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At least 73 records · Page 4Linked to original sources
PubMed2006

Photoexcitation of K-shell and L-shell hollow beryllium.

We have observed K-shell and L-shell hollow beryllium atoms (2s(2)2p3s and 1s3s(2)3p) created by photoexcitation using synchrotron radiation. Resonance shapes were fitted to the Fano profile and the parameters were deduced. A Dirac-Fock calculation was performed to identify the configuration of the peaks and to predict other hollow atomic peaks. The results of the calculation were in good agreement with the experimental data. The comparison of the transition strength has revealed that the three-electron photoexcitation to the 1s3s(2)3p configuration is stronger than the two-electron photoexcitation to the 2s(2)2p3s configuration. This is attributed to the large overlap between the 2s orbital of the ground state (1s(2)2s(2)) with the orbital of the L-shell hollow state (1s3s(2)3p).

Journal Article↗
PubMed2006

Shell-driven magnetic stability in core-shell nanoparticles.

The magnetic properties of ferromagnetic-antiferromagnetic Co-CoO core-shell nanoparticles are investigated as a function of the in-plane coverage density from 3.5% to 15%. The superparamagnetic blocking temperature, the coercivity, and the bias field radically increase with increasing coverage. This behavior cannot be attributed to the overall interactions between cores. Rather, it can be semiquantitatively understood by assuming that the shells of isolated core-shell nanoparticles have strongly degraded magnetic properties, which are rapidly recovered as nanoparticles come into contact.

Cobalt↗
PubMed2004

Determination of complex coefficients of radially polarized piezoelectric ceramic cylindrical shells using thin shell theory.

A method is presented to determine the complex coefficients eT33, sE11, sE12, and d31 of piezoelectric materials. The real parts of these coefficients are determined using axially polarized thin discs in the ANSI/IEEE Standard but are determined here using radially polarized cylindrical shells. The coefficients are determined by iteratively refining them until the values of the low-frequency complex admittance, three resonance frequencies, and three bandwidths computed using a thin-shell analytical model and the coefficients are very nearly equal to measured values. The accuracy of the method is determined by using quantities computed using a finite-element model in place of measured values. Measurement errors are accounted for by using a resolution of 10 Hz to compute the critical frequencies. The differences between the coefficients input to the finite-element model and those obtained using the iteration method are the errors. It is shown that the method is sufficiently accurate to use thin radially polarized cylindrical shells to determine the properties of new materials as well as characterize those used in hydrophones or other devices.

Journal Article↗
PubMed2006

Synthesis and properties of water-soluble core-shell-shell silica-CdSe/CdS-silica nanoparticles.

This paper describes the synthesis of highly water-soluble and fluorescent core-shell-shell silica-CdSe/CdS-silica nanoparticles (CSS silica-QDs-silica NPs). We used cadmium nitrate and 1,1-dimethyl-2-selenourea precursors to synthesize CdSe quantum dots (QDs) in aqueous solution under simultaneous illumination with a diode-pumped solid state green laser and a Xe-Hg lamp. After passivation of the CdSe QDs with CdS, the CdSe/CdS QDs were then conjugated covalently to (3-mercaptopropyl)trimethoxysilane (MPS); we call these nanoparticles "MPS-QDs". We mixed the MPS-QDs with tetraethoxysilane (TEOS), ethanol, and NH3. By controlling the concentrations of the reagents, the stirring speed, and the reaction time, we synthesized CSS silica-QDs-silica NPs having sizes ranging from 75 to 190 nm. The incubation time for preparing the MPS-QDs and their concentrations are important parameters in determining the morphologies of the CSS silica-QDs-silica NPs. When we mixed 50 nM MPS-QDs, 1.1 mM TEOS, and 78 mM NH3 and reacted them at a stirring speed of 750 rpm, we obtained 85-nm-diameter CSS silica-QDs-silica NPs having a QD shell thickness of about 20 nm. The CSS silica-QDs-silica NPs provide a strong photoluminescence intensity (quantum yield 88%) and exhibit enhanced stability both photochemically and in high-conductivity media (e.g., 1.0 M NaCl).

Cadmium Compounds↗
PubMed1982

Resistance of the shell membrane and mineral layer to diffusion of oxygen and water in flexible-shelled eggs of the snapping turtle (Chelydra serpentina).

At oviposition, flexible eggshells of many turtles have an outer mineral layer and an inner membrane layer of approximately equal thickness. We measured conductances of both layers to H2O and O2 at various levels of eggshell hydration. Both the mineral layer of the eggshell and the shell membrane offer significant resistance to diffusion of water vapor and oxygen in eggshells of the snapping turtle, Chelydra serpentina. Conductance to water vapor increases in both the membrane and mineral layer with increasing hydration of the eggshell, but conductance to oxygen decreases under similar conditions. Removal of the mineral layer increases conductance to oxygen in moist and dry eggshells, but decreases conductance at intermediate levels of dehydration. Removal of the mineral layer consistently increases conductance to water vapor. The eggshell membrane accounts for 24-76% of overall resistance to diffusion of water vapor. These results suggest that bulk flow of H2O or physical changes in the shell may interact with diffusion to limit gas exchange through the turtle eggshell.

Animals↗
PubMed1991

Uterine calcium-binding protein activity of nonlaying hens and hens laying hard-shelled or shell-less eggs.

Uterine calcium-binding protein (CaBP) activity was compared at oviposition and 14 h postoviposition in nonlaying (NL) hens and hens laying a high incidence of shell-less (SL) or hard-shelled (HS) eggs fed a commercial laying diet. Epithelial cells were removed from uteri of NL, SL, and HS hens and homogenized in Tris buffer, pH 7.4. Proteins in homogenate were fractionated by the addition of ammonium sulfate and centrifugation, separated by Sephadex chromatography, and measured by ultraviolet spectrophotometry at A280. Calcium-binding protein activity was determined using a 45Ca-chelex binding assay. Two peaks of CaBP activity were detected and designated CaBP-a (28,000 Da) and CaBP-b (16,000 Da). Uterine CaBP-b may differ from CaBP-a only in Ca content. The activities of CaBP (a and b) were higher (P less than or equal to .05) in HS and SL hens than in NL hens at 0 h. There was no significant difference (0 h) between CaBP activity (a versus b) of either HS or SL hens. There was also no difference (0 h) of CaBP-a or CaBP-b activity when SL and HS hens were compared. Uterine CaBP-a activity was lower (P less than or equal to .05) in SL hens than HS hens at 14 h. Under normal dietary conditions, the level of CaBP activity at 14 h may be associated with SL egg production.

Animals↗
PubMed2005

Type-II CdSe/CdTe/ZnTe (core-shell-shell) quantum dots with cascade band edges: the separation of electron (at CdSe) and hole (at ZnTe) by the CdTe layer.

The rational design and synthesis of CdSe/CdTe/ZnTe (core-shell-shell) type-II quantum dots are reported. Their photophysical properties are investigated via the interband CdSe-->ZnTe emission and its associated relaxation dynamics. In comparison to the strong CdSe (core only) emission (lambda(max) approximately 550 nm, Phi(f) approximately 0.28), a moderate CdSe-->CdTe emission (lambda(max) approximately 1026 nm, Phi(f) approximately 1.2 x 10(-3)) and rather weak CdSe-->ZnTe interband emission (lambda(max) approximately 1415 nm, Phi(f) approximately 1.1 x 10(-5)) are resolved for the CdSe/CdTe/ZnTe structure (3.4/1.8/1.3 nm). Capping CdSe/CdTe with ZnTe results in a distant electron-hole separation between CdSe (electron) and ZnTe (hole) via an intermediate CdTe layer. In the case of the CdSe/CdTe/ZnTe structure, a lifetime as long as 150 ns is observed for the CdSe-->ZnTe (1415 nm) emission. This result further indicates an enormously long radiative lifetime of approximately 10 ms. Upon excitation of the CdSe/CdTe/ZnTe structure, the long-lived charge separation may further serve as an excellent hole carrier for catalyzing the redox oxidation reaction.

Absorption↗
PubMed2006

Control of silica shell thickness and microporosity of titania-silica core-shell type nanoparticles to depress the photocatalytic activity of titania.

