Showing papers on "Relaxation (NMR) published in 1983"

Theory of the phase noise and power spectrum of a single mode injection laser

Abstract: Spontaneous emission alters the phase and amplitude of the laser field. The amplitude changes induce relaxation oscillations, which cause additional phase changes while restoring the field amplitude to the steady state value. It was previously shown that the additional phase changes greatly enhance the linewidth. We show here that the additional phase changes also give rise to line shape structure in the form of additional peaks separated from the main peak by multiples of the relaxation oscillation frequency. The calculated mean square phase change and power spectrum are in good agreement with published observations.

On the scattering of electrons by polar optical phonons in quasi-2D quantum wells

Abstract: Numerical calculations have been made of the rates of scattering and momentum relaxation of electrons caused by unscreened polar optical phonons in quasi-two-dimensional layers, using a simple infinite-depth square-well model. The results are compared with approximate analytic formulae, and with the situation in the bulk. Some of the major differences from scattering in the bulk are (i) an abrupt threshold for phonon emission appears, (ii) rates just above threshold are enhanced and (iii) the scattering just above the threshold of emission is more isotropic. These differences are intimately related to the energy dependence of the density of states and to the polar interaction, and are expected to be largely independent of the model. The form of the polar interaction strongly favours intra-sub-band scattering over inter-sub-band scattering and this effect has consequences for the luminescent spectrum expected from hot injected carriers. The large increase of the momentum relaxation rate at the emission threshold has consequences for transport phenomena.

Intramolecular vibrational relaxation from C-H stretch fundamentals

Abstract: Infrared fluorescence measurements have been used to determine the presence or absence of intramolecular vibrational relaxation (or more precisely state mixing) from an excited C–H stretch in 23 representative molecules varying in size from methane to nonbornene. Included are aliphatic hydrocarbons, aromatic molecules, ethers, and ketones, including cyclic molecules. The rate of resonance fluorescence, calculated relative to a nonrelaxing molecule, was used as the criterion of relaxation. Small rates imply state mixing and hence in the large molecule limit, relaxation. The primary correlation with the presence of mixing is with state density; about ten states per wave number (cm−1) are needed to insure mixing. The spread in threshold densities reflects variations in anharmonicity. For some molecules true relaxed (non C–H stretch) fluorescence is observed. These experiments were done in a molecular beam, using a pulsed optical parametric oscillator for excitation and a circular variable filter for spectral...

Collective effects in a random-site electric dipole system: KTaO3: Li

Abstract: Li, substituting for K in KTa${\mathrm{O}}_{3}$, creates a local electric dipole, due to its off-center position with respect to the cubic site. We have studied such crystals with different amounts of Li (and in a few cases also doped with Nb, substituting for Ta) by nuclear magnetic resonance, dielectric relaxation, pyroelectricity, ultrasound, and birefringence methods. Birefringence and dielectric susceptibility results show that collective effects between the Li dipoles occur below a fairly well-defined concentration-dependent temperature of the order of 50 K, but nuclear magnetic and dielectric relaxation indicate the absence of criticality at the onset of these effects. These collective effects are related to those arising in spin-glasses. We discuss the data in the light of theoretical models and computer simulations of systems of randomly interacting moments, which predict an apparent condensation into a system of metastable clusters without long-range order.

New view of lipid bilayer dynamics from 2H and 13C NMR relaxation time measurements

Abstract: Natural abundance 13C spin-lattice (T1) relaxation time measurements are reported for unilamellar vesicles of 1,2-dipalmitoylphosphatidylcholine (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), in the liquid crystalline phase, at magnetic field strengths of 1.40, 1.87, 2.35, 4.23, 7.05, 8.45, and 11.7 tesla (resonance frequencies of 15.0, 20.0, 25.1, 45.3, 75.5, 90.5, and 126 MHz, respectively), and the results are compared to previous 2H T1 studies of multilamellar dispersions. For both the 13C and 2H T1 studies, a dramatic frequency dependence of the relaxation was observed. At superconducting magnetic field strengths (4.23-11.7 tesla), plots of the 13C T1(-1) relaxation rates as a function of acyl chain segment position clearly reveal the characteristic "plateau" signature of the liquid crystalline phase, as found previously from 2H NMR studies. The dependence of T1(1) on ordering, determined previously from 2H NMR, and the T1(-1) dependence on frequency, determined from both 13C and 2H NMR studies, suggest that a unified picture of the bilayer molecular dynamics can be provided by a simple relaxation law of the form T1(-1) approximately equal to A tau f + BS2C-H omega -1/2(0). In the above expression, A and B are constants, SC-H (= SC-D) is the bond segmental order parameter, and omega 0 is the nuclear Larmor frequency. The first (A) term includes contributions from fast, local segmental motions characterized by the effective correlation time tau f, whereas the second (B) term describes slower, collective fluctuations in the local ordering. The value of tau f approximately equal to 10(-11) sec, obtained by extrapolating T1(-1) to infinite frequency, suggests that the segmental microviscosity of the bilayer hydrocarbon region does not differ appreciably from that of the equivalent n-paraffinic liquids of similar chain length.

Short time structural relaxation processes in liquids: Comparison of experimental and computer simulation glass transitions on picosecond time scales

Abstract: Recent Brillouin scattering studies of relaxation in the simple ion system Ca++–K+/NO−3 (two particles of argon structure and a small anion) provide the experimental basis for a direct comparison of glass transformation phenomenology in computer simulated LJ argon ‘‘glass,’’ with that in laboratory substances studied on the same time scale. Most of the attenuated glass transition characteristics observed for LJ argon are found in the ionic system, and the same relation of the dispersion midpoint to the so‐called ‘‘ideal’’ glass transition temperature is observed. Analysis of the real and imaginary parts of the complex longitudinal modulus shows that at high temperatures the relaxation function for Ca++–K+–NO−3, in strong contrast with that at normal low temperature behavior, closely approaches a simple exponential decay with Arrhenius form for the relaxation time. Furthermore, the high temperature Arrhenius plot extrapolates naturally to the reciprocal quasilattice vibration frequency determined by far in...

Clinical efficiency of nuclear magnetic resonance imaging.

Abstract: Advances in imaging technique have improved the efficiency of clinical nuclear magnetic resonance (NMR) imaging, and will allow total patient examination time that equals or is more favorable than that of x-ray computed tomography (CT). The whole head can be examined with NMR in a 6.5-minute imaging time with a spatial resolution of 1.7 mm. Fifteen sections in the body can be similarly imaged. Quantitative T2 ("spin-spin" relaxation time) information, as well as estimates of T1 ("spin-lattice" relaxation time) can be obtained in this time. Quantitative T1 information requires an additional procedure.

Cadmium-113 magnetic resonance spectroscopy

Abstract: Cadmium-113 nuclear magnetic resonance spectroscopy has been used in studies of the structure and dynamics of inorganic and bioinorganic molecules. Chemical dynamics play an important role in the analysis of relaxation and chemical shift data. Naive interpretations of relaxation data can be checked by performing these experiments at a variety of temperatures and magnetic field strengths. A combination of solid- and liquid-state nuclear magnetic resonance measurements can provide the user with unambiguous data on chemical shielding. These data can be used to characterize zinc and calcium ion binding sites in metalloproteins.

Carbon‐13 NMR of polyethylenes: Correlation of the crystalline component T1 with structure

Abstract: The carbon-13 spin-lattice relaxation times T1 of the crystalline portion of a set of polyethylenes have been studied. Chain structure and crystallization conditions have been varied over the widest possible extremes so that large differences are developed in the level of crystallinity, the supermolecular structure, and the crystallite thickness. Concomitantly, the observed crystalline T1 values cover the extraordinarily wide range of about 40–4500 s. They bear a one-to-one relation with the crystallite thickness, which is found to be the key structural variable determining this property. A correlation with the temperature for the α-transition can be established, which implies a similar type of segmental motions for the two phenomena. Major changes in the interfacial structure can also have a drastic influence on the value for the crystalline T1. Analysis of the magnetization decay curve also allows for a quantitative determination of the degree of crystallinity, which is found to be in excellent agreement with the corresponding value found from Raman spectroscopy.

Paramagnetic impurity effects in NMR determinations of hydrogen diffusion and electronic structure in metal hydrides. Gd 3 + in Y H 2 and La H 2.25

Abstract: Measurements are reported of the temperature dependence of the proton spin-lattice and spin-spin relaxation times ${T}_{1}$ and ${T}_{2}$ in yttrium and lanthanum dihydrides containing controlled levels of gadolinium as low as 50 ppm. The results demonstrate unambiguously that paramagnetic ions in concentrations so low as to have heretofore been regarded as insignificant have marked effects on the magnitude, frequency dependence, and temperature dependence of ${T}_{1}$ and to a lesser extent on ${T}_{2}$, and on the electronic structure and hydrogen diffusion parameters derived therefrom. The ${\mathrm{Gd}}^{3+}$ ion contributes an additional spin-lattice relaxation rate ${T}_{1p}$, which in these hydrides arises entirely from the dipolar coupling between impurity and proton moments. Proton magnetization is transported to the relaxation centers by spin diffusion at low temperatures and by hydrogen-atom diffusion at intermediate and high temperatures. The rate ${R}_{1p}$ is directly proportional to Gd-ion concentration at both low and high temperatures, but in the atom diffusion regime ${R}_{1p}$ is 20-25 times greater than for spin diffusion. The impurity-induced relaxation is shown to have profound effects on the apparent nuclear-nuclear dipolar relaxation rate ${R}_{1d}$ associated with hydrogen diffusion. At impurity levels as low as 10 ppm Gd, a secondary minimum appears in the temperature dependence of ${T}_{1}$ which may be readily misinterpreted in terms of a second motional process with lower activation energy. Even lower impurity levels yield a characteristic "slope-change" effect, which may be construed as indicating a change in the activation energy for hydrogen diffusion. At low temperatures ${R}_{1p}$ interferes with the determination of the conduction-electron contribution ${R}_{1e}$ and the Korringa product ${T}_{1e}T$. Separation of ${R}_{1e}$ and ${R}_{1p}$ is complicated by the fact the ${R}_{1p}$ is not temperature independent as has typically been assumed. Methods of achieving this separation are discussed, and it is shown experimentally that this difficulty can be circumvented by replacing the major part of the hydrogen with deuterium, thereby inhibiting spin diffusion. Measurement of ${T}_{1}$ as a function of resonance frequency and of ${T}_{2}$ can also be of value in separating the various sources of relaxation.

Exciton–phonon interaction in crystalline and vitreous SiO2

Abstract: An investigation of the Urbach rule in crystalline SiO2 (α-quartz), crystalline SiO2Ge, and vitreous SiO2 in the framework of the models of Dow and Redfield and of Toyozawa shows that the fundamental optical absorption edge of SiO2 is determined by strong exciton–phonon interactions. In the crystalline and vitreous SiO2 low-energy excitons with E = 9.1 eV (α-quartz) and E = 8.7 eV (glass) interact mainly with longitudinal optical phonons. In the crystalline SiO2Ge alloy the interaction with phonons is realized by excitons localized on germanium impurities. The exciton–phonon interaction in SiO2Ge and vitreous SiO2 is stronger than in crystalline SiO2 due to the higher degree of localization of low-energy excitons in these disturbed SiO2 forms. The strong exciton–phonon interaction leads to a momentary self-trapping of the low-energy excitons in crystalline and non-crystalline SiO2 and to a corresponding relaxation of the SiO2Ge excitons localized on Ge impurities. Numerical values of the fundamental exciton parameters are deduced and discussed. Eine Untersuchung der Urbach-Regel in kristallinem SiO2 (α-Quarz), kristallinem SiO2Ge und in SiO2-Glas im Rahmen der Modelle von Dow and Redfield sowie Toyozawa zeigt, das die fundamentale optische Absorptionskante von SiO2 durch starke Exziton–Phonon-Wechselwirkungen bestimmt wird. In kristallinem und glasernem SiO2 wechselwirken niederenergetische Exzitonen mit E = 9,1 eV (α-Quarz) und E = 8,7 eV (Glas) hauptsachlich mit longitudinalen optischen Phononen. In der kristallinen SiO2Ge-Verbindung wird die Wechselwirkung mit Phononen durch Exzitonen realisiert, die an Germaniumverunreinigungen lokalisiert sind. Die Exziton–Phonon-Wechselwirkung in SiO2Ge und glasernem SiO2 ist infolge der hoheren Lokalisierung der niederenergetischen Exzitonen in diesen gestorten SiO2-Formen starker als im kristallinen SiO2. Die starke Exziton–Phonon-Wechselwirkung fuhrt zu einem augenblicklichen Selbsteinfang der niederenergetischen Exzitonen im kristallinen und nichtkristallinen SiO2 und zu einer entsprechenden Relaxation der SiO2Ge-Exzitonen, die an Ge-Verunreinigungen lokalisiert sind. Numerische Werte der fundamentalen Exzitonparameter werden ermittelt und diskutiert.

