ESR and ENDOR of free radicals in X-irradiated barbituric acid

Free radicals from dihydrate and dideuterate crystals and from anhydrous powder of barbituric acid X-irradiated between 77 K and room temperature have been studied by ESR and ENDOR spectroscopy. Four different species were characterized. In dihydrate crystals, small amounts of molecular anions were found to be formed in addition to the major radical which results from elimination of a hydrogen from the C(₅)-methylene group. The α-hydrogen coupling in the latter as well as its weak intra- and intermolecular proton interactions were investigated in detail by ENDOR. The cation of barbituric acid was observed in dideuterate crystals between 77 K and 130 K. Isothermal decay curves indicated its transformation into the deuterated analog of the R₂Ċ(₅)-H fragment upon warming. The dose-yield relationship of dihydrate and anhydrous powder suggested a stabilization of this radical via interaction with the crystal water. Both the transformation and the stabilization mechanisms were supported by ENDOR-findings on weakly coupled protons.     

Hindered motions in a ketonea-urea inclusion compound. An ESR and ENDOR study

We have studied by ESR and ENDOR spectroscopy the free radicals produced in γ-irradiated inclusion compound formed between the ketone 10-nonadecanone and urea. Only one type of long lived radical is formed by the removal of an α-proton from the ketone. The hyperfine (hf) coupling constants of the α- and β-protons of the radicals have been measured by ESR at different temperatures in the range 110–292 K and at different orientations of the crystals. The hf coupling of the γ-protons of the radical and of the urea protons have been studied by ENDOR. The temperature and angular dependences of the coupling constants have been analyzed in terms of the internal and overall motions of the radical inside the hexagonal channels formed by the urea molecules. It has been found that the radical cannot perform complete reorientations around the long molecular axis, but it undergoes restricted rotational diffusion. This process is explained by assuming a coupling between the rotational and translational degrees of freedom of the radical inside the urea channels.     

EPR and ENDOR investigations of Sn impurities in CdS

Wurtzite type CdS single crystals with tin impurities have been reinvestigated by means of magnetic resonance. The by far strongest neighbour interaction with the rare isotope³³S is detectable by electron paramagnetic resonance (EPR), while nine weaker cadmium interactions can be resolved with the electron nuclear double resonance (ENDOR) technique. Three distinct shell symmetries are detected and can be explained by the wurtzite lattice symmetry. The parameters evaluated are interpreted in terms of the LCAO approach. An additional Sn-related spectrum, being not resolvable by ordinary EPR was identified by means of ENDOR-induced-EPR (EI-EPR) and turned out to be associated with Lithium.     

ESR and ENDOR studies of solitons and polarons in conjugated polymers

Nonlinear excitations such as solitons and polarons in conjugated polymers carry spins. In this case electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) provide unique methods to determine their wave functions. In this review article, the case of solitons in polyacetylene, CH_x, and polarons in an electroluminescent polymer, poly(paraphenylene vinylene) (PPV) are discussed as typical examples. High-resolution proton ENDOR spectra, obtained with stretch-oriented samples, yield the half extension of the excitations of 18 carbon atoms and 4 phenyl rings for CH_x and PPV, respectively. These extensions are well described by the theories in the case of finite electron correlation. In addition, light-induced ESR technique is shown to be useful in obtaining site-selective information of spin distribution in the case of PPV derivatives, as well as the excitation spectra of polarons.     

ESR study on intramolecular electron transfer rate of 1,3-dinitrobenzene radical anion in solution

The intramolecular electron transfer (ET) rates of the 1,3-dinitrobenzene radical anion in aprotic solvents were determined at various temperatures by means of ESR line-broadening method. The rates with negligible ion-ion interaction were determined in acetonitrile and N,N-dimethylformamide by addition of a cryptand. The obtained rates were 10 times or more compared with those in the presence of 0.1 mol dm⁻³ supporting electrolyte. Simple adiabatic ET model was used for the examination of solvent dependence of the ET rates.     

EPR and ENDOR investigations of Ge,Sn and Pb impurities in CdSe

Electron paramagnetic resonance (EPR) experiments of light activated Ge, Sn and Pb impurities in wurtzite type CdSe crystals are presented. These²S_1/2 centres exhibit a large hyperfine structure which shows a variation with temperature. 25 shells of neighbour nuclei are resolved for CdSeSn by means of the Electron nuclear double resonance (ENDOR) technique. All shells belong to one of three distinct shell symmetries of the lattice thus confirming a high symmetry defect. Additional Sn respectively Ge related spectra are described and interpreted as complex centres correlated with lithium.     

A contribution from exchange interaction to magnetization of a degenerate electron gas in a quantum cylinder

The exchange energy of an electron gas on a cylindrical surface in a constant magnetic field is calculated. Analytical formulas describing the contribution from exchange interaction to magnetization of a quantum cylinder are derived. It is shown that the magnetic response of the system undergoes the Aharonov-Bohm oscillations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 51–54, December, 2006.     

Solvent fluctuations and viscosity-dependent rates of solution reactions in a regime indescribable by the transition state theory

An organic molecule isomerizes in viscous solvents when appropriate cavities are formed around it in the course of slow diffusive thermal fluctuations of solvent molecules. The isomerization occurs when fast twisting (vibrational) fluctuations around a bond get to have large amplitudes in such cavities. This situation can be described by the two-reaction-coordinate model of Sumi and Marcus originally proposed for electron transfer reactions. In fact, the rate constant derived from this model fits nicely to that observed for thermal Z→E isomerization of substituted azobenzenes and N-benzylideneanilines. The rate constant is influenced by slow speeds of diffusive motions of solvent molecules, whose relaxation time τ is usually proportional to the solvent viscosity η. It has a form of k = 1/(k_TST⁻¹+k_f⁻¹), where k_TST, independent of τ, represents the rate constant expected from the transition state theory (TST), while k_f ∝ τ⁻ with 0 < ≤ 1 represents the part controlled by solvent fluctuations. An analytic expression of for the isomerization reactions is given in terms of physical parameters underlying the reaction mechanism with cavity formation.

This rate-constant formula is a general one applicable widely also to other solution reactions, covering from the TST-validated regime for a small τ to the TST-invalidated one for a large τ. In the former, k approaches k_TST since k_f k_TST, while in the latter, k approaches k_f since k_f k_TST, becoming decreasing with a decrease in the typical speed (∝ τ⁻¹) of solvent fluctuations. The dependence of k ≈ k_f ∝ η⁻ in the non-TST regime has often been observed also in biological reactions such as enzymatic ones. In this case, it is not appropriate to say that reactions are controlled by slow speeds of solvent fluctuations, but we should rather say that enzymes utilize this situation, which has been called conformational gating, in the course of solvent-fluctuation-driven conformational fluctuations of proteins. It has important meanings in protein functions.     

On the validity of marcus relationship for dissociative electron transfer reactions in solution

In this paper Monte Carlo simulations of a dissociative electron transfer reaction in solution have been performed in order to test to what extent the deviation of Marcus' relationship for this kind of reactions can be attributed to the features of the solute internal potential energy. It is shown that Marcus' equation does not apply to this kind of dissociative electron transfer processes if the ab initio solute internal potential energies are used. Conversely, if a Morse curve and its repulsive part are employed to describe the solute internal potential energies, Marcus' relationship is recovered. Then, the problem is reduced to what extent the ab initio solute internal potential energies can be represented by Morse curves.     

Dynamical exchange effects in the dielectric function of the two-dimensional electron gas

Dynamical exchange interactions can be introduced in the dielectric function via a dynamic local field factor. We study the effects of this inclusion on both the static and the frequency dependent dielectric function of a two-dimensional electron gas, using the dynamic local field factor that we derived recently via the dynamical exchange decoupling method. The results are compared with the dielectric function in the Random Phase Approximation and with different dynamic and static approximations of the local field factor.Received: 7 May 2003, Published online: 22 September 2003PACS: 71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons - 77.22.Ch Permittivity (dielectric function) - 77.55.+f Dielectric thin filmsJ.T. Devreese: , jtd@uia.ua.ac.be     

Localization and correlation effects on the magnetoresistance of a degenerate electron gas in InAs below the quantum limit

Summary Measurements of the resistivity tensor of a range of moderately dopedn-InAs samples were carried out at low temperatures in the ionized impurity scattering regime. Analyses were performed while the electron gas was in a degenerate state and below the quantum limit. The temperature and the magnetic-field dependences of the two components of the resistivity tensor, ϱ_‖ (longitudinal) and ϱ_⊥ (transverse), were discussed in terms of localization and correlation effects.     

A self-consistent theory for exchange and correlation effects in electron gas

A theory of electron correlations giving self-consistently the compressibility and spin susceptibility is formulated within the generalized random-phase approximation.     

ESR and ENDOR experiments using a disc-shaped resonator working in the Whispering Gallery Mode (WGM)

Different instrumental apparatuses are presented employing disc-shaped dielectric resonators working in the Whispering Gallery Mode (WGM). The conditions suitable to take the best from the microwave power obtained from a trade klystron in the X-band have been verified for the ESR part of the experiments. It is well known that a good ESR saturation condition is an essential prerequisite to have satisfactory ENDOR results. The WGM dielectric resonators seem to be very suitable for multiple irradiation purposes and in particular for ENDOR experiments which is the aspect we wanted to exploit here. Various radiofrequency ENDOR irradiation circuits have been tested and the obtained spectra of a γ-irradiated single crystal are presented and criticized. Some results for very high frequency ESR applications are also reported.     

Fermion number exchange processes and dynamical supersymmetry effects in the interaction of fermionic zero modes with matter

The influence of fermionic zero modes and fractional charges caused by a topological soliton on the dynamics of the system is considered.     

Single-particle spectrum of the degenerate electron gas IV. Ground state energy

Results for the ground state energy of the electron gas calculated from a spectrum containing plasmaron structure compare well with values from the dielectric formulation.     

Dispersion properties of electron acoustic waves in degenerate and non‐degenerate quantum plasmas

The longitudinal response functions are used to generalize the dispersion properties of electron acoustic waves (EAWs) in the presence of quantum recoil, for isotropic, non‐relativistic, degenerate/non‐degenerate plasmas. In order to study the EAWs, the constituents of non‐degenerate (thermal) plasma are considered to be of two groups of electrons having different number density and temperature, namely the cold electrons and the hot electrons. Similarly in degenerate (Fermi) plasma the two population of electrons are considered to be the thinly populated and the thickly populated electrons. The sparsely populated electrons are termed as cold electrons while the densely populated ones are termed as hot electrons. The ions are stationary which form the neutralizing background. The absorption coefficients for Landau damping with the inclusion of the quantum recoil in both plasmas are calculated and discussed. The results are discussed in the context of laser‐produced plasma.     

Polarization and exchange effects in elastic scattering of electron with atoms and ions

Laser-induced electron diffraction(LIED), in which elastic scattering of the returning electron with the parent ion takes place, has been used to extract atomic potential and image molecular structures with sub-?A precision and exposure time of a few femtoseconds. So far, the polarization and exchange effects have not been taken into account in the theoretical calculation of differential cross section(DCS) for the laser-induced rescattering processes. However, the validity of this theoretical treatment has never been verified. In this work, we investigate the polarization and exchange effects on electron impact elastic scattering with rare gas atoms and ions. It is found that, while the exchange effect generally plays a more important role than the polarization effect in the elastic scattering process, the exchange effect is less important on electron–ion collisions than on electron–atom collisions, especially for scattering in backward direction. In addition, our calculations show that, for electron–atom collisions at incident energies above 50 e V, both the polarization and exchange effects can be safely neglected, while for electron–ion collisions, both the polarization and exchange potentials do not make substantial contributions to the DCS at incident energies above 20 e V and scattering angles larger than 90?. Our investigation confirms the validity of the current LIED method.     

ESR investigation on the breather mode and the spinon-breather dynamical crossover in Cu benzoate

A "breather excitation" is observed directly by electron spin resonance in the quantum spin chain Cu benzoate, in which an unexpected field-induced gap has recently been found. The nonlinear field dependence of the resonance field agrees well with the formula based on a quantum sine-Gordon model. The power-law temperature dependence of the linewidth is observed in the gapless spinon regime while the width decreases exponentially for the gapped breather regime. In the intermediate range, a distinct anomaly is found, which is the manifestation of "the spinon-breather dynamical crossover."