650 research outputs found
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
Gaze-contingent flicker pupil perimetry detects scotomas in patients with cerebral visual impairments or glaucoma
The pupillary light reflex is weaker for stimuli presented inside as compared to outside absolute scotomas. Pupillograph perimetry could thus be an objective measure of impaired visual processing. However, the diagnostic accuracy in detecting scotomas has remained unclear. We quantitatively investigated the accuracy of a novel form of pupil perimetry. The new perimetry method, termed gaze-contingent flicker pupil perimetry, consists of the repetitive on, and off flickering of a bright disk (2 hz; 320 cd/m; 4° diameter) on a gray background (160 cd/m) for 4 seconds per stimulus location. The disk evokes continuous pupil oscillations at the same rate as its flicker frequency, and the oscillatory power of the pupil reflects visual sensitivity. We monocularly presented the disk at a total of 80 locations in the central visual field (max. 15°). The location of the flickering disk moved along with gaze to reduce confounds of eye movements (gaze-contingent paradigm). The test lasted ~5 min per eye and was performed on 7 patients with cerebral visual impairment (CVI), 8 patients with primary open angle glaucoma (age >45), and 14 healthy, age/gender-matched controls. For all patients, pupil oscillation power (FFT based response amplitude to flicker) was significantly weaker when the flickering disk was presented in the impaired as compared to the intact visual field (CVI: 12%, AUC = 0.73; glaucoma: 9%, AUC = 0.63). Differences in power values between impaired and intact visual fields of patients were larger than differences in power values at corresponding locations in the visual fields of the healthy control group (CVI: AUC = 0.95; glaucoma: AUC = 0.87). Pupil sensitivity maps highlighted large field scotomas and indicated the type of visual field defect (VFD) as initially diagnosed with standard automated perimetry (SAP) fairly accurately in CVI patients but less accurately in glaucoma patients. We provide the first quantitative and objective evidence of flicker pupil perimetry's potential in detecting CVI-and glaucoma-induced VFDs. Gaze-contingent flicker pupil perimetry is a useful form of objective perimetry and results suggest it can be used to assess large VFDs with young CVI patients whom are unable to perform SAP
Photoassociation spectroscopy of cold alkaline earth atoms near the intercombination line
The properties of photoassociation (PA) spectra near the intercombination
line (the weak transition between and states) of group
II atoms are theoretically investigated. As an example we have carried out a
calculation for Calcium atoms colliding at ultra low temperatures of 1 mK, 1
K, and 1 nK. Unlike in most current photoassociation spectroscopy the
Doppler effect can significantly affect the shape of the investigated lines.
Spectra are obtained using Ca--Ca and Ca--Ca short-range {\it ab initio}
potentials and long-range van der Waals and resonance dipole potentials. The
similar van der Waals coefficients of ground and
excited states cause the PA to differ greatly from
those of strong, allowed transitions with resonant dipole interactions. The
density of spectral lines is lower, the Condon points are at relatively short
range, and the reflection approximation for the Franck-Condon factors is not
applicable, and the spontaneous decay to bound ground-state molecules is
efficient. Finally, the possibility of efficient production of cold molecules
is discussed
Customized television: Standards compliant advanced digital television
This correspondence describes a European Union supported collaborative project called CustomTV based on the premise that future TV sets will provide all sorts of multimedia information and interactivity, as well as manage all such services according to each user’s or group of user’s preferences/profiles. We have demonstrated the potential of recent standards (MPEG-4 and MPEG-7) to implement such a scenario by building
the following services: an advanced EPG, Weather Forecasting, and Stock Exchange/Flight Information
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
Atom-molecule equilibration in a degenerate Fermi gas with resonant interactions
We present a nonequilibrium kinetic theory describing atom-molecule
population dynamics in a two-component Fermi gas with a Feshbach resonance. Key
collision integrals emerge that govern the relaxation of the atom-molecule
mixture to chemical and thermal equilibrium. Our focus is on the pseudogap
regime where molecules form above the superfluid transition temperature. In
this regime, we formulate a simple model for the atom-molecule population
