摘要:

The use of polarizability densities to calculate collision-induced polarizabilities is investigated. One method for computing polarizabilities of inert gas diatoms employs atomic polarizability densities from finite-field Hartree-Fock calculations, together with classical equations for the polarization of dielectrics. It is shown that this model gives inaccurate values for both the local fields and the local response to non-uniform fields. An alternative method incorporating the same physical effects is used to compute the pair polarizabilities to first order in the interatomic interaction. To first order the pair contribution to the trace of the polarizability is negative at short range. The calculated anisotropy does not differ significantly from the DID value, whereas the polarizability density calculation gives a substantial reduction in the anisotropy.

ISSN:0026-8976    

DOI:10.1080/00268977800101591

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The utility of multiple quantum transitions is discussed with regard to the elucidation of relaxation mechanisms in coupled spin systems. The information contained in the decay rates of multiple quantum coherence is analysed in terms of the Redfield formalism, and experimental schemes suitable for their measurement are presented. As an illustration, external paramagnetic relaxation in a two-spin system is investigated, based on measured multiple and single quantum relaxation rates.

ISSN:0026-8976    

DOI:10.1080/00268977800101601

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

An identity expressing formally the diagonal and off-diagonal elements of an inverse of a matrix is deduced employing operator techniques. Several well-known perturbation expressions for the self-energy are deduced as special cases. A new approximation and other applications following from the above formalism are briefly indicated through illustrations from a perturbed harmonic oscillator, chemisorption approximations and Kelly's result in the problem of electron correlation.

ISSN:0026-8976    

DOI:10.1080/00268977800101611

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A study is made of phonon-assisted transitions between closely spaced librational levels of an impurity molecule in a host crystal. The contributions to librational relaxation of direct, of Orbach-type and of Raman processes are evaluated as a function of temperature. It is found for a model case of NH(3π) in solid Ar, that belowT= 16 K, the direct process dominates, while forT> 20 K relaxation takes place mainly by the Raman mechanisms. The calculation predicts a nanosecond time scale for the relaxation at 4 K, and a picosecond time-scale for the relaxation atT= 24 K.

ISSN:0026-8976    

DOI:10.1080/00268977800101621

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A calculation of the depolarized Rayleigh scattering from a liquid of isotropically polarizable molecules is described. The spectrum reflects cooperative, many-particle, structural relaxation which is represented by a generalized Langevin equation for the bilinear number density fluctuations. The theory is applied to the particular case of liquid argon at the triple point and is shown to give a very close quantitative agreement with the experimental lineshape, using data from neutron-scattering experiments on liquid argon and no adjustable parameters. The various features of the observed spectra are related to the behaviour of the correlation functions of different Fourier components of the bilinear number density fluctuations which are interpreted in different limits of the molecular motion, diffusion, gas-like, etc. The sensitivity of the lineshape to modifications of the induced polarizability functional from DID form is investigated. Within the range of physically reasonable modifications little change in the lineshape is found, although the calculations suggest that the intensity, in the liquid, is more sensitive than in the gas phase. The theory is compared with Stephen's theory of collision-induced light scattering which uses a different approximation to the molecular dynamics. This theory gives a better overall description of the argon spectrum than had previously been realized but fails qualitatively to predict several features of the observed spectrum.

ISSN:0026-8976    

DOI:10.1080/00268977800101631

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The properties of a particle undergoing translational diffusion in one dimension, in the presence of a cosine potential, are investigated. Translational order parameters are defined and the neutron incoherent scattering law is calculated. The simple diffusion model and the jump model between equidistant sites are found as limiting cases of zero and infinite potential heights, respectively. This formalism is then used to re-analyse neutron quasi-elastic scattering data in smectic A terephthal-bis-4-n-butyl aniline. Combining these results and N.M.R. spin echo measurements of the long-range translational diffusion coefficient across the layers, it is found that the height of the barrier which maintains the molecules in the smectic planes, at 184°C, is about 1·2kbTan, whereTanis the smectic A-nematic transition temperature. This corresponds to a value of the translational order parameter of about 0·29.

ISSN:0026-8976    

DOI:10.1080/00268977800101641

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

An avoided crossing of the 786 and 815 cm-1vibronic bands in the absorption spectra of palladiumporphin (PdP) inn-alkane hosts in the presence of an external magnetic field is reported. A description of this effect is presented in terms of the Zeeman perturbation and intramolecular vibronic coupling. It appears to be closely connected with the orbital degeneracy of the first excited singlet state of the free PdP molecule. It is concluded that the anticrossing is due to a bilinear term in the nuclear potential energy, which gives rise to a vibronic matrix element of 4·8 cm-1.

ISSN:0026-8976    

DOI:10.1080/00268977800101651

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A theory for the isotropic Raman line shape (width and shift in particular) of diatomic molecules in the condensed state (liquid or solid) within the linear response formalism is presented. The prototypic system considered is that of a nitrogen argon mixture in any proportion. The two much discussed mechanisms which effect the lineshape (the vibrational modulation and the resonance transfer effects) appear naturally. The explicit concentration dependence of the width and shift is found while only the qualitative temperature dependence is determined. In particular, it is shown that the different temperature dependences of the line width in the liquid and solid states are simply related to the different types of collective modes which exist in the two states. It is also shown that the contributions to the width and shift of the above two mechanisms depend differently upon concentration and so can be separated experimentally.

ISSN:0026-8976    

DOI:10.1080/00268977800101661

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

We analyse, by cluster expansion methods, the behaviour of a classical system made up of different kinds of particles which can have, besides an electrical charge, an electrical dipole moment.

ISSN:0026-8976    

DOI:10.1080/00268977800101671

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The neutron-diffraction pattern of nitrogen gas at room temperature has been measured at densities between 0·4 and 0·8 × 1022molecules/cm3. These data are compared to model calculations to show that the anisotropic intermolecular potential probably reduces the height of the principal peak of the diffraction pattern, compared to its value for a spherical potential. By contrast the latter potential is shown to give good fit to the published data for liquid nitrogen at the triple point. It is concluded that this difference may be related to the difference in closest approach of molecules in these two states.

ISSN:0026-8976    

DOI:10.1080/00268977800101681

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor