摘要:

The vibrational dephasing of the fourA1modes of CH3CN has been calculated for a simulation of the liquid phase at 288 K assuming that it is caused by generalized forces interacting with the cubic anharmonicities of each mode. The forces along each mode were resolved into electrostatic and Lennard-Jones components and the mean values, probability distributions and time autocorrelation functions were calculated. Line shifts in the liquid state were evaluated from the mean values. Line shapes are related to the shift-shift time correlation functions and hence to force-force correlation functions using the cumulant approximation. This was found to be a good approximation and the vibrational correlation functions (Fourier transforms of the line shapes) were computed using two sets of anharmonicity constants. The time scale of the shift fluctuations was never slow, so that the line shapes are nearly lorentzian in this model.

ISSN:0026-8976    

DOI:10.1080/00268978700100791

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

A theoretical basis for the analysis of the influence of electron correlation upon two-photon transition amplitudes in rare-earth ions has been introduced. The approach is based on the standard perturbation theory and on application of effective operators. It has been shown that in third-order perturbation theory electron-correlation effects are represented by one- and two-particle effective operators. In order to estimate the relative importance of various effective operators, test calculations for the Pr+3ion have been performed.

ISSN:0026-8976    

DOI:10.1080/00268978700100801

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Single crystals of the title compound were studied by optical absorption, luminescence, Zeeman and MCD spectroscopy in the region of4A24A2→2E4A2,2T14A2transitions. The ground state exchange parametersJ, jcould be accurately determined from the luminescence spectra. From a complete analysis of the observed electronic transitions to the singly excited pair states2E4A2and2T14A2, the trigonal orbital exchange parametersJaandJecould be deduced.Jawas found to represent the dominant exchange pathway. This was rationalized by an Extended Hückel calculation. The exchange interaction leads to a strong mixing of the2E4A2and2T1a±4A2pair states. This is the main reason for the unusually largeg‖cvalues found in the lowest-energy excited states. The O-H stretching frequencies were found to play the major role as accepting modes in a multiphonon relaxation process.

ISSN:0026-8976    

DOI:10.1080/00268978700100811

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The results of Monte Carlo simulations to study liquid Cl2using the Price and Stone potential are reported. We discuss the orientational correlations together with the atom-atom correlation functions, and compare our results with those obtained using a two Lennard-Jones centres plus quadrupole model. The main results of this comparison show that the atom-atom distribution functions are practically the same for the two potential models, whereas the angular configurations set up by the interactions are very different. At short distances the Price-Stone potential increases the weight of the cross configurations, giving an orientational order closer to the one characteristic of a two Lennard-Jones centres model without a quadrupole. On the contrary, the addition of a point quadrupolar interaction has the effect of increasing the fraction of ‘T’ configurations.

ISSN:0026-8976    

DOI:10.1080/00268978700100821

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Reactive scattering of F atoms with Cl2molecules has been studied at an initial translational energyE∼ 32 kJ mol-1, using a supersonic beam of F atoms seeded in He buffer gas. Laboratory angular and velocity distributions of FCl product have been measured which show scattering peaking sharply in the forward direction with a smaller backward peak of relative height ∼0·3 ± 0·1. The product translational energy for scattering in the forward and backward directions is greater than that for the sideways scattering by a factor ∼2. Reaction proceeds via a short-lived collision complex with a lifetime ∼0·5 rotational periods for complexes rotating in the plane of collision. Bent structures are proposed for the collision complex and the reactant transition state, which relate the observed scattering to the initial orientation of the reactant Cl2molecule with respect to the incident F atom direction.

ISSN:0026-8976    

DOI:10.1080/00268978700100831

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Reactive scattering of F atoms with Br2molecules has been studied at an initial translational energyE∼ 33 kJ mol-1using a supersonic beam of F atoms seeded in He buffer gas. Laboratory angular and velocity distributions of FBr product have been measured which show scattering peaking sharply in the forward direction with isotropic wide angle scattering of relative intensity ∼0·15. The reaction dynamics follow the stripping limit of the short-lived complex mechanism displayed by the F + Cl2reaction. A bent structure is proposed for the transition state of the reaction which relates the observed scattering to the initial orientation of the reactant Br2molecule with respect to the incident F atom direction.

ISSN:0026-8976    

DOI:10.1080/00268978700100841

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The E.S.R. line shape of a triplet exciton moving in a coupled coherent and incoherent manner within a pair of differently oriented molecules is calculated. The hamiltonian describing the model includes the excitation energies of the identical, but differently oriented molecules, the interaction matrix element describing the coherent exciton transfer, the Zeeman energy, and the fine-structure terms of the molecules. Furthermore it contains the phonons which are treated quantum mechanically and their linear and local interaction with the exciton. Using a unitary transformation of the hamiltonian, the exciton-phonon interaction is transformed into a phonon modulated exciton transfer operator. Considering now the phonons a heat bath, with the help of the transformed hamiltonian the Liouville equation for the reduced density operator is derived, which contains only triplet exciton degrees of freedom. The eigenvalues and eigenvectors of the reduced density matrix are calculated numerically using parameters characteristic for naphthalene. From linear response theory the E.S.R. line shape is determined using the eigensolutions of the Liouville equation. These calculations show that in the high temperature limit a localization of the exciton either at moleculeAor moleculeBoccurs for all values of the exciton-phonon interaction. The E.S.R. line shape is also discussed in regions where level anticrossings occur.

ISSN:0026-8976    

DOI:10.1080/00268978700100851

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The statistical mechanical study of chemically reactive fluids via the analytic solution of integral equation approximations for such fluids (Cummings, P. T., and Stell, G., 1984,Molec. Phys.,51,253; 1985,Ibid.,55,33) is extended to a consideration of solvent effects. This is achieved by studying mixtures of molecular species some of which can undergo reaction (the solute(s)) and one of which is inert (the solvent). The results obtained near infinite dilution of the reacting solute in the inert solvent are similar in magnitude to those observed in experiments on the dissociation equilibrium of dinitrogen tetroxide.

ISSN:0026-8976    

DOI:10.1080/00268978700100861

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

A new approach is proposed to the introduction of long range electrostatic terms in the many-body expansion of the molecular potential. The method has been applied to the HO2and H2F2potentials.

ISSN:0026-8976    

DOI:10.1080/00268978700100871

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Fluorine spin lattice relaxation times per unit densityT1/ρ in pure SF6, MoF6, WF6and UF6gases are experimentally found to be proportional toT-1·5whereTis the temperature. These data can be analysed to obtain information on the anisotropic part of the intermolecular potential in these systems. Hard sphere, Lennard-Jones (6–12) and modified Buckingham (exp-6) potentials were used for the isotropic part of the intermolecular interaction. The analysis is made using the Bloom-Oppenheim theory, assuming that the correlation time of the spin rotation interaction can be approximated by the average lifetime of a molecule in the givenJstate. We have obtained the strengths of the repulsive and attractive terms in the anisotropic potential. From the strength of the attractive term in the anisotropic potential, the hexadecapole moments of all the four molecules are obtained. The hexadecapole moment of SF6obtained by us using the Lennard-Jones model is in good agreement with the values reported earlier, based on other techniques, while the values for MoF6, WF6and UF6are reported for the first time.

ISSN:0026-8976    

DOI:10.1080/00268978700100881

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor