Theoretical Treatment of Degenerate Electron Exchange and Dimerization in Spin Dynamics of Radical Ion Pairs as Observed by Magnetic Field Effects

In this work we have compared manifestations of degenerate electron exchange (DEE) and dimerization reactions in MARY (magnetically affected reaction yield) spectroscopy and time-resolved magnetic field effects (TR-MFE) of radical ion pairs (RIPs). It is shown that dimerization results in phase and frequency changes of the quantum beats in TR-MFE traces, whereas DEE leads to relaxation-type changes in the spin dynamics of RIPs. Dimerization does not change the width of MARY lines considerably, but leads to new lines corresponding to dimer radical ions. In contrast, DEE substantially changes the MARY line widths. Our treatment provides ways to discriminate between both processes from experimental MARY spectra and TR-MFE kinetics. An experimental example dealing with concentration-dependent broadening of the MARY lines is also discussed.     

A Low-Field Magnetically Affected Reaction Yield (MARY) Spectrometer with Spectral Fluorescence Resolution

A novel spectrometer for low-field studies in magnetically affected reaction yield (MARY) spectroscopy with fluorescence detection is described. The spectrometer is based on a yoke-free magnetic system containing no ferromagnetic elements, uses X-ray or optical excitation, and includes a monochromator to analyze the spectral composition of luminescence. Using the new setup, the effect of transversal residual magnetic field on zero field MARY line is illustrated, formation of exciplexes under X-irradiation in a naphthalene/N,N-dimethylaniline solution in alkane is demonstrated, a magnetic field effect on the emission spectrum is shown in field-cycling mode, and modulated MARY spectra in the exciplex and the intrinsic luminophor bands are compared to show that magnetic field sensitivity here is provided at the stage of the recombining radical ion pair, while exciplex formation only transforms the luminescence properties.     

Study of interaction of aliphatic alcohols with primary radical cations of n-alkanes using MARY spectroscopy

MARY spectroscopy (the radical ion pair level crossing technique) was employed to probe the reaction of proton transfer from primary radical cations of n-alkanes to alcohol molecules in liquid solution. Alcohols were demonstrated to react with the primary radical cation of the solvent, leaving the counterion of the radical ion pair unaffected. The broadening of the zero field MARY line, tentatively attributed to the proton transfer reaction, was found to be independent of the proton affinity of the species in the studied systems, estimated from gas-phase data. The rate constant of the reaction is close to the diffusion controlled limit within the experimental accuracy for all the studied alcohol/solvent combinations.     

Magnetic field effects on the pyrene—dicyanobenzene system: determination of electron self-exchange rates by MARY spectroscopy

An experimental determination has been made of electron self-exchange rates between the radical anions of 1,2-, 1,3- and 1,4-dicyanobenzene (DCB) and the respective neutral molecules applying steady-state field-modulated MARY (magnetic field effect on reaction yield) spectroscopy. For the first time this has been achieved successfully for compounds whose self-exchange rate constants can be obtained independently by alternative methods such as EPR linebroadening. In this study, pyrene was used as an electron donor to generate the spin-correlated radical ion pair (pyrene·⁺ DCB·^?) essential for MARY spectroscopy. The radical ion pair is in equilibrium with an exciplex whose magnetic field affected fluorescence was recorded as a function of the magnetic field to yield the MARY spectrum. Due to lifetime uncertainty energy broadening of spin levels caused by electron self-exchange, the characteristic B _1/2 value increases with the concentration of DCB in the sample. The rate constant of self-exchange was obtained from the slope of the linear part in the plot of B_1/2 versus DCB concentration. The values range between 6 x 10⁸ M^?1 s^?1 and 1.4 x 10¹⁰ M^?1 s^?1, depending on the DCB isomer and solvent. Comparison with literature data from EPR linebroadening measurements shows good agreement.     

ENDOR evidence for the quantum exchange of the methylene protons

ENDOR spectroscopy was used for studying methylene proton couplings in the 6-yl (or H-addition) radical in a single crystal of 1-methyluracil at 4.2K. The two methylene protons were magnetically equivalent in all crystal (radical) orientations in the magnetic field, indicating proton quantum exchange. The two protons compose magnetically single entity with spin I=1. The observed small ENDOR line splittings of about 0.60MHz agree well with the theoretical expectations for such a system.     

Steady state and transient Overhauser effect in systems of three spins

The rate equations describing spin polarization in a system of three spins are derived and solved for the case of a free radical dissolved in a solvent containing two nuclear spins. Triple irradiation experiments indicate that a nuclear spin A can be effectively coupled to an electron spin C via a second nuclear spin B and measurements of both the steady state and transient Overhauser effects are in accord with the theoretical predictions for a three-spin system. The ‘three spin effect’ is found to operate only in dilute solutions of free radicals in which case the probabilities for transitions between different nuclear or electronic energy levels can be determined. It was found to be effective for fluorine nuclei—in the presence of both protons and a free radical and for carbon [13] nuclei in the presence of either protons or fluorine nuclei and a free radical. Detailed measurements have been performed for CHFCl₂, para-difluorobenzene, and meta-fluorotoluene containing the tritertiary butyl phenoxyl radical.     

E.S.R. spectra of PF4 radicals produced in a single crystal of PF3

E.S.R. spectra of PF₄ radicals were investigated with single crystals of PF₃ made at a low temperature and irradiated with γ-rays. The angular dependence of the spectral lines was satisfactorily analysed in terms of the second-order equations for large couplings of one ³¹P nucleus and two magnetically equivalent ¹⁹F nuclei and of the first-order equation for two ¹⁹F nuclei with small couplings. We conclude that the two magnetically equivalent fluorine nuclei occupy the axial positions of the trigonal bipyramidal structure of the PF₄ radical and the other two fluorine nuclei are in the equatorial positions. Furthermore, the unpaired electron of the PF₄ radical was shown to occupy an orbital consisting mainly of the 2p orbital of each of the two axial fluorine atoms along the F-P-F axis and the 3s and 3p orbitals, directed towards the vacant equatorial position of the central phosphorus atom. The orbital of the unpaired electron can be reasonably represented as a Rundle three-centre non-bonding orbital. The results obtained for the PF₄ radicals not only strongly support the results given for POCl₃ ^- radicals, but also allow us to discuss the electronic structure of phosphoranyl radicals in more detail.     

Realization of a decoherence-free subspace using multiple quantum coherences

This Letter presents a two-dimensional nuclear magnetic resonance (NMR) approach for constructing a two-logical-qubit decoherence-free subspace (DFS) by using four multiple-quantum coherences of a CH3 spin system as logical qubits. The three protons in this spin system are magnetically equivalent and can only be used as a single qubit in one-dimensional NMR. We have experimentally demonstrated that our DFS can protect against more types of decoherences than those of the one composed of four noisy physical qubits all with different chemical shifts. This idea may provide new insights into extending qubit systems in the sense that it effectively utilizes the magnetically equivalent nuclei.     

Spin relaxation of quadrupole nuclei in paramagnetic and magnetically ordered insulators

The longitudinal and transverse spin relaxation through a (generally anisotropic) electron-nucleus interaction in paramagnetic and magnetically ordered insulators is theoretically studied for nuclei with a resolved quadrupole structure. Expressions are derived for the relaxation rates of both the transverse nuclear magnetization components when individual transitions are excited in the quadrupole structure and the total longitudinal nuclear magnetization component. These expressions are reduced to a form that contains the Fourier transforms of the time correlation functions only for the electron spins. Given the specific form of these correlation functions corresponding to different phase states of the electron spins and different origins of their fluctuations, the temperature dependences of the nuclear relaxation rates are ascertained in various cases, including those for dipole and isotropic hyperfine interactions. Calculations are performed for arbitrary electron and half-integer nuclear spins by taking into account the possible quadrupole splitting of the NMR spectrum without any restriction on the smallness of the ratio ?ω s/k_BT (ω_s is the resonance frequency of the electron spins). The derived expressions are compared with available experimental data on the longitudinal and transverse nuclear relaxation in colossal-magnetoresistance lanthanum manganites in the part of their phase diagram where the corresponding samples are either paramagnetic or magnetically ordered insulators and near the points of transition to an ordered state. Interpretations alternative to the existing ones are offered.     

Search for supersymmetry in light nuclei

The supersymmetry assumption based on a system of valence interacting bosons and odd nucleons has been checked in the second half of the nuclear sd-shell. The dynamical supersymmetric hamiltonian restricted to the linear combination of chosen second-order Casimir invariants has been applied to energy levels of several nuclei organized in supermultiplets. The supersymmetry predictions for nine nuclei are in accord, to a good approximation, with experimental energy levels up to 4–7 MeV.     

Typical applications of MARY spectroscopy: Radical ions of substituted benzenes

The paper describes the underlying principles and discusses the most important advantages and limitations of the experimental technique of magnetically affected reaction yield spectroscopy as developed in the authors’ laboratory and guides the reader step by step through a typical experimental sequence using as example the problem of short-lived radical cations of a series of methyl-substituted benzenes in X-irradiated nonpolar solutions. For two of the eight target substances — benzene itself and mesithylene — the paper reports the first unequivocal observation of their radical cations in liquid alkane solution at room temperature and provides a lower estimate of about 10 ns for their relaxation times in low magnetic field.     

Magnetic-field-induced orientation of photosynthetic reaction centers as revealed by time-resolved W-band EPR of spin-correlated radical paris: Development of a molecular model

Spin-correlated radical pairs are the short-lived intermediates of the primary energy conversion steps of photosynthesis. In this paper, we develop a comprehensive model for the spin-polarized electron paramagnetic resonance (EPR) spectra of these systems. Particular emphasis is given to a proper treatment of the alignment of the photosynthetic bacteria by the field of the EPR spectormeter. The model is employed to analyze time-resolved W-band (94 GHz) EPR spectra of the secondary radical pair P ₇₀₀ ⁺ A ₁ ^? in photosystem I formed by photoexcitation of the deuterated and¹⁵N-substituted cyanobacteriumSynechococcus lividus. Computer simulations of the angular-dependent EPR spectra of P^700/+A_1/? provide values for the order parameter of the cyanobacterial cells and for the orientation of the membrane normal in a molecular reference system. The order parameter from EPR compares favorably with corresponding data from electron microscopy obtained for theS. lividus cells under similar experimental conditions. It is shown that high-field EPR of a magnetically aligned sample in combination with the study of quantum beat oscillations represents a powerful structural tool for the short-lived radical pair intermediates of photosynthesis.     

用于暗物质探测实验的CsI(Tl)晶体探测器性能的实验研究

在直接测量暗物质的实验中,反冲核能量的Quenching Factor是一个重要参数.用低能X射线源对一套测量入射中子引起的反冲核能量Quenching Factor的系统进行了能量刻度,得到了这套系统的能量响应关系.PMT单光电子的发射对应于晶体中的能量沉积约为0.32keV.同时研究了不同能量的X射线引起的PMT输出电流信号的积分时间宽度与积分电荷的关系,得到最佳的PMT输出电荷收集条件.     

Electron-nuclear cross polarization

We describe the coherent polarization transfer from an unpaired electron to neighboring nuclei via electron-nuclear cross polarization (eNCP) in a doubly, tilted rotating frame. Although the experiment superficially resembles the well-known Hartmann-Hahn cross polarization (CP) process introduced by Pines et al., that is widely used in solid-state nuclear magnetic resonance (SSNMR), it differs in significant respects. In particular, eNCP requires an alternative treatment due to the very different sizes of the specific terms in the spin Hamiltonian. We derive analytical expressions for the matching condition for optimal polarization transfer and verify their correctness with experimental results obtained with electron detected CP experiments performed on powder samples of BDPA radical dispersed in a protonated polystyrene matrix and with numerical simulations. We use fully protonated BDPA as an example of polarization transfer to strongly coupled nuclei. In contrast, perdeuterated BDPA serves as an example of the transfer of polarization from electrons to weakly coupled nuclei. In addition, we performed CP on a paramagnetic crystal to determine the influence of resolved hyperfine structure on the CP process. It is shown that almost no structure is observed in the corresponding electron-(1)H CP matching curve. It appears that only a restricted number of hyperfine coupled (1)H's contribute to the observed signal intensities in the CP experiment.     

双核系统核子交换势能面

实验证实,用重离子碰撞合成超重原子核时,准裂变对熔合的抑制是非常重要的.碰撞中双核系统间核子转移所形成的势能面称之为驱动势,它制约核子转移,因而决定熔合与准裂变的竞争.双核系统势能面还提供重离子碰撞合成超重原子核的最佳激发能和最佳弹靶组合的信息.     

An algorithm to determine electron energy distributions from synchrotron radiation measurements

An algorithm to unfold the energy distribution function of a weakly relativistic, magnetically confined electron cloud from its emitted synchrotron radiation is described and applied to experimental data.     

Calculation of Neutron Resonance Spacing with Microscopic Theory

Nuclear level spacings calculated with a microscopic theory are compared with spacings determined fromneutron resonance experiment. The gross features of the experimental data due to nuclear shells are reproduced with themicroscopic theory. The experimental data for nuclei with statistically deformed nuclei have also been tested with leveldensity formula including low energy rotational levels. The experimental data for the actinide nuclei and the lanthanidenuclei are found to be consistent with the theory which includes collective rotational levels.     

Geschwindigkeitsverteilung anisotroper Deuteriumplasmen aus zeitaufgelösten Neutronenspektren

The ion energy in hot deuterium plasmas can be determined by using the Doppler broadening of thed-d-neutron line at 2.45 MeV. A method allowing time resolved measurement of neutron spectra in pulsed discharges is described. It utilizes the known energy dependence of the total neutron cross section of light to medium weight nuclei for energy selection of the neutrons. The neutrons are detected with plastic scintillators which afford both a high detection efficiency and a time resolution in the nanosecond range. The method is applied to a theta pinch for investigating the anisotropy and relaxation of the deuteron velocity distribution. It is shown that, in particular, the ion energy parallel to the magnetic field can readily be measured even at the low neutron yield available at present. Suitable combination of this procedure with known diagnostic methods allows deviation of the velocity distribution from the isotropic Maxwell distribution to be determined with sufficient accuracy. Comparison of the experimental relaxation rate with the theoretical predicted collision relaxation shows that Coulomb collisions are not a sufficient explanation. The short wave-length mirror instabilities also observed probably make a major contribution to the relaxation of the anisotropic velocity distribution.     

Magnetic Control of Diffuse Discharges

By application of a crossed magnetic field, the electron energy distribution in a gas discharge can be shifted to lower energy values, as demonstrated by means of Monte Carlo calculations for electrons in He:SF6 mixtures. Consequently, through the change in the rate coefficients for ionization and attachment, the sustaining field in the discharge plasma is increased. This magnetically induced voltage rise was studied in a low-pressure glow discharge. The cathode fall was found to be the dominant component in determining the characteristics of this magnetically controlled discharge. The drastic rise of the cathode fall above a threshold value could be utilized in operating a glow discharge as an opening switch for an inductive energy storage system.     

Determination of amplification factor and intensity of alternating magnetic field on a nucleus by Mössbauer spectroscopy

The Moessbauer effect 15 reported to be useful in precision measurements of alternating magnetic field intensity at a nucleus and in determining the PF field gain factor in magnetically ordered material’s. The scope of this method is recognized to manifest itself in a most clear way on pumping of the nuclear spin system by the The problem of Moessbauer lines shape is considered theoretically for the case of nuclei excited by linearpolarized RF field.