Light-Induced Spin Polarization in Porphyrin-Based Donor–Acceptor Dyads and Triads

The light-induced spin polarization generated by sequential electron transfer in an axially bound triad based on Al(III) porphyrin (AlPor) is discussed. In the triad, $\text {TTF} \!-\! \text {Ph} \!-\! \text {py}\!\to\!\text {AlPor}\! - \!\text {Ph}\! -\! \text {NDI}$ , the electron acceptor naphthalene diimide (NDI) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) via an appended pyridine (py) on the opposite face of the porphyrin ring. Excitation of the porphyrin at room temperature in solution leads to charge separation between the donor and acceptor. In the liquid crystalline solvent 5CB, a spin-polarized transient electron paramagnetic resonance spectrum of a weakly coupled radical pair is observed and is assigned to the state $ \text{TTF}^{\cdot + } \text{NDI}^{\cdot - } $ . In the absence of the donor, a spectrum of the triplet state of the strongly coupled radical pair $ \text{AlPor}^{ \cdot + } \text{NDI}^{ \cdot - } $ is obtained. The analysis of the spectra is described using a model developed by Kandrashkin et al. (Appl Magn Reson 15: 417–447, 1998). It is shown that in the triad the spectrum of $ \text {TTF}^{ \cdot + } \text {NDI}^{ \cdot - } $ shows evidence of the singlet–triplet mixing in the precursor $ \text{AlPor}^{ \cdot + } \text{NDI}^{ \cdot - } $ . At later time, singlet recombination leads to inversion of the spectrum, from which the singlet back reaction lifetime is estimated as 350 ns. The decay of the inverted spectrum yields a lifetime of 8.3 μs for the triplet back reaction lifetime.     

Electronic Energy Transfer in a Single Donor–Acceptor Pair with Triplet–Triplet Absorption

Optics and Spectroscopy - The protocol of dynamic decoupling using a sequence of resonance RF pulses with different shapes in a system of dipole-coupled electron/nuclear spins with inhomogeneous...     

Triplet‐Triplet Electron Excitation Energy Transfer in the Gas Phase for the Carbazol–Diacetyl Donor‐Acceptor Pair

For the carbazol–diacetyl donoracceptor pair, the features of exothermal triplettriplet (TT) transfer of electron excitation energy in the gas phase upon excitation of carbazol by the nitrogen laser radiation which is not absorbed by diacetyl have been investigated. The luminescence spectra of carbazol in the gas phase have been studied. It is shown that carbazol phosphorescence in the gas phase is absent. Therefore, for estimating the TT transfer efficiency and its temperature dependence, in the kinetic approximation the dependences of the intensity of timeresolved delayed fluorescence of carbazol and sensitized phosphorescence of diacetyl on diacetyl pressure have been analyzed. It has been found that in the gas phase the features of the electronic structure of the molecules under investigation are responsible for the low efficiency of transfer (2·10^–4 at 453 K), which increases by an order of magnitude with increasing temperature in the 453–513 K range despite the increasing influence of the back transfer.     

Electron–Nuclear Spin Dynamics in Semiconductor QDs

This work presents an overview of investigations of the nuclear spin dynamics in nanostructures with negatively charged InGaAs/GaAs quantum dots characterized by strong quadrupole splitting of nuclear spin sublevels. The main method of the investigations is the experimental measurements and the theoretical analysis of the photoluminescence polarization as a function of the transverse magnetic field (effect Hanle). The dependence of the Hanle curve profile on the temporal protocol of optical excitation is examined. Experimental data are analyzed using an original approach based on separate consideration of behavior of the longitudinal and transverse components of the nuclear polarization. The rise and decay times of each component of the nuclear polarization and their dependence on transverse magnetic field strength are determined. To study the role of the Knight field in the dynamic of nuclear polarization, a weak additional magnetic field parallel to the optical axis is used. We have found that, only taking into account the nuclear spin fluctuations, we can accurately describe the measured Hanle curves and evaluate the parameters of the electron–nuclear spin system in the studied quantum dots. A new effect of the resonant optical pumping of nuclear spin polarization in an ensemble of the singly charged (In,Ga)As/GaAs quantum dots subjected to a transverse magnetic field is discussed. Nuclear spin resonances for all isotopes in the quantum dots are detected in that way. In particular, transitions between the states split off from the ±1/2 doublets by the nuclear quadrupole interaction are identified.     

Determination of the Efficiency and Energy Transfer Rate in the Fluorescence of a Single Donor–Acceptor Pair Attached to a Biomolecule

Resonance energy transfer from a single donor molecule to a single acceptor molecule (Förster resonance energy transfer) is currently used to determine the microscopic parameters describing interconformational changes in a protein molecule to which this single donor–acceptor pair is attached. A recently developed method makes it possible to intricately search for such parameters using a formula for the efficiency of Förster resonance energy transfer, which is the sum of several Gaussians. Another simpler method for the determination of the desired parameters of interconformational transitions has been proposed in this work on the basis of statistical processing of fluctuating tracks of fluorescence of a single donor–acceptor pair attached to a protein molecule.     

