Bibliothek

Notizen: The optical-absorption spectrum of Ho3+:Y3Al5O12 (Ho:YAG) has been analyzed between 220 and 2160 nm under variable temperature conditions between 4 and 60 K. Measurements between 220 and 470 nm were obtained from a Ho:YAG crystal that contained 15 at. % holmium. A crystal containing 0.5 at. % holmium was used to obtain the spectrum between 470 and 2160 nm. Within the temperature and total spectral range, more than 1500 transitions due to Ho3+(4f10) were observed and attributed to transitions between individual crystal-field (Stark) levels of the ground-state multiplet 5I8, and 49 different excited multiplet manifolds, 4f10(2S+1LJ), of trivalent holmium. From an analysis of these transitions it was possible to establish 419 of the 486 Stark levels predicted below 45 000 cm−1. This large number of experimentally characterized energy levels provided a suitable basis for a detailed analysis of the 4f10(Ho3+) electronic state structure in Ho:YAG. The analysis was carried out using a model Hamiltonian that assumes D2 site symmetry for the Ho3+ ions and incorporates all of the interactions known to have a significant influence on the 4fN electronic state structure of trivalent lanthanide ions in crystalline hosts. The radial-dependent parts of the interaction terms in the model Hamiltonian are represented in parametric form, and the resulting parameters are used as variables in fitting calculated-to-experimental energy-level data.The parametric fits of energy-level data yield a rms deviation of ∼11 cm−1 between the calculated and observed locations of the 419 experimentally assigned levels (between 0 and 45 000 cm−1), and both the isotropic and anisotropic (crystal-field) interaction parameters of the model Hamiltonian are well characterized by these data fits. Among the 50 4f10[SL]J multiplet manifolds represented in the energy-level analysis, only three show major discrepancies between calculated and observed crystal-field splitting energies. The three problematic multiplet regions are: (5G6, 5F1), centered at 22 258 cm−1, 5G5, centered at ∼24 040 cm−1; and (5G3, 3L9), centered close to 28 950 cm−1. These same multiplet regions have posed problems in energy-level analyses of Ho3+(4f10) in other systems. The possible influence of correlation crystal-field (CCF) interactions on these multiplet manifolds is explored in the present study, but CCF effects appear to be small. Overall, the model Hamiltonian derived gives an excellent representation of 4f10(Ho3+) energy-level structure in Ho:YAG, and the eigenvectors of this Hamiltonian should provide a satisfactory basis for future calculations and modeling studies of Ho:YAG optical and magneto-optical properties. © 1995 American Institute of Physics.

Notizen: A case of deliberate ingestion of clothes washing liquid and fabric conditioner is described. Tracheal intubation and controlled ventilation became necessary because of respiratory muscle dysfunction associated with hypocalcaemia. We suggest that the phosphate component of the clothes washing liquid was responsible for the dramatic metabolic upset and subsequent need for admission to the Intensive Care Unit.

Notizen: African American students continue to drop out of school at a level higher than that of their White counterparts and perform lower, on average, in math and reading achievement. Research has documented influences at the family and school levels, yet little work has dealt with both simultaneously. Equally important, few analyses acknowledge that family and school influences are themselves embedded in, and partially a function of, broader structures and spatial variations in class- and race-based opportunity. I draw from the restricted-use National Educational Longitudinal Survey and 1990 Census data in addressing these concerns. Findings suggest that disparities in family background and educational resources influence racial gaps in achievement, and sometimes in an interrelated manner. These more proximate influences on achievement are, for all students, influenced by spatial patterns of general, class-based opportunity. Notably, Black disadvantage is exacerbated even further in areas of high racial inequality. Thus, broader structures of racial and class opportunity, rather than one or the other, are important determinants of educational stratification and achievement. Implications for future research are discussed.

Notizen: Abstract In this study, we explore the reproductive biology of Cercocarpus ledifolius (Rosaceae), a species showing extremely limited recruitment throughout its range in the Intermountain West, USA. We examined (1) self-compatibility, (2)the potential for wind pollination, (3) pollen limitation of seed production, and (4)loss of developing seeds. Through hand-pollination experiments, we determined that C. ledifolius flowers are highly self-compatible. Abundant seeds were produced from flowers pollinated with pollen from the same plant, and the level of seed production did not differ from control outcrossed treatments. Furthermore, we measured substantial airborne pollen loads within C. ledifolius stands, indicating transport of pollen via wind and a strong potential for wind pollination, despite apparent adaptations for insect pollination. Pollen did not limit seed production; branches on which all flowers were handpollinated with outcrossed pollen did not have greater seed production than control branches open only to natural pollination. Lastly, virtually all initiated seeds developed to maturity, indicating low seed abortion rates and minimal loss to pre-dispersal seed predators. Natural levels of fruit production were at or greater than mean fruit production for woody perennials. In summary, we found no evidence that the quantity of pollen, the source of pollen (self vs. outcrossed), or developing seed loss is seriously limiting recruitment of this species.

Notizen: Abstract Since the discovery that cells can activate their own suicide program, investigators have attempted to determine whether the events that are associated with this form of cell death are genetically determined. The discovery that the ced-3 gene of Caenorhabditis elegans encodes a cysteine protease essential for developmentally regulated apoptosis ignited interest in this area of research. As a result, we now know that cell death is specified by a number of genes and that this biologic process contributes significantly to development, tumorigenesis, and autoimmune disease. In this review I summarize what is currently known about signaling pathways involved in apoptosis, with particular emphasis on the function of the cysteine proteases known as caspases. However, there is also evidence that protease-independent cell death pathways exist. Is there a relationship between these two distinct mechanisms? If so, how do they communicate? Finally, even though the involvement of tumor necrosis factor/nerve growth factor family of receptors and cysteine proteases has been elegantly established as a component of many apoptotic signaling pathways, what happens downstream of these initial events? Why are only a selected group of cellular proteins-many nuclear-the targets of these proteases? Are nuclear events essential for apoptosis in vivo? Are the cellular genes that encode products involved in apoptotic signaling frequent targets of mutation/alteration during tumorigenesis? These are only a few questions that may be answered in the next ten years.