N.M.R. solid echoes in systems of dipolar-coupled inequivalent spins-1 or dipolar-coupled inequivalent pairs of spins-1/2

Proton solid-echo transverse relaxation functions for many thermotropic and lyotropic mesophases, mapped by measurement of the echo amplitude S_yx (t′ = τ) as a function of τ using a P_y (90°)-τ-P_x (90°)-t′ sequence, yield gaussian behaviour of the form exp [-½M ^E ₂τ²] for decays up to 20–30 per cent of the value at τ = 0. M ^E ₂, the second moment for the dipolar interactions between the spin-½ pairs, is related to the van Vleck second moment M ^VV ₂ through a factor f. Whilst experiments suggested a value of 0·70–0·72 for f = M ^E ₂/M ^VV ₂, simple models that ignored the non-equivalence of the dipolar-coupled spin-pairs had predicted f = 0·65. In this paper we derive an exact analytic expression for the spin response of a model of two dipolar-coupled inequivalent spins-1 to the pulse sequence P_y (90°)-τ-P _α(β)-t′, and show that the present model, with the quenching of the spin-flip terms of the dipolar hamiltonian, resolves the afore-mentioned discrepancy. We also reconcile the differences between the experimental and the earlier predicted values of f for deuteron N.M.R. spin echoes in perdeuterated solids.     

Contribution of proton NMR relaxation to the investigation of molecular dynamics in columnar mesophases of discotic and polycatenar molecules

We present in this work a review concerning wide frequency rangeT ₁ proton NMR relaxation studies performed in compounds exhibiting columnar mesophases, namely the Col_ho mesophase in the case of a liquid crystal of discotic molecules and the ø_h mesophase in the case of a liquid crystal of biforked molecules. These NMR relaxation studies were performed combining conventional and fast field cycling NMR techniques in a frequency range between 100 Hz and 300 MHz. The possibility of probing such a large frequency range has provided a way to effectively distinguish the influence, on theT ₁ relaxation profiles, of the different molecular movements observed in this type of mesophases. In addition, we present a comparison between the molecular dynamics in columnar (ø_h) and lamellar (SmC) mesophases exhibited by the same biforked compound.     

A theory of the dielectric loss in the aligned nematic mesophase

A theory of dielectric absorption and dispersion in the nematic phase is developed which does not rely on the use of a nematic “director”. The major features of the spectrum are reproduced by using the fact that the autocorrelation functions 〈μ(0) · υ(t)〉 and 〈υ(t) · μ(0)〉 are no longer symmetry disallowed when the overall sample is anisotropic, as in the aligned nematic. Here μ is the dipole vector of the diffusing molecule and υ the centre of mass linear velocity.     

Dielectric losses in statistical mixtures

The specific features of the dielectric spectra of statistical mixtures in the form of heterogeneous systems with spherical particles chaotically arranged in the space have been investigated. The distribution function of relaxation times f(τ) has been restored. It has been established that the relaxation times are continuously distributed within a wide interval [τ₁, τ₂]. Different methods for broadening the relaxation time distribution interval and approximating the relaxation time distribution function f(τ) have been analyzed. It has been demonstrated that f(τ) is a nonmonotonic function with two maxima at the boundaries and a minimum in the vicinity of the midpoint of the interval [τ₁, τ₂]. These features of the relaxation time distribution function are responsible for the large difference between the average relaxation frequencies of the permittivity and the dielectric loss (electrical conductivity).     

Solvation of large dipoles

A model system consisting of one strongly polar molecule dissolved in a pool of 499 moderately polar particles is studied in a molecular dynamics experiment. A pair potential of Stockmayer type is used. The structure of the solution has been analysed in terms of g(R), f(R), h _Δ(R), h _D(R), g(R, x), and Voronoi polyhedra. Solvent-solvent spatial correlations are presented and compared to solute-solvent properties already discussed in part I. Voronoi polyhedra analysis is also used to define structural relaxation around solute or solvent particles unambiguously. The dynamics of the system is described by means of angular momentum, orientational, and velocity autocorrelation functions. The solute's influence on the dynamics of the solvent is analysed by a polyhedral decomposition of time correlation functions.     

