Measurement of the asymmetry in the emission of bremsstrahlung by transversely polarized electrons

Bremsstrahlung emitted by transversely polarized electrons shows a “right-left” asymmetry in spatial distribution. Measurements and remeasurements of this asymmetry are presented using electrons of 300 and 128 keV, respectively, impinging on a gold target. Within the accuracy of the experiment there are no significant discrepancies to the partial wave calculations of Tseng and Pratt.     

Resonance structures in polarization bremsstrahlung from electron–atom collisions

Generalized results of the experimental and theoretical investigations on the resonance structure of polarization bremsstrahlung in the collisions of high-energy (nonrelativistic) and intermediate-energy electrons with atoms have been presented. The features of the gas-jet method of generating electromagnetic radiation used in the experimental investigation of polarization bremsstrahlung are considered. The features and regularities of the resonance structure in the differential spectra of X-ray bremsstrahlung have been analyzed as a function of the incoming electron energy.     

Nuclear spin polarization in radical reactions

An extension of the Closs-Kaptein-Oosterhoff theory concerning nuclear spin polarization resulting during free radical reactions is presented. This extension is based on the Merrifield model for the magnetic field dependence of triplet-triplet annihilation.     

Electron spin polarization in the photoexcited triplet state of porphyrins

Electron spin polarization in the photoexcited triplet state of tetraphenyl porphyrin was detected at 100°K using EPR technique. The zero field splitting parameters |D| and |E| the free base porphyrin were found to be 0.0369 ± 0.0005 and 0.0082 ± 0.0005 cm^?1, respectively.     

Electron spin polarization of the excited quartet state of strongly coupled triplet-doublet spin systems

The electron spin polarization associated with electronic relaxation in molecules with trip-quartet and trip-doublet excited states is calculated. Such molecules typically relax to the lowest trip-quartet state via intersystem crossing from the trip doublet, and it is shown that when spin-orbit coupling provides the main mechanism for this relaxation pathway it leads to spin polarization of the trip quartet. Analytical expressions for this polarization are derived using first- and second-order perturbation theory and are used to calculate powder spectra for typical sets of magnetic parameters. It is shown that both net and multiplet contributions to the polarization occur and that these can be separated in the spectrum as a result of the different orientation dependences of the +/-1/2<-->+/-3/2 and +1/2<-->-1/2 transitions. The net polarization is found to be localized primarily in the center of the spectrum, while the multiplet contribution dominates in the outer wings. Despite the fact that the multiplet polarization is much stronger than the net polarization for individual orientations of the spin system, the difference in orientation dependence of the transitions leads to comparable amplitudes for the two contributions in the powder spectrum. The influence of this difference on the line shape is investigated in simulations of partially ordered samples. Because the initial nonpolarized state of the spin system is not conserved for the proposed mechanism, the net polarization can survive in the doublet ground state following electronic relaxation of the triplet part of the system.     

The mechanism of spin polarization in aromatic free radicals

 The spin-polarization mechanism in aromatic systems is analyzed with reference to the prototypical phenoxyl, cyclohexadienyl and benzyl radicals. In particular, a decomposition into “first-order” and “second-order” contributions is proposed, which helps to rationalize the different nature of the spin density for atoms in α or in β positions with respect to the radical center. The different weights of the two contributions are discussed on the basis of Hartree–Fock and density functional computations. Received: 17 September 1999 / Accepted: 3 February 2000 / Published online: 29 June 2000     

Electron spin polarization in a three-electron spin system. An application to bacterial photosynthesis

Calculations on a system consisting of three electron spins and one nuclear spin are presented and their implications for bacterial photosynthesis discussed. Comparison with experimental measurements of electron spin polarization in pre-reduced photosynthetic reaction centres leads to conclusion that the exchange interaction within the primary radical pair is positive and less than 0.8 mT when the g values of the photoinduced radicals are taken to be those measured for the isolated radical species.     

Many-electron self-interaction and spin polarization errors in local hybrid density functionals

Errors for systems with noninteger occupation have been connected to common failures of density functionals. Previously, global hybrids and pure density functionals have been investigated for systems with noninteger charge and noninteger spin state. Local hybrids have not been investigated for either of those systems to the best of our knowledge. This study intends to close this gap. We investigate systems with noninteger charge to assess the many-electron self-interaction error and systems with noninteger spin state to assess the spin polarization error of recently proposed local hybrids and their range-separated variants. We find that long-range correction is very important to correct for many-electron self-interaction error in cations, whereas most full-range local hybrids seem to be sufficient for anions, where long-range-corrected density functionals tend to overcorrect. On the other hand, while all hitherto proposed long-range-corrected density functionals show large spin polarization errors, the Perdew-Staroverov-Tao-Scuseria (PSTS) functional performs best of all local hybrids in this case and shows an outstanding behavior for the dependence of the energy on the spin polarization.     

Insights on spin polarization through the spin density source function

Understanding how spin information is transmitted from paramagnetic to non-magnetic centers is crucial in advanced materials research and calls for novel interpretive tools. Herein, we show that the spin density at a point may be seen as determined by a local source function for such density, operating at all other points of space. Integration of the local source over Bader''s quantum atoms measures their contribution in determining the spin polarization at any system''s location. Each contribution may be then conveniently decomposed in a magnetic term due to the magnetic natural orbital(s) density and in a reaction or relaxation term due to the remaining orbitals density. A simple test case, ³B₁ water, is chosen to exemplify whether an atom or group of atoms concur or oppose the paramagnetic center in determining a given local spin polarization. Discriminating magnetic from reaction or relaxation contributions to such behaviour strongly enhances chemical insight, though care needs to be paid to the large sensitivity of the latter contributions to the level of the computational approach and to the difficulty of singling out the magnetic orbitals in the case of highly correlated systems. Comparison of source function atomic contributions to the spin density with those reconstructing the electron density at a system''s position, enlightens how the mechanisms which determine the two densities may in general differ and how diverse may be the role played by each system''s atom in determining each of the two densities. These mechanisms reflect the quite diverse portraits of the electron density and electron spin density Laplacians, hence the different local concentration/dilution of the total and (α–β) electron densities throughout the system. Being defined in terms of an observable, the source function for the spin density is also potentially amenable to experimental determination, as customarily performed for its electron density analogue.     

Electron and nuclear spin dynamics in the thermal mixing model of dynamic nuclear polarization

A novel mathematical treatment is proposed for computing the time evolution of dynamic nuclear polarization processes in the low temperature thermal mixing regime. Without assuming any a priori analytical form for the electron polarization, our approach provides a quantitative picture of the steady state that agrees with the well known Borghini prediction based on thermodynamic arguments, as long as the electrons-nuclei transition rates are fast compared to the other relevant time scales. Substantially different final polarization levels are achieved instead when the latter assumption is relaxed in the presence of a nuclear leakage term, even though very weak, suggesting a possible explanation for the deviation between the measured steady state polarizations and the Borghini prediction. The proposed methodology also allows us to calculate nuclear polarization and relaxation times, once the electrons/nuclei concentration ratio and the typical rates of the microscopic processes involving the two spin species are specified. Numerical results are shown to account for the manifold dynamic behaviours of typical DNP samples.     

On the pressure dependence of the muon spin polarization in mixtures of noble gases

In muon spin rotation (μSR) experiments, where spin-polarized positive muons are stopped in condensed matter, three magnetically distinguishable chemical environments can be observed. That is, the Larmor frequencies associated with diamagnetic environments and two types of paramagnetic environments (muonium and radicals) can be resolved. The chemical identities of the latter two are distinct since their Larmor frequencies are distinct, whereas the chemical identities of the possible diamagnetic species are not determined by the Larmor frequency since chemical shifts can not be resolved in μSR experiments. However, two different diamagnetic species have been observed in experiments performed on mixtures of noble gases. Their distinction arises through different thermal rate constants that lead to “fast” and “slow” relaxing components of the diamagnetic signal. The pressure dependencies of the amplitudes associated with these components are related to the stopping dynamics of muons in noble gas targets. A set of coupled rate equations for muon spin dynamics, based upon quantal Boltzmann equations, have been developed to describe this process in single component gases. This theory is now extended to mixtures. In particular, the dynamics of the muon spin is generated by the muonium hyperfine interaction and by time dependent rate constants for the various chemical species that are assumed to be present, namely, muonium and three diamagnetic species. Radicals have not yet been observed in low pressure gases. The coupled quantal rate equations are solved for two models of the stopping dynamics wherein the rates are taken as square box functions of time.     

Intuitive local picture for spin polarization of chemical bonds

The spin polarization of chemical bonds near radical centers is investigated by an analytical spin unrestricted Hartree–Fock model with numerical examples. The centroid analysis of localized molecular orbitals is also introduced to obtain an intuitive local picture for the spin polarization. The alternation of spin alignments in molecules are discussed with orbital symmetries and Hund's rule through chemical bonds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Electron spin polarization in photosynthetic bacteria. Anisotropic chemical reactivity

It is shown that electron spin polarization can be used to probe the anisotropy of singlet-triplet interconversion of radical pairs involved in photosynthetic charge separation. Anisotropic polarization may be observed with non-oriented reaction centres, provided an anisotropic interaction (e.g. zero-field splitting or g-tensor anisotropy) produces resolvable structure in the EPR spectrum of the reaction intermediates. Two examples, both for prereduced bacterial reaction centres, are discussed: (i) the triplet state of the primary donor (a bactenochlorophyll dimer) and (ii) the reduced secondary acceptor (a semiquinone). Computer simulations are used to understand the observed behaviour and yield information on the magnetic and electronic interactions involved in electron transport.     

Ferromagnetic intermolecular interaction in the galvinoxyl radical: Cooperation of spin polarization and charge-transfer interaction

The orbital energies and the intermolecular overlap integrals of the galvinoxyl radical are calculated. It is found that the ferromagnetic intermolecular interaction in galvinoxyl can be qualitatively interpreted by a combined effect of spin polarization within the radical and charge-transfer interaction between the radicals.     

Electronic structure, chemical bonding, and spin polarization in ferromagnetic MnAl

The electronic structure of ferromagnetic τ-MnAl has been calculated using density-functional techniques (TB-LMTO-ASA, FLAPW) and quantum-chemically analyzed by means of the crystal orbital Hamilton population tool. While all observable quantities are in good agreement with experiment, the tetragonal structure of ferromagnetic MnAl is interpreted to arise from a nonmagnetic cubic structure by two subsequent steps, namely (a) an electronic distortion due to spin polarization followed by (b) a structural distortion into the tetragonal system. The various strengths of interatomic bonding have been calculated in order to elucidate the competition between electronic and structural distortion.