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41.
Residual amplitude modulation (RAM) distorts saturated absorption signals, limiting the accuracy of optical frequency references based on modulation transfer spectroscopy (MTS). Described here are two independent means by which RAM is produced in these references: (1) by the modulator and (2) when the overlap of the optical fields in the saturable absorber is asymmetric. Methods to vary RAM generated by either mechanism will be outlined and these will be used to show how RAM arising from one effect can be cancelled by the other. A theoretical treatment of MTS signals in references containing RAM is given and used to evaluate the level of signal distortion allowing the conditions for RAM cancellation to be determined. This technique is applied to improve the frequency accuracy of a reference by an order of magnitude. 相似文献
42.
Binolfi A Rodriguez EE Valensin D D'Amelio N Ippoliti E Obal G Duran R Magistrato A Pritsch O Zweckstetter M Valensin G Carloni P Quintanar L Griesinger C Fernández CO 《Inorganic chemistry》2010,49(22):10668-10679
The aggregation of alpha-synuclein (AS) is a critical step in the etiology of Parkinson's disease (PD). A central, unresolved question in the pathophysiology of PD relates to the role of AS-metal interactions in amyloid fibril formation and neurodegeneration. Our previous works established a hierarchy in alpha-synuclein-metal ion interactions, where Cu(II) binds specifically to the protein and triggers its aggregation under conditions that might be relevant for the development of PD. Two independent, non-interacting copper-binding sites were identified at the N-terminal region of AS, with significant difference in their affinities for the metal ion. In this work we have solved unknown details related to the structural binding specificity and aggregation enhancement mediated by Cu(II). The high-resolution structural characterization of the highest affinity N-terminus AS-Cu(II) complex is reported here. Through the measurement of AS aggregation kinetics we proved conclusively that the copper-enhanced AS amyloid formation is a direct consequence of the formation of the AS-Cu(II) complex at the highest affinity binding site. The kinetic behavior was not influenced by the His residue at position 50, arguing against an active role for this residue in the structural and biological events involved in the mechanism of copper-mediated AS aggregation. These new findings are central to elucidate the mechanism through which the metal ion participates in the fibrillization of AS and represent relevant progress in the understanding of the bioinorganic chemistry of PD. 相似文献
43.
Cletus B Olds W Izake EL Sundarajoo S Fredericks PM Jaatinen E 《Analytical and bioanalytical chemistry》2012,403(1):255-263
A time-resolved inverse spatially offset Raman spectrometer was constructed for depth profiling of Raman-active substances
under both the lab and the field environments. The system operating principles and performance are discussed along with its
advantages relative to traditional continuous wave spatially offset Raman spectrometer. The developed spectrometer uses a
combination of space- and time-resolved detection in order to obtain high-quality Raman spectra from substances hidden behind
coloured opaque surface layers, such as plastic and garments, with a single measurement. The time-gated spatially offset Raman
spectrometer was successfully used to detect concealed explosives and drug precursors under incandescent and fluorescent background
light as well as under daylight. The average screening time was 50 s per measurement. The excitation energy requirements were
relatively low (20 mW) which makes the probe safe for screening hazardous substances. The unit has been designed with nanosecond
laser excitation and gated detection, making it of lower cost and complexity than previous picosecond-based systems, to provide
a functional platform for in-line or in-field sensing of chemical substances. 相似文献
44.
We consider the affine equivariant sign covariance matrix (SCM) introduced by Visuri et al. (J. Statist. Plann. Inference 91 (2000) 557). The population SCM is shown to be proportional to the inverse of the regular covariance matrix. The eigenvectors and standardized eigenvalues of the covariance matrix can thus be derived from the SCM. We also construct an estimate of the covariance and correlation matrix based on the SCM. The influence functions and limiting distributions of the SCM and its eigenvectors and eigenvalues are found. Limiting efficiencies are given in multivariate normal and t-distribution cases. The estimates are highly efficient in the multivariate normal case and perform better than estimates based on the sample covariance matrix for heavy-tailed distributions. Simulations confirmed these findings for finite-sample efficiencies. 相似文献
45.
We study parameterized families of orthogonal projections for which the dimension of the parameter space is strictly less than that of the Grassmann manifold. We answer the natural question of how much the Hausdorff dimension may decrease by verifying the best possible lower bound for the dimension of almost all projections of a finite measure. We also show that a similar result is valid for smooth families of maps from the n-dimensional Euclidean space to the m-dimensional one. 相似文献
46.
Antti J. Koivisto Maija M?kinen Elina M. Rossi Hanna K. Lindberg Mirella Miettinen Ghita C.- M. Falck Hannu Norppa Harri Alenius Anne Korpi Joakim Riikonen Esa Vanhala Minnamari Vippola Pertti Pasanen Vesa-Pekka Lehto Kai Savolainen Jorma Jokiniemi Kaarle H?meri 《Journal of nanoparticle research》2011,13(7):2949-2961
This study presents a novel exposure protocol for synthesized nanoparticles (NPs). NPs were synthesized in gas phase by thermal decomposition of metal alkoxide vapors in a laminar flow reactor. The exposure protocol was used to estimate the deposition fraction of titanium dioxide (TiO2) NPs to mice lung. The experiments were conducted at aerosol mass concentrations of 0.8, 7.2, 10.0, and 28.5 mg m?3. The means of aerosol geometric mobility diameter and aerodynamic diameter were 80 and 124 nm, and the geometric standard deviations were 1.8 and 1.7, respectively. The effective density of the particles was approximately from 1.5 to 1.7 g cm?3. Particle concentration varied from 4 × 105 cm?3 at mass concentrations of 0.8 mg m?3 to 12 × 106 cm?3 at 28.5 mg m?3. Particle phase structures were 74% of anatase and 26% of brookite with respective crystallite sized of 41 and 6 nm. The brookite crystallites were approximately 100 times the size of the anatase crystallites. The TiO2 particles were porous and highly agglomerated, with a mean primary particle size of 21 nm. The specific surface area of TiO2 powder was 61 m2 g?1. We defined mice respiratory minute volume (RMV) value during exposure to TiO2 aerosol. Both TiO2 particulate matter and gaseous by-products affected respiratory parameters. The RMV values were used to quantify the deposition fraction of TiO2 matter by using two different methods. According to individual samples, the deposition fraction was 8% on an average, and when defined from aerosol mass concentration series, it was 7%. These results show that the exposure protocol can be used to study toxicological effects of synthesized NPs. 相似文献
47.
Taras K. Antal Wayne Lo William H. Armstrong Esa Tyystjärvi 《Photochemistry and photobiology》2009,85(3):663-668
We measured the photosensitivity of an artificial tetranuclear oxo–Mn(IV) complex, [Mn4 O6 (bpea)4 ]Br4 , which has an adamantane-shaped {Mn4 O6 }4+ core. Illumination caused changes in the absorption spectrum of the compound consistent with a one-electron reduction in the compound. Bromide appears to be the most probable electron donor in the reaction system. Chemical modification of the cluster appears to destabilize it, predisposing it to reductive degradation. UV light was more efficient than visible light in causing the changes. The data support the suggestion that the natural oxygen-evolving Mn complex is photosensitive and can oxidize components of the oxygen-evolving complex in its excited state causing photoinhibition, and that photostability is an important issue in designing Mn complexes for artificial photosynthesis. Furthermore, light-induced oxidation of bromide by [Mn4 O6 (bpea)4 ]4+ may suggest that oxidation of chloride is involved in natural water splitting or has been involved during the evolution of the water-splitting enzyme. 相似文献
48.
Copper oxidation studies were carried out by means of field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) techniques. The growth of copper oxide occurs as a copper surface comes in an oxygen containing environment. The reaction sequence leading to oxidation of the copper surface is generally accepted to be oxygen chemisorption, nucleation and growth of the surface oxide and bulk oxide growth. HRTEM examination of the cross section of the oxidized copper sample revealed the interface region in between the copper and copper oxide. At high oxidation temperature, formation of micro-voids and separations were observed along this interface region. Poor adhesion at this interface region due to micro-voids and separation were found to be the root cause of delamination issue. EELS analysis determined that for regions with intact interface the oxidation system is Cu/CuO/Cu2O/CuO, however, in regions containing micro-voids or separation it is found to be Cu/Cu2O/CuO. 相似文献
49.
The Claisen rearrangements of reactants containing two fluorine atoms in either the allyl or vinyl fragment are described. 相似文献
50.
Peter Ekholm Petri Lehmus Esa Kokko Matti Haukka Jukka V. Seppl Carl‐Eric Wiln 《Journal of polymer science. Part A, Polymer chemistry》2001,39(1):127-133
The synthesis and characterization of rac‐[ethylenebis(1‐(tert‐butyldimethylsilyl)‐3‐indenyl)]zirconium dichloride ( 3 ) is reported. The silyl substituted 3 /MAO was compared to its siloxy substituted analogue ( 4 ) in ethylene homo‐ and in ethylene‐1‐hexene copolymerizations to elucidate the effect of the heteroatom on polymerization performance. The influence of monomer and cocatalyst concentration and the polymerization temperature was investigated. The oxygen between the indenyl ligand and the bulky tert‐butyldimethylsilyl group in the siloxy substituted 4 /MAO was found to have a positive influence on polymerization activity and copolymerization performance. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 127–133, 2001 相似文献