962 research outputs found
R-matrix calculation of electron collisions with electronically excited O2 molecules
Low-energy electron collisions with O molecules are studied using the
fixed-bond R-matrix method. In addition to the O ground
state, integrated cross sections are calculated for elecron collisions with the
and excited states of O molecules. 13
target electronic states of O are included in the model within a valence
configuration interaction representations of the target states. Elastic cross
sections for the and excited states are
similar to the cross sections for the ground state. As in
case of excitation from the state, the O
resonance makes the dominant contribution to excitation cross sections from the
and states. The magnitude of excitation
cross sections from the state to the
state is about 10 time larger than the corresponding cross sections from the
to the state. For this
transition, our cross section at
4.5 eV agrees well with the available experimental value. These results should
be important for models of plasma discharge chemistry which often requires
cross sections between the excited electronic states of O.Comment: 26 pages, 10 figure
Observations of the diffuse UV radiation field
Spectra are presented for the diffuse UV radiation field between 1250 to 3100 A from eight different regions of the sky, which were obtained with the Johns Hopkins UVX experiment. UVX flew aboard the Space Shuttle Columbia (STS-61C) in January 1986 as part of the Get-Away Special project. The experiment consisted of two 1/4 m Ebert-Fastie spectrometers, covering the spectral range 1250 to 1700 A at 17 A resolution and 1600 to 3100 A at 27 A resolution, respectively, with a field of view of 4 x .25 deg, sufficiently small to pick out regions of the sky with no stars in the line of sight. Values were found for the diffuse cosmic background ranging in intensity from 300 to 900 photons/sq cm/sec/sr/A. The cosmic background is spectrally flat from 1250 to 3100 A, within the uncertainties of each spectrometer. The zodiacal light begins to play a significant role in the diffuse radiation field above 2000 A, and its brightness was determined relative to the solar emission. Observed brightnesses of the zodiacal light in the UV remain almost constant with ecliptic latitude, unlike the declining visible brightnesses, possibly indicating that those (smaller) grains responsible for the UV scattering have a much more uniform distribution with distance from the ecliptic plane than do those grains responsible for the visible scattering
Soft-tissue specimens from pre-European extinct birds of New Zealand
We provide the first complete review of soft tissue remains from New Zealand birds that became extinct prior to European settlement (c. AD 1800). These rare specimens allow insights into the anatomy and appearance of the birds that are not attainable from bones. Our review includes previously unpublished records of ‘lost’ specimens, and descriptions of recently discovered specimens such as the first evidence of soft tissues from the South Island goose (Cnemiornis calcitrans). Overall, the soft tissue remains are dominated by moa (with specimens from each of the six genera), but also include specimens from Finsch's duck (Chenonetta finschi) and the New Zealand owlet-nightjar (Aegotheles novaezealandiae). All desiccated soft tissue specimens that have radiocarbon or stratigraphic dates are late Holocene in age, and most have been found in the semi-arid region of Central Otago
Weak line water vapor spectrum in the 13 200–15 000 cm−1 region
New Fourier transform spectra of water vapor are presented in the range 6500–16 400 cm−1 obtained using pathlengths of up to 800 m and long integration times. These spectra have a significantly higher signal-to-noise than previous measurements in this wavenumber range. Wavenumbers, absolute intensities and self-broadening coefficients, all with associated uncertainties, are presented for 3604 lines in the region 13 200–15 000 cm−1. Analysis of these lines using variational linelists, along with other unassigned lines from previous studies, has been conducted. This leads to 952 new line assignments to transitions involving 35 different vibrational states of H216O. A smaller number of lines are assigned to H218O and H217O
Atomic oxygen studies on polymers
The purpose was to study the effects of atomic oxygen on the erosion of polymer based materials. The development of an atomic oxygen neutral beam facility using a SURFATRON surface wave launcher that can produce beam energies between 2 and 3 eV at flux levels as high as approx. 10 to the 17th power atoms/cm (2)-sec is described. Thin film dielectric materials were studied to determine recession rates and and reaction efficiencies as a function of incident beam energy and fluence. Accelerated testing was also accomplished and the values of reaction efficiency compared to available space flight data. Electron microscope photomicrographs of the samples' surface morphology were compared to flight test specimens
Space environmental effects on polymer matrix composites as a function of sample location on LDEF
This paper presents results on the effect of circumferential location on the variation in solar absorptance (alpha(sub S)) and infrared emittance (epsilon) for five different polymer matrix composites (PMC), and variations in erosion depth due to atomic oxygen (AO) for fourteen different PMC materials. In addition, a chemical content design parameter (gamma) has been found that correlates well with the erosion yield obtained from space flight data and hyperthermal AO tests for hydrocarbon polymeric materials. This parameter defines the ratio of the total number of atoms in a repeat monomer unit to the difference between the total carbon content and the total number of intermolecular oxygen atoms in the same repeat unit
Early Miocene fossil frogs (Anura: Leiopelmatidae) from New Zealand.
Author version made available in accordance with publisher copyright policy.The first pre-Quaternary anurans from New Zealand are reported from the Early Miocene (19–16 Ma) St Bathans Fauna based on 10 fossil bones. Four bones representing two new species differing in size are described in Leiopelma: Leiopelmatidae, and are the first Tertiary records for the family. Six indeterminate frog fossils are morphologically similar to leiopelmatids and represent two species consistent in size with those known from diagnostic material. These records are highly significant, as minimally, they reduce the duration of the leiopelmatid ‘ghost lineage’ by c.20 million years and demonstrate that a diversity of leiopelmatids has long been present on New Zealand, supporting the ancient dichotomy of the extant species based on molecular data
A bittern (Aves: Ardeidae) from the Early Miocene of New Zealand
Author version made available in accordance with Publisher copyright policy
Stabilizing the intensity of a wave amplified by a beam of particles
The intensity of an electromagnetic wave interacting self-consistently with a
beam of charged particles as in a free electron laser, displays large
oscillations due to an aggregate of particles, called the macro-particle. In
this article, we propose a strategy to stabilize the intensity by re-shaping
the macro-particle. This strategy involves the study of the linear stability
(using the residue method) of selected periodic orbits of a mean-field model.
As parameters of an additional perturbation are varied, bifurcations occur in
the system which have drastic effect on the modification of the self-consistent
dynamics, and in particular, of the macro-particle. We show how to obtain an
appropriate tuning of the parameters which is able to strongly decrease the
oscillations of the intensity without reducing its mean-value
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