摘要:

The equations describing the spin-diffusion-coupled nuclear magnetic spin-lattice relaxation behaviour of a one-dimensional, two-region system are solved exactly. The solutions allow the calculation of the relaxation times and their associated amplitudes in terms of the values of the sizes, spin-diffusion coefficients and intrinsic relaxation times of each region. The morphology of the roots of the equation are represented graphically in a manner which illustrates their behaviour as a function of the system parameters. The results obtained from these exact calculations are compared with numerically simulated data which mimic experimental measurements and which are fitted to a maximum of four exponential processes. Good agreement is demonstrated, particularly for the longer relaxation components when there are several roots with significant amplitude. Preliminary consideration is given to the inverse problem of deriving system parameters from the observed relaxation behaviour.

ISSN:0026-8976    

DOI:10.1080/00268978700102601

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Statistical mechanical theory of inhomogeneous fluids is used to discuss the continuous growth of adsorbed fluids at wall-fluid interfaces. An ansatz for the form of the singular behaviour of the transverse surface structure factor leads to rederivations of all classes of continuous wetting behaviour expected in the absence of bulk critical phenomena, apart from the strong fluctuation regime of critical wetting, thereby providing one route by which previous results obtained from phenomenological interface hamiltonians and mean-field density functional theory may be based on a true many-body hamiltonian. Earlier reports dealt with systems involving power-law interactions. This paper discusses the complicated case of three dimensional systems in the absence of long-range forces. In particular, the relevant bulk correlation length, determining the appearance of various classes of behaviour, is identified with the asymptotic decay length of the bare profile of the fluctuating interface. This lends strong support to the view that continuous wetting systems can be viewed in terms of a delocalizing mean-field interface, renormalized by capillary-wave fluctuations. A discussion is given of the behaviour likely to be observed in three-dimensional systems involving liquid-vapour coexistence, with a view to possible experiments on liquid metal films.

ISSN:0026-8976    

DOI:10.1080/00268978700102611

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

We present the generalization of Wertheim's theory of associated fluids to multicomponent liquid mixtures. For a model binary fluid mixture with site-site coulombic interactions, the theory yields results in excellent agreement with Monte Carlo simulations. The simulations also provide pair distribution functions.

ISSN:0026-8976    

DOI:10.1080/00268978700102621

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The local structure of a hard sphere fluid near a hard wall and near a hard cylinder is investigated using the Monte Carlo method. Near the wall we find a reduction in the average fluid density, and in the normal diffusivity. The simulation results are compared with the predictions of a density functional model. In this model the local density profile of a hard sphere fluid can be calculated ind-dimensional symmetry for any real symmetry dimension.

ISSN:0026-8976    

DOI:10.1080/00268978700102631

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

A large number of anisotropic potential energy surfaces for the N2-He, Ne and Ar systems have been examined using the infinite-order sudden approximation (IOSA) to test their usefulness in predicting transport properties as functions of temperature. In particular, the binary diffusion, shear viscosity and thermal conductivity coefficients have been considered in detail. Six N2-He potentials, eight N2-Ne potentials and seven N2-Ar potentials have been considered in this study. For each system, only one of each set of potentials considered was found to predict adequately all three transport coefficients. Of the three transport properties examined, the thermal conductivity is the most sensitive to anisotropy in the potential energy surface. The calculation of this property should, however, be carried out using dynamical methods that are more accurate than the IOSA for the calculation of purely inelastic cross sections.

ISSN:0026-8976    

DOI:10.1080/00268978700102641

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The usefulness of available anisotropic potential energy surfaces for the calculation of collision cross sections governing relaxation phenomena is considered for binary mixtures of N2with the noble gases He, Ne and Ar. The relaxation phenomena specifically considered are collision broadening of the depolarized Raman and Rayleigh spectra of scattered light. In addition, the relaxation cross section associated with the position on the magnetic field by pressure,B/p, axis of the viscomagnetic effect in such binary mixtures is examined. Six N2-He potentials, eight N2-Ne potentials and seven N2-Ar potentials have been considered in the present study. Of the N2-He potentials, one is satisfactory as far as these relaxation phenomena are concerned; a set of six Hartree-Fock plus damped dispersion (HFD) potentials seem to be reasonable for the N2-Ne system, and three HFD potentials seem reasonable for the N2-Ar system. However, when the present results are combined with transport property requirements, only one of the N2-Ne and none of the N2-Ar potentials are able to predict all the transport coefficients and relaxation cross sections considered in our study.

ISSN:0026-8976    

DOI:10.1080/00268978700102651

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

We have investigated the use of translational lineshapes generated from information theory (IT) for collision induced spectroscopies. In several cases of general interest, we find that such IT lineshapes fail to adequately approximate exact lineshapes computed from fundamental theory. The IT profiles are associated with poorly defined parameters, and thus have at best a marginal ability to represent collision induced spectra; other lineshape models are known which provide better approximations to exact lineshapes.

ISSN:0026-8976    

DOI:10.1080/00268978700102661

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Four vibronic components of the (v1,v2,v3) = (020) - (010) hot band of CO2+have been measured by infrared diode laser absorption spectroscopy. The CO2+molecules were generated in a hollow cathode discharge of a dilute mixture of CO2in helium at a total pressure of a few hundred mTorr. The hamiltonian operator for a linear triatomic molecule in a2Π electronic state undergoing bending vibrational motion is described and the vibronic matrix elements given. The theory is used to fit the observed data together with previously reported infrared measurements of the bending fundamental and combination differences extracted from the electronic spectrum of the ion. The analysis yields rotational, spin-orbit, and vibronic parameters including those describing the Renner-Teller and Fermi resonance effects, and the bending potential function including anharmonicity.

ISSN:0026-8976    

DOI:10.1080/00268978700102671

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

In this paper we study the sorption of guest molecules in porous media of well-defined structure, specifically in zeoliteAand faujasite. We introduce the concept of a site-specific maximum sorption time τi, defined as the average time required for an internalized diffusing species, starting from any cavity sitei, to reach the cavity farthest removed from the surface of the zeolite, viz. the one at the geometric centre of the assembly. When averaged over all cavity sites accessible initially to the guest molecule, the consequent mean sorption time τ reflects the role of different spatial distributions of cavities and attendant channel patterns in influencing the efficiency of sorption in these systems. We study how this characteristic time τ changes as a function of the number of available cavities and the nature of the ambient conditions to which the guest/host system is exposed (two ‘boundary conditions’ are examined). We show that the enhancement in the efficiency of the sorption process when faujasite is replaced by zeoliteAis bracketed between the values ∼ 110 per cent (for the smallest crystallites) and 14.2 per cent (for the very largest ones). Further studies document how this picture changes when sorption in any polyhedral cavity can take place. Letsibe the probability of sorption of a diffusing species in cavityi; for the case, allsi≡s> 0, we show quantitatively that the characteristic, maximal sorption time τ behaves like τ ⋍ 1/sfor both zeolite structures, independent of the boundary (ambient) conditions. We explore quantitatively how the presorption of polar molecules (such as H2O and NH3) can influence the diffusivity of nonpolar molecules (such as Ar, H2, O2) by obstruction of blockage of the polyhedral cavities. Iffis the fraction of cavities that remain unblocked when presorption of polar molecules occurs, we illustrate the extent to which differences between zeolite structures are suppressed with increase in the fractionf. Finally, we comment on the implications of these results for the study of diffusion-controlled reactive processes in zeolites.

ISSN:0026-8976    

DOI:10.1080/00268978700102681

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Evidence is presented for the occurrence of a wetting transition in the adsorption of Kr and Xe on exfoliated graphite (Grafoil MAT). The estimated wetting temperatures are 82·95 ± 0·1 for Kr and 116·3 ± 0·1 K for Xe. In relation to the critical temperatureTcof the bulk adsorbate, these temperatures correspond toTw/Tc= 0·396 and 0·401 respectively. An earlier study of the adsorption of CH4on the same substrate yieldedTw= 75·5 ± 0·1 K andTw/Tc= 0·397.

ISSN:0026-8976    

DOI:10.1080/00268978700102691

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor