8,863 research outputs found
Properties of the ground F state and the excited P state of atomic thorium in cold collisions with He
We measure inelastic collisional cross sections for the ground F
state and the excited P state of atomic thorium in cold collisions with
He. We determine for Th (F) at 800 mK the ratio of the momentum-transfer to Zeeman relaxation cross sections for
collisions with He. For Th (P), we study electronic inelastic
processes and find no quenching even after collisions. We also determine
the radiative lifetime of Th (P) to be ms. This great
stability of the metastable state opens up the possibility for further study,
including trapping
Vibrational quenching of the electronic ground state in ThO in cold collisions with He
We measure the ratio of the momentum-transfer to the vibrational
quenching cross section for the X (), , state
of molecular thorium monoxide (ThO) in collisions with atomic He between
800 mK and 2.4 K. We observe indirect evidence for ThO--He van der Waals'
complex formation, which has been predicted by theory. We determine the 3-body
recombination rate constant at 2.4 K, and establish that the binding
energy E 4 K
Improving the Reliability and Modal Stability of High Power 870 nm AlGaAs CSP Laser Diodes for Applications to Free Space Communication Systems
The operating characteristics (power-current, beam divergence, etc.) and reliability assessment of high-power CSP lasers is discussed. The emission wavelength of these lasers was optimized at 860 to 880 nm. The operational characteristics of a new laser, the inverse channel substrate planar (ICSP) laser, grown by metalorganic chemical vapor deposition (MOCVD), is discussed and the reliability assessment of this laser is reported. The highlights of this study include a reduction in the threshold current value for the laser to 15 mA and a degradation rate of less than 2 kW/hr for the lasers operating at 60 mW of peak output power
The extraordinary mid-infrared spectral properties of FeLoBAL Quasars
We present mid-infrared spectra of six FeLoBAL QSOs at 1<z<1.8, taken with
the Spitzer space telescope. The spectra span a range of shapes, from hot dust
dominated AGN with silicate emission at 9.7 microns, to moderately obscured
starbursts with strong Polycyclic Aromatic Hydrocarbon (PAH) emission. The
spectrum of one object, SDSS 1214-0001, shows the most prominent PAHs yet seen
in any QSO at any redshift, implying that the starburst dominates the mid-IR
emission with an associated star formation rate of order 2700 solar masses per
year. With the caveats that our sample is small and not robustly selected, we
combine our mid-IR spectral diagnostics with previous observations to propose
that FeLoBAL QSOs are at least largely comprised of systems in which (a) a
merger driven starburst is ending, (b) a luminous AGN is in the last stages of
burning through its surrounding dust, and (c) which we may be viewing over a
restricted line of sight range.Comment: ApJ, accepte
A Bayesian Approach to Comparing Cosmic Ray Energy Spectra
A common problem in ultra-high energy cosmic ray physics is the comparison of
energy spectra. The question is whether the spectra from two experiments or two
regions of the sky agree within their statistical and systematic uncertainties.
We develop a method to directly compare energy spectra for ultra-high energy
cosmic rays from two different regions of the sky in the same experiment
without reliance on agreement with a theoretical model of the energy spectra.
The consistency between the two spectra is expressed in terms of a Bayes
factor, defined here as the ratio of the likelihood of the two-parent source
hypothesis to the likelihood of the one-parent source hypothesis. Unlike other
methods, for example chi^2 tests, the Bayes factor allows for the calculation
of the posterior odds ratio and correctly accounts for non-Gaussian
uncertainties. The latter is particularly important at the highest energies,
where the number of events is very small.Comment: 22 pages, 10 figures, accepted for publication in Ap
RNA polymerase III interferes with Ty3 integration
AbstractTy3, a gypsylike retrotransposon of budding yeast, integrates at the transcription initiation site of genes transcribed by RNA polymerase III (pol III). It was previously shown that integration in vitro requires intact promoter elements and the pol III transcription factors TFIIIB and TFIIIC. In order to test the effect of pol III on integration, increasing amounts of a pol III-containing fraction were added to Ty3 in vitro integration reactions. The pol III-containing fraction was inhibitory to integration. These results are consistent with a model where the Ty3 integration complex and pol III recognize similar features of the stable transcription complex and compete with each other for access to the transcription initiation site
High-power AlGaAs channeled substrate planar diode lasers for spaceborne communications
A high power channeled substrate planar AlGaAs diode laser with an emission wavelength of 8600 to 8800 A was developed. The optoelectronic behavior (power current, single spatial and spectral behavior, far field characteristics, modulation, and astigmatism properties) and results of computer modeling studies on the performance of the laser are discussed. Lifetest data on these devices at high output power levels is also included. In addition, a new type of channeled substrate planar laser utilizing a Bragg grating to stabilize the longitudinal mode was demonstrated. The fabrication procedures and optoelectronic properties of this new diode laser are described
Large spin relaxation rates in trapped submerged-shell atoms
Spin relaxation due to atom-atom collisions is measured for magnetically
trapped erbium and thulium atoms at a temperature near 500 mK. The rate
constants for Er-Er and Tm-Tm collisions are 3.0 times 10^-10 cm^3 s^-1 and 1.1
times 10^-10 cm^3 s^-1, respectively, 2-3 orders of magnitude larger than those
observed for highly magnetic S-state atoms. This is strong evidence for an
additional, dominant, spin relaxation mechanism, electrostatic anisotropy, in
collisions between these "submerged-shell" L > 0 atoms. These large spin
relaxation rates imply that evaporative cooling of these atoms in a magnetic
trap will be highly inefficient.Comment: 10 pages, 3 figure
The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field
The Hubble Deep Field (HDF) is the deepest set of multicolor optical
photometric observations ever undertaken, and offers a valuable data set with
which to study galaxy evolution. Combining the optical WFPC2 data with
ground-based near-infrared photometry, we derive photometrically estimated
redshifts for HDF galaxies with J<23.5. We demonstrate that incorporating the
near-infrared data reduces the uncertainty in the estimated redshifts by
approximately 40% and is required to remove systematic uncertainties within the
redshift range 1<z<2. Utilizing these photometric redshifts, we determine the
evolution of the comoving ultraviolet (2800 A) luminosity density (presumed to
be proportional to the global star formation rate) from a redshift of z=0.5 to
z=2. We find that the global star formation rate increases rapidly with
redshift, rising by a factor of 12 from a redshift of zero to a peak at z~1.5.
For redshifts beyond 1.5, it decreases monotonically. Our measures of the star
formation rate are consistent with those found by Lilly et al. (1996) from the
CFRS at z 2, and
bridge the redshift gap between those two samples. The overall star formation
or metal enrichment rate history is consistent with the predictions of Pei and
Fall (1995) based on the evolving HI content of Lyman-alpha QSO absorption line
systems.Comment: Latex format, 10 pages, 3 postscript figures. Accepted for
publication in Ap J Letter
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