1,266 research outputs found
Extending electron orbital precession to the molecular case: Can orbital alignment be used to observe wavepacket dynamics?
The complexity of ultrafast molecular photoionization presents an obstacle to
the modelling of pump-probe experiments. Here, a simple optimized model of
atomic rubidium is combined with a molecular dynamics model to predict
quantitatively the results of a pump-probe experiment in which long range
rubidium dimers are first excited, then ionized after a variable delay. The
method is illustrated by the outline of two proposed feasible experiments and
the calculation of their outcomes. Both of these proposals use Feshbach 87Rb2
molecules. We show that long-range molecular pump-probe experiments should
observe spin-orbit precession given a suitable pump-pulse, and that the
associated high-frequency beat signal in the ionization probability decays
after a few tens of picoseconds. If the molecule was to be excited to only a
single fine structure state state, then a low-frequency oscillation in the
internuclear separation would be detectable through the timedependent
ionization cross section, giving a mechanism that would enable observation of
coherent vibrational motion in this molecule.Comment: 9 pages, 10 figures, PRA submissio
A pump-probe study of the formation of rubidium molecules by ultrafast photoassociation of ultracold atoms
An experimental pump-probe study of the photoassociative creation of
translationally ultracold rubidium molecules is presented together with
numerical simulations of the process. The formation of loosely bound
excited-state dimers is observed as a first step towards a fully coherent
pump-dump approach to the stabilization of Rb into its lowest ground
vibrational states. The population that contributes to the pump-probe process
is characterized and found to be distinct from a background population of
pre-associated molecules.Comment: Accepted for publication in Phys. Rev. A (10 pages, 9 figures
A duchenne muscular dystrophy gene hot spot mutation in dystrophin-deficient Cavalier King Charles Spaniels is amenable to exon 51 skipping
BACKGROUND
Duchenne muscular dystrophy (DMD), which afflicts 1 in 3500 boys, is one of the most common genetic disorders of children. This fatal degenerative condition is caused by an absence or deficiency of dystrophin in striated muscle. Most affected patients have inherited or spontaneous deletions in the dystrophin gene that disrupt the reading frame resulting in unstable truncated products. For these patients, restoration of the reading frame via antisense oligonucleotide-mediated exon skipping is a promising therapeutic approach. The major DMD deletion "hot spot" is found between exons 45 and 53, and skipping exon 51 in particular is predicted to ameliorate the dystrophic phenotype in the greatest number of patients. Currently the mdx mouse is the most widely used animal model of DMD, although its mild phenotype limits its suitability in clinical trials. The Golden Retriever muscular dystrophy (GRMD) model has a severe phenotype, but due to its large size, is expensive to use. Both these models have mutations in regions of the dystrophin gene distant from the commonly mutated DMD "hot spot".
METHODOLOGY/PRINCIPAL FINDINGS
Here we describe the severe phenotype, histopathological findings, and molecular analysis of Cavalier King Charles Spaniels with dystrophin-deficient muscular dystrophy (CKCS-MD). The dogs harbour a missense mutation in the 5' donor splice site of exon 50 that results in deletion of exon 50 in mRNA transcripts and a predicted premature truncation of the translated protein. Antisense oligonucleotide-mediated skipping of exon 51 in cultured myoblasts from an affected dog restored the reading frame and protein expression.
