Electron Spin Relaxation Rate Research Articles

Spin relaxation inGaAs/AlxGa1−xAsquantum wells

Spin dynamics of photoexcited carriers in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells have been investigated in a wafer containing twelve different single quantum wells, allowing full investigation of well-width and temperature dependences with minimal accidental variations due to growth conditions. The behavior at low temperatures is dominated by excitonic effects, confirming results described in detail by others. Between 50 and 90 K there is a transition from excitonic to free-carrier-dominated behavior; both the temperature and time scale of the transition are in excellent agreement with a theoretical model for exciton dissociation. Above 90 K we find two-component spin decays consisting of an unresolved component (faster than 2 ps) associated with exciton dissociation and hole spin-relaxation and a longer-lived component that yields the electron spin-relaxation time. In the free-carrier regime, the electron spin-relaxation rate in wide wells follows that for bulk GaAs, which varies approximately as ${T}^{2}.$ For narrow wells the rate is approximately independent of temperature and varies quadratically with confinement energy. This behavior is consistent with dominance of the D'yakonov-Perel mechanism of electron-spin relaxation and the expected behavior of the electron mobility. The data show evidence of the influence of electron scattering by interface roughness.

The Accessibility of Iron at the Active Site of Recombinant Human Phenylalanine Hydroxylase to Water As Studied by 1H NMR Paramagnetic Relaxation: EFFECT OF l-PHE AND COMPARISON WITH THE RAT ENZYME

The high-spin (S = 5/2) Fe(III) ion at the active site of recombinant human phenylalanine hydroxylase (PAH) has a paramagnetic effect on the longitudinal relaxation rate of water protons. This effect is proportional to the concentration of enzyme, with a paramagnetic molar-relaxivity value at 400 MHz and 25 degrees C of 1. 3 (+/- 0.03) x 10(3) s-1 M-1. The value of the Arrhenius activation energy (Ea) for the relaxation rate was -14.4 +/- 1.1 kJ/mol for the resting enzyme, indicating a fast exchange of water protons in the paramagnetic environment. The frequency dependence of the relaxation rate also supported this hypothesis. Thus, the recombinant human PAH appears to have a more solvent-accessible catalytic iron than the rat enzyme, in which the water coordinated to the metal is slowly exchanging with the solvent. These findings may be related to the level of basal activity before activation for these enzymes, which is higher for human than for rat PAH. In the presence of saturating (5 mM) concentrations of the substrate L-Phe, the paramagnetic molar relaxivity for human PAH decreased to 0.72 (+/- 0.05) x 10(3) s-1 M-1 with no significant change in the Ea. Effective correlation times (tauC) of 1.8 (+/- 0.3) x 10(-10) and 1.25 (+/- 0.2) x 10(-10) s-1 were calculated for the enzyme and the enzyme-substrate complex, respectively, and most likely represent the electron spin relaxation rate (tauS) for Fe(III) in each case. Together with the paramagnetic molar-relaxivity values, the tauC values were used to estimate Fe(III)-water distances. It seems that at least one of the three water molecules coordinated to the iron in the resting rat and human enzymes is displaced from coordination on the binding of L-Phe at the active site.

EPR and DNP Properties of Certain Novel Single Electron Contrast Agents Intended for Oximetric Imaging

Parameters of relevance to oximetry with Overhauser magnetic resonance imaging (OMRI) have been measured for three single electron contrast agents of the triphenylmethyl type. The single electron contrast agents are stable and water soluble. Magnetic resonance properties of the agents have been examined with electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and dynamic nuclear polarization (DNP) at 9.5 mT in water, isotonic saline, plasma, and blood at 23 and 37°C. The relaxivities of the agents are about 0.2–0.4 mM−1s−1and the DNP enhancements extrapolate close to the dipolar limit. The agents have a single, narrow EPR line, which is analyzed as a Voigt function. The linewidth is measured as a function of the agent concentration and the oxygen concentration. The concentration broadenings are about 1–3 μT/mM and the Lorentzian linewidths at infinite dilution are less than 1 μT in water at room temperature. The longitudinal electron spin relaxation rate is calculated from the DNP enhancement curves. The oxygen broadening in water is about 50 μT/mM O2at 37°C. These agents have good properties for oximetry with OMRI.

