摘要:

Flipping ratios for 923 reflections were measured in a polarized neutron diffraction experiment at 1·5 K with a magnetic field of 4·6 T applied along the [100] and [011] directions of a crystal of Cs2KFe(CN)6, containing the low-spin [2T2g] hexacyanoferrate(III) ion. Various multipolar models of the noncollinear magnetization density were fitted to this and to the earlier [010] data. The expected transfer of spin by π covalence and σ spin polarization to the cyanide groups is observed. The field applied along [011] breaks the magnetic monoclinic symmetry of the crystal to give two independent anions. The direction of the net magnetization on each of the four independent ions in the three orientations can be accounted for by a crystal field of uniaxial symmetry with polar angles θ = 95·4(16)°, φ = 92·5(16)° relative toac*b. The magnetization is almost collinear with the applied field for the [100] and [010], but distinctly noncollinear for the [011] data. In addition, there are significant anisotropic noncollinear terms which make the magnetization direction within the Fe(CN)6unit nonuniform. The distribution of magnetization derived from all three data sets shows almost cylindrical symmetry about the ligand direction closest to the applied field for all four ions, and for both collinear and noncollinear components. It does not follow the applied field, nor is it fixed with respect to ligand axes. A cubic crystal field model, in which the magnetization is dominated by the orbital moment, reproduces these results.

ISSN:0026-8976    

DOI:10.1080/00268979300100521

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

At the magnetic field which makes the proton Larmor frequency equal to the rotational tunnelling frequency of hindered methyl groups at low temperatures, terms in the dipole-dipole interactions which are normally non-secular become secular and strongly influence the observed line shape. A study of the low and zero field NMR spectrum is reported for a sample containing groups whose tunnel frequency is 75 kHz. Broadening due to anticrossings occurs at 0·9 and 1·8 mT, in good agreement with calculated spectra. The possible modification of the tunnelling frequency at the resonances is discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300100531

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A proof of the theoretical equivalence between the E. Helfand, 1960,Phys. Rev.,119, 1 and the D. McQuarrie, 1976,Statistical Mechanics(Harper & Row), Chap. 21 equations for the calculation of thermal conductivity via the Einsteintype relations is presented here. Some theoretical implications of that equivalence are also discussed, such as the unification of the thermal conductivity expressions into one similar to that given for linear transport coefficients by F. C. Andrews, 1967,J. Chem. Phys.,47, 3161.

ISSN:0026-8976    

DOI:10.1080/00268979300100541

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Energy transfer in arrays of photosynthetic antenna pigments containing a photochemical trap is studied combining the transition probability matrix method with random walk theory and experimental data on fluorescence lifetimes and overall trapping efficiencies in systems with open and closed photochemical trap. New exact expressions for the fluorescence decay time and the trapping at infinite time are derived that contain the probability to escape from the trap as a single parameter. Two approximate relations for the fluorescence decay time, τe, andT, the probability that trapping has occurred at infinite time, are derived that can be applied to homogeneous and heterogeneous antenna arrays:

ISSN:0026-8976    

DOI:10.1080/00268979300100551

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The constant-volume specific heat and adiabatic sound velocity in liquid sulphur dioxide are studied by means of high resolution inelastic neutron and light scattering experiments. The generalized densities of statesZ(E) are obtained from the inelastic neutron scattering spectra and show a marked temperature dependence, evidenced as quasicrystalline behaviour at temperatures just above melting, which develops a substantial intensity at zero and low frequencies as the temperature is raised. The hypersonic velocity derived from the light scattering study is shown to be consistent with previous ultrasound and thermal neutron data.

ISSN:0026-8976    

DOI:10.1080/00268979300100561

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The fluorescence following excitation of BF3is studied using two techniques: (a) He* (23S) metastable excitation with dispersed fluorescence detection, (b) photon excitation using tunable vacuum UV radiation from a synchrotron source with undispersed detection. The He* experiment gives an extensive spectrum between 200 and 300 nm with two long progressions, each of separation 525 ± 30 cm-1. They are assigned to transitions to thev2bending mode of BF2[Xtilde]2A1, probably from the first excited state Ã2B1. Using photons in the energy range 10–28 eV two different fluorescence decay channels are observed: (1) BF2fluorescence for photon energies below 17eV, (2) BF3+fluorescence for energies > 21·5eV. The shapes of the excitation functions confirm that (1) is a resonant process via Rydberg states of BF3, whereas (2) is a non-resonant photoionization process. The emitting state in BF3+is the Ẽ2A′1state with a vertical ionization potential of 21·5eV. The two strongest resonant peaks at 13·1 and 14·0eV correspond to excitation of the 3s and 3p Rydberg states of BF3, but they show very different properties. The former is a continuous band, fluorescence being produced at λ > 300 nm, whereas the latter shows vibrational structure in thev4bending mode in the excitation spectrum, fluorescence now being predominantly in the range 200–350nm. The emissions are assigned to two different electronic transitions of the BF2radical. Using the synchrotron in its single-bunch mode, radiative lifetime of the two fluorescing states of BF2and BF3+Ẽ have been measured to be ca. 49, 11 and 10 ns respectively. The results are compared with recent vacuum UV photoabsorption studies of BF3, and the spectroscopy of the BF2radical is reviewed.

ISSN:0026-8976    

DOI:10.1080/00268979300100571

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Results of large-scale configuration interaction calculations of the potential surfaces for the doublet electronic states and of the corresponding transition moments in the BF2radical are presented. All computations are carried out in the C2vpoint group. Special attention is paid to the study of interactions between valence and Rydberg states.

ISSN:0026-8976    

DOI:10.1080/00268979300100581

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Potential energy surfaces for the 12A1, 12B1, and 22A1electronic states of BF2and the corresponding electronic transition moment functions obtained fromab initiocalculations are employed to calculate the vibronic structure of the u.v. absorption and emission spectra. The results of the calculations are employed to explain available experimental findings; they predict further spectral features not yet accessible by measurements.

ISSN:0026-8976    

DOI:10.1080/00268979300100591

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Transverse, longitudinal and rotating frame water proton relaxation time distributions in saturated, randomly packed silica suspensions are analysed using a simple one-dimensional numerical model of the microstructure which explicitly incorporates variable pore connectivity. Despite its simplicity the model provides a unified description not only of the relaxation behaviour but also of macroscopic diffusion measurements, magnetization transfer experiments and pulsed field gradient studies. The same approach should be useful in many heterogeneous biopolymer systems.

ISSN:0026-8976    

DOI:10.1080/00268979300100601

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Zero-valent tris(2,2′-bipyridine) aluminium ([Al(bpy)3]) is formulated as tris(2,2′-bipyridine (1-)) aluminium (III): each bipyridine ligand traps an electron. The ground-state doublet (2E) is more stabilized than the ground-state quartet (4A1) by a contribution of vacant aluminium 3dπorbitals. The filled ruthenium 4dπorbitals also take part in the spin pairing in the ground state of [Ru(bpy)3]-, a ruthenium (II) complex of three bipyridine monoanions. Both2E and4A1ground states of a system of three monoanions have a common spatial wavefunction but differ only in their spin function and thus they are inert for Jahn-Teller distortion and preferential solvation.

ISSN:0026-8976    

DOI:10.1080/00268979300100611

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor