Spin-lattice Relaxation Processes Research Articles

The EPR signals from the S0 and S2 states of the Mn cluster in photosystem II relax differently.

The oxygen evolving complex (OEC) of photosystem II (PSII) gives rise to manganese-derived electron paramagnetic resonance (EPR) signals in the S0 and S2 oxidation states. These signals exhibit different microwave power saturation behavior between 4 and 10 K. Below 8 K, the S0 state EPR signal is a faster relaxer than the S2 multiline signal, but above 8 K, the S0 signal is the slower relaxer of the two. The different temperature dependencies of the relaxation of the S0 and S2 ground-state Mn signals are due to differences in the spin-lattice relaxation process. The dominating spin-lattice relaxation mechanism is concluded to be a Raman mechanism in the S0 state, with a T(4.1) temperature dependence of the relaxation rate. It is proposed that the relaxation of the S2 state arises from a Raman mechanism as well, with a T(6.8) temperature dependence of the relaxation rate, although the data also fit an Orbach process. If both signals relax through a Raman mechanism, the different exponents are proposed to reflect structural differences in the proteins surrounding the Mn cluster between the S0 and S2 states. The saturation of SII(slow) from the Y(D)(ox) radical on the D2 protein was also studied, and found to vary between the S0 and the S2 states of the enzyme in a manner similar to the EPR signals from the OEC. Furthermore, we found that the S2 multiline signal in the second turnover of the enzyme is significantly more difficult to saturate than in the first turnover. This suggests differences in the OEC between the first and second cycles of the enzyme. The increased relaxation rate may be caused by the appearance of a relaxation enhancer, or it may be due to subtle structural changes as the OEC is brought into an active state.

Nuclear spin-spin coupling inLa2−xSrxCuO4studied by stimulated echo decay

We have performed copper NQR experiments in high temperature superconductors YBa_{2}Cu_{4}O_{8}, YBa_{2}Cu_{3}O_{7}, and La_{2-x}Sr_{x}CuO_{4} (x=0.12 and 0.15), using the stimulated echo technique. The stimulated echo intensity is analyzed by a model that includes the spin-lattice relaxation process (T_ {1 }-process) and the fluctuating local field due to nuclear spin-spin coupling. The model gives quantitative account of the experimental results in Y-based compounds using the known values of 1/T_{1} and 1/T_{2G}, the gaussian decay rate of the spin echo intensity. The same model applied to LSCO enables us to extract the value of T_{2G}. Our results indicate that T_{1}T/T_{2G} is independent of temperature, implying that the dynamic exponent is one in La_{2-x}Sr_{x}CuO_{4}.

Spin diffusion and nuclear spin-lattice relaxation in irradiated solids: a multiple-pulse NQR study

We present a detailed theoretical and experimental NQR multiple-pulse spin-locking study of spin-lattice relaxation and spin diffusion processes in the presence of paramagnetic impurities in solids. The relaxation function of the nuclear spin system at the beginning of the relaxation process is given by exp − t T 1ρ α , where T 1 ρ is spin-lattice relaxation time in rotating frame and α= d/6, d is the sample dimensionality. Then the relaxation proceeds asymptotically to an exponential function of time, which was attributed to the spin-diffusion regime. Using the experimental data obtained from the analysis of those two relaxation regimes in γ-irradiated powdered NaClO 3, spin diffusion coefficient has been determined and the radius of the diffusion barrier has been estimated.

Nuclear magnetic relaxation studies of 69Ga, 71Ga, and 75As nuclei in GaAs single crystals doped with paramagnetic impurities

The GaAs, GaAs:In,Cr, and GaAs:Mn single crystals were grown by the vertical gradient freeze (VGF) technique. The spin-lattice relaxation processes of 69Ga, 71Ga and 75As have been studied for the undoped GaAs single crystals and crystals doped with paramagnetic impurities Cr and Mn. The relaxation processes in all samples are well described by a single exponential function. The T 2 dependence of the relaxation rate was in accordance with that predicted for the Raman mechanism of nuclear spin-lattice relaxation for all crystals investigated. The manganese doping showed a tendency to shorten slightly the spin-lattice relaxation times.

Nuclear magnetic relaxation of 19F in fluoride glasses

Temperature and frequency dependence of the 19F nuclear spin relaxation of the fluoroindate glass, 40InF 3–20ZnF 2–20SrF 2–2GaF 3–2NaF–16BaF 2 and the fluorozirconate glass, 50ZrF 4–20BaF 2–21LiF–5LaF 3–4AlF 3, are reported. Measurements were undertaken on pure and Gd 3+ doped samples, in the temperature range of 185–1000 K, covering the region below and above the glass transition temperature, T g. The temperature and frequency dependence of the spin-lattice relaxation rate, T 1 −1, measured in the glassy state at temperature <300 K, is less than the observed dependence at higher temperatures. At temperatures > T g, the fluorine mobility increases, leading to a more efficient spin-lattice relaxation process. Activation energies, for F − motion, are 0.8 eV for the fluoroindate glass and 1 eV for the fluorozirconate glass. The addition of Gd 3+ paramagnetic impurities, at 0.1 wt%, does not alter the temperature and frequency dependence of T 1 −1, but increases its magnitude more than one order of magnitude. At temperatures <400 K, the spin–spin relaxation time, T 2 −1, measured for all samples, is determined by the rigid-lattice nuclear dipole–dipole coupling, and it is temperature independent within the accuracy of the measurements. Results obtained for the pure glass, at temperatures >400 K, show that T 2 −1 decreases monotonically as the temperature increases. This decrease is explained as a consequence of the motional narrowing effect caused by the onset of the diffusive motion of the F − ions, with an activation energy around 0.8 eV. For the doped samples, the hyperfine interaction with the paramagnetic impurities is most effective in the relaxation of the nuclear spin, causing an increase in the T 2 −1s observed at temperatures >600 K.

Selective-inversion experiments applied to chemical exchange in coupled spin systems

This paper addresses the problems of studying slow chemical exchange in spin systems that show scalar coupling. Line broadening due to exchange in this regime is minimal, so rates are often obtained by selective-inversion experiments. These are modified inversion-recovery experiments, in which only one part of the exchanging system is inverted. These spins can then return to equilibrium by exchange with the non-inverted sites, as well as by normal spin-lattice relaxation processes. This is a standard method for spin systems which show little or no scalar coupling. The problem is more complex for coupled spin systems, because the longitudinal magnetizations are more difficult to measure. There is no longer a one-to-one correspondence between the longitudinal magnetizations and the observable lines in the spectrum. The spectra depend on the flip angle of the observe pulse, there may be longitudinal magnetizations that can not be measured reliably, and zero-quantum transitions may interfere. These problems are addressed here. The flip angle dependence can be exploited to enhance the information collected, and a singular value decomposition (SVD) is used to obviate the numerical problems of poorly determined longitudinal magnetizations. The role of the zero-quantum interference is also discussed. With these methods, coupled spin systems can be treated like an uncoupled system. The technique is tested on two molecules: ReBr(CO)3-2,6-pyridinedicarboxylate, a simple three-spin exchange system; and 4-nitroso-N,N-dimethylaniline, a four-spin system.

Spin-lattice relaxation times in amorphous materials as effected by exchange interactions, tunneling level states (TLS) centres, and fractons

The processes of spin-lattice relaxation (SLR) in amorphous materials are reviewed. In particular, the mechanisms involving exchange interaction, tunneling level states (TLS) centers and fractons predominant in effecting electron SLR at low temperatures in amorphous materials are reviewed. It has been deducded that the former two lead to quadratic temperature dependences at very low temperatures and linear temperature dependences at intermediate temperatures of the relaxation rate in amorphous materials. As for relaxation due to fractons in fractal state, the theories proposed by Alexander, Entin-Wohlman and Orbach (Phys. Rev. B 32 (1985) 6447; Phys. Rev. B 33 (1986) 3935; J. Phys. Lett. 46 (1985) 1555) are reviewed and the temperature dependences of fracton-assisted relaxation rate in the fractal state for Kramers and non-Kramers ions are examined.

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.

13C NMR Investigations of Spin–Lattice Relaxation in 99%13C-Enriched Diamonds

The paper is devoted to investigations of spin–lattice relaxation processes in 99%13C-enriched diamonds. Relaxation time measurements were performed as a function of orientation, magnetic field, and temperature. Both experimental results and theoretical discussion are presented. Multiexponential behavior of nuclear magnetization recovery was observed. There was found no significant influence of the diamond orientation on the nuclear spin–lattice relaxation. The field dependence of the spin–lattice relaxation time was found to be proportional to the second power of the magnetic field. The temperature measurements showed a weak increase of the spin–lattice relaxation time with decreasing temperature. Possible mechanisms of impurity relaxation are considered and compared with the experimental data.

Investigation of Spin-Lattice Relaxation in the Excited Triplet States of Impurity Molecular Crystals Induced by the Translational Molecule Motions and Intramolecular Nuclei Vibrations

Direct one-phonon processes of spin-lattice relaxation are investigated in the excited triplet states of molecular crystals in high magnetic fields and at low temperatures. The mixing of the vibrational motions of the nuclei with the translational ones of the molecule is chosen as the major phenomenon, leading to the coupling between the spin system and acoustic vibrations. The angular, field and temperature dependences of the spin-lattice relaxation rate for an isotopic impurity in deuterobenzophenone crystal are calculated by the molecular orbitals and atom-atom potential methods.

Determination of Relaxation Paths in the Manifold of Excited States of Pt(2-thpy)2and [Ru(bpy)3]2+by Time-Resolved Excitation and Emission

Pt(2-thpy)₂ and [Ru(bpy)₃]²⁺, studied as representatives of transition metal complexes with zero-field splittings (zfs) of the lowest triplets of several cm⁻¹, exhibit a series of generally not-well known time dependencies of emission decay properties. These are strongly determined by relatively slow spin-lattice relaxation (slr) processes. Thus, one finds emission decays for [Ru(bpy)₃]²⁺ and Pt(2-thpy)₂ of 220 and 600 ns at T = 1.3 K, respectively, which are in both compounds controlled by relaxation processes from the second to the lowest excited state, while the lowest state itself emits with a long decay of 230 and 110 μs, respectively. According to these distinctly different emission decay times observed for the two lowest excited states (of the same compound), it is possible to gain a more detailed insight into the properties of the different states by applying the techniques of spectrally highly resolved and time-resolved emission spectroscopy. In particular, this deeper insight results from the possibility to register high-quality low-temperature emission spectra also of the second excited state, hitherto not known. Moreover, from the temperature dependencies of the slr rates in Pt(2-thpy)₂, it is concluded that at low temperature the direct process of slr dominates, while for T > 2.3 K the Orbach process becomes increasingly important. For [Ru(bpy)₃]²⁺ the situation is similar, but the Orbach process grows in for T > 6 K. It is the highlight of the present investigation that the specific properties of slr can be used to study details of relaxation paths in the manifold of the electronically and vibrationally excited states by introducing - for the first time - the method of time-resolved excitation spectroscopy. In particular, it can be shown - without applying a sub-picosecond time resolution - that after a pulsed excitation the relaxations occur within the vibrational potential hypersurfaces of each triplet sublevel. A crossing between the triplet sublevels does not occur via excited vibrational states, but it takes place after the zero-point vibrational levels are reached. However, this selectivity of the relaxation paths is lost when a higher lying singlet is excited. Moreover, this new method provides access to a series of further excited state properties.

Intraligand Charge Transfer in Pt(qol)(2). Characterization of Electronic States by High-Resolution Shpol'skii Spectroscopy.

Pt(qol)(2) (qol(-) = 8-quinolinolato-O,N) is investigated in the Shpol'skii matrices n-heptane, n-octane-h(18), n-octane-d(18), n-nonane, and n-decane, respectively. For the first time, highly resolved triplet phosphorescence as well as triplet and singlet excitation spectra are obtained at T = 1.2 K by site-selective spectroscopy. This permits the detailed characterization of the low-lying singlet and triplet states which are assigned to result mainly from intraligand charge transfer (ILCT) transitions. The electronic origin corresponding to the (3)ILCT lies at 15 426 cm(-)(1) (FWHM approximately 3 cm(-)(1)) exhibiting a zero-field splitting smaller than 1 cm(-)(1), which shows that the metal d-orbital contribution to the (3)ILCT is small. At T = 1.2 K, the three triplet sublevels emit independently due to slow spin-lattice relaxation (slr) processes. Therefore, the phosphorescence decays triexponentially with components of 4.5, 13, and 60 &mgr;s. Interestingly, two of the sublevels can be excited selectively, which leads to a distinct spin polarization manifested by a biexponential decay. At T = 20 K, the decay becomes monoexponential with tau = 10 &mgr;s due to a fast slr between the triplet sublevels. From the Zeeman splitting of the (3)ILCT the g-factor is determined to be 2.0 as expected for a relatively pure spin triplet. The (1)ILCT has its electronic origin at 18 767 cm(-)(1) and exhibits a homogeneous line width of about 12 cm(-)(1). This feature allows us to estimate a singlet-triplet intersystem crossing rate of about 2 x 10(12) s(-)(1). This relatively large rate compared to values found for closed shell metal M(qol)(n)() compounds displays the importance of spin-orbit coupling induced by the heavy metal ion. Moreover, this small admixture leads to the relatively short emission decay times. All spectra show highly resolved vibrational satellite structures. These patterns provide information about vibrational energies (which are in good accordance with Raman data) and shifts of equilibrium positions between ground and excited states. These shifts are different in the (1)ILCT and (3)ILCT states. The vibrational satellite structures support the assignment of ILCT character to the lowest excited states.

Investigation of Spin-Lattice Relaxation in the Excited Triplet States of Impure Molecular Crystals Induced by the Translational and Rotational Motions of the Molecule

Abstract The direct one-phonon processes of spin-lattice relaxation (SLR) in the excited triplet states of organic molecular crystals are investigated at low temperatures and high magnetic fields. The mixing of the translational and rotational motions of the molecule (TRM relaxation mechanism) is chosen as the major mechanism, leading to the coupling of the spin system with acoustic vibrations. The angular, field and temperature dependences of SLR rate are calculated for an isotopic impurity in deuterobenzophenone crystal using the atom-atom potentials method of the intermolecular interaction. The anisotropy of the electronic ĝ-tensor is analyzed. The obtained results are in accordance with available experimental data.

Fluid Structures ofCO2 andCO2-CH4 Mixture at Supercritical Fluid and Liquid Densities by Nuclear Spin-Lattice Relaxation Measurements

13C nuclear relaxation times were measured by a high-pressure sapphire NMR cell for neat CO2 and a CO2–CH4 mixture over a wide range that covers gas, liquid and supercritical densities. The nuclear spin–lattice relaxation times at liquid and supercritical densities for the CO2–CH4 mixture were obtained for the first time. Although the spin–rotation interactions still dominate the spin–lattice relaxation processes in CH4, the intramolecular dipolar interaction contributes approximately 5% to the overall relaxation rates at liquid densities for this mixture. The chemical shift anisotropy and the intermolecular contributions to the relaxation rates of CO2 were experimentally determined to be negligible. A movable wall cell model was modified and used to relate the relaxation time data to the fluid structures of the first solvation shell around the central molecules in the liquid and supercritical fluid states. The coordination number in the first shell was found to be 12 for both the neat CO2 and CO2–CH4 mixture at liquid and supercritical fluid densities.

Temperature dependence of 1H-NMR relaxation in hydrogenated amorphous carbon sample series

A two-exponential spin-lattice ( T 1) relaxation process of hydrogen nuclear magnetization was found in a series of hydrogenated amorphous carbon samples prepared under different self-bias conditions from benzene. The temperature dependence of these relaxation times reveals the different dynamic background of the two sub-systems. Low activation energies (depending on self-bias voltages) were measured for the longer relaxation time components; temperature independence was detected for the shorter relaxation time components.

Crystal structure, ESR and conductivity studies of bis(ethylenedithio)tetrathiafulvalene (ET) organic conductor (H 8-ET) 2[Hg(SCN) 2Br] and its deuterated analogue (D 8-ET) 2[Hg(SCN) 2Br

