5,150 research outputs found
Efficient creation of molecules from a cesium Bose-Einstein condensate
We report a new scheme to create weakly bound Cs molecules from an atomic
Bose-Einstein condensate. The method is based on switching the magnetic field
to a narrow Feshbach resonance and yields a high atom-molecule conversion
efficiency of more than 30%, a factor of three higher than obtained with
conventional magnetic-field ramps. The Cs molecules are created in a single
-wave rotational quantum state. The observed dependence of the conversion
efficiency on the magnetic field and atom density shows scattering processes
beyond two-body coupling to occur in the vicinity of the Feshbach resonance.Comment: 7 pages, 4 figures, submitted to Europhysics Letter
Observation of Feshbach-like resonances in collisions between ultracold molecules
We observe magnetically tuned collision resonances for ultracold Cs2
molecules stored in a CO2-laser trap. By magnetically levitating the molecules
against gravity, we precisely measure their magnetic moment. We find an avoided
level crossing which allows us to transfer the molecules into another state. In
the new state, two Feshbach-like collision resonances show up as strong
inelastic loss features. We interpret these resonances as being induced by Cs4
bound states near the molecular scattering continuum. The tunability of the
interactions between molecules opens up novel applications such as controlled
chemical reactions and synthesis of ultracold complex molecules
Metastable Feshbach Molecules in High Rotational States
We experimentally demonstrate Cs2 Feshbach molecules well above the
dissociation threshold, which are stable against spontaneous decay on the
timescale of one second. An optically trapped sample of ultracold dimers is
prepared in an l-wave state and magnetically tuned into a region with negative
binding energy. The metastable character of these molecules arises from the
large centrifugal barrier in combination with negligible coupling to states
with low rotational angular momentum. A sharp onset of dissociation with
increasing magnetic field is mediated by a crossing with a g-wave dimer state
and facilitates dissociation on demand with a well defined energy.Comment: 4 pages, 5 figure
Resolved Spectroscopy of the Narrow-Line Region in NGC 1068. I. The Nature of the Continuum Emission
We present the first long-slit spectra of the Seyfert 2 galaxy NGC 1068
obtained by the Space Telescope Imaging Spectrograph (STIS); the spectra cover
the wavelength range 1150 - 10,270 Angstroms at a spatial resolution of 0.05 -
0.1 arcsec and a spectral resolving power of 1000. In this first paper, we
concentrate on the far-UV to near-IR continuum emission from the continuum
``hot spot'' and surrounding regions extending out to +/- 6 arcsec (+/-432 pc)
at a position angle of 202 degrees In addition to the broad emission lines
detected by spectropolarimetry, the hot spot shows the ``little blue bump'' in
the 2000 - 4000 Ang. range, which is due to Fe II and Balmer continuum
emission. The continuum shape of the hot spot is indistinguishable from that of
NGC 4151 and other Seyfert 1 galaxies. Thus, the hot spot is reflected emission
from the hidden nucleus, due to electron scattering (as opposed to
wavelength-dependent dust scattering). The hot spot is ~0.3 arcsec in extent
and accounts for 20% of the scattered light in the inner 500 pc. We are able to
deconvolve the extended continuum emission in this region into two components:
electron-scattered light from the hidden nucleus (which dominates in the UV)
and stellar light (which dominates in the optical and near-IR). The scattered
light is heavily concentrated towards the hot spot, is stronger in the
northeast, and is enhanced in regions of strong narrow-line emission. The
stellar component is more extended, concentrated southwest of the hot spot,
dominated by an old (> 2 x 10 Gyr) stellar population, and includes a nuclear
stellar cluster which is ~200 pc in extent.Comment: 32 pages, Latex, includes 11 figures (postscript), to appear in the
Astrophysical Journa
Probing the Ionization Structure of the Narrow Line Region in the Seyfert 1 Galaxy NGC 4151
We present a study of the distribution of [O III] 5007 and [O II]
3727 emission in the Narrow Line Region (NLR) of the Seyfert 1 galaxy
NGC 4151. While the NLR of NGC 4151 exhibits an overall structure consistent
with the unified model of Seyfert galaxies, narrow-band [O III] and [O II]
images obtained with the Wide Field and Planetary Camera 2 aboard the Hubble
Space Telescope reveal significant emission from outside the the emission-line
bi-cone. The [O III]/[O II] ratios are lower in these regions, consistent with
a weaker ionizing flux. We performed a photoionization modeling analysis of the
emission-line gas within a series of annuli, centered on the the central
continuum source, with inner radii from 13 to 90 pc. The gas is ionized by
radiation that has been attenuated by a relatively highly-ionized absorber
(HABS), which completely covers the central source, and a lower-ionization
absorber (LABS), which has a covering factor ranging from 0 to 1. We found that
the [O III]/[O II] ratios are well fit by assuming that, within each segment of
an annulus, some fraction of the NLR gas is completely within the shadow of
LABS, while the rest is irradiated by the continuum filtered only by HABS. This
suggests that the structure of the NLR is due to filtering of the ionizing
radiation by ionized gas, consistent with disk-wind models. One possible
scenario is that the low-ionization absorbers are dense knots of gas swept up
by a wind.Comment: 43 pages, 18 figure Accepted for publication in the Astrophysical
Journa
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