20,665 research outputs found
The effect of parallel static and microwave electric fields on excited hydrogen atoms
Motivated by recent experiments we analyse the classical dynamics of a
hydrogen atom in parallel static and microwave electric fields. Using an
appropriate representation and averaging approximations we show that resonant
ionisation is controlled by a separatrix, and provide necessary conditions for
a dynamical resonance to affect the ionisation probability.
The position of the dynamical resonance is computed using a high-order
perturbation series, and estimate its radius of convergence. We show that the
position of the dynamical resonance does not coincide precisely with the
ionisation maxima, and that the field switch-on time can dramatically affect
the ionisation signal which, for long switch times, reflects the shape of an
incipient homoclinic. Similarly, the resonance ionisation time can reflect the
time-scale of the separatrix motion, which is therefore longer than
conventional static field Stark ionisation. We explain why these effects should
be observed in the quantum dynamics.
PACs: 32.80.Rm, 33.40.+f, 34.10.+x, 05.45.Ac, 05.45.MtComment: 47 pages, 20 figure
Elucidating the role of hyperfine interactions on organic magnetoresistance using deuterated aluminium tris(8-hydroxyquinoline)
Measurements of the effect of a magnetic field on the light output and
current through an organic light emitting diode made with deuterated aluminium
tris(8-hydroxyquinoline) have shown that hyperfine coupling with protons is not
the cause of the intrinsic organic magnetoresistance. We suggest that
interactions with unpaired electrons in the device may be responsible.Comment: Submitte
AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field
We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations
of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds.
Above the 210 microJy/beam (5sigma) detection level, the EVN clearly detects
two radio sources in a field that encompasses the HDF and part of the Hubble
Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013,
low-luminosity FR-I radio source located within the HDF itself) and VLA
J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system).
A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI
detections of all three sources suggest that most of the radio emission of
these particular sources (including the dusty starburst) is generated by an
embedded AGN.Comment: 4 pages, 1 figure; Accepted by Astron. & Astrophys Letters ... See
http://www.nfra.nl/~mag/hdf_evn.htm
A MERLIN Study of 6 GHz Excited-state OH & 6.7 GHz Methanol Masers in ON1
MERLIN observations of 6.668-GHz methanol and both 6.031- and 6.035-GHz
hydroxyl (OH) emission from the massive star-formation region ON1 are
presented. These are the first methanol observations made in full polarization
using 5 antennas of MERLIN, giving high resolution and sensitivity to extended
emission. Maser features are found to lie at the southern edge of the
ultra-compact HII region, following the known distribution of ground-state OH
masers. The masers cover a region ~1 arcsec in extent, lying perpendicular to
the H13CO+ bipolar outflow. Excited-state OH emission demonstrates consistent
polarization angles across the strongest linearly polarized features which are
parallel to the overall distribution. The linear polarizations vary between
10.0 and 18.5 per cent, with an average polarization angle of -60 deg +/- 28
deg. The strongest 6.668-GHz methanol features provide an upper limit to linear
polarization of ~1 per cent. Zeeman splitting of OH shows magnetic fields
between -1.1 to -5.8 mG, and a tentative methanol magnetic field strength of
-18 mG is measured.Comment: 10 Pages, 5 Figure
A projection method for statics and dynamics of lattice spin systems
A method based on Monte Carlo sampling of the probability flows projected
onto the subspace of one or more slow variables is proposed for investigation
of dynamic and static properties of lattice spin systems. We illustrate the
method by applying it, with projection onto the order-parameter subspace, to
the three-dimensional 3-state Potts model in equilibrium and to metastable
decay in a three-dimensional 3-state kinetic Potts model.Comment: 4 pages, 3 figures, RevTex, final version to appear in Phys. Rev.
Let
The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei
We perform a detailed investigation of moderate-to-high quality X-ray spectra
of ten of the most luminous active galactic nuclei (AGNs) known at z>4 (up to
z~6.28). This study includes five new XMM observations and five archived X-ray
observations (four by XMM and one by Chandra). We find that the X-ray power-law
photon indices of our sample, composed of eight radio-quiet sources and two
that are moderately radio loud, are not significantly different from those of
lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen
column densities, N_H<~10^{22}-10^{23} cm^{-2}, indicate that these AGNs are
not significantly absorbed. A joint fit performed on our eight radio-quiet
sources, with a total of ~7000 photons, constrains the mean photon index of z>4
radio-quiet AGNs to Gamma=1.97^{+0.06}_{-0.04}, with no detectable intrinsic
dispersion from source to source. We also obtain a strong constraint on the
mean intrinsic column density, N_H<~3x10^{21} cm^{-2}, showing that optically
selected radio-quiet AGNs at z>4 are, on average, not more absorbed than their
lower-redshift counterparts. All this suggests that the X-ray production
mechanism and the central environment in radio-quiet AGNs have not
significantly evolved over cosmic time. The mean equivalent width of a putative
neutral narrow Fe Ka line is constrained to be <~190 eV, and similarly we place
constraints on the mean Compton reflection component (R<~1.2). None of the AGNs
varied on short (~1 hr) timescales, but on longer timescales (months-to-years)
strong variability is observed in four of the sources. In particular, the X-ray
flux of the z=5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ~4
over a rest-frame period of 73 d. This is the most extreme X-ray variation
observed in a luminous z>4 radio-quiet AGN.Comment: 10 pages (emulateapj), 5 figures. Accepted by Ap
Report of the direct infrared sensors panel
The direct infrared sensors panel considered a wide range of options for technologies relevant to the science goals of the Astrotech 21 mission set. Among the technologies assessed are: large format arrays; photon counting detectors; higher temperature 1 to 10 micro-m arrays; impurity band conduction (IBC) or blocked impurity band (BIB) detectors; readout electronics; and adapting the Space Infrared Telescope Facility and Hubble Space Telescope. Detailed development plans were presented for each of these technology areas
Observations of neutral circulation at mid-latitudes during the Equinox Transition Study
Measurements of ion drift velocity made by the Millstone Hill incoherent scatter radar have been used to calculate the meridional neutral wind velocity during the Sept. 17 to 24, 1984 period. Strong daytime southward neutral surges were observed during the magnetically disturbed days of September 19 and 23, in contrast to the small daytime winds obtained as expected during the magnetically quiet days. The surge on September 19 was also seen at Arecibo. In addition, two approaches have been used to calculate the meridional wind component from the radar-derived height of the F-layer electron density peak. Results confirm the wind surge, particularly when the strong electric fields measured during the disturbed days are included in the calculations. The two approaches for the F-layer peak wind calculations are applied to the radar-derived electron density peak height as a function of latitude to study the variation of the southward daytime surges with latitude
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