Stimulated Electron Spin Polarization in Strongly Coupled Triplet–Doublet Spin Pairs

The possibility of stimulating electron spin polarization in a system consisting of a stable paramagnetic center and a chromophore that can be excited into its triplet state is discussed. In such systems, the doublet state of the paramagnetic center couples to the excited triplet state of the chromophore and if the coupling is larger than the difference in the precession frequencies of the doublet and triplet, the eigenstates of the coupled system are quartet and doublet states. The quartet state is usually the lowest energy excited state. Following light excitation, the initial electronic relaxation to the quartet state generates strong multiplet polarization if it is governed by the spin–orbit coupling that follows the molecular symmetry. It is shown that application of a selective π-pulse to the ±3/2 ↔ ±1/2 transitions of the quartet converts this multiplet polarization into net polarization. The magnitude and orientation dependence of the generated polarization is estimated on the basis of a simple analytical model. The experimental conditions required for this net polarization to be retained in the ground state after decay of the quartet state are discussed. The viability of using this as a method to enhance the signal strength of a spin label or metal center in selective excitation experiments is considered.     

Theoretical study of the intensity of chemically induce dynamic electron polarization of radical-triplet pairs

Considering the interaction between encited triplet molecule and cloublet radical,based on the second-order perturbation theroy and the motion equation of density matrix,the polarzation intersity of RTPM were theoretically calculated with the overpopulated doublet spin states and quartet spin states of radical0triplet paris as initial conditions the radical result from the zero-field-splitting(zfs)and the multiplet A/E and E/A polarization result from hyperfine (hf) interactions of the triplet molcule,The hyperfine ralated A A/E or E E/A CIDEP on the radical were the overpopulation of the net abscrptive or emissive polarization and multiple A/E or E/A polarization.     

Radiative decay of the metastable quartet state of copperporphin

In CuP the low temperature luminescence originates from eight transitions between a quartet (M = ±3/2, ±1/2) and a doublet (M′ = ±1/2) with M and M′ denoting the (approximate) eigenvalues of the spin angular momentum S_z along the fourfold axis. Here we report (1) the selection rules governing the polarizations of the transitions; (2) the zero-field splitting ξ between the ±3/2 and ±1/2 components of the quartet for CuP in an n-octane crystal (ξ = 1·1 ± 0·2 cm^-1); (3) a photo-selection experiment on CuP in an isopentane glass from which it is concluded that the ratio of in-plane to out-of-plane polarization in the 0-0 band at 2·1 K amounts to an intensity ratio I _‖/I _⊥ ≈ 2. The implications of these results for the different SOC pathways are analysed.     

Hyperfine decoupling in electron paramagnetic resonance as a powerful tool for unraveling complicated ESEEM spectra of S=1/2, I> or =1/2 systems

Hyperfine decoupling in electron paramagnetic resonance after strong microwave radiation is studied for S=1/2, I=1/2 and S=1/2, I=1 spin systems. A new 2D pulse sequence based on the hyperfine-decoupled DEFENCE (deadtime-free ESEEM by nuclear coherence-transfer echoes) experiment is introduced, which is distinguished by a remarkable reduction of the residual hyperfine coupling. The efficiency of this new decoupling experiment in comparison to the old pulse sequence is studied by means of numerical simulations. The advantages of the new decoupling experiment and its ability to simplify ESEEM spectra are experimentally demonstrated on two disordered systems.     

The Decrease of the ESEEM Frequency of $${\text{P}}_{700}^{ + } {\text{A}}_{1}^{ - }$$ Ion-Radical Pair in Photosystem I Embedded in Trehalose Glassy Matrix at Room Temperature can be Explained by Acceleration of Spin–Lattice Relaxation

The main observation in this work is a decrease in the modulation frequency of the primary electron spin-echo decay (ESEEM) of the \({\text{P}}_{ 7 0 0}^{ + }\) cofactor in the reaction center of Photosystem I (PS I) from cyanobacteria Synechocystis sp. PCC 6803 embedded in dry trehalose matrix as the temperature rises from 150 K to room temperature. From the previous studies of the EPR spectrum shape of this system, it is known that, in dry trehalose matrix at room temperature, the distance between \({\text{P}}_{ 7 0 0}^{ + }\) and \({\text{A}}_{ 1}^{ - }\) spins does not increase compared to the distance measured in glycerol–water solution at cryogenic temperature. From the present ESEEM study, we conclude that the decrease of modulation frequency with rising temperature in trehalose matrix can be fully attributed to the influence of accelerated spin–lattice relaxation of \({\text{A}}_{ 1}^{ - }\). Our calculations show that this requires a decrease in the spin–lattice relaxation time from 3 to 1 μs. To the best of our knowledge, this is the first time that a shift in the ESEEM frequency due to the dipole–dipole interaction between the spins is observed that is caused by spin–lattice relaxation. Based on the above-mentioned results, we formulate a model of the protective effect of trehalose matrix on the electron transfer in the reaction center of PS I that is based on different hydrogen-bond networks between trehalose, local water, and protein.     

Spin polarization in fullerene derivatives containing a nitroxide group. Observation of the intermediate photoexcited quartet state in radical triplet pair interaction

A series of C₆₀ fullerene derivatives containing a nitroxide group has been photoexcited by short LASER pulses in the microwave cavity of a cw-EPR spectrometer. Strongly spin polarized signals have been observed, in glassy matrix as well as in liquid solution, for both the ground electronic state and the excited quartet state. In the quartet state the excitation resides in the fullerene part and the molecule constitutes a triplet-radical pair with the partner covalently linked. The absorptive or emissive character of the transitions is explained in terms of the mechanism of radicaltriplet interaction producing spin polarization. Opposite initial sign and polarization patterns are observed for molecules with different spacer between nitroxide and fullerene. The time evolution of the relevant sublevel populations is fitted by a kinetic model taking into account quartet decay constants, quartet and doublet spin-lattice relaxation rates and branching ratios.     

OPTESIM, a versatile toolbox for numerical simulation of electron spin echo envelope modulation (ESEEM) that features hybrid optimization and statistical assessment of parameters

Electron spin echo envelope modulation (ESEEM) is a technique of pulsed-electron paramagnetic resonance (EPR) spectroscopy. The analyis of ESEEM data to extract information about the nuclear and electronic structure of a disordered (powder) paramagnetic system requires accurate and efficient numerical simulations. A single coupled nucleus of known nuclear g value (g_N) and spin I = 1 can have up to eight adjustable parameters in the nuclear part of the spin Hamiltonian. We have developed OPTESIM, an ESEEM simulation toolbox, for automated numerical simulation of powder two- and three-pulse one-dimensional ESEEM for arbitrary number (N) and type (I, g_N) of coupled nuclei, and arbitrary mutual orientations of the hyperfine tensor principal axis systems for N > 1. OPTESIM is based in the Matlab environment, and includes the following features: (1) a fast algorithm for translation of the spin Hamiltonian into simulated ESEEM, (2) different optimization methods that can be hybridized to achieve an efficient coarse-to-fine grained search of the parameter space and convergence to a global minimum, (3) statistical analysis of the simulation parameters, which allows the identification of simultaneous confidence regions at specific confidence levels. OPTESIM also includes a geometry-preserving spherical averaging algorithm as default for N > 1, and global optimization over multiple experimental conditions, such as the dephasing time (τ) for three-pulse ESEEM, and external magnetic field values. Application examples for simulation of ¹⁴N coupling (N = 1, N = 2) in biological and chemical model paramagnets are included. Automated, optimized simulations by using OPTESIM lead to a convergence on dramatically shorter time scales, relative to manual simulations.     

Mass measurements of 19Na and 23Al using the (3He, 8Li) reaction

The mass excesses of ¹⁹Na and ²³Al have been measured with the (³He, ⁸Li) reaction. The values obtained are 12928 ± 12 keV and 6767 ± 25 keV, respectively. The first excited state of ¹⁹Na, observed to lie at 120 ± 10 keV, completes a mass quartet which agrees with a quadratic isobaric multiplet mass equation.     

Time-Resolved EPR Spectra of Photoexcited Copper Porphyrin

Photoinduced spin-polarized transient electron paramagnetic resonance (EPR) spectra of copper 5,10,15,20-tetrakis(3-pyridyl)porphyrin (3PyNCu) in the frozen solution have been observed in the X-band. The time evolution and the temperature dependence of the spectra have been studied. The effect of molecular oxygen in the frozen solution on the polarization pattern has also been examined. The magnetic resonance parameters of the ground state of 3PyNCu have been obtained by comparing the experimental continuous-wave and echo-detected EPR spectra with the numerical computations. The magnetic resonance parameters of the excited states and the photoinduced polarizations have been investigated by time-resolved EPR (TREPR) spectroscopy and numerical analysis. The experimental spectra have been considered as a sum of the polarized spectra of the ground and excited states. Our analysis confirmed that the TREPR spectra consisted of two main patterns: the enhanced signal from the ground state and the multiplet contribution belonging to the excited quartet state.     

Electron Spin Echo of Photoinduced Spin-Correlated Polaron Pairs in P3HT:PCBM Composite

A variation of the electron spin echo (ESE) signal caused by laser pulse in a blend of [6,6]-phenyl C₆₁ butyric acid methyl ester and poly(3-hexylthiophene) (P3HT:PCBM) was detected. This variation was attributed to light-generated paramagnetic species in P3HT:PCBM blend, with non-equilibrium spin polarization. The echo-detected electron paramagnetic resonance (EPR) spectrum of these species closely resembles the time-resolved EPR spectrum of spin-correlated polaron pair PCBM^?/P3HT⁺ (Behrends et al. in Phys. Rev. B 85:125206, 2012) and was assigned to this pair. The characteristic times for polarization and coherence decay (9 ± 1 and 1.0 ± 0.2 μs, respectively) were measured for the PCBM^?/P3HT⁺ pair at 77 K. These times are long enough, which shows the possibility of the application of the ESE technique for studying spin evolution of light-generated charge transfer intermediates in composites of fullerenes and conductive polymers.     

Chiral-symmetry realization for even- and odd-parity baryon resonances

Baryon resonances with even and odd parity are collectively investigated from the viewpoint of chiral symmetry (ChS). We propose a quartet scheme where Delta's and N(*)'s with even and odd parity form a chiral multiplet. This scheme gives parameter-free constraints on the baryon masses in the quartet, which are consistent with observed masses with spin 1 / 2,3 / 2,5 / 2. The scheme also gives selection rules in the one-pion decay: The absence of the parity nonchanging decay N(1720)-->piDelta(1232) is a typical example which should be confirmed experimentally to unravel the role of ChS in baryon resonances.     

Production of a highly polarized sodium atomic beam

Two experimental techniques for preparing atoms in selected states were combined in order to obtain a highly polarized sodium beam: Firstly state selection by an inhomogeneous magnetic field and secondly optical pumping with laser light. This results in a dominating population of the 3s ² S _1/2 ground state levelF=2M _F=+2 (or alternativelyF=2M _F=−2) corresponding to high electron as well as nuclear spin polarization. Polarization values of 0.85±0.05 were easily obtained. The sign of the polarization can be reversed by changing the light polarization. The method can also be applied to other atoms. In addition, it is demonstrated that the optical pumping process allows a determination of the spin-selectivity of hexapole magnets.     

Continuous-Wave and Time-Resolved Electron Paramagnetic Resonance Study of Dimerized Aza-Crown Copper Porphyrins

The dimerization of 5-(4’-(aza-15-crown-5)-phenyl) copper porphyrin (CuP) upon the addition of the K(SCN) salt to a solution of the CuP monomer has been proven by electron paramagnetic resonance (EPR). The magnetic resonance parameters of the CuP monomer, the exchange interaction parameter, J = +0.25 cm^?1, and the Cu–Cu distance of the CuP dimer have been determined by comparing the experimental continuous-wave EPR spectra with the results of the numerical calculations. The photoexcited states have been studied in the time-resolved EPR experiments. It has been shown that the time-resolved EPR spectra of the dimerized porphyrins can be presented as a sum of two components that represent the spectra derived by integrating the dataset in the time windows of 1.1–1.3 and 2.1–2.3 μs. To describe the spectrum in the time window of 2.1–2.3 μs, it is assumed that there is an essential contribution to the signal from the excited state of the supramolecule, which is formed by the interaction between the photoexcited porphyrin in the quartet state and the neighboring non-excited porphyrin in the ground state.     

Electron spin polarization in an excited triplet-radical pair system: Generation and decay of the state

A computational model to simulate electron spin polarization in the three-spin-1/2 system composed of the molecular excited triplet state of (tetraphenylporphinato)zinc(II) (ZnTPP) and the doublet ground state of the 3-(N-nitronyl-notroxide) pyridine (3-NOPy) stable radical is proposed. The model is based on numerical solutions of the stochastic Liouville equation for the diffusively rotating system where the magnetic dipolar, isotropic Heisenberg exchange, and anisotropic Zeeman electron spin interactions are taken into account in a full measure, whereas the intersystem crossing processes between the singlet and triplet states of ZnTPP are considered in terms of kinetic equations for the relevant spin density matrices. Additional longitudinal and transversal paramagnetic relaxation caused by relative rotation motions of the ZnTPP and 3-NOPy moieties is taken into consideration in the form of the generalized relaxation operator.     

Nitrogen atoms encased in cavities within the beryl structure as candidates for qubits

Two new paramagnetic centers (N1 and N2) have been observed after γ-irradiation of a beryl single crystal. The observation of zero-field splitting from the electronic spin-state quartet and the isotropic hyperfine splitting with values similar to that of the free nitrogen atom enables us to suppose that the new paramagnetic centers are nitrogen atoms resulting from radiolysis of molecular nitrogen inside the structural channels of beryl. The following parameters of the spin Hamiltonian were determiend:g=2.0021±0.0003,A/h=13.7±0.1 MHz, andD/h=44.6±0.2 for N1 and 37.6±0.2 MHz for N2 with unique directions of zero-field splittingD along the optical axisc. An amazing resilience of the atomic properties of nitrogen trapped within the structural cavity has been experimentally observed. The N1 paramagnetic atom is stable up to 280°C. We suggest that the structural cavity in beryl could be a shelter for qubits encoded in electron spins of hydrogen or nitrogen atoms.     

