ISSN:0026-8976    

DOI:10.1080/002689797169745

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The spectral and temporal consequences of a non-zero differencedkj= µkk- µjjbetween the permanent dipole moments µjjof two states involved in a transition are investigated for two and three-level model molecular systems interacting with a continuous wave laser. The field strengths are such that the molecule–EMF couplings for the systems are near-nodal. The interesting spectral and dynamic effects of near-nodal molecule–EMF couplings are unique to transitions wheredkj≠ 0, relative todkj= 0. These effects are studied using exact Floquet calculations and analytical solutions in the two-level rotating wave approximation (RWA). The RWA discussion of near-nodal molecule–EMF couplings, how they are achieved, and their effects, is quite general and applies to any two-level model of a dipolar molecule. The actual values of the laser parameters required to achieve a molecule–EMF coupling node depend on the specific molecule under consideration. This is illustrated with explicit twoand three-level molecular models, based on the S0and S1states of 1-[p-(N,N-dimethylamino)phenyl]-4-(p-nitrophenyl)-1,3-butadiene. Although the field strengths required to achieve near-nodal couplings are too large for the quantitative application of the analytical results, the two-level RWA solutions are crucial for the qualitative interpretation and prediction of the exact laser–molecule behaviour. The examples illustrate that the near-nodal effects ofdkj= 0, as predicted by the two-level RWA and verified by the two- and three-level exact calculations, persist when neighbouring energy levels are present in the model.

ISSN:0026-8976    

DOI:10.1080/002689797169754

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The exp-6 potential fluids for four values of the softness parameter α, ranging from 11·5 to 14·5, have been investigated both by computer simulations and theory at supercritical temperatures and very high densities (pressures). Tables of the simulation data for pressure include altogether 141 thermodynamic state points generated at six temperatures for each α. In addition to computation of the thermodynamic properties, great attention has been paid to the structure, and an approximate location of the onset of freezing has been determined. To estimate the properties of the fluid from theory, a modified version of the Weeks–Chandler–Anderson (mWCA) theory and the optimized reference hypernetted chain (RHNC) theory were used. It is shown that for the thermodynamic properties mWCA theory performs very well under all stable fluid conditions considered, whereas RHNC theory, comparable with mWCA at the lower densities considered, gradually loses its accuracy in the very high density range. However, for structure the overall performance of RHNC theory is considerably better than that of mWCA, which points to possible cancellation of errors in the latter theory when applied to thermodynamic properties.

ISSN:0026-8976    

DOI:10.1080/002689797169763

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Direct molecular dynamics simulations of the coexisting liquid and vapour phases have been used to calculate the orthobaric curves of the three fluorocarbon refrigerants CH2F2, CHF3and CF4. By comparison with experiment we have derived a force-field for these molecules consisting of a 12-6 Lennard-Jones potential for each atom with additional point charges to model the Coulombic interactions. The orthobaric densities, vapour pressure and latent enthalpy of vaporization of the model fluids have been determined at a range of temperatures between 200 K and 300 K for CH2F2, between 180 K and 270 K for CHF3and between 120 K and 200 K for CF4, and compared with experiment. The radial distribution function of the coexisting liquids has been determined for the potential model and compared with the weightedg(r) obtained from coherent neutron scattering. The potential models are adequate for predicting the experimental orthobaric curves. The liquid structure of CHF3is predicted accurately but the experimentally determined weighted radial distribution for CH2F2could not be reproduced.

ISSN:0026-8976    

DOI:10.1080/002689797169772

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The results of numerical analysis performed for the electric dipole transitions observed in Eu3+ in the host with C symmetry are presented. The discussion is focused on the one-photon transitions [querry] However, to complete the investigations, numerical results obtained for [querry] are also reported. The approach is based on the second-order Judd-Ofelt theory modified by the third-order terms caused by electron correlation effects. In the case of the 0 1 transition, for the first time, the calculations are performed within the alternative formulation of second-order theory based on the velocity form of electric dipole radiation operator.

ISSN:0026-8976    

DOI:10.1080/002689797169781

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The role of three-particle effective operators in the theory of f↔f transitions in rare earth ions in crystals is analysed. The relative important of these operators is established through calculations performed for Eu3+ion in hosts of C2vand C3vsymmetry. The radial and angular terms of all the effective operators analysed are evaluatedab initio, while the matrix elements of the unit tensor operators are obtained with symmetry adapted wavefunctions of 4f6configuration. The approach is based on the alternative formulation of second-order theory where instead of the length formula of the electric dipole radiation operator its velocity version is taken as an approximation. The various effective operators contributing to the f↔f transition amplitude are reviewed.

ISSN:0026-8976    

DOI:10.1080/002689797169790

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Small molecules that form liquid crystals typically consist of a rigid core with flexible tails on one end or on both ends. To date, most computer simulation studies have used completely rigid models such as hard spherocylinders: cylinders, characterized by their length/diameter ratioL/D, with hemispherical end caps. We have studied a model consisting of spherocylinders withL/D= 4, with a flexible tail attached to each end. The tails are ‘ideal’ in the sense that they have no volume. Using Monte Carlo simulations the phase behaviour of this model was studied and, for comparison, the behaviour of hard spherocylinders withL/D= 4 without tails was studied as well. The addition of the tails is found to stabilize the smectic-A phase at a lower pressure, and the nematic phase disappears. In the smectic-A and crystal phases, the smectic layers are further apart when tails are added. The structure of the layers and the smectic-A–crystal transition pressure change only a little. For both models close to melting the crystal consists of ordered layers, but there is almost no correlation between particle positions in neighbouring layers. In fact, the layer coupling is so weak that in a long simulation the layers are found to glide over each other. As the pressure is increased the crystal gradually becomes more ordered and the crystalline layers ultimately ‘lock’ into place.

ISSN:0026-8976    

DOI:10.1080/002689797169808

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A theory for modelling electrolyte solutions which includes the formation of ion clusters of different size has been developed in the framework of the primitive model. In primitive models the solvent is described as a dielectric continuum and the solvent–solute interactions are neglected. For the dielectric constant the value of the pure solvent has been used. The ion cluster distribution is calculated from the mass action law. The association constants are related to integrals over the cluster distribution functions which are calculated with the Kirkwood superposition approximation from low density pair distribution functions. The ion clusters are defined by a certain distance which rules if two ions belong to the same cluster. This so-called Bjerrum distance is chosen according to fundamental investigations of the structure of ion cluster. All ion clusters are modelled as hard spheres. For the free ions and charged clusters the mean spherical approximation expression for the Coulomb interaction is added. The co-volumes of the ion clusters have been taken from the investigation of the ion cluster structures, and are consistent with the definition of an ion cluster chosen here.

ISSN:0026-8976    

DOI:10.1080/002689797169817

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A physically intuitive non-resonant local mode axis tilting model for the interpretation of higher overtone bands of XH2species is outlined and illustrated by reference to the near2 infrared of spectrum H2S. Quantitative connections with the conventional normal mode resonance coupled picture are derived and local mode based expressions for the vibrational–rotational wavefunctions are given. Constants are reported for the (311)–(212) and (302)–(203) bands at 11 008cm-1and 12 149cm-1, respectively.

ISSN:0026-8976    

DOI:10.1080/002689797169826

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The microdynamic motions of the clusters A[MCo3(CO)12] (A = H, NEt4; M = Fe, Ru) and HFeCo3(CO)9[P(OCH3)3]3have been studied for the first time using59Co NMR spectroscopy. The chemical shift anisotropies and the quadrupolar coupling constants known from solid state NMR results allow a direct determination of the correlation times in solution by measuring the relaxation times at different temperatures. Most of the clusters behave as rigid spherical rotors with an activation energy nearly equal to that of the solvents. The phosphite derivative is a good example of the case where the mechanical properties are spherical although the shape is not spherical. From the99Ru relaxation time measurements, the quadrupolar coupling constant of this nucleus is deduced.

ISSN:0026-8976    

DOI:10.1080/002689797169835

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor