Peak positions and shapes in neutron pair correlation functions from powders of highly anisotropic crystals

The effect of the powder average on the peak shapes and positions in neutron pair distribution functions of polycrystalline materials is examined. It is shown that for highly anisotropic crystals, the powder average leads to shifts in peak positions and to non-Gaussian peak shapes. The peak shifts can be as large as several percent of the lattice spacing

High Temperature Electron Localization in dense He Gas

We report new accurate mesasurements of the mobility of excess electrons in high density Helium gas in extended ranges of temperature $[(26\leq T\leq 77) K ]$ and density $[ (0.05\leq N\leq 12.0) {atoms} \cdot {nm}^{-3}]$ to ascertain the effect of temperature on the formation and dynamics of localized electron states. The main result of the experiment is that the formation of localized states essentially depends on the relative balance of fluid dilation energy, repulsive electron-atom interaction energy, and thermal energy. As a consequence, the onset of localization depends on the medium disorder through gas temperature and density. It appears that the transition from delocalized to localized states shifts to larger densities as the temperature is increased. This behavior can be understood in terms of a simple model of electron self-trapping in a spherically symmetric square well.Comment: 23 pages, 13 figure

CENTRES COLORÉS, PROPRIÉTÉS SPECTROSCOPIQUESDÉTECTION OPTIQUE DE LA RÉSONANCE ÉLECTRONIQUE DE L'EXCITON AUTOPIÉGÉ DANS LES HALOGÉNURES ALCALINS ; OBSERVATION D'UN CROISEMENT DE NIVEAUX

La résonance dc l'exciton autopiégé dans KBr, RbBr, NaCl a été observée par détection optique. Dans le cas de KBr, les constantes de l'hamiltonien de spin ont été déterminées. Les spectres de résonance montrent qu'à 1,6 K, l'équilibre de Boltzmann n'est pas atteint. L'ensemble de ces résultats est décrit dans une publication récente et sera seulement brièvement résumé ici. Les résultats obtenus sur KBr sont comparés à ceux prédits dans un modèle simple. L'observation d'un effet de croisement de niveau est décrite.Electron Paramagnetic Resonance of the self trapped exciton was observed in NaCl, RbBr and KBr. The parameters defining the spin hamiltonian were determined for KBr. The appearance of the EPR spectra shows that at low temperature (1.6 K) the populations of the Zeeman sublevels are not in thermal equilibrium. These experimental results are described in a recent publication and they will only be briefly recalled. The parameters determined for KBr are compared to those predicted within a simple model. The observation of a level crossing effect is described

Détection optique de la résonance électronique de l'exciton autopiégé dans les halogénures alcalins ; observation d'un croisement de niveaux

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