Showing papers on "Relaxation (NMR) published in 1985"

Specific-Heat Spectroscopy of the Glass Transition

Abstract: The glass transition has historically been viewed as an anomaly involving the specific heat of supercooled liquids. It is also associated with a divergence of the relaxation times for liquid rearrangements. We have developed a new spectroscopy to study the frequency dependence of the specific heat of liquids which connects these two approaches. We report measurements as a function of temperature of the distribution of relaxation times which are directly responsible for the glass transition in glycerol.

Light-induced excited-spin-state trapping in iron(II) spin-crossover systems. Optical spectroscopic and magnetic susceptibility study

Abstract: Single-crystal optical absorption spectra of the iron(I1) sph-crossover compounds [Fe(pt~)~]@F~), (ptz = 1-propyltetrazole) and [Fe(2-pi~)~]Cl,.EtOH (2-pic = 2-(aminomethy1)pyridine) have been.measured between room temperature and 8 K. The wellestablished thermally induced spin transition high spin (?,& + low spin (lAIg) as well as the recently discovered phenomenon of 'light-induced excited-spin-state trapping (LIWT)" is followed and analyzed. A kinetic study by optical spectroscopy of the relaxation from the light-induced high-spin (HS) state to the low-spin (LS) state has been carried out for [Fe(ptz),](BF,), in the temperature range 53-61 K. The magnetic susceptibility of the light-induced HS state of this compound has also been measured.

Single-particle relaxation time versus scattering time in an impure electron gas.

Abstract: Relative magnitudes of the single-particle relaxation time and the scattering time that enters in conductivity are given for two- and three-dimensional electron gases in the presence of random distributions of charged Coulomb scattering centers. We find that for accessible electron densities in the usual three-dimensional metallic systems the scattering time is at most a factor of \ensuremath{\sim}2 larger than the single-particle relaxation time whereas in high-mobility GaAs-based heterojunctions the spatial separation between the impurities and the carriers gives rise to scattering times which can be as much as two orders of magnitude larger than the corresponding single-particle relaxation times.

Facilitated kinetic Ising models and the glass transition

Abstract: The class of ‘‘facilitated’’ kinetic Ising models introduced in a recent letter is investigated in greater detail An interpretation of the models in terms of relaxation processes in dense fluids is described Various types of kinetic behavior are predicted for the different spin models: (A) Arrhenius temperature dependence of the average structural relaxation time, (B) non‐Arrhenius temperature dependence with a divergence of the relaxation time at a nonzero temperature, and (C) non‐Arrhenius temperature dependence with a divergent relaxation time only at zero temperature All of the models show nonexponential decay of equilibrium time correlation functions, consistent with the Kohlrausch–Williams–Watts empirical form The nature of the glass transitions exhibited by the various models is discussed

Influence of vibrational motion on solid state line shapes and NMR relaxation

Abstract: The influence of vibrational motion on bond lengths and quadrupole constants obtained from dipolar and quadrupolar solid state line shapes is considered. It is shown that such motions average both the magnitude and the orientation of the intrinsic interaction tensor. Explicit expressions for the effective coupling constants that can be conveniently evaluated using the results of a normal mode analysis are derived. When the vibrationally averaged interaction tensor is axially symmetric, it is shown that the effect of vibrational motion on relaxation can be rigorously incorporated into an effective coupling constant which is formally identical to the one that determines the line shape. Illustrative calculations for several alkanes, in both the gas and solid phases, are presented. The relative contributions of stretching and bending vibrations and the effect of anharmonicity on the effective C–H bond lengths and deuterium quadrupole constants are examined. The influence of vibrational averaging on the magnitude of the dipolar coupling is shown to be essentially independent of the nature of the molecule and its environment and to be quite small (i.e., for C–H bonds, the coupling constant decreases by 3% and hence the effective bond length increases by 1%). Vibrational averaging of the orientation of the dipolar interaction vector, on the other hand, depends on the size of the molecule and its environment because of the predominant role played by low frequency bending and torsional modes. The implication of these results for the value of the effective internuclear distance that should be used for the interpretation of the dipolar relaxation experiments is considered.

Brownian motion and vibrational phase relaxation at surfaces: CO on Ni(111).

Abstract: The temperature dependence of the internal stretch vibrational mode of CO chemisorbed on Ni(111) has been studied with the use of infrared spectroscopy. The width of the absorption peak of the bridge-bonded molecules exhibits a strong temperature dependence. A theory is developed which accounts well for the experimental results. It shows that the peak broadening is caused by anharmonic coupling to one particular low-frequency mode, namely a hindered rotation.

Modulation induced transient chirping in single frequency lasers

Abstract: Wavelength excursions with magnitudes as large as 6 A are seen to occur in single frequency lasers (both C3and DFB) during a transition from one power level to another. The wavelength shifts briefly toward shorter wavelengths and then back to the equilibrium value during turn-on and toward longer wavelengths and back during turn-off. These excursions, which are well explained by a model in which the carrier density is temporarily driven out of equilibrium by a change in injection current, last for hundreds of picoseconds or about one half of the relaxation resonance period. This time dependent behavior gives rise to a dramatic degradation of lightwave system performance with increasing bit rate. Laser structures which heavily damp the relaxation resonance peak are seen to exhibit the least chirp and to perform best in high speed transmission systems.

A Method for Tlp Imaging

Abstract: The spin lattice relaxation time (T1) is dependent on the strength of the polarizing magnetic field. The relaxation at low field strengths provides information from the processes at macromolecular level. However, the decrease of the polarizing magnetic field decreases the signal-to-noise ratio that determines the resolution of magnetic resonance images. In this report we describe a method for T1 rho imaging. The method possesses the relaxation time contrast of low field strengths with signal-to-noise ratio provided by the higher polarizing field. The relaxation time T1 rho is obtained under spin lock conditions. The spin system relaxes toward thermal equilibrium along the locking field. This process is analogous to the spin lattice relaxation at low field strength and characterized by the time constant T1 rho. T1 rho and T1 rho-dispersion may provide new imaging parameters for noninvasive tissue characterization.

Mode-coupling theory of the glass transition.

Abstract: The mode-coupling theory of the glass transition recently presented by Leutheusser is generalized so that some of the important wave-number dependence neglected in his theory is taken into account. As a consequence the correlation functions which appear in the theory presented here involve a continuous range of relaxation times rather than the single relaxation time obtained by Leutheusser. However, the important exponents describing the divergence in the shear viscosity and the vanishing of the self-diffusion coefficient are very similar in the two theories.

Ultrafast relaxation dynamics of photoexcited carriers in GaAs and related compounds

Abstract: The femtosecond intraband relaxation of hot carriers in GaAs, Al0.32Ga0.68As, and the multiple-quantum-well structure is studied using the equal-pulse optical-correlation technique. An overview of the experimental application of this technique to semiconductors is presented. A detailed theoretical analysis of the coherent-artifact contribution to the transmission-correlation peak in the geometry of parallel copropagating beams and a calculation of the saturable-absorption symmetry coefficients for GaAs are given. The relaxation time of carriers from their initially excited states was measured to be in the range 50–100 fsec for the materials studied. The interpretation of the measured relaxation time in terms of electron and hole response functions is discussed. The relevant scattering processes and rates and the corresponding relaxation times calculated from these rates are given.

Pressure broadening of rotational bands. I - A statistical theory

Abstract: Absorption of electromagnetic waves by rotational transitions of molecules is formulated for the case in which the wave frequency is displaced from resonance by an amount large compared to the reciprocal duration of a typical binary collision, and also large compared to the differences between frequencies of the strong resonances of the gas. In this far-wing limit, Fano's relaxation operator is reduced to a scalar parameter which depends on the frequency displacement. This relaxation parameter is not symmetric with respect to reflection about resonance, but becomes symmetric when multiplied by the factor exp (h/2pi) (omega sub d)/2kT where omega sub d is the frequency displacement. The theory applies to dipolar molecules of any shape, in collisions with either dipolar or quadrupolar molecules.

Optically induced magnetization in a diluted magnetic semiconductor: Hg1-xMnxTe.

Abstract: This paper reports the observation of light-induced magnetization in ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ by a novel technique combining optical pumping and superconducting quantum interference detection. We show that the magnetization is due to orientation of Mn ions caused by spin transfer from polarized electrons and holes to Mn centers in the process of spin-flip exchange scattering. This interpretation goes beyond a mean-field approach. The effect furnishes direct information on the spin-lattice relaxation in dilute magnetic semiconductors.

NMR Line Shape-Spin-Lattice Relaxation Correlation Study of Portland Cement Hydration

Abstract: The proton free induction decay of a portland cement paste in an advanced stage of hydration can be roughly divided into three main components: (1) a component with a very short spin-spin relaxation time, T2, representing the protons of the solid OH groups and the water of crystallization, (2) a component with an intermediate T, representing the bonded water in the gel phase, and (3) a third component with a relatively long T2 representing the water in the micropores and layers. The dependences of the intensities, T2's, and spin-lattice relaxation times (T1's) of these three components on the cement hardening time have been determined. The proton spin-lattice relaxation time of the “solid” component increases with hardening time whereas T1 decreases for the other two components. The observed time dependence of the diffusion coefficient, D, of water in a tricalcium silicate paste supports the findings of the above correlation study.

31P spin-lattice relaxation times and resonance linewidths of rat tissue in vivo: dependence upon the static magnetic field strength.

Abstract: Phosphorus-31 NMR spin-lattice relaxation times and resonance linewidths of rat leg muscle, brain, and liver metabolites in vivo have been examined at 1.9-, 4.7-, and 8.5-T static magnetic field strengths. The resonance linewidths expressed in ppm that have been measured are independent of field strength. The spin-lattice relaxation times of muscle and brain phosphorus metabolites decrease linearly with increasing field strength while those of liver are constant over the range of static fields examined.

Can the Bloch equations describe the vibrational spectra of a reacting molecule

Abstract: The vibrational spectra of molecules that are rapidly interconverted among distinct species by very fast motions, for example, during ordinary chemical reactions or during the rotation of a molecule between different sites in a solid, are considered. The question of the title is addressed in a number of distinct stages. First, the spectra predicted by Bloch equations with the inclusion of exchange terms are derived. The results differ from those familiar from magnetic resonance spectroscopy, since the vibrational transition moment can have a different magnitude and orientation in each site. Next, the question of whether a reaction can be fast enough on the time scale required for the simple vibrational Bloch equations to be valid is addressed, and it is concluded that this is unlikely. The observed spectrum may be fit with the result of the Bloch equation analysis (as has been done often in the past), but we conclude that the rate of the reaction cannot be simply extracted from the parameters used in this...

Protonic conductivity of hydrated lysozyme powders at megahertz frequencies.

Abstract: Dielectric losses were measured for lysozyme powders of varied hydration level by a dielectric-gravimetric technique in the frequency range of 10 kHz to 10 MHz. The relaxation showed an isotope effect and pH dependence, indicating that the inferred conductivity is protonic. The transport process is likely restricted to the surface of individual macromolecules and involves shifting of protons between ionizable side chain groups of the protein. The time constant of the relaxation shows cooperativity in its seventh-order dependence on bound protons. The process develops in the hydration region critical for the onset of the catalytic properties of the enzyme. The binding of a substrate increases the relaxation time by a factor of 2. These observations suggest that the megahertz dispersion reflects behavior at the protein surface, specifically the cooperative channeling of proton flow through the active site, that may be of particular significance for the enzymatic and other functional properties of proteins.

Strategies and tactics in NMR imaging relaxation time measurements. I. Minimizing relaxation time errors due to image noise--the ideal case.