Photoinduced Intramolecular Electron Transfer and Electronic Coupling Interactions in π- Expanded Imidazole Derivatives

An intramolecular excited charge transfer (CT) analysis of imidazole derivatives has been made. The determined electronic transition dipole moments has been used to estimate the electronic coupling interactions between the excited charge transfer singlet state (¹CT) and the ground state (S₀) or the locally excited state (¹LE). The properties of excited ¹CT state imidazole derivatives have been exploited by the significant contribution of the electronic coupling interactions. The excited state intramolecular proton transfer (ESIPT) analysis has also been discussed.     

Influence of SiC Electron Acceptor–Donor Modification on Thermal and Physical Properties of Carbon Fiber/SiC/Epoxy Composites

In this work, the effects of electron acceptor–donor modification on the surface properties of SiC were investigated in the mechanical interfacial properties of carbon fibers-reinforced SiC-impregnated epoxy matrix composites. The surface properties of the SiC were determined according to acid/base values and FT-IR, and contact angle measurements. The thermal and mechanical interfacial properties of the composites were evaluated using a thermogravimetric analysis, critical strain energy release rate mode II (G _IIC), and impact strength testing. As a result, the electron acceptor-treated SiC had a higher acid value and polar component in surface free energy than did the untreated SiC or the electron donor-treated SiC. The G _IIC and impact strength mechanical interfacial properties of the composites had been improved in the specimens treated by acidic solutions due to the good wetting and a high degree of adhesion with electron donor characteristic epoxy resins.     

Features of the Coupled Nuclear–Electron Spin Precession in the Bose–Einstein Condensate of Magnons

JETP Letters - The experimental detection of the Bose-Einstein condensate of magnons in coupled nuclear-electron spin precession in antiferromagnets brings the prospect of its use for magnonics and...     

Luminescence of Donor‐Acceptor Pairs in Compounds of Yttrium and Scandium Oxides

The luminescence spectra of Sc₂O₃, Y₂O₃, and Y₂GeO₅ ceramics and thin films exposed to laser and cathode excitation were investigated. The investigation of the properties of longwave luminescence bands in Sc₂O₃ with maxima at 2.65, 2.35, and 2.05 eV, in Y₂O₃ with maxima at 2.60, 2.35, and 2.10 eV, and also in Y₂GeO₅ with maxima at 2.55, 2.25, and 2.00 eV point to the fact that they are caused by radiative recombination of the excited donoracceptor pairs Sc³⁺ (or Y³⁺)O^2–.     

Coulomb Drag of Dipole Excitons in a Hybrid Exciton–Electron System

The effect of Coulomb drag on a gas of dipole excitons in spatially separated two-dimensional quantum wells containing electron and exciton gases is studied theoretically. The Coulomb drag of excitons can be used to control exciton transport in transistor structures whose active element is a two-dimensional gas of dipole excitons. Expressions for the exciton cross conductivity as a function of temperature are obtained for the diffusion and ballistic transport regimes. For each regime, the limiting cases in terms of the ratio of the Coulomb interaction screening length to the distance between the gases are analyzed. It is shown that, at temperatures exceeding considerably the exciton-gas degeneracy temperature, the cross conductivity is independent of the temperature, while in the opposite case it vanishes exponentially.     

Anderson Localization in a Two-Dimensional Electron–Hole System

JETP Letters - Anderson localization is discovered in a highly disordered two-dimensional electron–hole system in a HgTe quantum well. The behavior of this localization is fundamentally...     

Electron Spin Resonance–Native Signal in Thermally Treated Dental Tissue

Although the dosimetric Electron Spin Resonance (ESR) signal of hard tissues, particularly enamel, has been extensively studied, little attention has been paid to the native signal. This signal is known to be affected by the health of the tissue, as well as by socio–economic factors. In dental applications several clinical procedures, including the use of laser irradiation, can heat the tissue locally with side effects that must be studied. The purpose of the present work is to study the ESR signals in enamel and dentin tissues after thermal treatment with temperatures in the range of 100°C–300°C. Non‐irradiated permanent bovine teeth were studied. ESR measurements were performed with a Varian E‐4 ESR spectrometer operating in the X band range. Progressively larger ESR signals were produced in dentin tissues previously heat treated at and above 100°C. No detectable signals were observed in similarly treated enamel. The signal shows partial decay at four and six months after thermal treatment. The experimental data for dentin show a correlation with the Arrhenius function with an activation energy of (41 ± 2)10³ J/mol. After six months, the ESR signal shows a higher activation energy (67 ± 3)10³ J/mol and the decay shows a activation energy of (38 ± 2)10³ J/mol. A possible assignment of the signal origin in dentin is difficult. The water lost during thermal treatment and reincorporated during the following six months correlates with the signal gain and subsequent decay. The water lost can produce point defects in the hydroxyapatite, or structural changes in the collagen structure. The results observed here are useful for understanding the thermal effects produced in dentin by infrared laser irradiation, and provides a cautionary warning that annealing conditions in ESR studies of biological tissues should be standardized.     

