5,479 research outputs found
Angle-Resolved Photoemission of Solvated Electrons in Sodium-Doped Clusters
Angle-resolved photoelectron spectroscopy of the unpaired electron in
sodium-doped water, methanol, ammonia, and dimethyl ether clusters is
presented. The experimental observations and the complementary calculations are
consistent with surface electrons for the cluster size range studied. Evidence
against internally solvated electrons is provided by the photoelectron angular
distribution. The trends in the ionization energies seem mainly determined by
the degree of hydrogen bonding in the solvent and the solvation of the ion
core. The onset ionization energies of water and methanol clusters do not level
off at small cluster sizes, but decrease slightly with increasing cluster size
Electron mean free path from angle-dependent photoelectron spectroscopy of aerosol particles
We propose angle-resolved photoelectron spectroscopy of aerosol particles as
an alternative way to determine the electron mean free path of low energy
electrons in solid and liquid materials. The mean free path is obtained from
fits of simulated photoemission images to experimental ones over a broad range
of different aerosol particle sizes. The principal advantage of the aerosol
approach is twofold. Firstly, aerosol photoemission studies can be performed
for many different materials, including liquids. Secondly, the size-dependent
anisotropy of the photoelectrons can be exploited in addition to size-dependent
changes in their kinetic energy. These finite size effects depend in different
ways on the mean free path and thus provide more information on the mean free
path than corresponding liquid jet, thin film, or bulk data. The present
contribution is a proof of principle employing a simple model for the
photoemission of electrons and preliminary experimental data for potassium
chloride aerosol particles
Albedo and Reflection Spectra of Extrasolar Giant Planets
We generate theoretical albedo and reflection spectra for a full range of
extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects.
Our albedo modeling utilizes the latest atomic and molecular cross sections, a
Mie theory treatment of extinction by condensates, a variety of particle size
distributions, and an extension of the Feautrier radiative transfer method
which allows for a general treatment of the scattering phase function. We find
that due to qualitative similarities in the compositions and spectra of objects
within each of five broad effective temperature ranges, it is natural to
establish five representative EGP albedo classes: a ``Jovian'' class (T K; Class I) with tropospheric ammonia clouds, a ``water
cloud'' class (T K; Class II) primarily affected by
condensed HO, a ``clear'' class (T K; Class III)
which lacks clouds, and two high-temperature classes: Class IV (900 K
T 1500 K) for which alkali metal absorption
predominates, and Class V (T 1500 K and/or low surface
gravity ( 10 cm s)) for which a high silicate layer
shields a significant fraction of the incident radiation from alkali metal and
molecular absorption. The resonance lines of sodium and potassium are expected
to be salient features in the reflection spectra of Class III, IV, and V
objects. We derive Bond albedos and effective temperatures for the full set of
known EGPs and explore the possible effects of non-equilibrium condensed
products of photolysis above or within principal cloud decks. As in Jupiter,
such species can lower the UV/blue albedo substantially, even if present in
relatively small mixing ratios.Comment: revised LaTeX manuscript accepted to Ap.J.; also available at
http://jupiter.as.arizona.edu/~burrows/paper
A Metric Framework for quantifying Data Concentration
Poor performance of artificial neural nets when applied to credit-related classification problems is investigated and contrasted with logistic regression classification. We propose that artificial neural nets are less successful because of the inherent structure of credit data rather than any particular aspect of the neural net structure. Three metrics are developed to rationalise the result with such data. The metrics exploit the distributional properties of the data to rationalise neural net results. They are used in conjunction with a variant of an established concentration measure that differentiates between class characteristics. The results are contrasted with those obtained using random data, and are compared with results obtained using logistic regression. We find, in general agreement with previous studies, that logistic regressions out-perform neural nets in the majority of cases. An approximate decision criterion is developed in order to explain adverse results
Modern microwave methods in solid state inorganic materials chemistry: from fundamentals to manufacturing
No abstract available
The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey
We present the design and performance of the multi-object fiber spectrographs
for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon
Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999
on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the
spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II
surveys, enabling a wide variety of Galactic and extra-galactic science
including the first observation of baryon acoustic oscillations in 2005. The
spectrographs were upgraded in 2009 and are currently in use for BOSS, the
flagship survey of the third-generation SDSS-III project. BOSS will measure
redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha
absorption of 160,000 high redshift quasars over 10,000 square degrees of sky,
making percent level measurements of the absolute cosmic distance scale of the
Universe and placing tight constraints on the equation of state of dark energy.
The twin multi-object fiber spectrographs utilize a simple optical layout
with reflective collimators, gratings, all-refractive cameras, and
state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in
two channels over a bandpass covering the near ultraviolet to the near
infrared, with a resolving power R = \lambda/FWHM ~ 2000. Building on proven
heritage, the spectrographs were upgraded for BOSS with volume-phase
holographic gratings and modern CCD detectors, improving the peak throughput by
nearly a factor of two, extending the bandpass to cover 360 < \lambda < 1000
nm, and increasing the number of fibers from 640 to 1000 per exposure. In this
paper we describe the original SDSS spectrograph design and the upgrades
implemented for BOSS, and document the predicted and measured performances.Comment: 43 pages, 42 figures, revised according to referee report and
accepted by AJ. Provides background for the instrument responsible for SDSS
and BOSS spectra. 4th in a series of survey technical papers released in
Summer 2012, including arXiv:1207.7137 (DR9), arXiv:1207.7326 (Spectral
Classification), and arXiv:1208.0022 (BOSS Overview
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Recommended from our members
A Search for Dark Higgs Bosons
Recent astrophysical and terrestrial experiments have motivated the proposal
of a dark sector with GeV-scale gauge boson force carriers and new Higgs
bosons. We present a search for a dark Higgs boson using 516 fb-1 of data
collected with the BABAR detector. We do not observe a significant signal and
we set 90% confidence level upper limits on the product of the Standard
Model-dark sector mixing angle and the dark sector coupling constant.Comment: 7 pages, 5 postscript figures, published version with improved plots
for b/w printin
The Baryon Oscillation Spectroscopic Survey of SDSS-III
The Baryon Oscillation Spectroscopic Survey (BOSS) is designed to measure the
scale of baryon acoustic oscillations (BAO) in the clustering of matter over a
larger volume than the combined efforts of all previous spectroscopic surveys
of large scale structure. BOSS uses 1.5 million luminous galaxies as faint as
i=19.9 over 10,000 square degrees to measure BAO to redshifts z<0.7.
Observations of neutral hydrogen in the Lyman alpha forest in more than 150,000
quasar spectra (g<22) will constrain BAO over the redshift range 2.15<z<3.5.
Early results from BOSS include the first detection of the large-scale
three-dimensional clustering of the Lyman alpha forest and a strong detection
from the Data Release 9 data set of the BAO in the clustering of massive
galaxies at an effective redshift z = 0.57. We project that BOSS will yield
measurements of the angular diameter distance D_A to an accuracy of 1.0% at
redshifts z=0.3 and z=0.57 and measurements of H(z) to 1.8% and 1.7% at the
same redshifts. Forecasts for Lyman alpha forest constraints predict a
measurement of an overall dilation factor that scales the highly degenerate
D_A(z) and H^{-1}(z) parameters to an accuracy of 1.9% at z~2.5 when the survey
is complete. Here, we provide an overview of the selection of spectroscopic
targets, planning of observations, and analysis of data and data quality of
BOSS.Comment: 49 pages, 16 figures, accepted by A
Combined search for the quarks of a sequential fourth generation
Results are presented from a search for a fourth generation of quarks
produced singly or in pairs in a data set corresponding to an integrated
luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in
2011. A novel strategy has been developed for a combined search for quarks of
the up and down type in decay channels with at least one isolated muon or
electron. Limits on the mass of the fourth-generation quarks and the relevant
Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a
simple extension of the standard model with a sequential fourth generation of
fermions. The existence of mass-degenerate fourth-generation quarks with masses
below 685 GeV is excluded at 95% confidence level for minimal off-diagonal
mixing between the third- and the fourth-generation quarks. With a mass
difference of 25 GeV between the quark masses, the obtained limit on the masses
of the fourth-generation quarks shifts by about +/- 20 GeV. These results
significantly reduce the allowed parameter space for a fourth generation of
fermions.Comment: Replaced with published version. Added journal reference and DO
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