dynamics. The model predicts the saturation of molecule formation that has been
observed in recent experiments, and indicates that a dramatic enhancement of
the atom-molecule conversion efficiency occurs at low temperatures.Comment: Updated manuscript on July 5, 2004. Four pages with three embedded
figure
Spectroscopy of the a^3\Sigma_u^+ state and the coupling to the X^1\Sigma_g^+ state of K_2
We report on high resolution Fourier-transform spectroscopy of fluorescence
to the a^3\Sigma_u^+ state excited by two-photon or two-step excitation from
the X^1\Sigma_g^+ state to the 2^3\Pi_g state in the molecule K_2. These
spectroscopic data are combined with recent results of Feshbach resonances and
two-color photoassociation spectra for deriving the potential curves of
X^1\Sigma_g^+ and a^3\Sigma_u^+ up to the asymptote. The precise relative
position of the triplet levels with respect of the singlet levels was achieved
by including the excitation energies from the X^1\Sigma_g^+ state to the
2^3\Pi_g state and down to the a^3\Sigma_u^+ state in the simultaneous fit of
both potentials. The derived precise potential curves allow for reliable
modeling of cold collisions of pairs of potassium atoms in their ^2S ground
state
Spin-axis relaxation in spin-exchange collisions of alkali atoms
We present calculations of spin-relaxation rates of alkali-metal atoms due to
the spin-axis interaction acting in binary collisions between the atoms. We
show that for the high-temperature conditions of interest here, the spin
relaxation rates calculated with classical-path trajectories are nearly the
same as those calculated with the distorted-wave Born approximation. We compare
these calculations to recent experiments that used magnetic decoupling to
isolate spin relaxation due to binary collisions from that due to the formation
of triplet van-der-Waals molecules. The values of the spin-axis coupling
coefficients deduced from measurements of binary collision rates are consistent
with those deduced from molecular decoupling experiments. All the experimental
data is consistent with a simple and physically plausible scaling law for the
spin-axis coupling coefficients.Comment: text+1 figur
Seasonal variation in the correlation of airglow temperature and emission rate
The hydroxyl (OH) rotational temperature and band emission rate have been derived using year-round, ground-based measurements of the infrared OH nightglow from Sweden from 1991 to 2002. Recent work has suggested that, during the winter, all scales of dynamical variations of radiance and temperature arise from vertical motions, implying that the effective source concentrations of atomic oxygen are constant. The present data show correlations between temperature and radiance both during winter and summer that are consistent with those observed in that previous work. However, during the transition to summer there is a rapid decrease in the temperature and its variation that is not reflected in the band radiance, suggesting that only the shorter-scale variations are accompanied by significant vertical motion. This indicates that the shorter-scale dynamical variations occur against an independent, seasonally changing background temperature profile in a way that is consistent with that predicted by gravity-wave models
Laser Intensity Dependence of Photoassociation in Ultracold Metastable Helium
Photoassociation of spin-polarized metastable helium to the three lowest
rovibrational levels of the J=1, state asymptoting to 2SP is studied using a second-order perturbative
treatment of the line shifts valid for low laser intensities, and two variants
of a non-perturbative close-coupled treatment, one based upon dressed states of
the matter plus laser system, and the other on a modified radiative coupling
which vanishes asymptotically, thus simulating experimental conditions. These
non-perturbative treatments are valid for arbitrary laser intensities and yield
the complete photoassociation resonance profile. Both variants give nearly
identical results for the line shifts and widths of the resonances and show
that their dependence upon laser intensity is very close to linear and
quadratic respectively for the two lowest levels. The resonance profiles are
superimposed upon a significant background loss, a feature for this metastable
helium system not present in studies of photoassociation in other systems,
which is due to the very shallow nature of the excited state potential.
The results for the line shifts from the close-coupled and perturbative
calculations agree very closely at low laser intensities.Comment: 14 pages, 7 figures, title altered, text reduce
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