CONCLUSIONS/SIGNIFICANCE
Given the small size of the breed, the amiable temperament and the nature of the mutation, we propose that CKCS-MD is a valuable new model for clinical trials of antisense oligonucleotide-induced exon skipping and other therapeutic approaches for DMD
NH3 in the Central 10 pc of the Galaxy I: General Morphology and Kinematic Connections Between the CND and GMCs
New VLA images of NH3 (1,1), (2,2), and (3,3) emission in the central 10
parsecs of the Galaxy trace filamentary streams of gas, several of which appear
to feed the circumnuclear disk (CND). The NH3 images have a spatial resolution
of 16.5''x14.5'' and have better spatial sampling than previous NH3
observations. The images show the ``southern streamer,'' ``50 km/s cloud,'' and
new features including a ``western streamer'', 6 parsecs in length, and a
``northern ridge'' which connects to the CND. NH3(3,3) emission is very similar
to 1.2 mm dust emission indicating that NH3 traces column density well. Ratios
of the NH3(2,2) to (1,1) line intensities give an estimate of the temperature
of the gas and indicate high temperatures close to the nucleus and CND. The new
data cover a velocity range of 270 km/s, including all velocities observed in
the CND, with a resolution of 9.8 km/s. Previous NH3 observations with higher
resolution did not cover the entire range of velocities seen in the CND. The
large-scale kinematics of the CND do not resemble a coherent ring or disk. We
see evidence for a high velocity cloud within a projected distance of 50'' (2
pc) which is only seen in NH3(3,3) and is likely to be hot. Comparison to 6 cm
continuum emission reveals that much of the NH3 emission traces the outer edges
of Sgr A East and was probably pushed outward by this expanding shell. The
connection between the northern ridge (which appears to be swept up by Sgr A
East) and the CND indicates that Sgr A East and the CND are in close proximity
to each other. Kinematic evidence for these connections is presented in this
paper, while the full kinematic analysis of the central 10 pc will be presented
in Paper II.Comment: 16 pages (containing 6 figures), 8 additional JPEG figures. Accepted
for publication in ApJ. For full resolution images, see
http://cfa-www.harvard.edu/~rmcgary/SGRA/nh3_figures.htm
Tumor Vasculature in Young and Old Hosts: Scanning Electron Microscopy of Microcorrosion Casts with Microangiography, Light Microscopy and Transmission Electron Microscopy
Tumor growth in vivo is dependent upon new blood vessel formation. When B16-F10 melanoma cells are implanted subcutaneously in young (3 mo) and old (24 mo) C57BL/6 mice the rate of growth is dependent on the age of the mice. This study involved a wide range of histological and microscopic techniques but was limited primarily to the initial phase of tumor growth. Stereological point counting from light microscopy (LM) of standard histological sections has been used to yield data regarding blood content. Tumor-bearing mice were perfused through the aorta with a fixation solution and were infused with a low-viscosity radiopaque gel (Microfil) or resin (Mercox). Soft x-rays of the whole animal were used for identifying the feeding vessels to the tumor. Tumors with Microfil were sliced and used for microangiography and light-microscopic observation while those with resin were used to make corrosion casts for scanning electron microscopy (SEM). The different characteristics of the tumor blood vessels in different aged mice were most obvious through SEM of vascular corrosion casts. In comparison with tumors in young mice those of similar size in old hosts had more necrosis, reduced presence of angiogenic features, decreased vessel density, reduced penetration into the tumor, and enhanced tortuosity of the vessel lumen. Transmission electron microscopy (TEM) revealed incompletely developed wall structure of the vessels regardless of host. The above results are consistent with the hypothesis that retarded angiogenesis may be responsible in part for the limited growth of tumors in old hosts
Demonstrating coherent control in 85Rb2 using ultrafast laser pulses: a theoretical outline of two experiments
Calculations relating to two experiments that demonstrate coherent control of
preformed rubidium-85 molecules in a magneto-optical trap using ultrafast laser
pulses are presented. In the first experiment, it is shown that pre-associated
molecules in an incoherent mixture of states can be made to oscillate
coherently using a single ultrafast pulse. A mechanism that can transfer
molecular population to more deeply bound vibrational levels is used in the
second. Optimal parameters of the control pulse are presented for the
application of the mechanism to molecules in a magneto-optical trap. The
