The transient EPR spectra and spin dynamics of coupled three-spin systems in photosynthetic reaction centres

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an “observer”. The spin polarized transient EPR spectra are investigated using the coupled three-spin system P⁺I⁻Q⁻ _A in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of P⁺I⁻Q⁻ _A and Q⁻ _A following recombination of the radical pair (P = primary donor, I = primary acceptor, Q_A = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state. The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, J_PI, in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of J_PI.     

Electron distribution function and recombination coefficient in ultracold plasma in a magnetic field

The electron distribution function and diffusion coefficient in energy space have been calculated for the first time for a weakly coupled ultracold plasma in a magnetic field in the range of magnetic fields B = 100?50000 G for various temperatures. The dependence of these characteristics on the magnetic field is analyzed and the distribution function is shown to depend on the electron energy shift in a magnetic field. The position of the “bottleneck” of the distribution function has been found to be shifted toward negative energies with increasing magnetic field. The electron velocity autocorrelators as a function of the magnetic field have been calculated; their behavior suggests that the frequency of collisions between charged particles decreases significantly with increasing magnetic field. The collisional recombination coefficient α _B has been calculated in the diffusion approximation for a weakly coupled ultracold plasma in a magnetic field. An increase in magnetic field is shown to lead to a decrease in α _B and this decrease can be several orders of magnitude.     

Temperature dependence of low field CIDNP and time resolved SNP in cyclic ketones

¹H CIDNP and time resolved Stimulated Nuclear Polarization (SNP) in the photolysis of 2,2,12,12-tetramethylcyclododecanone have been investigated at low magnetic field in the temperature range from ?70 to +90°C. Considerable differences for protons of different reaction products are seen in the CIDNP field dependences and SNP decay times at high temperatures. For disproportion products of acyl-alkyl biradicals the width of the CIDNP field dependence and the position of the emission maximum exhibit a pronounced temperature dependence with the largest shift towards low field around ?30°C. At lower temperatures a significant slowing down of the SNP decay is observed. The results are discussed in terms of a superposition of contributions from acyl-alkyl and bis(alkyl) biradicals to the nuclear spin polarization.     

CIDNP及其在光化学中的应用

化学诱导动态核极化(Chemically Induced Dynamic Nuclear Polarization,简称CIDNP)是一种在化学反应体系中观察到的NMR谱线强度反常的现象(常称之为极化谱,包括增强吸收和发射等)。1967年由Bargon和Fischer^[1] (西德)以及Ward和Lawler^[2] (美国)首先发现的。由于他们所研究的反应体系中涉及自由基,因此最初认为这种谱线强度反常现象是由于自由基中间体中的电子-核交叉弛豫引起的。这种机理通常称为:Overhauser型的动态核极化(Dynamic Nnclear Polarization简称DNP)。但很快就发现它不足以解释实验得到的结果,而CIDNP的命名却是这样因袭下来了。     

Field cycling by fast NMR probe transfer: Design and application in field-dependent CIDNP experiments

A novel field-cycling unit with fast digital positioning of a high-resolution nuclear magnetic resonance probe in a spatially varying magnetic field is described and used to measure CIDNP spectra of the amino acid-dye (histidine-bipyridyl) photoreaction system in the range between 0 and 7 T. The pattern of nuclear polarization varies with the magnetic field. In particular, strong polarization with an emission/absorption pattern (multiplet effect) is found at low field for two histidine ringprotons with scalar coupling below 3 Hz visible only because of the high resolution made possible by the new field-cycling setup. Also for the CH₂ protons in the β-position a multiplet effect is observed having a pattern changing with magnetic field. By analysis of the spin nutation the non-Boltzmann population differences of the nuclear levels have been determined.     

Spin Chemistry Investigation of Peculiarities of Photoinduced Electron Transfer in Donor–Acceptor Linked System

Photoinduced intramolecular electron transfer in linked systems, (R,S)- and (S,S)-naproxen-N-methylpyrrolidine dyads, has been studied by means of spin chemistry methods [magnetic field effect and chemically induced dynamic nuclear polarization (CIDNP)]. The relative yield of the triplet state of the dyads in different magnetic field has been measured, and dependences of the high-field CIDNP of the N-methylpyrrolidine fragment on solvent polarity have been investigated. However, both (S,S)- and (R,S)-enantiomers demonstrate almost identical CIDNP effects for the entire range of polarity. It has been demonstrated that the main peculiarities of photoprocesses in this linked system are connected with the participation of singlet exciplex alongside with photoinduced intramolecular electron transfer in chromophore excited state quenching.     

Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data

In this work we demonstrate that low-field chemically induced dynamic nuclear polarization (CIDNP) is strongly affected by re-distribution of polarization, which is formed in the course of spin evolution in transient radical pairs, in diamagnetic reaction products. This phenomenon is of importance when the spins of the reaction product are coupled strongly meaning that spin–spin interactions between them are comparable to the differences in their Zeeman interactions with the external magnetic field. In this case, polarization transfer relies on a coherent mechanism; as a consequence, spins can acquire significant polarization even when they have no hyperfine coupling to the electron spins in the radical pairs, i.e., cannot be polarized directly by CIDNP. This is demonstrated by taking CIDNP of n-butylamine as an example: in this case only the α-CH₂ protons are polarized directly, which is confirmed by high-field CIDNP, whereas the β-CH₂, γ-CH₂ and δ-CH₃ protons get polarized only indirectly due to the transfer of polarization from the α-CH₂ protons. These results show that low-field CIDNP data should be interpreted with care to discriminate between the effects of spin evolution in transient radical pairs and in diamagnetic reaction products.     

CIDNP in photolysis of acetone solutions in plastic crystals of cyclohexane-d12

The emissive CIDNP was observed during photolysis of acetone solutions in solid cyclohexane-d₁₂. The polarization is assumed to arise in contact triplet radical pairs as a result of their T_-S, conversion and their recombination.     

Quantitative Approach to CIDNP in Proteins with Several Polarizable Residues on the Surface

A quantitative theoretical approach to protein chemically induced dynamic nuclear polarization (CIDNP) is formulated in the present work, which is based on the theory of sterically specific chemical reactions in liquids. Kinetic equations for polarization of all CIDNP-active accessible amino acid residues are presented. Relations between the kinetic parameters and the total side-chain accessibility (TSA) values are established. Numerical calculations of the CIDNP kinetics are also shown to demonstrate how the time behavior of polarization depends on TSA. The present theoretical approach was applied to model time-resolved protein CIDNP data obtained for Tyr59 and His68 of ubiquitin in the native state. Comparison of theoretical predictions with the experimental data confirms the accuracy of the approach.     

Parameter-dependent third-order optical nonlinearity in a CdSe/ZnS quantum dot quantum well in the vicinity of a gold nanoparticle

The nonlinear optical properties of the CdSe/ZnS quantum dot quantum well (QDQW) in the vicinity of a spherical metal nano-particle (MNP) have been described. The third-order nonlinear optical susceptibility induced by the transition between E₁ (inside the well) and E₂ (outside the well) has been calculated for the third-harmonic generation (THG) under the effective mass approximation and modified by the local field theory. The parameters-dependent third-order nonlinear optical susceptibility for the THG has been specifically explored and the influence of the distance between the QDQW and the MNP on the third-order susceptibility for the THG in the system has been shown and analyzed.     

Recombination properties of Rydberg nonequilibrium plasma created by dye laser

Ultracold Rydberg plasma formed by a dye laser has been studied theoretically. Recombination properties in the presence of three-body collisions in hydrogen-like plasma have been investigated. We have shown that the existing calculation models for three-body recombination (Thomson model, diffusion approximation, and modified diffusion approximation) work only in the high-electron-temperature region—more than or equal to 1 eV. When the electron temperature is substantially less 1 eV, we have found the strong dependence of process probability on the energy of free particle. As a result, there is a drastic slowdown recombination in the region, where the three-body recombination dominates. Another reason for this slowdown of the recombination deals with the increment nonideality parameter. Effect of slowdown of recombination with free-body collisions can be reduced to the situation when the thermal equilibrium is set earlier than the ionization equilibrium, and, hence, the formation of metastable structure is possible in the plasma. Also this effect can cause the inverse population on Rydberg levels.     

Oscillations of echo amplitude in glasses in a magnetic field induced by nuclear dipole-dipole interaction

The influence of a magnetic field on the dipole echo amplitude in glasses (at temperatures of about 10 mK) induced by the dipole-dipole interaction of nuclear spins has been theoretically studied. It has been shown that a change in the mutual position of nuclear spins at tunneling and the Zeeman energy E _H of their interaction with the external magnetic field lead to a nonmonotonic magnetic-field dependence of the dipole echo amplitude. The approximation that the nuclear dipole-dipole interaction energy E _d is much smaller than the Zeeman energy has been found to be valid in the experimentally important cases. It has been shown that the dipole echo amplitude in this approximation may be described by a simple universal analytic function independent of the microscopic structure of the two-level systems. An excellent agreement of the theory with the experimental data has been obtained without fitting parameters (except for the unknown echo amplitude).     

Magnetocaloric effect in Sr2TiMoO6 double perovskite

Environmental friendly double perovskite Sr₂TiMoO₆ has been studied by using the density functional theory with generalized gradient approximation (GGA+U) approaches, and the Monte Carlo simulation in the framework of Ising model. The parameter of the unit cell has been optimized. The value of the crystal field and the exchange coupling have been estimated from the partial electronic density of states and the energy calculations, respectively. The magnetic properties and the magnetocaloric effect of the compound have been also studied. The Relative cooling power has been performed. It is found that Sr₂TiMoO₆ is an interesting material for the magnetic refrigeration applications.     

