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When an impurity in a crystal is impact-excited by hot electrons, its level occupancies and hence the emission intensities depend on a convolution of the impact cross sections and the hot-electron distribution function. In special circumstances the impact cross section is proportional to the inverse of the electric dipole radiative lifetime. Information about the hot-electron distribution can then be obtained. Some of the conditions to be fulfilled are discussed.

It is argued that the very large variation in the electroluminescence emission intensities from a given type of rare-earth ion indicates a hot-electron energy distribution in which the probability of finding an electron of energy E has an exponential dependence on E. Means of ascertaining the precise nature of this energy dependence are briefly described.

The author comments on a question raised by Choy and Sherrington (see ibid., vol.17, p.739, 1984). He compares experimentally and numerically the mean-field models of van Hemmen (1982) and of Sherrington and Kirkpatrick (1975), in an attempt to assess their validity.

The magnetic properties of the organic superconductor beta -(BEDT-TTF)₂AuI₂ (BEDT-TTF=bis(ethylenedithio)tetrathiofulvalene) have been studied from 300 to 4.2K by ESR spectroscopy and bulk susceptibility measurements using a Faraday balance. The Pauli susceptibility is independent of temperature and has a value of 3.4(3)*10^-4 emu mol^-1. The temperature dependence of the ESR linewidth is consistent with the behaviour expected for conduction electrons although there is a difference in the angular dependence of the linewidth between 290 and 15K.

The Mossbauer recoil-free fraction for a general non-Euclidean dimensionality is calculated for the first time. The result is useful for the study of the Mossbauer effect in proteins, glues, gels etc.

X-ray absorption spectroscopy is now being applied to the study of a wide range of surface structures. The modulation of the absorption coefficient as a function of photon energy, caused by photo-electron scattering, contains information on the local geometric structure around the absorbing atom. Far above an absorption edge, in the EXAFS (extended X-ray absorption fine structure) regime, single scattering usually applies and near-neighbour distances (R) and coordination numbers (N) can readily be obtained. Some of the factors influencing the scattering and thus an accurate determination of R and N are critically discussed. The usefulness of surface EXAFS (SEXAFS) is illustrated by reviewing measurements on the oxidation of aluminium, the adsorption of Te and I on silicon and germanium, the clean surface structure of ion-bombarded silicon, and the ionic-covalent transition in submonolayer coverages of caesium adsorbed on silver. Near to the absorption edge, the XANES (X-ray absorption near edge structure), also called NEXAFS (near-edge X-ray absorption fine structure), is often complicated by multiple-scattering effects and may in consequence produce a spectrum rich in structure but awkward to interpret. The atomic adsorption of oxygen on nickel is given as an example. However, for molecular adsorbates, the XANES is often dominated by intra-molecular scattering, and may then quite simply yield the molecular orientation and intra-molecular distances, and hence details of the bonding to the surface. These ideas are illustrated by the adsorption of various carbon- and oxygen-containing small molecules.

A molecular dynamics simulation has been performed for the system of an Ar atom in 216 KCl ions at 1173K. The chemical potential of the Ar atom has been calculated by using three methods based on the Widom prescription. Information on the magnitudes of various contributions to the solubility of Ar gas in molten KCl has been obtained. A slight sign of a solvation effect between the solute and the solvent particles, which was induced by the different sizes of the charged particles, has been observed in their pair distribution functions.

The orientationally disordered phase of carbon tetrabromide at 345K has been studied using the molecular dynamics simulation technique. A realistic inter-molecular potential has been developed, which was parametrized by fitting to some experimental data and tested by comparing the results of the simulations with other independent experimental data. This model has been used to study various properties generally associated with the behaviour of single molecules and their neighbouring molecules: thermodynamic properties, the orientational order of single molecules, correlations between the orientations of neighbouring molecules, and single-molecule dynamics. The results are compared with previous work, and it is shown that the molecular orientations are more uniform and their dynamics more complicated than previously thought.

For pt.I see ibid., vol.19, p.3325 (1986). Molecular dynamics simulation techniques have been used to study the orientationally disordered phase of carbon tetrabromide at 345K, with particular emphasis placed on analysing the coupling between orientations and translations. The wave-vector dependence of this coupling has been calculated, showing that there are no couplings of any sort at k=0, and that over the surface of the Brillouin zone boundary there is a strong coupling between the transverse acoustic modes and the orientational variables that order in the low-temperature phase. The phase transition is discussed within the context of this coupling. The collective dynamics have been studied by calculations of various correlation functions, which become long-lived when the orientational-translation coupling becomes strong. It is shown that a simple Mori theory that includes this coupling is able to reproduce the observed behaviour. In particular, it has been found that when the coupling is strong, the acoustic modes appear in a coherent inelastic neutron scattering experiment as quasielastic scattering with full width at half-maximum height of less than 0.03 THz.

One-dimensional monatomic and diatomic chains with harmonic coupling between neighbouring sites and an on-site anharmonic potential V( phi )=C phi ²ⁿ⁺²+B phi ⁿ⁺²+A phi ²+D are examined in the displacive limit, which serves as a model for a structural phase transition. Kink solutions are obtained at the T=0 first-order phase transition point for B²=4AC, A, C>0, B<0 and D=0 for arbitrary values of n. Non-linear periodic solutions (non-linear phonons) are obtained for n=1, and these are also found to be the solutions of discrete equations of motion. The solutions are shown to be stable and have finite energy.

The local static and dynamic properties of the mixed crystals Na(CN)_xCl_1-x are studied by ²³Na NMR over a wide concentration range. Special emphasis is laid upon systems which transform into a low-temperature glass state (x<or approximately=0.7). The strong anisotropy of the lineshapes could be accounted for by assuming that the principal axes of the electric field gradient tensors are oriented parallel to the axes of the average cubic structure with the principal components distributed according to a gaussian law. Between 200 and 100K, the width of this distribution increases by one order of magnitude indicating considerable local changes, which are attributed to a transition into an orientational glass state. From the temperature dependence of the ²³Na spin-lattice relaxation time, the temperature dependence of the correlation times describing the motions of the CN ions in their local potentials are deduced for various concentrations x. In the temperature region where the transition to the glass state takes place the motions remain fast compared with the timescale of the NMR experiment (reciprocal Larmor frequency) but the type of motions changes gradually. A qualitative model is developed which basically assumes that decreasing temperature deviations from the cubic symmetry of the local potentials occur progressively as a consequence of cooperative phenomena. Measurements of the ²³Na satellite T₁ exclude slow fluctuations in kHz regime.

Recent experiments on superionic CaF₂ and other fluorites using coherent inelastic neutron scattering have revealed a strong quasi-elastic peak in the cross section. This is associated with collective fluctuations which decay on a time scale of order 1 ps. The author describes here an extensive series of molecular dynamics simulations on CaF₂ over a wide range of temperatures spanning the transition to the superionic state. The simulations have been used to calculate the dynamical structure factors, which are directly related to the coherent inelastic cross section. He shows the existence of a quasi-elastic peak in the structure factors and demonstrates that the dependence on temperature and wave-vector of the intensity and frequency width of this peak are in reasonable agreement with the experimental results. He concludes that the collective motions responsible for the peak are essentially the same in the simulated system and in the real material.

The authors study a long-range quantum spin glass model with randomly mixed local uniaxial anisotropies. Two types of magnetic impurities are considered, and the case of competing anisotropies will be discussed in particular. They calculate the phase diagrams of these systems as a function of impurity concentrations and strengths of the anisotropies. Depending on the parameters of the system, interesting transitions from longitudinal to transverse freezing and/or condensation in a non-magnetic state are observed.

The changes in the magnetic resonance spectra at the transition from the incommensurate plane wave to the incommensurate multi-soliton lattice modulation limits are analysed for different relations between the magnetic resonance frequency and the nuclear displacements. The temperature dependence of the soliton density in Rb₂ZnCl₄ is redetermined for different crystal orientations in the external magnetic field and the results for the 'linear' and the 'quadratic' case are found to be in good agreement. Whereas comparison of experimental and theoretical lineshapes allows for a rather precise determination of the soliton density, a determination based only on the intensity of the 'commensurate' peaks-as used in early studies-may lead to quantitatively inconsistent results.

An analysis of ¹H and ¹⁹F NMR spectra recorded over the entire Co²⁺ concentration region makes it possible to determine the interactions between resonant nuclei and paramagnetic ions in the system. The paramagnetic broadening observed for the ¹H NMR is interpreted in terms of purely dipolar interaction. Fast flipping of the Co²⁺ ions inhibited by short-range interactions is suggested. The ¹⁹F spectra reveal three lines reflecting three different paramagnetic environments. Hyperfine interactions are predominant for the nearest paramagnetic neighbours whereas dipolar interactions are predominant for those more distant.

The structure and the free energy of domain walls in order-disorder ferroelectrics have been studied for an ANNNI-type pseudo-spin model exhibiting (for a suitable choice of parameters) ferroelectric, paraelectric, or modulated structures. Non-zero tunnelling terms are allowed for in the Hamiltonian. The interactions are confined to values which lead to a ferroelectric ground state. The four-dimensional mapping resulting from a mean field treatment of plane wall structures is linearised about the ferroelectric fixed point, and analytical expressions are derived for eigenvalues and eigenvectors. A fixed point analysis is presented and used to characterise wall profiles in different regions of parameter space. Quantitative results for such profiles are given which were obtained by combining the linearised mapping valid in the asymptotic region with the non-linear equations needed near the centre of the wall. Both for the minimum and the saddle point configurations the free wall energies were calculated and their relation to the phase diagram is discussed.

Matrix elements describing one- and two-photon magneto-optical transitions in InSb-type semiconductors have been calculated by treating the cubic nonsphericity and inversion asymmetry of the crystal within perturbation theory. Selection rules are obtained for these transitions as a function of angle between the magnetic field and the (001) axis in (001) and (110) planes for sigma _L, sigma _R and pi optical polarisations.

The wavenumber, nu ₁, of the symmetric chain-stretching mode of linear-chain, halogen-bridged mixed-valence palladium complexes of the type (Pd(LL)₂)(Pd(LL)₂X₂)(ClO₄)₄, where LL=1,2-diaminoethane (en), 1,2-diaminopropane (pn) or 1,3-diaminopropane (tn) for X=Cl, and LL=en for X=Br, increases as the wavenumber of the excitation line ( nu ₀) increases. For all four complexes studied nu ₁ consists of several components whose relative intensities change with a change in nu ₀. Possible explanations for the dependence of the components of nu ₁ on nu ₀ are considered.

The authors report low-frequency Raman measurements on the plastic phase, metastable supercooled phase and glassy phase of 1-cyanoadamantane. The main feature is the change of the shape of the Raman spectra below T_g. Comparison with coherent neutron scattering suggests the existence of ordered clusters in the quenched matrix of the glassy phase.

The M_4,5 photo-absorption spectra (XAS) of pure CeO₂ and oxidised cerium have been studied. The spectrum of CeO₂ shows two prominent peaks in both the M₅ and M₄ regions. The stronger peaks, which occur at the energies 883.6 and 901.7 eV, are attributed to the transitions 3d¹⁰4f¹ to 3d⁹4f² and the other peaks which appear at the energies 888.6 and 906.5 eV are attributed to the transitions 3d¹⁰4f⁰ to 3d⁹4f¹. Comparison of the intensity ratio I(f¹)/(I(f¹)+I(f²)) of the peaks with recent theoretical results indicates only 0.33 for the f⁰ weight, c(f⁰), in CeO₂. That is, the number of 4f electrons, n_f, of CeO₂ is found to be 0.67 in its ground state. Measurements of the M_4,5 XAS spectra of oxidised metallic cerium absorbers show that CeO₂ results when the metal is oxidised in air, but oxidation in oxygen results in either Ce₂O₃ or a metallic sample covered with a protective layer of Ce₂O₃. The difference between the M_{alpha , beta} photo-emission spectrum (XES) and the M_4,5 XAS of CeO₂ is attributed to changes in the sample as a result of heating during the experiment.