1,028 research outputs found
Building library skills in the seventh grade
Thesis (M.A.)--Boston University, 1948. This item was digitized by the Internet Archive
Quantitative modeling of \textit{in situ} x-ray reflectivity during organic molecule thin film growth
Synchrotron-based x-ray reflectivity is increasingly employed as an
\textit{in situ} probe of surface morphology during thin film growth, but
complete interpretation of the results requires modeling the growth process.
Many models have been developed and employed for this purpose, yet no detailed,
comparative studies of their scope and accuracy exists in the literature. Using
experimental data obtained from hyperthermal deposition of pentane and
diindenoperylene (DIP) on SiO, we compare and contrast three such models,
both with each other and with detailed characterization of the surface
morphology using ex-situ atomic force microscopy (AFM). These two systems each
exhibit particular phenomena of broader interest: pentacene/SiO exhibits a
rapid transition from rough to smooth growth. DIP/SiO, under the conditions
employed here, exhibits growth rate acceleration due to a different sticking
probability between the substrate and film. In general, \textit{independent of
which model is used}, we find good agreement between the surface morphology
obtained from fits to the \insitu x-ray data with the actual morphology at
early times. This agreement deteriorates at later time, once the root-mean
squared (rms) film roughness exceeds about 1 ML. A second observation is that,
because layer coverages are under-determined by the evolution of a single point
on the reflectivity curve, we find that the best fits to reflectivity data ---
corresponding to the lowest values of --- do not necessarily yield
the best agreement between simulated and measured surface morphologies.
Instead, it appears critical that the model reproduce all local extrema in the
data. In addition to showing that layer morphologies can be extracted from a
minimal set of data, the methodology established here provides a basis for
improving models of multilayer growth by comparison to real systems.Comment: 34 pages (double-spaced, including figures and references), 10
figures, 3 appendice
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Transition Probabilities Of Astrophysical Interest In The Niobium Ions Nb+ And Nb2+
Aims. We attempt to derive accurate transition probabilities for astrophysically interesting spectral lines of Nb II and Nb III and determine the niobium abundance in the Sun and metal-poor stars rich in neutron-capture elements. Methods. We used the time-resolved laser-induced fluorescence technique to measure radiative lifetimes in Nb II. Branching fractions were measured from spectra recorded using Fourier transform spectroscopy. The radiative lifetimes and the branching fractions were combined yielding transition probabilities. In addition, we calculated lifetimes and transition probablities in Nb II and Nb III using a relativistic Hartree-Fock method that includes core polarization. Abundances of the sun and five metal-poor stars were derived using synthetic spectra calculated with the MOOG code, including hyperfine broadening of the lines. Results. We present laboratory measurements of 17 radiative lifetimes in Nb II. By combining these lifetimes with branching fractions for lines depopulating the levels, we derive the transition probabilities of 107 Nb II lines from 4d(3)5p configuration in the wavelength region 2240-4700 angstrom. For the first time, we present theoretical transition probabilities of 76 Nb III transitions with wavelengths in the range 1430-3140 angstrom. The derived solar photospheric niobium abundance log epsilon(circle dot) = 1.44 +/- 0.06 is in agreement with the meteoritic value. The stellar Nb/Eu abundance ratio determined for five metal-poor stars confirms that the r-process is a dominant production method for the n-capture elements in these stars.Integrated Initiative of Infrastructure RII3-CT-2003-506350Swedish Research CouncilKnut and Alice Wallenberg FoundationBelgian FRS-FNRSFRIAUS National Science Foundation AST-0607708, AST-0908978Astronom
Development of high-gain gaseous photomultipliers for the visible spectral range
We summarize the development of visible-sensitive gaseous photomultipliers,
combining a semitransparent bi-alkali photocathode with a state-of-the-art
cascaded electron multiplier. The latter has high photoelectron collection
efficiency and a record ion blocking capability. We describe in details the
system and methods of photocathode production and characterization, their
coupling with the electron multiplier and the gaseous-photomultiplier operation
and characterization in a continuous mode. We present results on the properties
of laboratory-produced KCsSb, CsSb and NaKSb photocathodes and
report on their stability and QE in gas; KCsSb photocathodes yielded QE
values in Ar/CH(95/5) above 30% at wavelengths of 360-400 nm. The novel
gaseous photomultiplier yielded stable operation at gains of 10, in
continuous operation mode, in 700 Torr of this gas; its sensitivity to single
photons was demonstrated. Other properties are described. The successful
detection of visible light with this gas-photomultiplier pave ways towards
further development of large-area sealed imaging detectors, of flat geometry,
insensitive to magnetic fields, which might have significant impact on light
detection in numerous fields.Comment: 22 pages, 12 figures, for submission to JINS
Reduced Deadtime and Higher Rate Photon-Counting Detection using a Multiplexed Detector Array
We present a scheme for a photon-counting detection system that can be
operated at incident photon rates higher than otherwise possible by suppressing
the effects of detector deadtime. The method uses an array of N detectors and a
1-by-N optical switch with a control circuit to direct input light to live
detectors. Our calculations and models highlight the advantages of the
technique. In particular, using this scheme, a group of N detectors provides an
improvement in operation rate that can exceed the improvement that would be
obtained by a single detector with deadtime reduced by 1/N, even if it were
feasible to produce a single detector with such a large improvement in
deadtime. We model the system for continuous and pulsed light sources, both of
which are important for quantum metrology and quantum key distribution
applications.Comment: 6 figure
Orbit structure and (reversing) symmetries of toral endomorphisms on rational lattices
We study various aspects of the dynamics induced by integer matrices on the
invariant rational lattices of the torus in dimension 2 and greater. Firstly,
we investigate the orbit structure when the toral endomorphism is not
invertible on the lattice, characterising the pretails of eventually periodic
orbits. Next we study the nature of the symmetries and reversing symmetries of
toral automorphisms on a given lattice, which has particular relevance to
(quantum) cat maps.Comment: 29 pages, 3 figure
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