Titania is of potential interest as an ultraviolet (UV) radiation blocking material in personal care products because of its excellent UV light absorption properties. Its high photocatalytic activity, however, facilitates the generation of reactive oxygen species, which can oxidize and degrade other ingredients during its formulation, raising safety concerns. Dense coating of titania nanoparticles with a silica layer could help in depression of their photocatalytic activity by disturbing the formation of radicals produced by the reaction of oxygen and/or water with the electron-hole pair. Depression of the high photocatalytic activity of titania necessitates that the silica shell has to be thick, with minimum microporosity. Coating parameters were optimized to attain greater amounts of precipitated silica and thicker shells with lower microporosity, which in turn resulted in great depression of photocatalytic activity. Silica-coated titania nanoparticles were characterized by TEM, XPS, FT-IR, EDX, and microporosity measurements. The photocatalytic activity was evaluated for the coated powder to investigate the efficiency of the silica coating as well.

Journal Article↗
PubMed2006

Optical oscillator strengths for valence-shell and Br-3d inner-shell excitations of HCl and HBr.

Absolute optical oscillator strength density spectra for valence-shell excitations of HCl and HBr, as well as for Br-3d inner-shell excitations of HBr, have been determined by high-resolution electron-energy-loss-spectroscopy method in the dipole limit. Absolute optical oscillator strengths for the discrete transitions of HCl and HBr are reported and compared with the previous results determined by the photoabsorption method.

Journal Article↗
PubMed2000

Stringent tests of shell model calculations in fp shell nuclei (46, 48)Ti and (50,52)Cr from measurements of g factors and B(E2) values

Measurements of magnetic moments and lifetimes of 2(+)(1) and 4(+)(1) states of (46,48)Ti and (50,52)Cr were performed with high accuracy via projectile Coulomb excitation and the technique of transient magnetic fields. The high quality of the data allows for the first time to establish stringent constraints on large scale shell model calculations. Whereas the global behavior of the data is well explained by full fp shell model calculations, distinct deviations in the g factors and B(E2) values of (46,48)Ti from theoretical predictions can be attributed to excitations of the 40Ca core. This suggestion is supported by recent Monte Carlo calculations which provide evidence that 48Ca is a better inert core.

Journal Article↗
PubMed2002

Ab initio shell model calculations with three-body effective interactions for p-shell nuclei.

We present a qualitative improvement of the ab initio no-core shell model (NCSM) approach by implementing three-body interaction capability for p-shell nuclei. We report the first calculations using three-body effective interactions derived from realistic nucleon-nucleon potentials for 6Li, 8Be, and 10B and demonstrate that the use of three-body effective interactions speeds up the convergence of the NCSM approach. For 10B, we predict JpiT = 1(+)0 ground state, contrary to the experimental observation of 3(+)0, when the AV8(') potential is used, indicating the need for true three-body forces.

Journal Article↗
PubMed2000

Atomic scattering factors for K-shell and L-shell ionization by fast electrons

Atomic scattering factors have been calculated for K-shell ionization for elements in the range Z = 6 (carbon) to Z = 50 (tin) and for L-shell ionization in the range Z = 20 (calcium) to Z = 60 (neodymium). The calculations are based on relativistic Hartree-Fock wave functions for the atomic bound states and Hartree-Slater wave functions for the continuum wave functions. The results are presented in tabular form such that accurate values of the scattering factors can be obtained by cubic spline interpolation for incident electron energies between 50 and 400 keV and for scattering vectors with magnitude s = sin straight theta / lambda up to 2.5 A(-1) (2straight theta is the scattering angle and lambda the wavelength of the incident electrons). A separate parameterization of the form factors is given for 2.5 </= s </= 20 A(-1), where they are small. In addition, a simpler but less accurate parameterization of the atomic scattering factors in an exponential form has been obtained by fitting the calculated form factors in the region s </= 2.5 A(-1). The scattering factors are suitable for the calculation of ionization cross sections for use in atom location by channelling-enhanced microanalysis (ALCHEMI).

Journal Article↗
PubMed2001

The complete primary structure of molluscan shell protein 1 (MSP-1), an acidic glycoprotein in the shell matrix of the scallop Patinopecten yessoensis.

The complete primary structure of MSP-1, a major water-soluble glycoprotein in the foliated calcite shell layer of the scallop Patinopecten yessoensis, is reported. The full-length complementary DNA for MSP-1 isolated by polymerase chain reaction contained a sequence for a signal peptide of 20 amino acids followed by a polypeptide of 820 amino acids with calculated molecular mass of 74.5 kDa. The deduced amino acid sequence of MSP-1 includes a high proportion of Ser (32%), Gly (25%), and Asp (20%), and the predicted isoelectric point is 3.2; in these respects, MSP-1 is a typical acidic glycoprotein of mineralized tissues. A repeated modular structure characterizes MSP-1, with a sequence unit between 158 and 177 amino acids in length being repeated 4 times in tandem in the middle part of the protein. The repeated unit comprises 3 modules (SG, D, and K domains), each having a distinct amino acid composition and sequence. The SG domain is almost exclusively composed of Ser and Gly residues. The D domain is rich in Asp residues, potential N-glycosylation and phosphorylation sites. The K domain is rich in Gly residues and has a core of basic residues. The Asp residues are arranged more or less regularly in the D domains, exhibiting some repeated motifs such as Asp-Gly-Ser-Asp and Asp-Ser-Asp. Further, the 4 D domains indicate remarkable overall sequence similarities to each other. These observations suggest that the regular arrangements of COO(-) groups in the D domain side chains may be important for specific control of crystal growth.

Journal Article↗
PubMed1994

Different modifications of the dopamine metabolism in the core and shell parts of the nucleus accumbens following CCK-A receptor stimulation in the shell region.

After the injection of CCK8 into the posterior N. Acc. of rats DA, DOPAC HVA contents were determined from punches of the anterior and posterior N. Acc. and VTA. CCK8 (20 pmol/side) modified these levels only in the posterior N. Acc. and these responses were inhibited by the CCK-A antagonist devazepide. Five min after treatment, DA, DOPAC and HVA were increased in the N. Acc.shell and 10 min later they were decreased in the N. Acc.core. These data suggest that in these regions CCK8 could both abolish the influence of DA from the core on the transmission of motor information and favor that of DA from the shell on emotional-like responses.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1997

Computational Comparison of S(N)()2 Substitution Reactions of CHX(*-)() and CH(2)()X(-)() with CH(3)()X (X = Cl, Br). Do Open-Shell and Closed-Shell Anions React Differently?

The S(N)2 displacement reaction of the radical anions (CHCl(*-) and CHBr(*-)) and the closed-shell anions (CH(2)Cl(-) and CH(2)Br(-)) with CH(3)Cl and CH(3)Br were studied with density functional theory. It was determined that the anions CH(2)Cl(-) and CH(2)Br(-) were more reactive than the radical anions CHCl(*-) and CHBr(*-), in agreement with experiment. The degree of charge transfer in the reactant complex is the best indicator of the reactivity trend. It was found that two reactions (CH(2)Cl(-) with CH(3)Cl and CH(3)Br) proceeded without an activation barrier at the B3LYP/6-31+G(d) level. The backside transition states (leading to inversion of configuration at the methyl group) were 20-30 kcal/mol lower in energy than the frontside transition states (leading to retention of configuration). Interestingly, it was found that the mechanism of backside and frontside attack by radical anions was different. In backside attack, the radical anion leads with the lone pair directed toward the methyl group to form an incipient two-center, two-electron bond. In frontside attack, the radical anion leads with the unpaired electron directed toward the methyl group to form an incipient two-center, one-electron bond.

Journal Article↗
PubMed1958

Studies on shell formation. VII. The submicroscopic structure of the shell of the oyster Crassostrea virginica.

The submicroscopic structure of the growing surface of the shell of the oyster, Crassostrea virginica, was studied by means of shadowed replicas. The outer edge of the prismatic region consists of a fine grained matrix enclosing crystals, the surfaces of which show a finely pebbled structure. Crystal size varies continously from 0.01 micro to 8 micro. The matrix surface shows no evidence of fibrous structure. The outer portions of the prismatic region exhibit a tile-like arrangement of large crystals separated by granular matrix 0.02 to 0.08 micro in thickness. The exposed crystal surfaces have indentations of varying form which appear as roughly parallel grooves spaced at intervals of approximately 0.3 micro. The final form of this region is believed to result from the random distribution of crystal seeds, which grow without orientation and through coalescence and growth come into contact, producing polygonal areas. The crystal arrangement of the nacreous region is one of overlapping rows of crystals in side to side contact, and with one end of each crystal free, permitting continued increase in length. Crystal angles and plane indices are presented.

Animals↗