Studies of molecular motions and interactions in acetonitrile

Microwave and far-infrared absorption coefficients are reported for acetonitrile over the whole concentration range in carbon tetrachloride solution. The data have been used to compute dipole reorientational correlation functions and relaxation times as a function of concentration. These show that τ_1R ^T increases non-linearly as the acetonitrile concentration increases (in contrast to the values of τ_2R ^T obtained from light scattering measurements) and as the viscosity decreases. This behaviour is examined in terms of intermolecular correlations between rotating dipoles. Intensity data are used to provide a check on the importance of induced dipole absorption in the far-infrared spectra and to calculate ‘static’ correlation factors g ⁽¹⁾. Both g ⁽¹⁾ and the dynamic correlation factor f ⁽¹⁾ reflect the effects of strong molecular interactions in these solutions.     

Magnetic and microstructural properties of the assembly of Ni(C) nanoparticles

Superparamagnetic properties of the assembly of carbon encapsulated Ni nanoparticles (Ni(C) nanoparticles) with an average particle size of 10.5 nm are studied, the blocking temperature (T_B) is determined to around 115 K at l000 Gs applied field. Above T_B, the magnetization M(H,T) can be described by the standard Langevin function L using the relation M/M_S(T=0)=coth(μH/kT)−kT/μH. Magnetization measurements suggest, this assembly of carbon encapsulated Ni nanoparticles have been exhibited typical single-domain, field-dependent superparamagnetic relaxation properties.     

Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

The equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece-Gouldwave. The electrons obtain the energy by linear collisionless wave absorption. The relaxation phenomena of density and energy are explained in terms of two relaxation times τ_E, τ₁ and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. In the experiment they are taken from the transient curves of Langmuir-probe current, optical line radiation and the noise power at the electron cyclotron frequency. The experimental conditions are: Helium-gas, p = 1 …? 5 Pa, T_e = 4 eV, n = 1 …? 5 · 10¹⁰ cm^?3, B = 6,5 · 10^?2 T, 27 MHz rf plasma source, low frequency fluctuation level < 1%, classical losses. The energy relaxation time …?_E = 10 …? 15 μs is given by inelastic collision losses. The ionization time constant τ₁ is related to the instantaneous ionization frequency during the transient state. It shows a high value at the very beginning of the pulse which must be explained by a tail formation in the distribution function and enhanced radial losses becoming Bohm-like in the transition phase.     

New pulse sequences for T1− and T1/T2−contrast enhancing in NMR imaging

Improved pulse sequences DIFN (abbreviation of the words: DIFferentiation by N pulses), 90° − τ₁ − 180° − τ₁ − … 180° − τ_n, with optimised time intervals τ₁ for T₁ measurement and contrast enhancing in NMR imaging are presented. The pulse sequences DIFN have a better sensitivity to T₁ than the well-known pulse sequence SR. In contrast to the IR pulse sequence, the information given by the DIFN pulse sequence is more reliable, because the NMR signal does not change its sign. For a given time interval τ₀ ≤ (0.1 − 0.3) T₁′ the DIFN pulse sequences serve as T₁-filters. They pass the signal components with relatively short T₁ < T₁′ and suppress the components with relatively long T₁ < T₁′. The effects of the radiofrequency field inhomogeneity and inaccurate adjusting of pulse lengths are also considered. It is also proposed in this work to use the joint T₁T₂-contrast in NMR imaging obtained as a result of applying the DIFN pulse sequences in combination with the well-known Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. The region of interest, where the contrast should be especially enhanced, is specified by the two times at which measurements are performed, which allow the amplitudes of pixels to reach some defined levels by spin-lattice and spin-spin relaxation.     

Theoretical reason for the lack of influence of 1H–14N cross-relaxation on the water proton T 1 NMRD profile in slow tumbling proteins

For immobilized protein the water proton T ₁-NMRD profile displays three enhanced relaxation peaks (QP). For slow tumbling proteins these relaxation peaks are not experimentally observed. However, the theoretically determined QP effect on the amide proton T ₁-NMRD profile displays a distorted Lorentzian dispersion profile. The question arises as to whether there is also a distortion of the water-proton T ₁-NMRD profile due to QP. The model of Sunde and Halle [J. Magn. Reson. 203, 257 (2010)] predicts a decreasing QP relaxation contribution and, with the aid of a model for tumbling proteins [P.-O. Westlund, Phys. Chem. Chem. Phys, 12, 3136 (2010)], it is shown that the QP effect is absent in water-proton T ₁-NMRD profiles for slow tumbling proteins with τ_R?I.     

Model-Free Approach beyond the Borders of Its Applicability

Model calculations presented in this article show that commonly used methodology of¹⁵N relaxation data analysis completely fails in detecting nanosecond time scale motions if the major part of the molecule is involved in these motions. New criteria are introduced for the detection of such cases, based on the dependence of the apparent overall correlation time, derived from theT₁/T₂ratio, on the spectrometer frequency. Correctly estimating the overall rotation correlation time τ_Rwas shown to play the key role in model-free data analysis. It is found, however, that in cases of slow internal motions with characteristic times of more than 3–4 ns, the effective τ_Rprovided by theT₁/T₂ratio for individual amide nitrogens can be used for the characterization of the fast picosecond internal dynamics.     

Energy relaxation of two-dimensional electrons in the quantum Hall effect regime

The mm-wave spectroscopy with high temporal resolution is used to measure the energy relaxation times τ_e of 2D electrons in GaAs/AlGaAs heterostructures in magnetic fields B=0–4 T under quasi-equilibrium conditions at T=4.2 K. With increasing B, a considerable increase in τe from 0.9 to 25 ns is observed. For high B and low values of the filling factor ν, the energy relaxation rate τ _e ^?1 oscillates. The depth of these oscillations and the positions of maxima depend on the filling factor ν. For ν>5, the relaxation rate τ _e ^?1 is maximum when the Fermi level lies in the region of the localized states between the Landau levels. For lower values of ν, the relaxation rate is maximum at half-integer values of τ _e ^?1 when the Fermi level is coincident with the Landau level. The characteristic features of the dependence τ _e ^?1 (B) are explained by different contributions of the intralevel and interlevel electron-phonon transitions to the process of the energy relaxation of 2D electrons.     

Controlling the width of picosecond laser pulses

The relaxation time τ_R of the saturable dye used to mode-lock a Nd: YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Δω changed by the insertion of an etalon. The pulse duration τ_p was approximately transform-limited for τ_R<2π/Δω but increased to about twice this value when 2π/Δω<τ_p<τ_R. No significant increase in pulse duration was observed for τ_R?2π/Δω but multiple pulses were generated within each round-trip-transit-time.     

Auditory detection of paired pulses by a dolphin in the presence of a pulse jam

From behavioral studies of a bottlenose dolphin (Tursiops truncatus), the audibility thresholds were measured for a single pair of equal-amplitude pulses, i.e., clicks, presented to the dolphin in combination with a pulse jam. The pulse jam consisted of pairs of identical pulses with a pulse spacing τ_j within the pairs and a pair repetition rate f _j. Series of pulses were interrupted by a pause R>1/f _j, within which the pulse jam was absent while a pair of test pulses was supplied to one of the two channels at random. Each series had a duration T, and the total stimulation cycle was J=T+R. The dependence of the test pair detection threshold on the pulse spacing τ_j was studied at different fixed values of the pulse spacing in the test pair: τ_t=50, 100, 200, and 500 µs. Preliminary measurements performed with τ_j=τ_t=100 µs were used to adjust the parameters of the pulse jam. The threshold shift at τ_j=τ_t=100 µs reached 35 dB above the audibility threshold of the test pair in the absence of the pulse jam. On both sides of the point τ_j=τ_t=100 µs the thresholds decreased with varying τ_j to approximately 20 dB above the detection threshold of the test pair in the absence of the jam. However, in the course of training, the threshold curves gradually shifted downwards approaching the detection level of the test pair in the absence of the jam and becoming progressively flatter (the selectivity with respect to the pulse jam vanished). A decrease in the pause duration R restored the dependence of the test pair detection threshold on τ_j. In this case, a statistically significant maximum was obtained at τ_j=τ_t for τ_j within the critical interval (for τ_t<500 µs). Beyond the critical interval (for τ_t>500 µs), even with the smallest pause duration (R=15 ms), no dependence of the test pair detection thresholds on τ_j could be observed.     

Molecular motion in liquid benzene by nuclear magnetic resonance

The proton spin-lattice relaxation time, T ₁, has been measured for a series of mixtures of benzene in perdeuterobenzene for the liquid in equilibrium with its vapour over the temperature range from below the normal freezing point up to the critical temperature. The two contributions to T ₁ due to interactions within the molecule (T _{1 intra}) and between molecules (T _{1 inter}) have been separated and are found to be very different in magnitude and in variation with temperature. The variation and magnitude of T _{1 inter} correlates well with other translational motion dependent properties such as self diffusion and viscosity. The correlation of T _{1 intra} with other re-orientation dependent properties such as deuteron T ₁ and Rayleigh scattering is poor.