A determination of individual spectral densities in a smectic liquid crystal from angle dependent nuclear spin relaxation measurements

Abstract: Deuterium relaxation experiments have been carried out at 38.4 MHz from 29.9 to 44.3 °C on p‐butoxybenzylidene‐p‐n‐octylaniline (40⋅8), a smectic B liquid crystal, which had been deuterated in the aniline ring and along the octyl chain. The Jeener–Broekaert pulse sequence was used to determine the spectral densities of motion J1(ω) and J2(ω), and by rotating the sample such that the optic axis was oriented at three different angles to the external magnetic field we are able to obtain information about the frequency dependence not just of J1 and J2 but also of J0. We report here the first measurements of the latter parameter at frequencies other than zero.

NMR study of spectral densities over a large frequency range for intermolecular relaxation in liquids: Pair correlation effects

Abstract: In order to study the dynamic effects of pair correlation functions on intermolecular translational relaxation in liquids, we have measured the relaxation times T1 of the protons in neopentane in the presence of di‐t‐butylnitroxide free radicals with concentrations varying up to 6.5×1020 paramagnetic centers × cm−3. Under these conditions, the dominant relaxation mechanism for the resonating protons arises from their interactions with the electronic spins of the free radicals. NMR experiments were performed between 2.2 and 250 MHz and these allowed us to obtain information about the behavior of the relevant spectral densities over a very wide range of frequencies 0<ωSτ≲50, where ωS is the electronic resonance frequency of the radical and where τ is the translational correlation time. It is shown that a model in which the effects of eccentricity of the spins are included and where the translational motion of the molecules is described by a Smoluchowski diffusion equation, (i.e., taking into account their i...

Proton nuclear magnetic resonance investigation of the conformation and dynamics in the synthetic deoxyribonucleic acid decamers d(ATATCGATAT) and d(ATATGCATAT)

Abstract: A variety of one-dimensional proton NMR methods have been used to investigate the properties of two synthetic DNA decamers, d(ATATCGATAT) and d(ATATGCATAT). These results, in conjunction with the results of two-dimensional NMR experiments, permit complete assignment of the base proton resonances. Low-field resonances were assigned by sequential "melting" of the A . T base pairs and by comparison of the spectra of the two decamers. Below 20 degree C spin-lattice relaxation is dominated by through-space dipolar interactions. A substantial isotope effect on the G imino proton relaxation is observed in 75% D2O, confirming the importance of the exchangeable amino protons in the relaxation process. A somewhat smaller isotope effect is observed on the T imino proton relaxation. At elevated temperatures spin-lattice relaxation of the imino protons is due to proton exchange with solvent. Apparent activation energies for exchange vary from 36 kcal/base pair for base pairs (3,8) to 64 kcal/mol for the most interior base pairs (5,6), indicating that disruption of part, or all, of the double helix contributes significantly to the exchange of the imino protons in these decamers. By contrast, single base pair opening events are the major low-temperature pathways for exchange from A X T and G X C base pairs in the more stable higher molecular weight DNA examined in other studies. The temperature dependence of the chemical shifts and line widths of certain aromatic resonances indicates that the interconversion between the helix and coil states is not in fast exchange below the melting temperature, Tm. Within experimental error, no differential melting of base pairs was found in either molecule, and both exhibited melting points Tm = 50-52 degrees C. Spin-spin and spin-lattice relaxation rates of the nonexchangeable protons (TH6, AH8, and AH2) are consistent with values calculated by using an isotropic rotor model with a rotational correlation time of 6 ns and interproton distances appropriate for B-family DNA. The faster decay of AH8 compared with GH8 is attributed to an interaction between the thymine methyl protons and the AH8 protons in adjacent adenines (5'ApT3'). The base protons (AH8, GH8, and TH6) appear to be located close (1.9-2.3 A) to sugar H2',2" protons.(ABSTRACT TRUNCATED AT 400 WORDS)