calculations make use of an experimental determination of the initial state of
molecules photoassociated by the trapping lasers in the magneto-optical trap
and use shaped pulses consistent with a standard ultrafast laser system.Comment: 8 pages, 9 figures, PRA, 80, 033403 (2009
Systematic Molecular Differentiation in Starless Cores
(Abridged) We present evidence that low-mass starless cores, the simplest
units of star formation, are systematically differentiated in their chemical
composition. Molecules including CO and CS almost vanish near the core centers,
where the abundance decreases by one or two orders of magnitude. At the same
time, N2H+ has a constant abundance, and the fraction of NH3 increases toward
the core center. Our conclusions are based on a study of 5 mostly-round
starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have
mappedin C18O(1-0), C17O(1-0), CS(2-1), C34S(2-1), N2H+(1-0), NH3(1,1) and
(2,2), and the 1.2 mm continuum. For each core we have built a model that fits
simultaneously the radial profile of all observed emission and the central
spectrum for the molecular lines. The observed abundance drops of CO and CS are
naturally explained by the depletion of these molecules onto dust grains at
densities of 2-6 10^4 cm-3. N2H+ seems unaffected by this process up to
densities of several 10^5, while the NH3 abundance may be enhanced by reactions
triggered by the disappearance of CO from the gas phase. With the help of our
models, we show that chemical differentiation automatically explains the
discrepancy between the sizes of CS and NH3 maps, a problem which has remained
unexplained for more than a decade. Our models, in addition, show that a
combination of radiative transfer effects can give rise to the previously
observed discrepancy in the linewidth of these two tracers. Although this
discrepancy has been traditionally interpreted as resulting from a systematic
increase of the turbulent linewidth with radius, our models show that it can
arise in conditions of constant gas turbulence.Comment: 25 pages, 9 figures, accepted by Ap
Two massive star-forming regions at early evolutionary stages
We report sensitive ATCA radio-continuum observations toward IRAS 15596-5301
and 16272-4837, two luminous objects (> 2x10^4 Lsun) thought to represent
massive star-forming regions in early stages of evolution (due to previously
undetected radio emission at the 1-sigma level of 2 mJy per beam). Also
reported are 1.2-millimeter continuum and a series of molecular-line
observations made with the SEST telescope. For IRAS 15596-5301, the
observations reveal the presence of three distinct compact radio-continuum
sources associated with a dense molecular core. We suggest that this core
contains a cluster of B stars which are exciting compact HII regions that are
in pressure equilibrium with the dense molecular surroundings. No radio
continuum emission was detected from IRAS 16272-4837 (3-sigma limit of 0.2
mJy). However, a dense molecular core has been detected. The high luminosity
and lack of radio emission from this massive core suggests that it hosts an
embedded young massive protostar that is still undergoing an intense accretion
phase. This scenario is supported by the observed characteristics of the line
profiles and the presence of a bipolar outflow detected from observations of
the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star-
forming region in a very early evolutionary stage, being the precursor of an
ultra compact HII region.Comment: 25 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Avalanche Photo-Detection for High Data Rate Applications
Avalanche photo detection is commonly used in applications which require
single photon sensitivity. We examine the limits of using avalanche photo
diodes (APD) for characterising photon statistics at high data rates. To
identify the regime of linear APD operation we employ a ps-pulsed diode laser
with variable repetition rates between 0.5MHz and 80MHz. We modify the mean
optical power of the coherent pulses by applying different levels of
well-calibrated attenuation. The linearity at high repetition rates is limited
by the APD dead time and a non-linear response arises at higher photon-numbers
due to multiphoton events. Assuming Poissonian input light statistics we
ascertain the effective mean photon-number of the incident light with high
accuracy. Time multiplexed detectors (TMD) allow to accomplish photon- number
resolution by photon chopping. This detection setup extends the linear response
function to higher photon-numbers and statistical methods may be used to
compensate for non-linearity. We investigated this effect, compare it to the
single APD case and show the validity of the convolution treatment in the TMD
data analysis.Comment: 16 pages, 5 figure
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