STRUCTURE OF SODIUM ALUMINOSILICATE GLASSES : T1 LUMINESCENCE SPECTROSCOPY

Les spectres optiques des ions Tl+, correspondant à une émission à 350 nm, ont été mesurés dans des verres d'aluminosilicate de sodium. Les spectres mettent en évidence deux contributions dues respectivement à T1+ agissant comme modifieur du réseau ou comme compensateur de charge pour l'aluminium. Quand le rapport Al/Na devient plus grand que 1, le spectre de compensation de charge est seul observé. En dessous de 1, le spectre de modifieur du réseau prend de plus en plus d'importance quand Al/Na décroît. Ceci est bien en accord avec le modèle traditionnel des structures d'aluminosilicate alcalins, où la composition critique pour la disparition des oxygènes non-pontants est donnée par Al/Na = 1.Optical excitation spectra of Tl+ ions, corresponding to emission at 350 nm, have been measured in Na aluminosilicate glasses. The excitation spectra are shown to be superpositions of two primary spectra, which are identified with T1+ acting as network modifiers or as charge compensators for network aluminums. When Al/Na ≥ 1, only the charge compensator spectrum can be observed. As Al/Na decreases below unity, the fraction of the charge compensator spectrum decreases rapidly, and the fraction of the network modifier spectrum increases correspondingly. These results strongly support the traditional model of alkali aluminosilicate structure, in which the critical compositions for (dis)appearance of nonbridging oxygens are given by Al/Na = 1 ; they contradict reports of XPS measurements from which it had been concluded that the critical compositions are given by Al/Na ≈ 0.7. The network modifier spectra do not depend strongly on glass composition, whereas the charge compensator peaks vary significantly with composition. These results are fully consistant with new oxygen is XPS spectra

STRUCTURE OF SODIUM ALUMINOSILICATE GLASSES : X-RAY PHOTOELECTRON SPECTROSCOPY

Plusieurs séries de verre d'aluminosilicate de sodium ont été étudiées par spectroscopie des photoélectrons X (XPS) de l'oxygène. Les spectres sont interprétés sur la base de l'existence de 3 types d'environnement de l'oxygène : des oxygènes pontants symétriques BO1 (Si-O-Si), des oxygènes pontants asymétriques BO2 (Si-O-Al) et des oxygènes non-pontants NBO (Si-O-, Na+) . Les résultats sont compatibles avec la théorie classique qui prédit que la fraction d'oxygène non-pontant tend vers zéro quand le rapport Al/Na croît de 0 à 1, et que le rapport BO2/BO1 croît constamment.X-ray photoelectron spectroscopy (X.P.S) of the oxygen O1S spectra has been measured in several series of sodium alumino silicate glasses. The spectra are interpreted in terms of 3 types of oxygen environment : - a symmetric bridging oxygen - BO1 (Si-O-Si), an assymmetric bridging oxygen - BO2 (Si-O-Al), and a non-bridging oxygen - NBO (Si-O-, Na+). The results are in agreement with the classical theory which predicts that the fraction of non-bridging oxygen atoms fNBO → 0 as Al/Na increases from zero to unity, and the ratio BO2/BO1 steadily increases

The impact of the sandeel fishery closure in the northwestern North Sea on seabird food consumption, distribution and productivity

In the North Sea, the lesser sandeel (Ammodytes marinus) is the target of an industrial fishery and the principal prey of many top predators. Because of concerns about potential effects on predators, the sandeel fishery off eastern Scotland was closed in 2000, and local sandeel abundance increased subsequently. To examine whether closure benefitted sandeel-dependent seabirds, we compared summer sandeel consumption, at-sea distributions, and breeding success of seven species with fishery removals and abundance of older (1+ group) and young-of-the-year (0 group) sandeels from 1996 to 2003. Breeding success of black-legged kittiwake (Rissa tridactyla), a species that has declined in recent decades, was related to abundance of both 1+ group (the age class targeted by the fishery) and 0 group sandeels. The proportion of 0 group consumed by kittiwakes and proportion of the kittiwake population foraging in the area were linked to 0 group abundance. None of these parameters in the other seabird species were associated with sandeel abundance. Our results suggest that fishery closure can have a beneficial impact on top predators sensitive to variation in abundance of the target species, although environmental conditions before and after closure are also likely to be critically important