Nuclear magnetic resonance studies of wheat starch pastes

Abstract: Nuclear magnetic resonance measurements have been made on wheat starch pastes with contrasting rheological properties. Carbon-13 measurements indicate that about 60% of the polysaccharide in the pastes exists as mobile liquid-like polymer while the remaining 40% is in an immobile crystalline form. The ratio of mobile to crystalline polymer is the same for all pastes. Proton (1H) relaxation measurements show that in contrast to other gel systems the observed water relaxation times are not related to rheological properties and are governed by exchange between bulk and mobile polymer-associated water. Exchange at crystalline polymer sites is either nonexistent or too slow to be of significance on the NMR time scale. Measurements have also been made of water diffusion coefficients for pastes and for amylopectin-water systems. The modified Wang equation when fitted to the diffusion data gives shape factors that are consistent with an oblate ellipsoidal shape for the amylopectin molecule in solution.

Fast and exceptionally slow vibrational energy transfer in acetylene and phenylacetylene in solution

Abstract: The excitation of the CH‐stretching mode (ν3) of C2H2 in CCl4 leads to fast population (<2 ps) and very slow depopulation (240 ps) of the symmetric CC‐stretching mode (ν2). The long lifetime of ν2 is due to the absence of anharmonic interaction to neighboring states (symmetry forbidden). The corresponding CC‐stretching mode of C6H5–C2H has a lifetime of 15 ps. The phenyl group relaxes the symmetry restrictions and introduces new decay channels.

Spin–spin and spin‐lattice contributions to the rotating frame relaxation of 13C in L‐alanine

Abstract: The spin‐lattice relaxation times in both the Zeeman (TC1) and rotating (TC1ρ) frames were determined for three chemically distinct carbon atoms (13C) in polycrystalline L‐alanine by combining crosspolarization and magic angle spinning techniques together with proton decoupling. The spin‐lattice and spin–spin contributions to the experimentally measured TC*1ρ could be separated by an experiment in which the 13C spin‐locking field was varied. The spin‐lattice contributions (TC1ρ), which contain motional information, were determined to be 21.7, 23.4, and 138 ms for the Cα, CH3, and COO− carbons, respectively. The spin–spin contribution (TDCH) was found to be exponential, namely, (TDCH)−1 ∝exp(−2πνeCτD) in the low 13C spin‐locking field. Therefore, the assumption of a Lorentzian correlation function for the proton dipolar fluctuations is adequate for L‐alanine. Furthermore, the proton dipolar correlation times τD were found to be the same (31±1 μs) for all three carbons in L‐alanine. The spin‐lattice relaxat...

Highly excited states of nitric oxide studied by two-color double resonance spectroscopy

Abstract: Double resonance enhanced four‐photon ionization spectra and fluorescence dip spectra of nitric oxide have been simultaneously measured using two tunable dye lasers. The spectra showed clear rovibronic structure of the 2Σ+ and 2Π states in the third‐photon region (65 000∼70 000 cm−1). Laser power dependencies of the fluorescence dip and the ionization signals have been analyzed by kinetic rate equations and the absorption cross section of the transition between the second resonance state and the A 2Σ+ state has been obtained. Finally, by varying the delay time between the pulses from the first and second dye lasers, we have observed collision induced rotational relaxation in the A 2Σ+ state. The relaxation rate was found to be comparable to the electronic quenching rate.