The observed variation in T _{1 intra} and in particular the broad maximum at higher temperatures is then interpreted as due to a combination of dipolar and spin-rotation effects. This interpretation results in good agreement between the activation energies for re-orientational molecular motion deduced from proton T ₁ and deuteron T ₁. It supports the Hubbard theory for the relation between the dipolar and spin-rotation correlation times τ_d and τ_sr. It gives a rough value, 3·8 kc/s, for the spin-rotation interaction constant for protons in benzene. Reasonable values for τ_d and τ_sr are predicted and for all temperatures τ_sr < τ_d as expected.

There is clearly a considerable difference between the re-orientational and translational motion of the molecules in liquid benzene but the exact nature of the difference cannot be elucidated.     

Molecular field theory analysis of R3Co11B4 compounds

The temperature dependence of magnetization of the R₃Co₁₁B₄ compounds has been analysed using the two-sublattice molecular field theory. The molecular field coefficients, n_CoCo, n_RCo, n_RR, have been calculated by a numerical fitting process. The analytic form of the exchange field H_R(T) varying with temperature for each of the R₃Co₁₁B₄ compounds is presented, and some results are discussed.     

The influence of CO adsorption on the galvanomagnetic and magnetic properties of evaporated nickel films

The influence of CO adsorption on the ordinary Hall coefficient, R_HO, the extraordinary Hall coefficient, R_HE, the perpendicular magnetoresistivity ΔR_{mag O} and the saturation magnetization, B_S, of Ni films with thicknesses between 1 and 200 nm has been studies as a function of the CO coverage at 77 and 273 K. There is a maximum in ΔR_HE at a coverage of about half a monolayer. ΔR_HO, ΔΔR_{mag O} and ΔB_S exhibit an oscillating behaviour.     

Influence of Thomson effect on the resultant local Seebeck coefficient in thermoelectric composite materials

The resultant local Seebeck coefficient α _R (=α _S−α _T) at the interface of a thermoelement has not yet been measured, although it is an important factor governing the thermoelectric efficiency, where α _S is the local Seebeck coefficient and α _T is the one caused by the Thomson effect. It is shown in this paper that α _S, α _T, and α _R of the p- and n-type Cu/Bi–Te/Cu composites are obtained analytically and experimentally on the assumption that the local temperature of the composite on which the temperature difference ΔT is imposed varies linearly with changes in position along the composite. They were indeed estimated as a function of position from the local experimental data of R,ΔI,ΔT, and V generated by applying an additional current of ±I to the composite, where R is the electrical resistance and ΔI is a current generated by the composite. As a result, it was found that the absolute values of α _S at the hot interface of the p- and n-type composites are approximately 1.5 and 1.4 times higher than their lowest values in the middle region of the composite, respectively, while those of α _T are less than 8% of α _S all over the composite and are so small that the relation α _R≈α _S can be held. We thus succeeded in measuring α _R at the interfaces of the composite.     

Relaxation mechanism in γ-ray-irradiated L-alanine studied by transfer saturation EPR and pulse EPR

Stable paramagnetic centers in γ-ray-irradiated L-alanine dosimeters exhibit a maximum in relaxation rate in the vicinity of 190 K. The mechanism of this relaxation rate has been investigated on the first stable alanine radical center, SARI, by employing continuous-wave transfer saturation electron paramagnetic resonance and pulse electron paramagnetic resonance techniques. The detected in-phase and out-of-phase spectra as well as phase memory times,T _M, indicate that besides the well-known τ_p of the CH₃ group of SAR1 an additional correlation time, τ_l (ΔElk=2689±50 K and 0 τ₁₀ = 0.15 ± 0.03 ps), is involved in the transverse relaxation process and effects the SAR1 center. For the SAR1 center this mechanism originates from the hindered motion of undamaged CH₃ and NH ₃ ⁺ groups in the lattice. The motion of these groups additionally effects the spectrum of the SAR1 center through averaging out of the anisotropic splitting.     

Fast electro-optical response of a cell with a homeoplanar layer of a nematic liquid crystal

The refraction of light, i.e., the turn of an extraordinary ray in the liquid crystal layer similar to total internal reflection at an interface between two media, has been studied in a cell with the homeoplanar orientation of the director. The rise, τ_on, and decay, τ_off, times of optical responses have been obtained for various angles of incidence of light on a liquid crystal layer subjected to an electric field. The times τ_on and τ_off of optical responses for the angles of incidence much larger than the angle of total internal reflection are 1–2 ms, which is three orders of magnitude smaller than the relaxation time of an optical response in the case of normal incidence of the ray.     