Circular polarization of ion fluorescence completing the analysis of resonant Xe* 4d(-1)(5/2)6p Auger decay

The relative contributions of the partial electron waves emitted in the Auger decay of the Xe* 4d(-1)(5/2)6p(J(*)=1) resonance have been determined by fluorescence polarimetry after excitation with circularly polarized synchrotron radiation. The analysis of circularly polarized fluorescence of the photoion leads to an independent determination of the orientation parameters for all states of the Xe II 5p(4)6p multiplet. The present study provides, in combination with data on the angular distribution and spin polarization of the Auger electrons, complete quantum mechanical information on the resonant Auger decay, i.e., branching ratios and relative phases of the Auger decay amplitudes.     

Theoretical models for the interpretation of E.S.C.A. spectra

Three features of E.S.C.A. spectra are discussed: the chemical shift, multiplet splitting, and satellite bands. In the chemical shift effect a perturbation theory treatment shows that the electronic relaxation energy in the hole state of the molecules can be attributed to a flow of electrons on to the ionized atom. Thus the relative abilities of surrounding atoms to “feed” electrons to the ionized atom determines the relative importance of the relaxation energy contribution to the chemical shift. This explains why neutral atom charges don't always determine the direction of the chemical shift. In multiplet splitting the same perturbation treatment shows that the sign and magnitude of the relaxation energy contribution to the multiplet splitting is determined by the direction and magnitude of flow of majority electron spin density in going to the relaxed hole state. Unrestricted Hartree-Fock calculations on the series MF ₂, with M first row transition metals, are compared with E.S.C.A. experiments on the corresponding octahedral complexes to show that complex ion formation has only a very small effect on core level multiplet splitting. Relaxed hole state calculations on a series of first row atom (C,N,O,F) containing radicals leads to the conclusion that in these cases correlation energy effects are larger for the high spin multiplets than for the low spin multiplets. The assignments of satellite structure in transition metal complexes is reviewed and multiconfiguration self-consistent field results are presented and discussed for the satellite structure     

Efficient spin filtering in a disordered semiconductor superlattice in the presence of Dresselhaus spin-orbit coupling

The influence of the Dresselhaus spin-orbit coupling on spin polarization by tunneling through a disordered semiconductor superlattice was investigated. The Dresselhaus spin-orbit coupling causes the spin polarization of the electron due to transmission possibilities difference between spin up and spin down electrons. The electron tunneling through a zinc-blende semiconductor superlattice with InAs and GaAs layers and two variable distance In_xGa_(1−x)As impurity layers was studied. One hundred percent spin polarization was obtained by optimizing the distance between two impurity layers and impurity percent in disordered layers in the presence of Dresselhaus spin-orbit coupling. In addition, the electron transmission probability through the mentioned superlattice is too much near to one and an efficient spin filtering was recommended.     

Refocused primary echo: A zero dead time detection of the electron spin echo envelope modulation

We report on the two-dimensional (2D) implementation of the refocused primary electron spin echo envelope modulation (ESEEM) technique, its theory and experimental application to a model system and a system of biological interest. This technique is virtually free of dead time along one time coordinate. The ESEEM obtained by integration of the 2D time-domain data of the refocused primary ESEEM over one of the time coordinates shows the intensity of the sum combination harmonics proportional to k² for k 1 and proportional to k for k 1 (k is a usual notation for the modulation amplitude factor). This feature, in combination with the adjustment of k by means of variation of the operational frequency of the spectrometer, was found to be very useful for detection of protons with distributed hyperfine interaction parameters situated close to the electron spin.     

ESR, ENDOR, and ESEEM investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) measurements were carried out for UV-irradiated 2,4,6-tri-tert-butyl phenol in the polycrystalline state. The radical produced in the crystal was detected by ESR and identified to be the corresponding phenoxyl radical, which is well characterized in the chemical oxidations in solutions. ENDOR and ESEEM spectra were unambiguously analyzed in terms of the hyperfine coupling constants determined from well-resolved ESR in solutions. Radical pairs in the crystals were also ascertained, and together with the single-crystal study the analysis disclosed zero-field splitting parameters in the triplet states. ESEEM time decays gave relaxation timesT ₁ = 5.94 andT ₂ = 1.12 μs at room temperature. These appropriate values permit an easy detection of the spin echoes, and therefore this radical matrix can be used as a useful standard for pulsed ESR investigations.