Exciton luminescence polarization decay in type II semiconductor heterostructures

In type II semiconductor heterostructures, one can reduce the electron–hole overlap, without changing much the oscillator strength, so that the space indirect exciton luminescence polarization will decay according to the exciton-bound single-particle spin flip. The exciton spin relaxation rate is then limited by the slower single-particle spin-flip rate which is typically the electron one. We discuss the microscopic theory of the exciton-bound electron spin relaxation driven by the spin–orbit k e 3 splitting in the conduction band. The exciton-bound electron spin relaxation rate ( W e) is obtained as a function of the material and structure parameters. The exciton luminescence polarization decay rate in AlSb/GaSb/AlSb/InAs/AlSb heterostructures, where space indirect excitons can be created and controled, is estimated. The InAs electron well width L e dependence is also discussed.

Electron Spin Relaxation Rates for High-Spin Fe(III) in Iron Transferrin Carbonate and Iron Transferrin Oxalate

To optimize simulations of CW EPR spectra for high-spin Fe(III) with zero-field splitting comparable to the EPR quantum, information is needed on the factors that contribute to the line shapes and line widths. Continuous wave electron paramagnetic resonance (EPR) spectra obtained for iron transferrin carbonate from 4 to 150 K and for iron transferrin oxalate from 4 to 100 K did not exhibit significant temperature dependence of the line shape, which suggested that the line shapes were not relaxation determined. To obtain direct information concerning the electron spin relaxation rates, electron spin echo and inversion recovery EPR were used to measure T(1) and T(m) for the high-spin Fe(III) in iron transferrin carbonate and iron transferrin oxalate between 5 and 20-30 K. For comparison with the data for the transferrin complexes, relaxation times were obtained for tris(oxalato)ferrate(III). The relaxation rates are similar for the three complexes and do not exhibit a strong dependence on position in the spectrum. Extrapolation of the observed temperature dependence of the relaxation rates to higher temperatures gives values consistent with the conclusion that the CW line shapes are not relaxation determined up to 150 K.

Interspin distances determined by time domain EPR of spin-labeled high-spin methemoglobin

Continuous wave (CW) EPR spectra of the nitroxyl signal in high-spin fluoro- and aquo-methemoglobin spin-labeled at the β-93 cysteines exhibited line broadening as the temperature was reduced from 20 to 5 K. In the temperature interval where the CW spectra exhibited broadening, electron spin echo data showed a dramatic increase in the phase memory relaxation rate, 1/ T m, for the nitroxyl. The lineshape changes and changes in 1/ T m are attributed to iron—nitroxyl interaction in the regime where the iron relaxation rate in s −1 is of the same order of magnitude as the iron—nitroxyl spin—spin splitting expressed in s −1. Saturation recovery data for the nitroxyl signal in spin-labeled fluoro- and aquo-methemoglobin between 15 and 150 K demonstrated that interaction with the high-spin iron(III) enhanced the electron spin relaxation rate of the spin label. Electron spin relaxation rates for the m s = ± 1/2 transitions of the high-spin iron(III) were determined by electron spin echo and inversion recovery at 4.2–6 K and by analysis of the temperature-dependent contribution to the iron CW linewidths between about 20 and 150 K. The saturation recovery data for the nitroxyl in the spin-labeled methemoglobins were analyzed to determine the interspin distances, using a modified version of the Bloembergen equation and experimentally determined iron relaxation rates. Interspin distances in the high-spin methemoglobins (15.8–17.5 Å) are systematically longer than in the low-spin analogs (15–15.5 Å).

A low-field paramagnetic nuclear spin relaxation theory

A low-field theory of paramagnetically enhanced proton spin-lattice relaxation is developed resulting in closed analytical expressions. The theory describes paramagnetically enhanced water proton spin-lattice relaxation in complexes of transition metal ions with electron spin quantum number S=1. In the low-field regime the electron spin system is dominated by a static or permanent zero-field splitting interaction which is much larger than the Zeeman interaction. The electron spin is thus quantized in the molecular fixed principal frame of the static ZFS-interaction rather than in the laboratory fixed frame. In the molecular fixed frame the zero-field splitting Hamiltonian is characterized by an axial parameter (D) and a rhombic parameter (E). It is shown how the relative magnitude of D and E strongly influence the magnitude of the paramagnetically enhanced proton spin lattice relaxation rate. In describing electron spin relaxation in the molecular fixed frame, we consider a transient ZFS-interaction and a reorientation modulated Zeeman interaction as the two main relaxation mechanisms. Then, using Redfield theory we derive expressions for the relevant electron spin relaxation rates in terms of spectral densities. In contrast to the Zeeman or high field regime, the electron spin system in the low field limit does not have a permanent magnetic moment. The lack of this magnetic moment implies that electron spin-lattice relaxation processes do not influence the paramagnetically enhanced proton spin-lattice relaxation rates. Instead three spin–spin relaxation rates determine the enhanced proton spin-lattice relaxation profile. We express the electron spin–spin relaxation rates in terms of spectral densities determined by the mean square value of the transient zero-field splitting interaction, its characteristic correlation time, and the reorientational correlation time of rank 1.

Determination of High-Spin Iron(III)–Nitroxyl Distances in Spin-Labeled Porphyrins by Time-Domain EPR

Continuous wave EPR spectra of the nitroxyl signals for four spin-labeled high-spin (h.s.) Fe(III) porphyrins showed partially resolved splittings at temperatures near 4 K. Axial ligands were fluoride, chloride, or bromide. As temperature was increased to 20 to 30 K the iron–nitroxyl splitting collapsed due to increasing rates of iron relaxation. Electron spin-echo (ESE) spectroscopy showed that above about 6 K collapse of the iron–nitroxyl spin–spin splitting caused a dramatic increase in the nitroxyl phase memory relaxation rates. Electron spin relaxation rates were determined for Fe(tetratolylporphyrin)X,X= F, Cl, Br, in toluene solution by ESE or inversion recovery at 4.5 to 6 K and by analysis of the temperature-dependent contributions to the continuous wave EPR linewidths between about 10 and 120 K. Above about 10 K iron relaxation rates increase in the orderX= F < Cl < Br, which is the order of increasing zero-field splitting. Saturation recovery data for two spin-labeled h.s. iron(III) porphyrins between about 15 and 120 K and for two additional spin-labeled h.s. iron(III) porphyrins between about 85 and 120 K demonstrated that interaction with the h.s. iron enhanced the electron spin relaxation rate of the spin label. The saturation recovery curves for the nitroxyl were analyzed to determine interspin distances using a modified version of the Bloembergen equation and independently determined iron relaxation rates. Interspin distances were between 11.6 and 15.0 Å, were independent of axial ligand, and were in good agreement with values obtained previously for low-spin Fe(III) and Cu(II) analogs.

Ultrafast spin dynamics in diluted magnetic semiconductor quantum wells

We investigated the well-width dependence of the carrier spin relaxation in CdTe/Cd 1− x Mn x Te quantum wells. We found that the electron-spin relaxation time decreases dramatically with decreasing well width, while the heavy-hole spin relaxation is not changed so much with the well width. The electron-spin relaxation rate is fairly proportional to the overlap between the electron wave function and the Mn ions in the magnetic barrier layer. This suggests that the electron spin scattering is caused by the interaction with Mn 2+ ions. From the analysis of the spin-dependent spectral change, it was found that the peak shift reflects the population dynamics more directly.

Polarization properties of optically pumped VCSELs in a transverse magnetic field

It is shown that the polarization ellipticity of light emitted by a VCSEL, which is optically pumped with circularly polarized light, depends significantly on the strength of the transverse magnetic field. This dependence can be used for a precise determination of the electron spin relaxation rate under lasing conditions at room temperature.

Enhancement of electron spin relaxation rates of metalloporphyrins due to interaction with a faster relaxing metal bound to an appended bipyridyl

Complexes of 5,10,15-tri- p-tolyl 20-4-(4-methyl) bipyridyl) porphyrin were prepared with Zn(II), Cu(II) or vanadyl coordinated to the porphyrin and bis(hexafluoroacetylacetonato)copper(II), Cu(hfac) 2, or bis(ethylxanthato)nickel(II), Ni(ex) 2, bound to the bipyridyl. Continuous wave EPR, electron spin echo and saturation recovery measurements were performed to determine the effect of metal-metal interaction on the EPR lineshapes between 6 K and room temperature and on the electron spin relaxation rates between about 10 and 120 K. Metal-metal exchange was larger for the Cu(II) porphyrins than for the vanadyl porphyrins. For the Cu(hfac) 2 adducts the spin-spin interaction caused the relaxation rate for the slower relaxing Cu(II) or vanadyl in the porphyrin to become approximately equal to that for the faster relaxing Cu(hfac) 2 moiety. The zero-field splitting for the Ni(II) complex was too large to permit detection of Ni(II) EPR signals at X-band, but the effect of the Ni(II) on the slower relaxing Cu(II) or vanadyl provided information on the relaxation rates for the Ni(II). Although the electron spin relaxation rates for the Ni(II) were orders of magnitude faster than for the Cu(II) in the Cu(hfac) 2 moiety, this did not result in proportionately larger effects of the Ni(II) on the relaxation rates for the slower relaxing centers because of the large separation between the Ni(II) and Cu(II) or vanadyl transition energies.

Exciton spin dynamics in GaAs quantum wells

We have investigated the temperature dependence of the exciton spin relaxation between 10 and 300 K in undoped GaAs quantum wells by using the nonlinear transmission pump-probe and the four-wave-mixing techniques. The electron spin relaxation rate above 40 K is found to be proportional to the product of the temperature and the momentum relaxation time, which indicates that the D'yakonov-Perel' interaction governs the spin-flip dynamics. The temperature independence below 30 K is considered to be due to the band-mixing effect that determines the hole spin relaxation, and the spin exchange is effective to the exciton spin relaxation in these temperatures.

Hydrogenase from the thermophilic bacterium Thermococcus stetteri: isolation and characterisation of EPR-detectable redox centres

The hydrogenase of the extremely thermophilic archaebacterium Thermococcus stetteri has been isolated. The procedure involved four steps, the last stage being preparative electrophoresis, with an overall yield of 40%. The molecular mass was estimated to be approx. 106 kDa. Analysis for metal content found iron and nickel in the ratio 13:1. Flavin was also detected in the enzyme and identified as FMN. The hydrogenase activity was relatively insensitive to inhibition by carbon monoxide, Ki 67 μM. This behaviour is indicative of a nickel-iron type hydrogenase. Electron paramagnetic resonance spectroscopy of the enzyme showed signals characteristic of iron-sulfur clusters. The spectra of the hydrogen-reduced T. stetteri hydrogenase, recorded at 70 K, indicated the presence of a single type of [2Fe-2S]cluster. Below 10 K, the spectra were complex, and indicated the presence of other iron-sulfur clusters with extremely fast electron-spin relaxation rates, interpreted as [4Fe-4S] clusters, and minor amounts of [3Fe-4S]clusters. These properties indicate that T. stetteri hydrogenase is related to the hydrogenases of Pyrococcus furiosus, a hyperthermophilic archaebacterium, and Alcaligenes eutrophus.

Hydrogenase from the thermophilic bacteriumThermococcus stetteri: isolation and characterisation of EPR-detectable redox centres

Abstract The hydrogenase of the extremely thermophilic archaebacterium Thermococcus stetteri has been isolated. The procedure involved four steps, the last stage being preparative electrophoresis, with an overall yield of 40%. The molecular mass was estimated to be approx. 106 kDa. Analysis for metal content found iron and nickel in the ratio 13:1. Flavin was also detected in the enzyme and identified as FMN. The hydrogenase activity was relatively insensitive to inhibition by carbon monoxide, K i 67 μM. This behaviour is indicative of a nickel-iron type hydrogenase. Electron paramagnetic resonance spectroscopy of the enzyme showed signals characteristic of iron-sulfur clusters. The spectra of the hydrogen-reduced T. stetteri hydrogenase, recorded at 70 K, indicated the presence of a single type of [2Fe-2S]cluster. Below 10 K, the spectra were complex, and indicated the presence of other iron-sulfur clusters with extremely fast electron-spin relaxation rates, interpreted as [4Fe-4S] clusters, and minor amounts of [3Fe-4S]clusters. These properties indicate that T. stetteri hydrogenase is related to the hydrogenases of Pyrococcus furiosus , a hyperthermophilic archaebacterium, and Alcaligenes eutrophus .

Ionic Association and Electron Spin Relaxation Rates in Aquo Gadolinium(III) Complexes

The electron paramagnetic resonance linewidth of aquo gadolinium(III) ion changes with the counter-ion identity and concentration in aqueous solutions. The EPR linewidth of 2 mMgadolinium(III) chloride increases from 49.2 to 89.0 mT when carbonate ion is added and decreases to 17.3 mT when nitrite ion is added. These observations suggest association reactions between aquo gadolinium(III) ion and anions that change the electron spin relaxation rates of the aquo ion. The concentration dependence of the gadolinium(III) EPR linewidth is consistent with binding constants for nitrite and nitrate ion with the aquo gadolinium(III) ion of 37 ± 6 and 2.3 ± 0.3 L mol−1respectively. The decreases in the EPR linewidth factors with these association reactions are difficult to understand unless the anion reactions increase the symmetry of the metal center. Although first-coordination reactions may not be ruled out, the decrease in EPR linewidth is more consistent with an outer-sphere association reaction that also reduces the coordination number of the metal center from 9 to 8.

Characterization of the motion of spin probes and spin labels in amorphous polymers with two-dimensional field-step ELDOR

The motion of nitroxide spin probes and spin labels in amorphous polymers is studied below the glass transition temperature with a two-dimensional pulsed electron double-resonance experiment. Polystyrene and a liquid crystalline side group polymer are studied using both spin probes and spin labels covalently bound to specific sites along the polymer chain. Two methyl acrylic polymers differing only in their side group structure and polyvinylacetate are compared and large differences in the molecular dynamics deduced from both the nuclear and the electron spin relaxation rates are observed as the glass transition is approached. The results demonstrate the complexity of small amplitude motion in simple polymers below the glass transition temperature and show that it is very sensitive to the packing in the polymer. © 1996 John Wiley & Sons, Inc.

Magnetic-field dependence of exciton spin relaxation in GaAs/AlxGa1-xAs quantum wells.

The magnetic-field dependence of spin relaxation of heavy-hole excitons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells is investigated at low temperatures. The variation of circular polarization of cw photoluminescence is modeled using steady-state solutions of rate equations describing the population dynamics of the exciton spin levels. Transitions between the levels are assumed to proceed via one-phonon ``direct'' processes, as well as zero-phonon processes when the levels are degenerate. Estimates of the exciton, hole, and electron spin-relaxation rates in units of the exciton population decay rate are obtained from least-squares fitting. We conclude that the exchange-driven exciton spin relaxation, involving simultaneous electron and hole spin flip, which dominates over hole or electron spin flips in zero field, is greatly reduced by a small applied field, which causes a Zeeman splitting and so suppresses the zero-phonon transitions. Level crossing signals in the data also show an enhancement of relaxation via zero-phonon hole spin flip when the Zeeman energy cancels the exchange energy. \textcopyright{} 1996 The American Physical Society.

Temperature and Orientation Dependence of Electron-Spin Relaxation Rates for Bis(diethyldithiocarbamato)copper(II)

Spin–lattice relaxation rates were measured by saturation recovery for bis(diethyldithiocarbamato)copper(II), Cu(dtc)2, dissolved in glassy 2:1 toluene:chloroform between 25 and 140 K and doped into Ni(dtc)2between 25 and 298 K. The temperature dependence is consistent with that expected for a two-phonon Raman process. For Cu(dtc)2in glassy solution and doped into Ni(dtc)2, 1/T1at 50 or 100 K is greater when the magnetic field is in the molecular plane than when it is perpendicular to the molecular plane, which is attributed to orientation dependence of intramolecular vibrations. The smaller orientation dependence of 1/T1for Cu(dtc)2doped into Zn(dtc)2than into Ni(dtc)2is attributed to the lower symmetry of the Cu(II) center in the Zn(dtc)2host. The phase-memory relaxation rate, 1/Tm, measured by electron spin echoes, exhibits a temperature dependence near 100 K that is characteristic of a dynamic process. This is attributed to rotation of the ligand methyl groups. The orientation dependence of 1/Tmat 50 K in glassy solution is that expected for low-amplitude molecular motion. Faster relaxation is observed for orientations at which the resonance field is more strongly dependent on small changes in molecular orientation.

Electron-electron spin-spin interaction in spin-labeled low-spin methemoglobin

Nitroxyl free radical electron spin relaxation times for spin-labeled low-spin methemoglobins were measured between 6 and 120 K by two-pulse electron spin echo spectroscopy and by saturation recovery electron paramagnetic resonance (EPR). Spin-lattice relaxation times for cyano-methemoglobin and imidazole-methemoglobin were measured between 8 and 25 K by saturation recovery and between 4.2 and 20 K by electron spin echo. At low temperature the iron electron spin relaxation rates are slow relative to the iron-nitroxyl electron-electron spin-spin splitting. As temperature is increased, the relaxation rates for the Fe(III) become comparable to and then greater than the spin-spin splitting, which collapses the splitting in the continuous wave EPR spectra and causes an increase and then a decrease in the nitroxyl electron spin echo decay rate. Throughout the temperature range examined, interaction with the Fe(III) increases the spin lattice relaxation rate (1/T1) for the nitroxyl. The measured relaxation times for the Fe(III) were used to analyze the temperature-dependent changes in the spin echo decays and in the saturation recovery (T1) data for the interacting nitroxyl and to determine the interspin distance, r. The values of r for three spin-labeled methemoglobins were between 15 and 15.5 A, with good agreement between values obtained by electron spin echo and saturation recovery. Analysis of the nitroxyl spin echo and saturation recovery data also provides values of the iron relaxation rates at temperatures where the iron relaxation rates are too fast to measure directly by saturation recovery or electron spin echo spectroscopy. These results demonstrate the power of using time-domain EPR measurements to probe the distance between a slowly relaxing spin and a relatively rapidly relaxing metal in a protein.

Reversion of magnetic field effects under large magnetic fields observed in the photochemical hydrogen abstraction reactions of fluorinated acetophenone and benzophenone in micellar solutions

Magnetic field effects (MFEs) on the dynamic behaviour of radical pairs generated by the photochemical hydrogen abstractions of 2′, 3′, 4′, 5′, 6′-pentafluoroacetophenone and decafluorobenzophenone in micellar solutions were investigated at large magnetic fields up to 10 T using a superconducting magnet. These MFEs were compared with the MFEs observed in the reactions of acetophenone and benzophenone. The decay rate of a radical pair generated in the reaction of acetophenone in a Brij 35 micellar solution decreased with increasing magnetic flux density (B) from 0 T, and the decrease was saturated at 0·34 T. On the other hand, the decay rate of the pair in the reaction of pentafluoroacetophenone decreased steeply with increasing B from 0 to 2 T, but increased gradually from 2 to 10 T. Such reversions of MFEs were observed for benzophenone and decafluorobenzophenone in Brij 35 micellar solutions, and the degree of the reversion for decafluorobenzophenone was larger than that of benzophenone. These new reverted MFEs were explained in terms of the relaxation mechanism. The reverted MFEs can be observed when the escape rate of a radical from a radical pair is much slower than the electron spin relaxation rates.