Hyperfine interactions at 139La nuclei in La1-x SrxCoO3 (x = 0.25, 0.50) perovskites

The hyperfine fields B ₀, enhancement factors η, and transverse relaxation times T ₂ for ¹³⁹La nuclei in La_0.75Sr_0.25CoO₃ and La_0.5Sr_0.5CoO₃ perovskites at 4.2 K are measured by pulsed nuclear magnetic resonance spectroscopy. The hyperfine coupling constants P for ¹³⁹La nuclei in the perovskites under investigation do not depend on the composition or the crystal lattice type. The local anisotropy fields are evaluated from the data on the enhancement factor η. The dependence of the echo amplitude decay on the delay time τ of the second radio-frequency pulse does not exhibit an exponential behavior. The amplitude decay rate increases with an increase in the delay time τ. This suggests that the Suhl-Nakamura interaction contributes to the transverse relaxation time T ₂.     

Generalized RF Pulse Train with Insensitivity to B 1 Inhomogeneity

Adiabatic inversion recovery radiofrequency (RF) pulse techniques are used to address B ₁ inhomogeneity; however, the specific absorption rates of these techniques are significantly higher than that of non-adiabatic RF pulse techniques. In addition, time efficiency is poorer because of the required longer inversion recovery time. Therefore, an RF pulse train with three subpulses was previously developed and reported. The purpose of this article was to generalize the RF pulse train for tissues with different T ₁ relaxation times and in a different application. The RF pulse train B ₁ insensitivities and frequency responses were calculated with different T ₁ relaxation times and different subpulse durations using the Bloch equation. The previously reported optimal flip angle (FA) combination was used. When using the optimal FA combination, the RF pulse train B ₁ insensitivity did not change even if the T ₁ relaxation times and the subpulse durations did change. In other words, the optimal FA combination does not require adjustments according to the T ₁ and subpulse duration. The RF pulse train frequency responses with these subpulses can be dramatically improved even if the inherent subpulse frequency response is poor. This finding will facilitate RF pulse train technique implementation on magnetic resonance imaging scanners.     

Numerical Determination of the Relaxation Parameters of NMR in Complex Heterogeneous Systems

The methods of mathematical processing of the envelopes of spin-echo signals have been considered within the framework of the multiphase relaxation theory. A mathematical model for separation of multiexponential relaxation curves into individual exponential components of spin-spin relaxation times T _2i and amplitudes I _i is described. The multiphase nature of the relaxation of protons in complex heterogeneous systems has been revealed, and the relaxation characteristics of individual components — spin-spin relaxation times and amplitudes — have been determined.     

Proton spin-lattice relaxation in liquid water and liquid ammonia

The proton spin-lattice relaxation time has been measured at 20·8 Mc/s for a series of solutions of water in heavy water and solutions of ammonia in heavy ammonia for the temperature range from the melting point to the liquid-vapour critical temperature. Measurements have also been made for water over limited temperature ranges at several fixed densities.

The contributions to the spin-lattice relaxation time from direct dipolar and spin-rotation interactions have been separated. The spin-rotation interaction contribution appears to be the same for H₂O as for HDO and also as between NH₃, NH₂D and NHD₂ and this result is justified. The correlation times for molecular re-orientation, τ_d, and for molecular angular velocity, τ_sr, are derived from the results and in so doing some support for the Hubbard [12] relation betweent τ_sr and τ_d is adduced. It is found that at the critical temperature τ_sr?τ_d which contrasts with other liquids for which it is usually found that τ_sr??τ_d. The spin-rotation interaction constants in the water and ammonia molecules are found to be approximately 120 kc/s and 80 kc/s, respectively.

An attempt to separate the inter- and intra-molecular contributions to the dipolar spin-lattice relaxation time is possible in principle, in spite of the rapid proton exchange, but is frustrated by the fact that the equilibrium constants are little different from their statistical values. Nevertheless there is evidence that the two interactions vary in much the same way with temperature.

The correlation times deduced from the dipolar relaxation time show close relationship with dielectric, self diffusion and deuteron relaxation time data.

It is suggested that the re-orientation of both water and ammonia molecules may be by a small angle Brownian diffusion even near the critical temperature.