摘要:

Molecular dynamics simulations on Xe in NaY and Ar in NaCaA zeolite are reported. Rates of cage-to-cage crossovers in the two zeolites exhibit trends which are contrary to that expected from geometrical considerations. The results suggest the important role of the sorbate-zeolite interactions in determining the molecular sieve properties of zeolites for small sized sorbates. The results are explained in terms of the barrier height for cage-to-cage crossover in the two zeolites.

ISSN:0026-8976    

DOI:10.1080/00268979300100011

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A random central force network model of a gel is constructed by relaxation of a bond diluted simple cubic lattice of Hooke's law springs under tension. The bond dilution procedure, which defines the model, involves a two stage process. First, the random removal of bonds connecting nodes at least one of which has a connectivity greater than a prescribed maximum value. Then the random removal of bonds to obtain any desired value of the mean node connectivity. The fracture of such a network under an incrementally applied uniaxial strain is studied by including a maximum value,Lb, for the extension of any spring before irreversible breakage. Random networks with maximally fourfold and maximally threefold connected nodes show qualitatively similar stress-strain curves, but quantitatively differ even when the mean node connectivities of the networks are the same. For sufficiently largeLb, the maximum value of the tension is linearly proportional toLbfor random networks with either maximally fourfold or maximally threefold connected nodes.

ISSN:0026-8976    

DOI:10.1080/00268979300100021

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Third, fourth and fifth virial coefficients are reported for isotropic 2-dimensional fluids of hard convex molecules of elliptical and spherocylindrical shapes, covering a wide range of elongations. Similar results are also included for hard lines of unit length, to give values in the limit of extreme elongation. Comparison with recent results forB3andB4confirms their values. BothB4andB5become negative when the elongation is large. ForB4this occurs only at very extreme elongations, around 50:1 for ellipses, and an even larger value for spherocylinders, whileB5becomes negative at much lower elongations, around 6:1 for ellipses.

ISSN:0026-8976    

DOI:10.1080/00268979300100031

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We show how to modify homonuclear cross polarization or mixing sequences to compensate for a range of spin coupling constants. The basis of the method is an analogy between complete polarization transfer, represented as a 180° pulse in a zero-quantum frame, and normal spin inversion pulses. The analogy lets us adapt existing compensation schemes for radiofrequency field inhomogeneity to the problem ofJcompensation in homonuclear cross polarization. We find that, inAXspin systems, complete polarization transfer can be made to occur for a broad range ofJvalues, providing the chemical shift difference between the spins is known. A contour plot of cross polarization efficiency as a function of pulse flip angle and precession angle under the chemical shift difference shows the degree of compensation for different combinations of these parameters. In arbitraryAMXspin systems, the formal analogy breaks down, but someJcompensation can still be achieved. These results can play an important role in maximizing cross peak intensity for 2D and 3D nuclear magnetic resonance applications.

ISSN:0026-8976    

DOI:10.1080/00268979300100041

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The21Ne,83Kr and131Xe NMR spectra of these noble gases dissolved in various thermotropic liquid crystals were recorded in the nematic phases at varying temperatures. The spectra display splittings due to the interaction between the nuclear quadrupole moment and the electric field gradient (EFG) at the nuclear site and, consequently, allow one to investigate electric field gradients. The temperature dependence of the EFG is found to be such that the field gradient arising from the electric multipoles of the liquid crystal molecules is not sufficient alone to explain the observations, but one additional contribution has to be taken into account. The quadrupole couplings were analysed in terms of a model based on the contributions from the liquid crystal molecules and from the deformation of the electron cloud due to the collisions of the atoms with liquid crystal molecules. The model provides fairly detailed information on the EFG contributions as well as on the liquid crystal order.

ISSN:0026-8976    

DOI:10.1080/00268979300100051

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Thev1band of the NO dimer has been studied using a long-path (84 m) low-temperature (83·5 K) gas cell and a Fourier transform infrared spectrometer. Over 440 lines of (NO)2have been measured in the 1840 to 1900 cm-1region, with a resolution of 0·008 cm-1, and assigned to perpendicular (Δka= ± 1) transitions withJandKavalues up to 35 and 16, respectively. These measurements were analysed, along with the four previous microwave transitions, using ans-reduced asymmetric rotor Hamiltonian in order to obtain accurate rotational and quartic centrifugal distortion parameters for (NO)2in its ground andv1= 1 vibrational states. As far as this experiment is concerned, the NO dimer behaves as a normal, planar, semi-rigid molecule, with no observed perturbations, only modest centrifugal distortion effects, and only small changes in parameters due to vibrational excitation. The observed intensities of (NO)2lines were used to estimate a value for the dissociation energy of the dimer ofD0= 639 ± 35 cm-1, where the indicated uncertainty depends mostly on an assumption that the vibrational transition moment of NO changes by 40% or less upon complex formation.

ISSN:0026-8976    

DOI:10.1080/00268979300100061

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Mono- and submonolayers of hexafluoroethane C2F6physisorbed on graphite have been studied by X-ray diffraction. At low temperatures the adlayer crystallizes in the commensurate 2 × 2 structure. At higher temperatures the layers expand into a triangular incommensurate state which gradually loses translational coherence with increasing temperature.

ISSN:0026-8976    

DOI:10.1080/00268979300100071

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Phase stability properties of fluid mixtures composed of hard sphere particles interacting via an attractive Yukawa tail are studied in the mean spherical approximation. The theory shows how the occurrence of either liquid-liquid or liquid-vapour spinodal decomposition correlates with the interparticle potential parameters and the density ρ* of the system. It appears that consolute-type decomposition is favoured at high ρ* or when the ‘unlike’ to ‘like’ interaction strength ratiovis small; by contrast, liquid-vapour type decomposition takes place at low ρ* or highvvalues. A numerical estimate of the threshold density separating the two regimes of phase stability is made on the basis of a very simple modelling of the interaction energies. An analysis is also made of theq→ 0 limit of the partial structure factorsSij(q), which gives an insight into the modifications occurring in the microscopic structural arrangement of the system at the ‘crossover’ point. Possible applications to real fluid mixtures are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300100081

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report for the first time an experimental realization of a ‘quantum time-translation machine’ via superpositions of time evolutions as suggested by Aharonov, Anandan, Popescu and Vaidman (1990,Phys. Rev. Lett.,64,2965). The physical system consists of two coupled spins 1/2 and the amplification scheme is applied to the heteronuclear coupling between the two spins. We give a simple explanation of the phenomenon and suggest possible classical analogues.

ISSN:0026-8976    

DOI:10.1080/00268979300100091

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Low temperature (T= 10 K) luminescence decay curves of A2SeCl6(A = Rb, Cs) are recorded which follow optical excitation at λexc= 458 nm applying different pulse lengths between 90 ns and 100 μs. For short excitation pulses a biexponential decay is observed, for pulse widthstexc> 10 μs a single exponential decay occurs. The experimental results can be explained from two rate equations which describe the (de)population of the lowest excited states (of octahedral s2systems) within the kinetics in a three-level system. The present compounds are obviously examples where thermally nonequilibrated populations of close-lying excited states can be observed within moderate time scales.

ISSN:0026-8976    

DOI:10.1080/00268979300100101

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor