Differential Cross Section and Analyzing Power of p(p,pi+)d Near Threshold

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Compilation and Network Analyses of Cambrian Food Webs

A rich body of empirically grounded theory has developed about food webs—the networks of feeding relationships among species within habitats. However, detailed food-web data and analyses are lacking for ancient ecosystems, largely because of the low resolution of taxa coupled with uncertain and incomplete information about feeding interactions. These impediments appear insurmountable for most fossil assemblages; however, a few assemblages with excellent soft-body preservation across trophic levels are candidates for food-web data compilation and topological analysis. Here we present plausible, detailed food webs for the Chengjiang and Burgess Shale assemblages from the Cambrian Period. Analyses of degree distributions and other structural network properties, including sensitivity analyses of the effects of uncertainty associated with Cambrian diet designations, suggest that these early Paleozoic communities share remarkably similar topology with modern food webs. Observed regularities reflect a systematic dependence of structure on the numbers of taxa and links in a web. Most aspects of Cambrian food-web structure are well-characterized by a simple “niche model,” which was developed for modern food webs and takes into account this scale dependence. However, a few aspects of topology differ between the ancient and recent webs: longer path lengths between species and more species in feeding loops in the earlier Chengjiang web, and higher variability in the number of links per species for both Cambrian webs. Our results are relatively insensitive to the exclusion of low-certainty or random links. The many similarities between Cambrian and recent food webs point toward surprisingly strong and enduring constraints on the organization of complex feeding interactions among metazoan species. The few differences could reflect a transition to more strongly integrated and constrained trophic organization within ecosystems following the rapid diversification of species, body plans, and trophic roles during the Cambrian radiation. More research is needed to explore the generality of food-web structure through deep time and across habitats, especially to investigate potential mechanisms that could give rise to similar structure, as well as any differences

Sensitisation of Eu(III)- and Tb(III)- based luminescence by Ir(III) units in Ir/lanthanide dyads: evidence for parallel energy-transfer and electron-transfer based mechanisms

A series of blue-luminescent Ir(III) complexes with a pendant binding site for lanthanide(III) ions has been synthesized and used to prepare Ir(III)/Ln(III) dyads (Ln = Eu, Tb, Gd). Photophysical studies were used to establish mechanisms of Ir→Ln (Ln = Tb, Eu) energy-transfer. In the Ir/Gd dyads, where direct Ir→Gd energy-transfer is not possible, significant quenching of Ir-based luminescence nonetheless occurred; this can be ascribed to photoinduced electron-transfer from the photo-excited Ir unit (*Ir, 3MLCT/3LC excited state) to the pendant pyrazolyl-pyridine site which becomes a good electron-acceptor when coordinated to an electropositive Gd(III) centre. This electron transfer quenches the Ir-based luminescence, leading to formation of a charge-separated {Ir4+}•—(pyrazolyl-pyridine)•− state, which is short-lived possibly due to fast back electron-transfer (<20 ns). In the Ir/Tb and Ir/Eu dyads this electron-transfer pathway is again operative and leads to sensitisation of Eu-based and Tb-based emission using the energy liberated from the back electron-transfer process. In addition direct Dexter-type Ir→Ln (Ln = Tb, Eu) energytransfer occurs on a similar timescale, meaning that there are two parallel mechanisms by which excitation energy can be transferred from *Ir to the Eu/Tb centre. Time-resolved luminescence measurements on the sensitised Eu-based emission showed both fast and slow rise-time components, associated with the PET-based and Dexter-based energy-transfer mechanisms respectively. In the Ir/Tb dyads, the Ir→Tb energy-transfer is only just thermodynamically favourable, leading to rapid Tb→Ir thermally-activated back energy-transfer and non-radiative deactivation to an extent that depends on the precise energy gap between the *Ir and Tb-based 5D4 states. Thus, the sensitised Tb(III)-based emission is weak and unusually short-lived due to back energy transfer, but nonetheless represents rare examples of Tb(III) sensitisation by a energy donor that could be excited using visible light as opposed to the usually required UV excitation

Ga-Modified (Si-Ca-P) Sol-Gel Glasses: Possible Relationships Between Surface Chemical Properties and Bioactivity

In vitro bioactivity features of a Ga-modified sol gel Si-Ca-P glass(SGGa) were investigated, in comparison with a plain ternary Si-Ca-P system(SG). Reaction/dissolution of the glass at increasing soaking times in simulatedbody fluids (SBF) and the consequent growth of an apatite-like layer, monitoringbioactivity, were studied by employing a variety of chemical and physical techniques.The growth of a crystalline apatitic layer at the Ga-modified-glass/SBFinterface is severely delayed with respect to the Ga-free glass, and the reasons for ithave been looked for in the dramatic changes induced, at the glass/SBF interface, bythe presence of the Ga2O3 component. In situ Fourier transform infraredspectroscopy allowed to describe the nature/structure of surface terminations forthe two glasses and to reveal/quantify the acidic strength of different Ga speciesexposed at the SGGa glass surface. 2,6-Dimethylpyridine and carbon monoxidewere employed as molecular probes to reveal Brønsted and Lewis acidity. At thesurface of the Ga-modified glass, both Brønsted and strong Lewis acidic sites are present. The enhanced surface acitiy of SGGa glass,with respect to the plain glass SG, has been proposed to be responsible for the slower glass dissolution in SBF and for the delayeddeposition/crystallization of an apatite-like layer at the glass/SBF interface