine the properties of primary donor and acceptor of electron in the reaction center of photosystem II (PSII RC) are studied. The results of calculations using stationary and time-dependent density functional theory indicate an important role of protein environments of chlorophylls P_A, P_B, B_A, and B_B and pheophytins H_A and H_B in the area with a radius of no greater than ≤10 Å in the formation of excitonic states of PSII RC. When the neighboring elements are taken into account, the wavelength of long-wavelength Q _y transition of chlorophyll molecules is varied by about 10 nm. The effect is less developed for pheophytin molecules (Δλ ? 2 nm). The following elements strongly affect energy of the transition: His^A198 and His^D197 amino-acid residues that serve as ligands of magnesium atoms affect P_A and P_B, respectively; Met^A183 affects P_A; Met^A172 and Met^D198 affect B_A; water molecules that are located above the planes of the B_A and B_B macrocycles form H bonds with carbonyl groups; and phytol chains of P_A and P_B affect B_A, B_B, H_A, and H_B. The analysis of excitonic states, mutual positions of molecular orbitals of electron donors and acceptors, and matrix elements of electron transfer reaction shows that (i) charge separation between B_A and H_A and P_B and B_A is possible in the active A branch of cofactors of PSII RC and (ii) electron transfer is blocked at the B_B - H_B fragment in inactive B branch of PSII RC.     

Hydrogen maser atomic beam resonances

Au^? ions at anionic places are formed in gold doped crystals by a reducing treatment withF centers. The ultraviolet absorption consists of 4 bands, which are namedA, B, C, andD in analogy to the isoelectronic centers of the s² type, like Tl⁺. TheB band oszillator strength strongly increases with temperature in accordance with a phonon allowed transition. The ratio of the dipole strength of theC band to that of theA band as a function of the relative position of theB band is compared with Suganos prediction. Zero phonon lines are found at helium temperatures for theA band in NaCl (2,985 Å), KCl (3,068 Å), and KBr (3,145 Å) and for theC band in KCl (2,329 Å). In KCl the Huang-Rhys factor isg=3.4 for theA band. The vibronic structure comes from the relatively large radius 6s ² state of the negative ion. Uniaxial stress splits the zero phonon line. The results definitely agree with the stress splitting behaviour of a degenerateΓ ₁→Γ ₄ transition. Inversion symmetry of the center is confirmed by the absence of a linear Stark effect.     

Ion-electron coincidence study of the thermal He(23S) + H2 Penning reaction

A new Penning-electron-Penning-ion coincidence method is described. It is applied to the study of the thermal reaction of He(2³S) with H₂. The main results reported are separate electron energy spectra that are coincident with the three different ions formed: HeH₂⁺, HeH⁺ and H₂⁺. Based on these results it is shown that the Penning reaction of the He(2³S)/H ₂ system proceeds in two well-separated steps: (i) ionization at distances R (HeH₂) ? 6a₀ in which H₂⁺ (v) is formed in different vibrational states; and (ii) reactive collision of H₂⁺ (v) with He. For the second step the variation of the branching ratios with vibrational quantum numbers v = 0 to v = 10 is derived, and it is shown that these branching ratios may be regarded as relative vibrational-energy-dependent cross-sections for the collision of H₂⁺ (v) with He at an average relative kinetic energy of ～20 meV.     

Angular correlation measurement in the B11(d,pγ)B12 reaction

Thep-γ angular correlations in B¹¹(d, p)B¹² reaction have been measured in both reaction and azimuthal planes for proton angles 35° at E_D=1.60 and 2.40 MeV. The correlation coefficient A ₂ ⁰ and the distortion parameterλ were calculated. The obtained values of the shift of the symmetry axis from the recoil direction, Φ₀ and the parameterλ are far from there plane wave limits. The anisotropy of the angular correlation gives two possible values, 1⁺ and 2⁺, for the spin of the first excited state of B¹².     

Photoinduced dichroism in additively coloured KCl: Na at room temperature

Spectral and kinetic investigations of the reversible photoinduced dichroism in KCl : Na containing F_A and F_A + F_B centers are done. It is suggested that in presence of F_B centers the observed 〈100〉 and 〈110〉 dichroism is connected with the F_B(II) centers.     

The study of the ground state rotational bands in U 233 and U 235 by the inelastic scattering of deuterons

The ground state rotational bands in the odd isotopes of uranium U 233 and U 235, were studied by the inelastic scattering of 13.1 MeV deuterons. Seven members of these bands were seen in both nuclei. By fitting the experimental energies of the levels to the relation E(I)= =AI(I+1)+B[I(I+1)]², the parametersA andB were determined. Their values and the upper limits of the quadrupole reduced transition probabilities determined from the cross sections were: U 233:A=(5.93±0.10)keV,B=(?0.002±0.001)keV,B(E2,5/2→7/2)= =(6.51±0.66)×10^?48 e² cm⁴,B(E2,5/2→9/2)=(2.80±0.37) X 10^?48 e² cm⁴. U 235:A=(5.36±0.04)keV,B=(?0.0017±0.0004) keV,B(E2,7/2→9/2)=(8.05±0.71) × × 10^?48 e² cm⁴,B(E2,7/2→11/2)=(2.17±0.39) X 10^?48 e² cm⁴.     

Techniques to Treat the Continuum Applied to Electromagnetic Transitions in 8Be

Bremsstrahlung emission in collisions between charged nuclei is equivalent to nuclear gamma decay between continuum states. The way the continuum spectrum can be treated is not unique, and efficiency and accuracy of cross section calculations depend on the chosen method. In this work we describe, relate, and compare three different methods in practical calculations of inelastic cross sections, that is, by (i) treating the initial and final states as pure continuum states on the real energy axis, (ii) discretizing the continuum states on the real energy axis with a box boundary condition, and (iii) complex rotation of the hamiltonian (complex scaling method). The electric quadrupole transitions, 2⁺ → 0⁺ and 4⁺ → 2⁺, in α + α scattering are taken as an illustration.     

Cl(2P)+D2→DCl+D反应中波恩-奥本海默近似有效性的交叉分子束研究

本文利用高分辨的里德堡态氘原子标识-交叉分子束装置，研究了碰撞能为4.5∽6.5 kcal/mol范围内Cl(²P)[Cl(²P_3/2)和Cl^*(²P_1/2)]与D₂的反应. 虽然自旋轨道激发态反应Cl^*(²P_1/2)+D₂在波恩-奥本海默(B-O)近似下本应是禁阻的，但实验中观测到了该反应的贡献. 通过测量靠近后向的碰撞能相关的微分散射截面连线，发现低碰撞能下的产物主要来自于B-O近似禁阻的反应Cl^*+D₂. 随着碰撞能的提高，自旋轨道基态反应Cl+D₂的反应性增加明显要比自旋轨道激发态反应Cl^*+D₂更快，并且在高碰撞能下成为产物的主要来源. 实验结果表明：在低碰撞能下，Cl^*中自旋轨道激发态的额外能量，可以帮助B-O近似禁阻的反应Cl^*+D₂越过势垒；然而当碰撞能接近和高于反应势垒时，B-O近似允许的反应Cl+D₂占主导地位. Cl/Cl^*+D₂反应中B-O近似有效性的特征与其同位素反应Cl/Cl^*+H₂是一致的.     

The ion-molecule reaction O+ (4S) + N2(X1Σ+) → NO+ (X1Σ +, v′) + N(4S) and the predissociation of the A2Σ+ and B2Π states of N2O+

The potential energy surfaces (PESs) for several electronic states involved in the reaction O⁺ (⁴S) + N₂(X¹Σ⁺) → NO⁺ (X¹Σ ⁺, v′) + N(⁴S) and the role of the ionic N₂O⁺ intermediate have been investigated by ab initio calculations. The ⁴A″ PES, which correlates with the ground state educts, has a barrier of about 1 eV, and therefore at low collision energies the reaction cannot take place adiabatically on this surface. However, the spin-orbit coupling in the entrance channel allows the system to pass into the Renner-Teller system of the X² Π electronic ground state of the N₂O⁺ intermediate. The reaction then proceeds on these surfaces up to the region in the exit channel where a similar coupling allows it to reach the product quartet asymptote. At collision energies higher than about 1 eV, the reaction proceeds mainly on the adiabatic PES of the ⁴A″ state. The A²Σ⁺ state of N₂O⁺ predissociates via a vibronic coupling with the B²Π state, and in bent structures via a spin-orbit coupling with the ⁴A″ component of the ⁴II state. The electronic structure of the B²Π state is found to be of crucial importance for the understanding of the reactive processes in low lying electronic states of N₂O⁺.     

On the role of indirect processes in the inelastic excitation of low lying 0+ states

Using simple microscopic form factors, we have studied the indirect excitation of low lying 0⁺ states by inelastic scattering through the lowest 2⁺ states in the case of α scattering from ²⁴Mg and ²⁸Si and ³He scattering from ⁴⁶Ti. Scaling amplitudes according to the inelastic scattering and B(E2) data we found that only for α scattering from ²⁴Mg was the indirect excitation small. For the other cases studied, our estimate yields indirect cross sections which were significant in comparison with the data, although this has been well described by the direct one-step process alone. The shape of the indirect cross section may be either strongly oscillatory or flat, depending on the radius of the real optical potential.     

Fine Structure of Levels and Piezospectroscopy of A<Superscript>+</Superscript> Centers in GaAs/AlGaAs Quantum Wells

Experimental and theoretical piezospectroscopic investigation of A⁺ centers in GaAs/AlGaAs quantum wells doped with beryllium is presented. Spectra of linearly polarized photoluminescence are studied experimentally depending on applied uniaxial pressure. A model of the A⁺ center in the quantum well in the presence of uniaxial deformation in the plane of the quantum well has been constructed. Analytical expressions for the level energy, optical transition intensities, and polarization ratio have been obtained. In the framework of the proposed theory, the experimentally observed change in the polarization ratio depending on pressure and the shift of the line maximum towards short waves are explained.