Dielectric relaxation and delayed-collection field experiments in amorphous semiconductors

Delayed-collection field (DCF) experiments, reported recently, were interpreted by the authors as giving evidence for rapid recombination in several amorphous semiconductors including aSi:H and a-As₂Se₃. An analysis is made of the DCF experiment for such materials which display dispersive transport because of multiple trapping (MT). It is shown that when the delay time is long compared with the zero-delay transit time, the effective transit time becomes equal to the delay time. Under such circumstances, a decrease of the collected charge may be a result of MT alone, and does not necessarily provide evidence for rapid recombination. It is shown that the process of dielectric relaxation is modified in a MT system. The mechanism by which the electric field is screened is closely related to the DCF experiment.     

One-loop approximation of Møller scattering in generalized Krein-space quantization

It has been shown that the negative-norm states necessarily appear in a covariant quantization of the free minimally coupled scalar field in de Sitter spacetime. In this processes ultraviolet and infrared divergences have been automatically eliminated. A natural renormalization of the one-loop interacting quantum field in Minkowski spacetime (λφ ⁴) has been achieved through the consideration of the negative-norm states defined in Krein space. It has been shown that the combination of quantum field theory in Krein space together with consideration of quantum metric fluctuation, results in quantum field theory without any divergences. Pursuing this approach, we express Wick’s theorem and calculate Møller scattering in the one-loop approximation in generalized Krein space. The mathematical consequence of this method is the disappearance of the ultraviolet divergence in the one-loop approximation.     

The CIDNP effect in plastic crystals

The CIDNP effects in the photolysis reactions of some aldehydes and ketones (including linear, aromatic and cyclic) in plastic crystals of cyclohexane have been discovered and studied. In going from liquid to solid solutions, the change of polarization sign is observed for some substances investigated. The CIDNP effect for a simple model for a relative motion of atoms of radicals possessing spin density (translational diffusion in a restricted volume) has been calculated. The results observed are interpreted in the framework of the radical pair mechanism of CIDNP effect formation.     

应变纤锌矿GaN/A10.15Ga0.85N柱形量子点的光学性质:流体静压力效应

在有效质量近似下,考虑强的内建电场和应变对材料参量的影响,变分研究了流体静压力对有限高势垒应变纤锌矿GaN/Al_0.15Ga_0.85N柱形量子点中重空穴激子的结合能、发光波长和电子空穴复合率的影响.数值结果表明,激子结合能和电子空穴复合率随流体静压力的增大而近线性增大,发光波长随流体静压力的增大而单调减小.在量子点尺寸较小的情况下,流体静压力对激子结合能和电子空穴复合率的影响更明显.由于应变效应,为了获得有效的电子-空穴复合过程,GaN量子点的高度必须小于5.5 nm.     

CIDNP investigation of electron excitation energy transfer in the photolysis of isotope-mixed molecules of benzophenone in cyclohexane-D12

Isotopic effects arising in the photolysis of benzophenones-H₁₀/D₁₀ mixtures in cyclohexane-D₁₂ at room temperature were studied by CIDNP method. Nonlinear dependences of these effects on a relative concentration of benzophenone-D and benzophenone-H molecules have been obtained. To interpret the results, a theoretical model involving intermolecular and intramolecular energy transfers and radical-pair mechanism of CIDNP formation has been proposed. Nonlinearity of isotopic effects is determined by the difference in electron excitation energy between benzophenone-D and benzophenone-H molecules.     

Theory of single-photon echo (SP-echo) and thepossibility of its experimental study in the gamma-region

The single-photon echo (SP-echo) effect is predicted to appear in the case of three-level medium excitation by means of a single photon propagating to the medium along two optical paths with a mutual time delay surpassing the temporal duration of the photon wave packet. The quantum electrodynamical theory describing this interaction is presented and the S-matrix of the field is shown for infinite time (t=∞). Using the S-matrix approach, physical properties of the scattering field are studied. Hence, it is shown that the field has an echo signal at the ω ₃₂ ⁰ carrier frequency. It has been shown that the echo signal exists only in the field amplitude while being absent in its intensity behaviour. Thus, SP-echo is an interference effect and is not influenced by the energy irradiation. The problems of SP-echo detection in the gamma-region (where special generation difficulties appear) are discussed. The influence of the additional detection of theω ₂₁ ⁰ frequency field on the echo signal has been shown. A special case is the EPR-paradox which can appear within the echo phenomenon. This revised version was published online in August 2006 with corrections to the Cover Date.