Investigation of Electron Injection Parameters in Vacuum Laser Acceleration

We investigate the parameters such as the energy gains, comparisons for linearly and circularly polarized laser fields, the electron incident polarization azimuth angle, the electron incident crossing angle, and the electronincident momentum, etc., which are crucial for experimental test of the vacuum laser acceleration scheme in capture and acceleration scenario (CAS) [see, e.g., Phys. Rev. E 66 (2002) 066501]. Physical explanation of these features has been presented based on the CAS scheme. The results show that all those parameters are in the reasonable ranges obtainable by presently experimental facilities.     

Mechanisms of Fluorescence Quenching in Donor—Acceptor Labeled Antibody—Antigen Conjugates

The cyanine dyes Cy5 and Cy5.5 are presented as a new long wavelength-excitable donor-acceptor dye pair for homogeneous fluoroimmunoassays. The deactivation pathways responsible for the quenching of the fluorescence of the antibody-bound donor are elucidated. Upon binding of the donor dye to the antibodies at low dye/protein ratios, its fluorescence quantum yield rises to unity. Higher dye/protein ratios lead to progressive aggregation of the dyes, which results in quenching of monomer fluorescence due to resonance energy transfer (RET) from the monomers to the nonfluorescent dimers. The dependence of the quenching efficiency on the labeling ratio is described quantitatively by assuming a Poisson distribution of the dyes over the antibodies. The maximum fluorescence intensity per antibody is obtained at a labeling ratio of 4. Upon formation of the antibody-antigen complex, electron transfer and RET to the antigen-bound acceptor dye occur. Steady-state and time-resolved fluorescence measurements reveal that approximately 50% of the donor quenching is due to RET, while the residual quenching effect is caused by the static quenching process.     

Investigation of the Three-Nucleon System Dynamics in the Deuteron–Proton Breakup Reaction

Precise and large sets of cross section, vector A _x , A _y and tensor A _xx , A _xy , A _yy analyzing power data for the ¹ H(d, pp)n breakup reactions were measured at 100 and 130 MeV deuteron beam energies with the SALAD and BINA detectors at KVI and the Germanium Wall setup at FZ-Jülich. Results are compared with various theoretical approaches which model the three-nucleon system dynamics. The cross section data reveal a sizable three-nucleon force (3NF) and Coulomb force influence. In case of the analyzing powers very low sensitivity to these effects was found and the data are well describe by 2N models only. For A _xy at 130 MeV, serious disagreements were observed when 3NF models are included in the calculations.     

Investigation of Elliptical Gaussian Beam in Optical Disk System

1.IntroductionLaserdiodehasbeenx`idelyusedinfibrc-opticcommunicat:onandopticaldisk$yctems.LaserbeamIsradiationangle(fullangu1aratha1f-maximumintensitypoint)ismeasuredbothparllelandPerpendiculartothcjunctionp1anclgenera1ly.o//=6,~1o",oJ-=2o"~5O':Inerasableopticaldisksystemsasmagneto-optica1disksystems,thelaserbcamiscollimatedandshapedtobecircularsymInetricbeaminordcrtoaquireahighintensityandhighqualityscanningbeamiPot,butitsoptica1systemiscomplicatedandeasyaddingassocialaberration.usingaellip…     

Observation of Phase Transitions in an Electron–Hole System Associated with Dislocation Cores in Cadmium Telluride

Bulletin of the Lebedev Physics Institute - Evolution of the fine structure of dislocation microphotoluminescence lines in cadmium telluride with a decrease in the optical-excitation power density...     

Generalized Rashba Coupling Approximation to a Resonant Spin Hall Effect of the Spin–Orbit Coupling System in a Magnetic Field

We introduce a generalized Rashba coupling approximation to analytically solve confined two-dimensional electron systems with both the Rashba and Dresselhaus spin–orbit couplings in an external magnetic field. A solvable Hamiltonian is obtained by performing a simple change of basis, which has the same form as that with only Rashba coupling. Each Landau state becomes a new displaced-Fock state instead of the original Harmonic oscillator Fock state. Analytical energies are consistent with the numerical ones in a wide range of coupling strength even for a strong Zeeman splitting, exhibiting the validity of the analytical approximation. By using the eigenstates, spin polarization correctly displays a jump at the energy-level crossing point, where the corresponding spin conductance exhibits a pronounced resonant peak. As the component of the Dresselhaus coupling increases,the resonant point shifts to a smaller value of the magnetic field. In contrast to pure Rashba couplings, we find that the Dresselhaus coupling and Zeeman splittings tend to suppress the resonant spin Hall effect. Our method provides an easy-to-implement analytical treatment to two-dimensional electron gas systems with both types of spin–orbit couplings by applying a magnetic field.     

Isolated (Quantum) Emitters Generated with the Participation of Donor–Acceptor Pairs in the ZnSe/ZnMgSSe Heterostructure

Measurements of low-temperature (5 K) microphotoluminescence reveal the existence of isolated (quantum) emitters in ZnSe/ZnMgSSe quantum well with a radiation line experiencing abrupt displacements of its spectral position by several meV within 1–10 min. The unusual properties of the detected emitters are explained on the basis of a system with a large dipole moment in its ground state, e.g., a single donor–acceptor pair.