Protonated (I) and deuterated (II) crystals of (ET) 2[Hg(SCN) 2Br] (ET = bis(ethylenedithio)tetrathiafulvalene) are isostructural with B2/ b space group and Z = 4. Crystal structure consists of (ET) 2 + dimers forming a κ-type motif and arranged in layers parallel to the bc-plane which are separated by anionic sheets of polymeric anions. A two-positional disorder exists in ethylene groups of deuterated ET molecules, while such disorder appears only in one of the ethylene groups in the hydrogenated salt. The crystals exhibit semimetallic behaviour at high temperatures with a transition to non-metallic state at 100 K for salt I and 60 K for salt II as is shown from d.c. conductivity measurements. Electron spin resonance (ESR) g factor and linewidth and their temperature behaviour are extensively discussed on the basis of existing theories and experimental data for low-dimensional organic conductors. The linewidth is determined by spin-phonon scattering driven relaxation processes which are mediated by Peierls fluctuations in the conducting phase and by spin-lattice relaxation processes below the critical temperature. The first-order phase transition in hydrogenated salt at 100 K is associated with a localization of electrons on ET dimers, whereas the transition of semiconductor-semiconductor type with possible magnetic ordering is suggested for deuterated salt. A considerable isotope effect in (ET) 2[Hg(SCN) 2Br] is considered to be related to a weak hydrogen bond between the terminal ethylene group of ET dot;+ and the bromine atom of the anion in hydrogenated salt. A discussion of the nature of the phase transitions is given.

Quantification of thaumasite in cementitious materials by 29Si {1H} cross-polarization magic-angle spinning NMR spectroscopy

29Si {1H} cross-polarization (CP) magic-angle spinning (MAS) nuclear nagnetic resonance (NMR) is shown to be a valuable tool for detection and quantification of thaumasite in cementitious systems. A 29Si {1H} CPIMAS NMR method for quantitative analysis of thaumasite employing 1H and 29Si rotating-frame relaxation times TH1p and TSi1p and the 1H—29Si CP time TSiH is introduced. The method is illustrated by determination of the thaumasite content in five laboratory samples containing carbonates and sulphates from different sources. The use of 29Si{lH} CP/MAS rather than 29Si MAS NMR for the detection of thaumasite is shown to reduce the instrument time by up to a factor of IOOO because of the large difference in 1H and 29 spin-lattice relaxation times (TH1 and TSi1) and the 29Si intensity gain by CP for this mineral. The non-standard behaviour of the 1H and 29 spin-lattice relaxation processes observed for some samples is shown to follow a decay exponential in t with time t. 27Al MAS NMR is used to quantify ettringite for some of the samples. In contrast to previous proposals on the mechanism for the formation of thaumasite, it is demonstrated using 29Si{lH} CP/MAS and 27Al MAS NMR that considerable quantities of thaumasite can be produced in virtually aluminium-free materials. 13C {lH} CP/MAS NMR of pure thaumasite shows its potential in characterizing the carbonate anions in this mineral.

NMR relaxation caused by fluctuating flux lines

Based on a model of harmonic fluctuations, we study the nuclear spin-lattice relaxation (NSLR) process due to the fluctuating flux lines (FLs) in a clean type-II superconductor. At low fields, the calculated longitudinal relaxation rate T 1 −1 is enormously enhanced at temperatures just below T c. At intermediate fields, the resulting T 1 −1 exhibits a peak structure as a function of temperature, which is eventually suppressed as the field is increased.

Spin dynamics in RENi5 ferromagnets byμSR measurements

We report a zero fieldμSR study of single crystals of theRENi5 ferromagnets where RE=Gd, Er, Dy or Tm. Our work points out the strong influence of the crystal electric field on the rare earth total momentum dynamics: whereas the spin-lattice relaxation process belowTc is controlled by a two magnon mechanism for a weakly anisotropic magnet (GdNi5), the phonons drive the relaxation for a strongly anisotropic magnet such asErNi5. ForTmNi5 a comparison is made between the fluctuation time measured byμSR and169Tm Mossbauer spectroscopy.

Oxygen and solvent intermolecular effects on the 13C spin-lattice relaxation process of C 60 in deuterated toluene at 303 K

13C spin-lattice relaxation time measurements of buckminsterfullerene were performed in mixtures of protonated and deuterated toluene to determine the magnitude of the intermolecular contribution. Results show that a small but measurable contribution can be attributed to this mechanism. Inclusion of this effect in the analysis of the overall spin-lattice relaxation rate provided a slight improvement in the agreement between the measured reorientational time and a value predicted by a modified Stokes-Einstein-Debye equation for molecular rotation. Relaxation measurements conducted in the presence of molecular oxygen showed a significant paramagnetic effect. This observation indicates that complete removal of molecular oxygen is extremely important when measuring the 13C relaxation time of buckminsterfullerene in condensed phases.