摘要:

The Jahn-Teller (J-T) effect in systems of four-fold symmetry is well known to differ from that in all other point groups with respect to the nature of the J-T active normal modes of vibration. The present report addresses some previously unnoticed features which are of intrinsic importance in recognizing and understanding the unique manifestations of quadrate symmetry in both the static and dynamic Jahn-Teller effects. We first consider the nature of the static J-T potential surfaces when coupling to and strains in two modes,b1andb2, are included in the hamiltonian.

ISSN:0026-8976    

DOI:10.1080/00268977800100671

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The energy dependence of the differential scattering of metastable Ar*(3P) by ground-state CO2(X1∑g+) has been studied at relative kinetic energies from 58 to 126 meV over an angular range of 5–160° c.m. using crossed molecular beams. The position and curvature of rainbow maxima, which are observed at each energy, are used to obtain parameters for a Lennard-Jones (12, 6) spherically symmetric potential. The position of the minimum,rm= 5·02 ± 0·65 Å, is identical to that for K + CO2and the well depth, ε = 16·3 ± 0·8 meV, is about 10 per cent greater. The scattered intensity shows a distinct fall-off on the dark side of the rainbow compared to that expected for elastically scattered Ar*. This depletion, caused primarily by the quenching of Ar*, is analysed in terms of the optical-shadow model to determine the energy dependence of the observed quenching cross section, which is predicted to have a maximum of 67 Å2at 193 meV.

ISSN:0026-8976    

DOI:10.1080/00268977800100681

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Measurements of the nuclear spin lattice relaxation times of the protons in solid samples of the di-t-butylnitroxide radical have been made. The relaxation times have been measured at different Zeeman frequencies as a function of concentration and of temperature. The primary mechanism for nuclear relaxation in these samples is dipolar coupling between the electron and nuclear spins which is modulated by either electron spin exchange or by electron spin lattice relaxation. Analysis of the N.M.R. data yields the correlation time for the exchange interaction in the concentrated samples and the electron's spin lattice relaxation time in the most dilute samples. Electron-proton separations are also determined from the relaxation data. The correlation time for spin exchange is found to depend strongly on the concentration but to be almost independent of temperature. The electron spin lattice relaxation time exhibits an exponential temperature dependence.

ISSN:0026-8976    

DOI:10.1080/00268977800100691

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The application of Regge pole techniques to rainbow scattering in atomic collisions is investigated. Semiclassical calculations of Regge pole positions and residues are carried out for Lennard-Jones potentials based on the collision systems CH4-Ar, H+-Ar and He-Ne. The residues of maximum modulus have values in the range 1–1013. The larger values are associated with pronounced rainbow maxima in the scattering. Decaying surface waves, propagating around the repulsive core of the potential, are used for the interpretation of rainbow scattering. The background integral is approximated by the direct scattering from the repulsive core. The dark side of a rainbow can be characterized by the Regge poles lying closest to the real axis in the complex angular momentum plane. In favourable cases, such as the differential cross section possessing widely spaced oscillations, a single Regge pole is sufficient to characterize the scattering on both the bright and dark side of the classical rainbow angle.

ISSN:0026-8976    

DOI:10.1080/00268977800100701

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Studies of the first triplet absorption and emission spectra ofp-dichlorobenzene-p-dibromobenzene (DCB-DBB) mixed crystals agree generally with the data, indicating that this system forms solid solutions. However, the spectral properties are found to be very different from that of isotopically mixed crystals of comparable concentrations. The singlet-triplet absorption spectra of the DCB-DBB mixtures are generally broad and not amenable to detailed analysis indicating significant site dependent perturbations of the DCB and DBB triplet energies. Energy migration enhances emission from the lower energy sites and in consequence a red shift in the position of the emission (0,0) is observed, which is a maximum at equimolar concentrations. The emission bands are generally much broader than found for isotopically mixed crystals, indicating that the many types of sites in the DCB-DBB system remained uncoupled. This is a direct consequence of exciton trapping by inhomogeneous energy broadening caused by the site energy disorder being greater than the triplet exciton band width. Long-range triplet energy migration is not observed at DCB concentrations less than 99 per cent. This high limit is only expected for near one-dimension energy migration topologies. New structure observed at low DCB concentrations is tentatively interpreted as being due to the formation of DCBn-mers. The observed splittings indicate that the gas-to-crystal shift of the DCBT1state increases (becomes more negative) by 11 cm-1when a neighbouring DBB molecule is replaced by DCB.

ISSN:0026-8976    

DOI:10.1080/00268977800100711

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The translational itinerant oscillator model for molecular motion in fluids is developed in terms of two phenomenological parameters. These are : (a) the retarding frictional force acting on a rigid cage which undergoes translational Brownian motion, and (b) the angular frequency of oscillation of a particle hamonically bound within the cage. The equations of motion contain fewer parameters than those of Damleet al.[6] and may be solved using Laplace transforms. The underlying probability density functions in phase and configuration space may then be calculated.

ISSN:0026-8976    

DOI:10.1080/00268977800100721

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Absoluteg-tensor calculations for planar hydrocarbon and for non-planar phenyl substituted hydrocarbon radicals are reported. The relevant interactions determininggare discussed. Calculations are performed on the basis of a second-order perturbation expansion. The electronic wavefunctions are obtained from a simplified version of Hoffmann's extended Hückel model (SEH), where all valence electrons are taken into account explicitly. For planar systems the observed linear dependence ofgon the energy of the half filled π orbital is well reproduced. A qualitative analysis of this dependence, making restrictive assumptions about the σ electrons, was given earlier by Stone. The calculations for non-planar model systems reproduce theg-factor anomalies which are observed for highly twisted phenyl substituted hydrocarbon radicals. The results show the necessity of direct π-σ mixing and are consistent with recent investigations of the proton hyperfine couplings in such systems.

ISSN:0026-8976    

DOI:10.1080/00268977800100731

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Nuclear relaxation rates of13C and14N have been measured as a function of temperature for neat liquid pyrimidine and pyridazine. Nuclear Overhauser enhancements, also measured as a function of temperature, were used to separate dipolar and spin rotational contributions to13C relaxation.13CH dipolar relaxation rates were combined with quadrupolar14N relaxation rates to determine rotational correlation times for motion about each principal molecular axis. The accuracy of the results is limited primarily by uncertainty in liquid-state values of the14N quadrupole coupling parameters. The temperature and viscosity dependence of the rotational correlation times is compared with those of other six-membered aromatic molecules, and is discussed in terms of the relative influence of molecular shape, attractive forces of dipolar origin and self association.2H and14N relaxation rates and the2H quadrupole coupling constant for pyrazine are also reported here, but the motion of this molecule could not be analysed.

ISSN:0026-8976    

DOI:10.1080/00268977800100741

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The radial distribution functions for the dipolar and the quadrupolar hard-sphere fluids are calculated by means of statistical-mechanical perturbation theory to third order using the hypernetted-chain (HNC) theory to approximate the perturbation terms. For both fluids the perturbation expansions through third order disagree with computer simulations, indicating that the expansion converges slowly. On the other hand, a Padé approximant, constructed from the second and third-order HNC terms, for each radial distribution function agrees excellently with the computer simulations.

ISSN:0026-8976    

DOI:10.1080/00268977800100751

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The magnetic and dynamic properties of bromo and chloro substituted benzonitriles, anisoles and anilines were studied by means of E.P.R. and zero-field odmr at 1·2–4·2 K.

ISSN:0026-8976    

DOI:10.1080/00268977800100761

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor