摘要:

In this work the four-centre Jahn-Teller effect and its influence on the structure and magnetic properties of Cu4OX6L4copper tetraclusters is investigated. The resulting adiabatic potential in the subspace ofEvibrations is qualitatively similar in shape to theE-eproblem. In the strong Jahn-Teller coupling limit, when the splitting of the degenerate electronic states of the Cu2+ions is large enough, the Heisenberg spin hamiltonian is valid for the exchange interaction. The spin energy spectrum is determined, taking account of nuclear dynamics, and the temperature dependence of the effective magnetic moment μeffis evaluated. The exchange and vibronic parameter values, producing different types of dependencies μeff(T), including the anomalous, non-monotonic one, are discussed. The effect of the spin-orbit interaction on the temperature dependence of the μeffvalues is determined. For a number of copper tetraclusters, good agreement is obtained between the theoretical curves μeff(T), resulting from the vibronic model in the strong Jahn-Teller coupling case, with the experimental data. It is shown that the X-ray, E.P.R. and μeff(T) experimental data for Cu4OCl6(TPPO)4tetraclusters can be understood within the above vibronic approach with a given relation between the crystal field, tunnelling and exchange parameters.

ISSN:0026-8976    

DOI:10.1080/00268978400101931

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Extensive variational calculations based on previously elaborated superposition of correlated configurations method have been performed for the ground state of H3+. The resulting upper bound to the non-relativistic energyE= -1·343422Ehis the best variational result so far reported in the literature. One can expect that the corresponding variational wavefunction is very accurate. The possibility of further improvements of the results is discussed. The kinetic energy, interelectronic repulsion energy, nuclear-electronic attraction energy, virial and squared electronic position expectation values, as well as electric quadrupole momentQzz= -0·92303ea02and the Larmor contribution to the mean diamagnetic susceptibilityxL= -3·65633 × 10-5a03where computed.

ISSN:0026-8976    

DOI:10.1080/00268978400101941

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

We present the results of a detailed Monte Carlo study of the phase diagram of infinitely thin hard platelets. A weak first order isotropicnematic transition is observed. The equation of state in the isotropic regime is compared with several current theories, none of which is found to be fully satisfactory. The density dependence of the nematic order parameter is found to be compatible with a ‘critical’ exponent β=0·25. A study of the fluctuations of the order parameter in the isotropic phase casts doubt on the applicability of the Landau-de Gennes expression for the free energy. We observe that the relation between the nematic order parameters <P2> and <P4> is compatible with the predictions of mean-field theory. Practical aspects of the computation are discussed. A novel method to compute the pressure in a constant-volume Monte Carlo run is presented.

ISSN:0026-8976    

DOI:10.1080/00268978400101951

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The usual semiquantitative evaluations of temperature-dependent dipolar13C spin-lattice relaxation times of liquid methylbenzenes (toluene, xylenes, tri- and tetramethylbenzenes), which assume isotropic molecular motion, do not allow any reliable statements to be made about the various hindered rotations of the methyl groups, and about preferred molecular rotation axes. The quantitative evaluation of the experimental data using a new formalism (complete anisotropic motion with arbitrary position of the internally rotating methyl groups) gave the following: physically meaningless results were obtained in all cases for the optimizations for one of the three possible axially symmetrical ellipsoidal motions, and in most cases for complete anisotropic motion. A quantitative evaluation of dipolar relaxation times which takes complete anisotropic motion into account is—at least for the methylbenzenes—only promising if the present 10 per cent level of experimental error in the dipolar relaxation times can be reduced to at least 1 per cent. However, the results do indicate clearly an anisotropic motion which is dependent on the molecular geometry. The degree of motional anisotropy is not only determined by effects of inertia: the activation parameters for the molecular motion do not increase continually with increasing molecular mass, as would be expected if the molecular motion is determined by inertial effects alone. At higher temperature, the suitability of the rotational diffusion model becomes increasingly dubious with decreasing molecular mass. An activation parameter was found from the calculated methyl jumping rates, which characterized the internal rotation barriers of the methyl groups. It became evident that the isotropic approximation of the overall molecular motion usually employed is insufficient, and can simulate hindered rotations for methyl groups which are not present according to experience. However, the values of the activation parameters for methyl groups which are known to show hindered rotation are independent of the model used for the molecular motion.

ISSN:0026-8976    

DOI:10.1080/00268978400101961

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The relationship between the pressure in a fluid and the density-functional which controls the density profile of a fluid confined between two walls is examined. As a result, the conditions which must be fulfilled by an approximate density functional to yield a bulk pressureP, a normal pressure,PWat the interface between a hard wall and the fluid, and a fluid density, ρW, adjacent to the wall which satisfy the exact relationsPW=P=kBTρW, are established. The density-functional which yields the HNC closure for a hard-sphere fluid near a hard wall is modified so that the modified functional yields the Carnahan-Starling bulk pressure and hence fulfils the necessary conditions. The density profile to which this gives rise is compared with the results of computer simulation for various bulk densities when the fluid is bounded by two hard walls separated by a distance equal to 8 diameters of the hard sphere. The agreement is found to be very good even at a bulk reduced density of 0·91.

ISSN:0026-8976    

DOI:10.1080/00268978400101971

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The variational method for the determination of ro-vibrational energy levels has been extended to the ro-vibronic energy levels of Renner-Teller triatomic molecules. A previously derived hamiltonian ℋ(R1,R2, θ, [bcirc]n) has been used, with [bcirc]nzreplaced by [bcirc]nz+Ḽz. Expansion functions having the correct symmetry inD∞h(Σg+, Σg-, Σu+, Σu-) orC∞v(Σ+, Σ-) are selected, for anyJvalue, from a recipe which is general for any type of Renner-Teller system.

ISSN:0026-8976    

DOI:10.1080/00268978400101981

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

A method is described by which second virial coefficient data from an anisotropic potential can be used to define an effective spherical potential. Equilibrium liquid state properties are calculated using this effective spherical potential and found to agree closely with those obtained from the original anisotropic potential.

ISSN:0026-8976    

DOI:10.1080/00268978400101991

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Time-resolved methods are used to study scattering and trapping processes in a one-dimensional triplet exciton system. Electron spin echo experiments indicate that intraband scattering in the lowest triplet exciton band in 1,2,4,5-tetrachlorobenzene is independent of chemical purity and certain forms of structural damage. Optical measurements between 0.35 K and 1.25 K demonstrate the importance of an 8 cm-1trap at low temperature, and show further that trapping rates are independent of band statekfor deepX-traps. The results suggest that theX-traps are not directly incorporated into the band structure of the host.

ISSN:0026-8976    

DOI:10.1080/00268978400102001

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

We generalize Wertheim's renormalized perturbation theory for polar, polarizable fluids to mixtures. The generalized theory is used to calculate the thermodynamic excess functions and phase equilibria for a variety of polar/non-polar and polar/polar mixtures. The results demonstrate the importance of including induction effects in a proper description of mixtures, and show the superiority of the 1-R form of the theory (which includes higher order multibody effects) over the 0-R form. Comparisons are made with experimental data for methyl chloride/xenon mixtures.

ISSN:0026-8976    

DOI:10.1080/00268978400102011

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

TheA3Π-X3Σ-emission systems of SH+and SD+have been observed free of impurity lines in a special d.c. discharge source. Seven bands have been analysed and accurate rotational constants have been obtained for the levelsv= 0 and 1 of theA3Π andv= 0, 1 and 2 of theX3Σ-states of SH+and SD+. The fine structure of the two states, characterized by (i) large (∼ 5 cm-1) spin-splitting of theXstate, (ii) large Λ doubling of theA3Π0component and (iii) anomalous spin-orbit splitting of theAstate, has been shown to originate mainly from second-order spin-orbit interactions. Finally, the radiative and non-radiative properties of theAstate of SH+are discussed in the framework of a crude quantum calculation of the predissociation rates for the lowest vibrational levels, in connection with the laser predissociation spectrum data described in the following paper (Edwards, Maclean, Sarre).

ISSN:0026-8976    

DOI:10.1080/00268978400102021

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor