Stimulated Electron Spin Research Articles

X- and Q-Band Electron Spin Echo Study of Stochastic Molecular Librations of Spin Labels in Lipid Bilayers

Electron spin echo (ESE) of nitroxide spin labels allows detecting fast nanosecond stochastic restricted rotations (stochastic molecular librations), which is a common property of molecules in disordered media including biological systems. Under the typical experimental conditions, the anisotropic electron paramagnetic resonance (EPR) spectrum of a nitroxide is only partly excited by microwave pulses, which allows selecting an anisotropic contribution to the transverse spin relaxation by comparing echo decays at different spectral positions. On the other hand, for low-amplitude orientational motion, the excitation bandwidth is large enough to cover the range of spectral diffusion occurring during the echo formation. To verify that the two-pulse echo decay is indeed related to fast motions, the stimulated electron spin echo can be used. In addition, theory predicts an increase of the relaxation rates at higher microwave resonance frequency. To check this prediction, in the present work we performed a comparative study of ESE decays at microwave X- and Q-bands, for spin-labeled lipids in the gel phase of a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. A good agreement found between experimental data and computer simulation provides additional justification for the model of fast stochastic molecular librations.

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.

Fast stochastic librations and slow small-angle rotations of molecules in glasses observed on nitroxide spin probes by stimulated electron spin echo spectroscopy

Abstract Electron spin echo (ESE) spectroscopy as applied to nitroxide spin probes in glasses is sensitive to molecular motions of two types. The first type of motion is fast stochastic librations, with correlation times on the scale of nanoseconds. In this work, these librations were found in glassy glycerol above ∼190 K and in glassy o-terphenyl above ∼250 K (glass transition temperatures for these two solvents are Tg = 185 K and Tg = 243 K, respectively). Most likely, these librations are of the same origin as anharmonic atomic vibrations, as seen in glasses by neutron scattering above a so-called dynamical transition temperature. The second type of motion is slow millisecond inertial rotations, which are observed in the experimentally accessible microsecond time scale as developing within an angular range of ∼0.1–2°. Over the temperature range studied, the square of the characteristic rotation rate was found to be proportional to the dielectric α-relaxation rate that is known for glycerol and o-terphenyl from literature. This empirical fact probably means that small-angle inertial rotations of spin probes and dielectric α-relaxation in the solvent are interrelated phenomena.

Nuclear spin relaxation in free radicals as revealed in a stimulated electron spin echo experiment

Electron spin echo envelope modulation (ESEEM) in a three-pulse stimulated echo experiment, when the time interval between the first and second pulses τ is varied, is attributed to a spontaneous change of the electron spin Larmor frequency in the time intervalT between the second and third pulses, due to the longitudinal relaxation of nearby nuclei. It is observed for nitroxide radicals in glassy matrices in the temperature range of 130–240 K. Nuclear relaxation is assumed to arise from fluctuation of the proton hyperfine interaction, due to fast rotation of the methyl groups. This contribution to ESEEM and the conventional one that is induced by the simultaneous excitation of allowed and forbidden electron spin transitions were found to be multiplicative. As the latter does not depend on the timeT, both contributions can be easily separated. The rate of nuclear spin relaxation was determined, and correlation time of methyl group rotation was estimated by Redfield theory of spin relaxation. Arrhenius parameters of this motion were estimated on the basis of these data and those at 77 and 90 K, where the previously developed approach was used (L.V. Kulik, I.A. Grigor’ev, E.S. Salnikov, S.A. Dzuba, Yu.D. Tsvetkov, J. Phys. Chem. A 106, 12066–12071, 2003).

Restricted orientational motion of nitroxides in molecular glasses: Direct estimation of the motional time scale basing on the comparative study of primary and stimulated electron spin echo decays

A comparative study of anisotropic relaxation in two-pulse primary and three-pulse stimulated electron spin echo decays provides a direct way to distinguish fast (correlation time tau(c)<10(-6) s) and slow (tau(c)>10(-6) s) motions. Anisotropic relaxation is detected as a difference of the decay rates for different resonance field positions in anisotropic electron paramagnetic resonance spectra. For fast motion anisotropic relaxation influences the primary echo decay and does not influence the stimulated echo decay. For slow motion it is seen in both two-pulse echo and three-pulse stimulated echo decays. For nitroxide spin probes dissolved in glassy glycerol only fast motion was found below 200 K. Increase of temperature above 200 K results in the appearance of slow motion. Its amplitude increases rapidly with temperature increase. While in glycerol glass slow motion appears above glass transition temperature T(g), in ethanol glass it is observable below T(g). The scenario of motional dynamics in glasses is proposed which involves the broadening of the correlation time distribution with increasing temperature.

Electron Spin Echo Study of Molecular Structure and Dynamics: New Approaches Based on Spontaneous Fluctuations of Magnetic Interactions

Modulation phenomena that take place during electron spin echo signal decay have long been used in structural studies of free radicals and their environment. These phenomena are based on coherent dynamic effects, arising from simultaneous excitation (by microwave pulses) of two or more transitions in the EPR spectrum. Recently, a new source of stimulated electron spin echo (ESE) modulation was discovered due to spontaneous changes in the magnetic parameters of radicals during the operation of the pulse sequence. For monoradicals, these changes are caused by intramolecular motions. For radical pairs, additional mechanisms are longitudinal relaxation of spin counterparts and transformations of the paramagnetic partners during chemical reactions. Promising applications of this phenomenon to structural studies of radicals and radical pairs in solids and to investigations of their mobility and chemical transformations are considered.

Electron spin relaxation due to small-angle motion: Theory for the canonical orientations and application to hierarchic cage dynamics in ionomers

Analytical expressions for transverse electron spin relaxation induced by small angle motion were derived for the first time within an anisotropic model for rotational diffusion by using an approximation of the spin Hamiltonian and its variation during reorientation that is valid close to the canonical orientations. The dependence of the decay of the stimulated echo on such motion was studied by extensive Monte Carlo simulations and regimes were identified in which the time constant of this decay is related to parameters of the anisotropic diffusion model by simple equations. For testing these theoretical findings and obtaining insight into hierarchical cage dynamics in soft matter, high-field electron paramagnetic resonance (EPR) measurements were performed at a frequency of 94 GHz where the canonical orientations for nitroxide spin labels are well resolved. A combination of continuous wave EPR, saturation recovery measurements, and measurements of the decay of primary and stimulated electron spin echoes was employed to cover time scales from a few picoseconds up to several microseconds. Ionic spin probes attached by electrostatic interactions to the surface of ionic clusters in ionomers were used as a model system in which slow cage reorientation can be studied in the glass transition region of the polymer (0.64&amp;lt;T/Tg&amp;lt;1.05). Three hierarchical reorientation processes of the spin probe were observed on different time scales. The spin probe undergoes fast intramolecular libration on the time scale of a few picoseconds, it experiences a local rearrangement of the cage on the time scale of hundreds of nanoseconds and it performs cooperative reorientation coupled to the structural relaxation of the glassy matrix over time scales comparable to or longer than several microseconds in the glass transition region.

Out-of-Phase Stimulated Electron Spin−Echo Appearing in the Evolution of Spin-Correlated Photosynthetic Triplet-Radical Pairs

A strong out-of-phase stimulated electron spin-echo is observed for the Q A - radical in the spin-correlated triplet-radical pair 3 PQ A - in photosynthetic bacterial reaction centers. The formation of this echo is shown tobe induced by spin polarization of Q A - and by decay of the triplet state. The out-of-phase and in-phase echoes show deep envelope modulation induced by electron-electron dipole interaction between the partners in the pair. The analysis of this modulation provides dipolar frequencies. The interspin distance in 3 PQ A - is shown to be the same as in the radical pair P + Q A -. This new type of experiment appears to be widely applicable for the study of chemical reactions and intermolecular distances in solids.

Structure of the Jahn-Teller distorted Cr2+ defect in SrF2:Cr by electron-spin-echo envelope modulation.

Electron spin echoes have been obtained by two-pulse (\ensuremath{\pi}/2-\ensuremath{\tau}-\ensuremath{\pi}) and three-pulse (\ensuremath{\pi}/2-\ensuremath{\tau}-\ensuremath{\pi}/2-T-\ensuremath{\pi}/2) experiments in the |+1〉\ensuremath{\leftrightarrows}|-1〉 EPR resonance transitions of the Jahn-Teller orthorhombically distorted ${\mathrm{Cr}}^{2+}$ in ${\mathrm{SrF}}_{2}$:Cr. Particular emphasis is placed on the stimulated electron spin envelope modulation (ESEEM). Since this system has a spin S=2 and the zero-field-splitting contribution is of the same order of magnitude as the electronic Zeeman term, a new formalism for analysis of the ESEEM has been developed, allowing us to extend the Mims formalism for S=1/2 to more complex systems. Applying it to our results we have determined the superhyperfine (shf) interaction tensors of ${\mathrm{Cr}}^{2+}$ with the nearest-neighboring (NN), next-nearest-neighboring (NNN), and third-shell-neighboring fluoride ions, estimating the ${\mathrm{Cr}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ distances with the orthogonalized envelope function method. The defect model consists of four fluoride ions in the yz (110) defect plane which relax inwards to touch the ${\mathrm{Cr}}^{2+}$ ion. The remaining four fluoride ions relax outwards to fit into the next ${\mathrm{Sr}}^{2+}$ ions cage. \textcopyright{} 1996 The American Physical Society.

Investigation of silicon surfaces by optically stimulated electron spin resonance

Various kinds of as-chemically etched and heat-treated Si surfaces have been investigated by means of optically stimulated electron spin resonance (ESR) method. It is clarified that HNO 3 containing etchant is not necessary to obtain the photoinduced ESR lines. Their intensity is enhanced by the heat treatment in wet ambient in contrast to the results obtained by Caplan et al. It is found that the paramagnetic centers attributable to the photoinduced lines are created by the reaction of water or hydrogen with the very thin surface film on silicon substrates.

Dielectric effects in the optically stimulated electron spin resonance in silicon

The appearance of the photoinduced ESR lines is directly responsible for the dielectric effect caused by a change in spin-dependent photoconductivity at resonance point. This appearance mechanism proposed in the optically stimulated ESR method is quite different from that of an ordinary ESR absorption line based on the paramagnetic absorption loss. A simple calculation to justify this model is made and typical features of the photoinduced ESR lines are explained in connection with the appearance mechanism.

Stimulated electron spin echo of captured electrons in frozen aqueous solutions

Stimulated electron spin echo of captured electrons in frozen aqueous solutions

Modulation effects in the stimulated electron spin echo of free radicals

Modulation effects in the stimulated electron spin echo of free radicals