Abstract: The effect of NMR image noise on errors in calculated values of relaxation times is quantitatively assessed by use of relaxation time noise figures, which are derived on the basis of statistical principles as functions of pulse delay, repetition, and recovery intervals for several types of pulse sequences Two strategies for determining relaxation times are considered: two point (ratio of intensities for two experiments) and multipoint (least-squares fit of intensities to pulse-sequence functions for n experiments) For given total measurement times, values of pulse interval times are found which give minimum relaxation time noise figures A comparison of ratio methods shows that the best is a combination saturation-recovery, inversion-recovery (SR/IR) technique For short measurement times (less than about 10T1) this optimized SR/IR ratio determination is also superior to the best multipoint method, a series of inversion-recovery experiments with equally incremented inversion-recovery times An examination of the effect of signal averaging on the relaxation time noise shows that up to a measurement time characteristic of the particular method used (eg, for times up to about 5T1 for the SR/IR ratio determination, 100T1 for the multipoint inversion-recovery method), increased measurement time is more effectively allotted to longer pulse intervals than to signal averaging Numerical examples are tabulated which can help one to set optimum values for pulse intervals, given a rough estimate of the relaxation time to be determined

Electron T1 measurements in short-lived free radicals by dynamic polarization recovery

Abstract: A dynamic polarization recovery method for measurement of electron spin T1 relaxation times in free radicals in liquids is described, which is valid even in the presence of chemically induced dynamic electron polarization (CIDEP) and fast chemical decay of the radicals. The method is based on pulsed microwave perturbation and detection of transient magnetization following radical creation in a short pulse. Analysis of the experimental approach and a theoretical description of the method is presented together with a detailed discussion of the advantages and the limitations of the technique. Electron T1 measurements are presented for 14 short‐lived free radicals generated in aqueous solution. The magnitudes of the observed relaxation times, which range from 0.1 to 4 μs, are discussed within the framework of current theories of relaxation for small radicals in liquids. It is tentatively concluded that the spin rotation mechanism is responsible for the very short T1’s in this series of radicals.

Excited state dynamics and photophysics of aggregated dye chromophores in solution

Abstract: We have studied the excited state relaxation processes of monomeric and aggregated diethylthiadicarbocyanine iodide (DTDCI) by means of picosecond absorption spectroscopy. The strongly interacting chromophores of dimers and trimers are found to have very fast and efficient radiationless processes, whose rates are independent of temperature and viscosity, but strongly dependent on solvent dielectric constant. These results and similar ones for the covalently linked bis‐DTDCI dimer allow us to identify the relaxation mechanism and suggest a theoretical model which accounts for the observations. We suggest that the radiationless processes are induced by the very strong dipole–dipole interactions between the units of the aggregate; analogies to the so called medium induced radiationless processes are also made. Estimates of the relaxation rates in the aggregated dye yields values which are in order‐of‐magnitude agreement with the measured values.

Quantum calculations of rotational and NMR relaxation, depolarized Rayleigh and rotational Raman line shapes for H2(HD)−He mixtures

Abstract: The two ab initio vibrotor potentials of H2−He previously published by Meyer, Hariharan, and Kutzelnigg have been composed in one potential grid. After using this potential in close coupled scattering calculations we have subsequently obtained rotational relaxation cross sections for para- and ortho-H2, NMR relaxation times for ortho-H2, depolarized Rayleigh line broadening cross sections for para-, ortho-, and normal-H2, and rotational Raman line width and shift cross sections for S0(0), S0(2) transitions, at temperatures between 20 and 450 K. The agreement with previous and more recent measurements has been found completely satisfying, with the exception of the rotational relaxation cross sections of para-H2. Throughout the paper previous results derived from the multiple property fit of Shafer and Gordon have been included for comparison. The new ab initio potential is obviously better, except for the rotational relaxation cross sections. The interaction potential of HD−He transformed from the new H2−He potential is also presented in this paper. It has been used recently in a very successful application of the Waldmann-Snider kinetic theory describing transport, relaxation, and reorientation phenomena in magnetic fields. Therefore, only the rotational relaxation cross sections, converged up to about 400 K, have been presented in this paper.

Magnetic field dependence of solvent proton relaxation in aqueous solutions of Fe3+ complexes.

Abstract: It might appear that the Fe3+ ion would be particularly useful as an agent for enhancing contrast in NMR images since it has a relatively large magnetic moment and occurs in vivo in a variety of forms. Moreover, the concentration of Fe3+ changes locally in certain disease states (e.g., beta-thalassemia) and in trauma (formation of methemoglobin), and can be altered in the gastrointestinal tract by the ingestion of readily available dietary supplements. However, the Fe3+ ion is insoluble above pH approximately 4, and soluble chelate and protein complexes of Fe3+ tend to sequester the ions from solvent; hence, the efficacy of Fe3+ ions for relaxing water protons ought to be low under typical physiological conditions. We report the magnetic field dependence of the relaxation rate of solvent protons (NMRD profiles) for solutions of a variety of Fe3+ complexes to demonstrate the phenomenology relevant to NMR imaging. From these data we make some estimates to show that, despite the low relaxation rates of solvent protons in solutions of Fe3+ complexes, certain observed changes in image contrast are consistent, quantitatively, with inferences that can be drawn from solution data.

Carbon-13 NMR relaxation studies demonstrate an inverse temperature transition in the elastin polypentapeptide

Abstract: Carbon-13 NMR longitudinal relaxation time and line-width studies are reported on the coacervate concentration (about 60% water by weight) of singly carbonyl carbon enriched polypentapeptides of elastin: specifically, (L-Val1-L-[1-13C]Pro2-Gly3-L-Val4-Gly5)n and (L-Val1-L-Pro2-Gly3-L-Val4-[1-13C]Gly5)n. On raising the temperature from 10 to 25 degrees C and from 40 to 70 degrees C, carbonyl mobility increases, but over the temperature interval from 25 to 40 degrees C, the mobility decreases. The results characterize an inverse temperature transition in the most fundamental sense of temperature being a measure of molecular motion. This transition in the state of the polypentapeptide indicates an increase in order of polypeptide on raising the temperature from 25 degrees C to physiological temperature. This fundamental NMR characterization corresponds with the results of numerous other physical methods, e.g., circular dichroism, dielectric relaxation, and electron microscopy, that correspondingly indicate an increase in order of the polypentapeptide both intramolecularly and intermolecularly for the same temperature increase from 25 to 40 degrees C. Significantly with respect to elastomeric function, thermoelasticity studies on gamma-irradiation cross-linked polypentapeptide coacervate show a dramatic increase in elastomeric force over the same interval that is here characterized by NMR as an inverse temperature transition. The temperature dependence of mobility above 40 degrees C indicates an activation energy of the order of 1.2 kcal/mol, which is the magnitude of barrier expected for elasticity.

Nanosecond fluctuations of the molecular backbone of collagen in hard and soft tissues: a carbon-13 nuclear magnetic resonance relaxation study.

Abstract: We have determined the amplitude of nanosecond fluctuations of the collagen azimuthal orientation in intact tissues and reconstituted fibers from an analysis of 13C NMR relaxation data. We have labeled intact rat calvaria and tibia collagen (mineralized and cross-linked), intact rat tail tendon and demineralized bone collagen (cross-linked), and reconstituted lathyritic (non-cross-linked) chick calvaria collagen with [2-13C]glycine. This label was chosen because one-third of the amino acid residues in collagen are glycine and because the 1H-13C dipolar coupling is the dominant relaxation mechanism. Spin-lattice relaxation times (T1) and nuclear Overhauser enhancements were measured at 15.09 and 62.98 MHz at 22 and -35 degrees C. The measured NMR parameters have been analyzed by using a dynamic model in which the azimuthal orientation of the molecule fluctuates as a consequence of reorientation about the axis of the triple helix. We have shown that if root mean square fluctuations in the azimuthal orientations are small, gamma rms much less than 1 rad, the correlation function decays with a single correlation time tau and T1 depends only upon tau and gamma rms and not the detailed model of motion. Our analysis shows that, at 22 degrees C, tau is in the 1-5-ns range for all samples and gamma rms is 10 degrees, 9 degrees, and 5.5 degrees for the non-cross-linked, cross-linked, and mineralized samples, respectively. At -35 degrees C, gamma rms is less than 3 degrees for all samples. These results show that mineral and low temperature significantly restrict the amplitude of nanosecond motions of the collagen backbone.(ABSTRACT TRUNCATED AT 250 WORDS)