The analytical potential energy function of flue gas SO2（X1A1）

The equilibrium structure of flue gas SO2 is optimized using the density functional theory （DFT）/B3P86 method and CC-PV5Z basis. The result shows that it has a bent （C2v, X1A1） ground state structure with an angle of 119.1184°. The vibronic frequencies and the force constants are also calculated. Based on the principles of atomic and molecular reaction statics （AMIIS）, the possible electronic states and reasonable dissociation limits for the ground state of SO2 molecule are determined. The potential functions of SO and 02 are fitted by the modified Murrell-Sorbie＋c6 （M-S＋c6） potential function and the fitted parameters, the force constants and the spectroscopic constants are obtained, which are all close to the experimental values. The analytic potential energy function of the SO2 （X1A1） molecule is derived using the many-body expansion theory. The contour liues are constructed, which show the static properties of SO2 （XIA1）, such as the equilibrium structure, the lowest energies, the most possible reaction channel, etc.     

Phase Behavior of Bow-Shaped Hard Particles in Two Dimensions

The phase behavior of a two-dimensional hard-particle model is studied via Monte Carlo simulations using the grand canonical, the isobaric and the canonical ensembles. This model consists of a three-segmented line whose geometry resembles a bow shape. The model reduces to some limiting cases: hard needles and bent-core particles. Manipulating the molecular parameters, a variety of molecular shapes were generated. As a result, several liquid crystalline structures like nematic and tetratic were obtained. Additionally, there are some other regions where the molecules form curvilinear paths. As the density increases, the formation of clusters of two or more particles is observed, producing assemblies with different shapes depending on the particular values of the molecular parameters. One interesting example is when the clusters have chiral features despite the particles are achiral. The two-dimensional tetratic, nematic and polar order parameters as well as the orientational correlation functions g ₂(r ^?) and g ₄(r ^?) and the distribution functions g _∥ and g _⊥ were calculated to describe the resulting mesophases. Besides this, the Gibbs ensemble was used to investigate some cases where indications of first-order phase transitions appeared. The mesophases diagrams are also reported.     

Investigation of ferroelectric properties of strontium barium niobate crystals by second harmonic generation technique

The ferroelectric properties of Sr _x Ba_{1 − x} Nb₂O₆ (SBN-x) crystals have been investigated using noncollinear (diffuse) second harmonic generation of IR radiation converted by a disordered domain structure. The switching characteristics have been determined from the dependences of the second harmonic intensity I _2ω(t) and I _2ω(E) for the SBN-0.75 crystals. These characteristics are in good agreement with the results of measurements by traditional methods and exhibit a number of specific features (long polarization relaxation times, irreproducibility of the shape of hysteresis loops, low-frequency dependence of the quantity E _c ) associated with the relaxor nature of SBN solid solutions. The kinetics of the diffuse second harmonic generation can be described in the framework of the phenomenological approach to polarization relaxation processes in relaxor ferroelectrics. The intensity of the diffuse second harmonic in switching is determined by the parameters (duration, amplitude) of the switching field pulse.     

Anomalies in the magnetic behavior of a rapidly cooled chromium mesogen

This paper reports on the results of investigations into the magnetic behavior of a chromium mesogen containing the liquid-crystal columnar phase LCrCl₃, where L is an azacyclononane ligand. It is found that the magnetic susceptibility of LCrCl₃ remains constant in the temperature range from 4.2 to 10 K. This effect manifests itself upon rapid cooling of an LCrCl₃ sample in the mesophase and exhibits relaxation behavior. The relaxation magnetoelectric effect is explained in terms of the multiwell potential of the system and a thermally nonequilibrium orientational distribution of electric dipole moments of the Cr-Cl bonds involved.     

Effect of high electric fields on the nematic to isotropic transition in a material exhibiting large negative dielectric anisotropy

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (Δn) and the dielectric constant (epsilon_⊥) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (T_PN) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near T_PN, indicating the presence of polarised domains. Our measurements show that ΔT_PN(= T_PN(E)-T_NI(0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E². It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition.     

取向和非取向In2O3纳米线的场发射研究

用自制的设备制备了取向和无取向氧化铟纳米线，并研究了In₂O₃纳米线的场发射性质，发现取向纳米线比非取向纳米线有着更好的场发射特性.取向纳米线的开启和阈值场强明显低于非取向纳米线，这可能是由于取向纳米线之间的场屏蔽效应较弱以及取向纳米线有较多的顶部发射端的缘故. 关键词： 场发射 纳米线 取向 非取向     

Principles of generalized spin-echo spectroscopy

The concept is proposed for determining the total dynamic scattering function of an object under study, representing a sum of odd and even parts measured by the generalized neutron spin-echo method in the form of the signals S _odd(q, t) ～ ΣS(ω, q)sin(ωt)dω and S_even(q, t) ～ ΣS(ω,q)cos(ωt)dω as functions of the momentum q transferred to the neutron and the time t corresponding to the frequency ω and the transferred energy ?ω. The principle of the generalized spin echo and the results of mathematical modeling are confirmed in experiments on inelastic scattering on magnetic fluids and polymer solutions. The developed method makes it possible to study the features of the dynamics of atomic and molecular systems, e.g., to analyze soft vibrational spectra of nanoparticle ensembles against the background of intense relaxation processes, which is inaccessible for classical spin-echo spectrometry.     

Neutron diffraction studies of liquid carbon suboxide

The structure factor S _m(Q) for liquid carbon suboxide has been determined for a Q-value range of 0·4 to 60 Å^-1 by neutron diffraction measurements using a steady-state (reactor) and a pulsed (linac) neutron source. The bond lengths of the molecule have been determined from the data and give good agreement with the results of electron diffraction measurements on the vapour phase after application of a molecular recoil correction term. The quasi-linear nature of the molecule is confirmed but the shape of the form factor indicates that large amplitude bending motion probably occurs in the liquid phase.

Oscillations in the intermolecular pair correlation function are observed to have a regular periodicity extending to 12 Å but details of orientational effects cannot be established from a single diffraction measurement.     

Photon distribution function in blinking fluorescence of individual molecules

A photon distribution function w_N(T) for blinking fluorescence with bright on- and dark off-intervals is derived. The function w_N(T) is expressed via few Poissonian functions each of which relates to corresponding exponential process in quantum dynamics of a given individual molecule. The distribution of photons is calculated for short, middle and long time intervals as compared to off-intervals. The distributions are much broader than Poissonian distribution and have rather complicated shape. If time resolution of an experiment does not permit us to see off-interval and, therefore, fluorescence looks like CW emission, the distribution of photons gives a signal about existence of hidden off- intervals in such CW fluorescence.     

Vibrational relaxation in liquids

A new theory is presented for vibrational energy relaxation in a liquid. It is shown that a vibrationally excited probe molecule relaxes through interaction with the density fluctuations in the surrounding solvent fluid. This interaction occurs through a potential V(k), which is expressed in terms of the intermolecular force between the excited probe molecule and the surrounding fluid molecules. By assuming spherically symmetric solvent particles the T ₁ energy relaxation time for direct V-T processes is related to the translational dynamic structure factor for the fluid S(k, ω_v), evaluated at the vibrational resonance frequency. It is shown that this is described by gas-like particle motions on a very short distance scale corresponding to k vectors lying well beyond the first or second peaks of the fluid structure factor S(k). Such motions can be pictured as high-frequency, short-distance distortions of the local equilibrium configuration of the solvent particles around the probe. T ₁ ^-1 is found to be proportional to ρ_e T ^1/2 ω_v ^-3. The V-V energy exchange relaxation time is also calculated. This is found to be proportional to S(k, ω′) evaluated at a frequency ω′, corresponding to the vibrational energy missmatch. An energy gap law for the V-V process is derived.     

Magnetization of optically polarized gas atoms by an optical pulse train

The properties of the density matrix and the multipole moments arising in oriented and aligned atoms with zero nuclear spin through the interaction with strong resonant ultrashort pulses with wave vector k ₀ and circular or linear polarization have been found. Calculations have been made for the time-dependent light-induced magnetization μ(t′) of a gas of pre-oriented and prealigned atoms following the passage of a weak resonant elliptically polarized pulse with frequency ω and wave vector k collinear with k ₀. It is shown that for oriented atoms, μ(t′) is an even function of the detuning from resonance, ω-ω _ba, and can be split into two terms whose directions are a consequence of symmetry and are determined by the vectors k ₀ and k as well as by the direction of rotation of the electric fields corresponding to the pulses. For aligned atoms the vector μ(t′) is collinear with k, and the first term is an even function of ω-ω _ba. However, the second term is an odd function of ω-ω _ba and reverses direction when the sign of ω-ω _ba changes, as well as when the orientation of the axes of the polarization ellipse is changed. It is shown that if a series of weak linearly polarized pulses pass through the gas, the light-induced magnetization of the oriented and aligned gas atoms can be decomposed into three factors: the first determines the direction and is a consequence of the symmetry; the second (with the dimensions of magnetic moment) depends on the characteristics of the resonant transitions; and the third is a universal function of t′ and ω-ω _ba that does not depend on the underlying characteristics of the resonant transition. These vector factors and the universal functions are in principle different for oriented and aligned atoms. Zh. éksp. Teor. Fiz. 111, 63–92 (January 1997)     

Fine structure of strength functions for beta decays of atomic nuclei

The β transition strength function S _β(E) is the nuclear excitation energy distribution of moduli squared of the β-decay-type matrix elements. The function S _β(E) determines the characteristics of β decay, the spectra of accompanying radiation, and the probabilities of delayed processes following the β decay. Until recently, tools widely used for experimental investigation of the S _β(E) structure have been total absorption gamma spectrometers and total absorption gamma-ray spectroscopy (TAGS) which could not provide high energy resolution. Development of experimental techniques allows nuclear spectroscopy methods with high energy resolution to be used for studying the fine structure of S _β(E). A thorough investigation of this kind has been carried out for a number of nuclei produced at the YASNAPP-2 facility in Dubna. In this review, studies involving works on measuring the fine structure of S _β(E) in spherical and deformed nuclei are analyzed. Modern nuclear spectroscopy methods made it possible to identify the splitting of peaks from nuclear deformation in S _β(E) for Gamow-Teller (GT) transitions. The resonance nature of S _β(E) for first-forbidden (FF) transitions in both spherical and deformed nuclei is experimentally proven. It is shown that for some nuclear excitation energies, FF transitions can be comparable in intensity with GT transitions. Criteria for verifying the completeness of nuclear decay schemes are considered. The S _β(E) functions obtained by TAGS and by the high-resolution spectroscopy are compared.     

Optimal echo spacing for multi-echo imaging measurements of Bi-exponential T2 relaxation

Calculations, analytical solutions, and simulations were used to investigate the trade-off of echo spacing and receiver bandwidth for the characterization of bi-exponential transverse relaxation using a multi-echo imaging pulse sequence. The Cramer–Rao lower bound of the standard deviation of the four parameters of a two-pool model was computed for a wide range of component T₂ values and echo spacing. The results demonstrate that optimal echo spacing (TE_opt) is not generally the minimal available given other pulse sequence constraints. The TE_opt increases with increasing value of the short T₂ time constant and decreases as the ratio of the long and short time constant decreases. A simple model of TE_opt as a function of the two T₂ time constants and four empirically derived scalars is presented.     

Detecting columnar deformations in a supermesogenic octapode by proton NMR relaxometry

We used proton ( ¹H nuclear magnetic relaxation (NMR) dispersions to study the molecular dynamics in the isotropic phase and mesophases (nematic and columnar hexagonal) of a supermesogenic octapode formed by laterally connecting calamitic mesogens to an inorganic silsesquioxane cube through flexible spacers. The dispersions of the spin-lattice relaxation time (T₁) are interpreted through relaxation mechanisms used for the study of molecular dynamics in low-molar-mass liquid crystals but adapted to the case of liquid crystalline supermolecules. At high frequencies (above 10MHz) the behaviour of the T₁ with the Larmor frequency is similar for all phases and is ascribed to local reorientations and/or rotations. At intermediate and low frequencies (below 10MHz) our results show notable differences in the T₁ behaviour with respect to the mesophases. The nematic (N) and isotropic (Iso) phases’ low-frequency results are similar and are interpreted for both phases in terms of order director fluctuations (ODF), revealing that even in the isotropic phase local nematic order is detected by proton NMR relaxometry. Local nematic order in the Iso phase is interpreted in terms of the presence of nematic cybotactic clusters induced by the interdigitation of mesogens that is promoted by the silsesquioxane octapode molecular structure. In the columnar hexagonal (Col _h phase, the T₁ dispersions show that elastic columnar deformations (ECD) dominate the nuclear magnetic relaxation below 10MHz. This result shows that the columnar packing of the octapode clearly restricts the collective fluctuations of the mesogenic units inspite of their local nematic order.