4,529 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
On The Reddening in X-ray Absorbed Seyfert 1 Galaxies
There are several Seyfert galaxies for which there is a discrepancy between
the small column of neutral hydrogen deduced from X-ray observations and the
much greater column derived from the reddening of the optical/UV emission lines
and continuum. The standard paradigm has the dust within the highly ionized gas
which produces O~VII and O~VIII absorption edges (i.e., a ``dusty warm
absorber''). We present an alternative model in which the dust exists in a
component of gas in which hydrogen has been stripped, but which is at too low
an ionization state to possess significant columns of O~VII and O~VIII (i.e, a
``lukewarm absorber''). The lukewarm absorber is at sufficient radial distance
to encompass much of the narrow emission-line region, and thus accounts for the
narrow-line reddening, unlike the dusty warm absorber. We test the model by
using a combination of photoionization models and absorption edge fits to
analyze the combined ROSAT/ASCA dataset for the Seyfert 1.5 galaxy, NGC 3227.
We show that the data are well fit by a combination of the lukewarm absorber
and a more highly ionized component similar to that suggested in earlier
studies. We predict that the lukewarm absorber will produce strong UV
absorption lines of N V, C IV, Si IV and Mg II. Finally, these results
illustrate that singly ionized helium is an important, and often overlooked,
source of opacity in the soft X-ray band (100 - 500 eV).Comment: 17 pages, Latex, includes 1 figure (encapsulated postscript), one
additional table in Latex (landscape format), to appear in the Astrophysical
Journa
Spectroscopy of Ultracold, Trapped Cesium Feshbach Molecules
We explore the rich internal structure of Cs_2 Feshbach molecules. Pure
ultracold molecular samples are prepared in a CO_2-laser trap, and a multitude
of weakly bound states is populated by elaborate magnetic-field ramping
techniques. Our methods use different Feshbach resonances as input ports and
various internal level crossings for controlled state transfer. We populate
higher partial-wave states of up to eight units of rotational angular momentum
(l-wave states). We investigate the molecular structure by measurements of the
magnetic moments for various states. Avoided level crossings between different
molecular states are characterized through the changes in magnetic moment and
by a Landau-Zener tunneling method. Based on microwave spectroscopy, we present
a precise measurement of the magnetic-field dependent binding energy of the
weakly bound s-wave state that is responsible for the large background
scattering length of Cs. This state is of particular interest because of its
quantum-halo character.Comment: 15 pages, 12 figures, 4 table
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
- …