摘要:

A realistic model of a monolayer molecular crystal of methane adsorbed on the (100) surface of MgO has been studied by means of molecular dynamics. The model treates the atomicity of the MgO substrate explicitly. Large corrugations in energy are found for both the translational and rotational motion of methane across the substrate. The preferred configurations of adsorption is over an Mg2+ion in a tripod-down orientation. The orientationally ordered low-energy states of the monolayer have been found. Finite-size-scaling calculations on systems of different sizes indicate, however, that the ordered state exhibits only quasi-long-range order. The disclinations in the ordered phase exist in low concentrations and in bound pairs. Evidence is presented for a Kosterlitz-Thouless-type unbinding transition occurring atTc≈ 27 K. No evidence is found for a first-order transition. The specific heat exhibits an anomaly atT≈ 32 K. The disclination density in the region of the transition is reported; belowTc, this density can be fitted to an Arrhenius-type law, yielding an estimate of the core enegy for a bound vortex-antivortex pair.

ISSN:0026-8976    

DOI:10.1080/00268979000102421

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Monte Carlo simulations are performed for a system of hard Gaussian-overlap molecules with embedded permanent quadrupole moment. Two shapes of the hard bodies are considered, with characteristic parameterk= 1·792 and 2·2 and three values of the quadrupole moment. The results obtained for the residual internal energy, Helmholtz function and compressibility factor at three densities fork= 1·792 and one density fork= 2·2 are used to test the recently proposed perturbation theory of polar non-spherical-molecule fluids.

ISSN:0026-8976    

DOI:10.1080/00268979000102431

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Self-diffusion coefficients for trichloromethane are reported for the temperature range 5–75°C at pressures up to 400 MPa, having been measured by both diaphragm cell and NMR spin-echo techniques. Shear viscosities, determined with a falling-body viscometer, are reported in the range 5–50°C and up to 300 MPa. The results are analysed with the aid of models derived from computer simulation molecular dynamics data.

ISSN:0026-8976    

DOI:10.1080/00268979000102441

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The Gibbs-ensemble Monte Carlo simulation method is used to predict the liquid-vapour coexistence of a fluid whose molecules interact with a cut and shifted Gay-Berne pair potential with elongation κ = 3 and well-depth ratio κ′ = 5. From these simulation results we estimate the temperature, density and pressure at the critical point to bekTc/εo= 0·489 ± 0·005, ρcσ3o= 0·096 ± 0·004,pcσ3o/εo= 0·0138 ± 0·0014. We also present evidence of the existence of the vapour-isotropic-liquid-solid triple point. Comparison with the density-functional approximation shows that this theoretical method predicts coexistence and critical temperatures that are too low.

ISSN:0026-8976    

DOI:10.1080/00268979000102451

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The permutation symmetry groups introduced by Longuet-Higgins are employed to solve the group theory for molecules of C3vand Tdsymmetry rotating in crystal fields of C3vand Tdsymmetry. The qualitative appearances of the energy level diagrams for the groups (C3v, Ȼ3v), (C3v, Ŧd) and (Td, Ȼ3v) are deduced for the range of barrier heights from the free-rotor limits to the pinned limits, for the ground state multiplet only. The correlations between the groups derived here and the group (Td, Ŧd) treated by Miller and Decius are considered and the alteration in the ground state splitting pattern in going from a tetrahedral molecule in a tetrahedral field through to a trigonal molecule in a trigonal field is illustrated. The group (C3v, Ȼ3v) is treated in detail and applied to the tunnelling states of monodeuteromethane adsorbed on the surface of graphite. The rotational Hamiltonian matrix is constructed in the pocket state formalism and diagonalized using the character table for (C3v, Ȼ3v). The quantitative appearance of the energy level diagram for the ground librational state of CH3D on graphite is shown.

ISSN:0026-8976    

DOI:10.1080/00268979000102461

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The Coulombic attraction theory of colloid stability proposed by Sogami and Ise is applied to the one-dimensional problem of the electrostatic interaction of plate macro-ions. The total electrostatic energy of the macro-ionic solution is calculated by solving the Poisson-Boltzmann equation with the linearisation approximation. It is proved that the Helmholtz free energy of interaction derived from this quantity is identical to the effective repulsive potential obtained in DLVO theory for the interaction of two flat double layers, in the limit where the Debye approximation holds. It is demonstrated that Overbeek's criticism of the Sogami theory violates the fundamental thermodynamic relations and that the Gibbs pair potential that is appropriate for describing the interaction of macro-ionic particles under isobaric conditions is equal to the sum of the pair term in the electrostatic energy and the Helmholtz pair potential. The Gibbs free energy of interaction between plate macro-ions is calculated as an analytic function of their separationX, the inverse Debye screening length κ and the plate thickness 2a. This pair potential leads to a repulsion at small interparticle distances and to an attraction at large interparticle separations, creating a ‘secondary’ minimum without the need for a van der Waals attraction. It is shown that this allows for the thermodynamic stability of lyophobic colloids, in contrast with DLVO theory. The position of the secondary minimumXminis calculated as an analytic function of κ anda. For dilute solutions the term containingais negligible and the Coulombic attraction theory reduces to the simple equation κXmin= 4, in contrast with the rapid variation ofXminwith κ predicted by DLVO theory. These predictions are compared with the observed interplate separations in n-butylammonium vermiculite gels. There is no agreement with DLVO theory, even when the Hamaker constant is treated as an adjustable parameter to fit the data. There is qualitative agreement with the Coulombic attraction theory, but the new theory predicts spacings systematically lower than those observed.

ISSN:0026-8976    

DOI:10.1080/00268979000102471

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A systematic approach is developed for the ZFS (zero-field splitting) parametersDandEin slightly distorted tetrahedral and octahedral complexes with spatially non-degenerate ground states. This approach is illustrated by treatment of the normal coordinate system. The ZFS part of a spin Hamiltonian is expressed in terms of the distortion along normal coordinates and the coefficients characteristic of a metal-ligand combination. This expression for a spin Hamiltonian clarifies the relation between the ZFS and the distortions of the coordination geometry from Tdand Ohsymmetries, and is particularly useful in the calculation of the ZFS under non-axial distortions. Some of the coefficients are derived for the ground A2state (tetrahedral d7and octahedral d3) ions in terms of theeσandeπparameters of the angular overlap model. Satisfactory agreement is obtained in the application of the approach to Cs3CoCl5, Cs2CoCl4and ruby.

ISSN:0026-8976    

DOI:10.1080/00268979000102481

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Rotational and vibrational operators of a molecule possessing an ℜ symmetry element, consisting of either a binary rotation axis normal to the angular momentum quantizationzaxis or of a reflection plane containingz, are classified according to the irreducible co-representations of the group (E, ℜ′, Θ, ℜ′Θ), Θ being the time-reversal operator. Using this classification, it is shown that the matrix elements of the vibration-rotation Hamiltonian, in the usual vibration-rotation basis, can be made all real for all classes of molecules possessing the ℜ′ symmetry element, by appropriate rotational and vibrational phase conventions, which are defined and recommended. With the same phase conventions that render the matrix elements of the vibration-rotation Hamiltonian all real, the matrix elements of the electric dipole vibration-rotation transition moment can be made all real if ℜ′ is a reflection plane, and all imaginary if ℜ′ is a binary rotation axis, by appropriate additional conventions about the relative phases of |JkM> basis functions differing by the values ofJandM.

ISSN:0026-8976    

DOI:10.1080/00268979000102491

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The effect of perturbations on the values of vibration-rotation transition wave-numbers of symmetric-top molecules is investigated, and it is shown that perturbations caused by interactions that can induce so-called ‘forbidden transitions’ yield informations useful to the determination of the ground stateK-structure parametersA0,D0K,H0K…, even if ‘forbidden transitions’ are not observed. The use of such additional information in the analysis of vibration-rotation spectra is discussed.

ISSN:0026-8976    

DOI:10.1080/00268979000102501

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Optical-optical double-resonance multiphoton ionisation spectroscopy is used to probe then*s andn*d Rydberg series converging to the first rotational levels of NO+X1Σ+,v= 0. Intermediate values of the principal quantum number probed are 5 ⩽n* ⩽ 2>5. The initial excitation occurs via one of two rotational-parity levels of the C2Π state (N′= 2 andN′= 6). A description of the rotational-electronic structure of the observed supercomplexes is proposed in terms of multichannel quantum defect theory (MQDT). The observation of ‘forbidden’n*f levels is tentatively explained both in terms of d-f mixing in the upper levels and of a d-wave contribution to the 3pπ orbital of the C state. Rydberg-valence interactions show up as predissociation, which strongly competes with ionisation in the intermediaten*d supercomplexes with 8 ⩽n* ⩽ 1>2. This work complements the previous analysis of the s and d Rydberg states of NO corresponding ton* ⩽ 8> and 25 ⩽n* ⩽ 4>0 [15].

ISSN:0026-8976    

DOI:10.1080/00268979000102511

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor