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At least 343 records · Page 19Linked to original sources
PubMed2001

Binary hard-sphere crystals with the cesium chloride structure.

The possibility of binary hard-sphere colloids crystallizing with the cesium chloride (CsCl) structure was examined experimentally using poly (methyl methacrylate) particles dispersed in organic solvents. Towards this end, two dispersions were prepared that contained particles with a radius ratio alpha=R(B)/R(A), where A refers to the large particles and B the small, of 0.736. This is close to the optimum ratio of 0.732 at which this structure is predicted to form as determined by space-filling calculations. The phases found within the binary mixture were examined using laser light crystallography and scanning electromicroscopy, and some were shown to have the CsCl structure. Over a period of time some of the CsCl crystals disappeared indicating that they were metastable and that this structure may not be the most enduring phase at this size ratio.

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PubMed2002

Nature of the liquid crystalline phase transitions in the cesium pentadecafluorooctanoate-water system: the nematic-to-smectic-A transition.

Measurements of the diamagnetic susceptibility chi(a), the rotational viscosity coefficient gamma(1), and the micellar orientational order parameter S have been made as a function of temperature across the entire nematic phases of cesium pentadecafluorooctanoate (CsPFO)-D2O samples over the concentration range weight fraction CsPFO w from 0.350 to 0.500. This has been chosen so as to embrace the pseudo-nematic phase-lamellar tricritical point that has been purported to occur at w=0.43 (volume fraction phi=0.26). The values of gamma(1) vary from about 0.1 kg m(-1) s(-1) to 1000 kg m(-1) s(-1) as the temperature decreases across the nematic phase, covering a much larger range than those reported for thermotropic liquid crystals. Measurements of gamma(1) have been made much closer to the critical temperature T(LN) than have previously been reported [(T-T(LN))/T(LN)=2 x 10(-5) for the w=0.350 sample]. The rotational viscosity critical exponents x calculated from the divergence of gamma(1) on approaching the nematic-phase-to-lamellar transition are found to be constant at 2/3 independent of the concentration. These values differ from those of 1/3 and 1/2, respectively, predicted by the He-like three-dimensional XY and mean-field models, in combination with mean-field dynamical relaxation. However, the former would be supported if complete Fisher renormalization (at alpha=1/2) was appropriate over an extended concentration range. Alternatively, this could be an indication of the importance of coupling between the micellar rotational dynamics and critical fluctuations since the latter are seen to be predominant across the entire temperature range of the nematic phase as revealed in the temperature dependence of gamma(1).

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PubMed2000

Beyond optical molasses: 3D raman sideband cooling of atomic cesium to high phase-space density

We demonstrate a simple, general purpose method to cool neutral atoms. A sample containing 3x10(8) cesium atoms prepared in a magneto-optical trap is cooled and simultaneously spin polarized in 10 ms at a density of 1.1x10(11) cm (-3) to a phase space density nlambda(3)(dB) = 1/500, which is almost 3 orders of magnitude higher than attainable in free space with optical molasses. The technique is based on 3D degenerate Raman sideband cooling in optical lattices and remains efficient even at densities where the mean lattice site occupation is close to unity.

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PubMed2000

Measurement of the hydrogen 1S- 2S transition frequency by phase coherent comparison with a microwave cesium fountain clock

We report on an absolute frequency measurement of the hydrogen 1S-2S two-photon transition in a cold atomic beam with an accuracy of 1.8 parts in 10(14). Our experimental result of 2 466 061 413 187 103(46) Hz has been obtained by phase coherent comparison of the hydrogen transition frequency with an atomic cesium fountain clock. Both frequencies are linked with a comb of laser frequencies emitted by a mode locked laser.

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PubMed2000

Model for the extreme wetting hysteresis of liquid helium on cesium

Although the liquid- 4He-cesium system is a nearly ideal one for studying wetting phenomena, it can show extreme hysteresis which is profoundly nonideal in behavior. It is suggested that this is due to the roughness of these Cs surfaces. We show that stable micropuddles of liquid 4He can form in shallow cavities on a Cs surface. It is the potential to form micropuddles, as the liquid tries to recede, which pins the contact line due to the large energy needed to create the surface of a micropuddle. This model also accounts for the memory that these surfaces have of being in contact with liquid 4He.

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PubMed2003

Sensitive detection of cold cesium molecules formed on Feshbach resonances.

We observe the dynamic formation of quasibound Cs2 molecules near Feshbach resonances in a cold sample of atomic cesium. Using an external probe beam, more than 15 weakly coupled molecular states are detected with high sensitivity, whose collisional formation cross sections are as small as sigma=2 x 10(-16) cm(2). By modeling the molecule formation and dissociation processes with rate equations, we conclude that at an atomic density of 10(13) cm(-3) and temperature of 5 microK, more than 5(1)x10(5) Cs2 molecules in a single rovibrational state coexist with 10(8) Cs atoms in our trap.

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PubMed2003

Time-resolved dynamics of thermal isomerization in cesium-halide cluster anions.

We have performed time-resolved studies of the dynamics of thermal isomerization occurring in certain cesium-halide cluster anions. Using a pump-probe technique, we have observed the repopulation of a photodepleted isomer within an ensemble as a function of time by monitoring the photoelectron spectrum. The rates of isomerization increase and the isomer lifetimes decrease as functions of temperature. The clusters undergo a gradual phase transition from solid-like to liquid-like states with liquid-like behavior obtained at approximately 500 K, much lower than the bulk melting temperatures of approximately 900 K.

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PubMed2003

New manifestation of atomic parity violation in cesium: a chiral optical gain induced by linearly polarized 6S-7S excitation.

We have detected, by using stimulated emission, an atomic parity violation (APV) in the form of a chiral optical gain of a cesium vapor on the 7S-6P(3/2) transition, consecutive to linearly polarized 6S-7S excitation. We demonstrate the validity of this detection method of APV, by presenting a 9% accurate measurement of expected sign and magnitude. We stress several advantages of this new approach which fully exploits the cylindrical symmetry of the setup. Future measurements at the percent level will provide an important cross-check of an existing more precise result obtained by a different method.

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PubMed2003

Bound states of 3He at the edge of a 4He drop on a cesium surface.

We show that small amounts of 3He atoms, added to a 4He drop deposited on a flat cesium surface at zero temperature, populate bound states localized at the contact line. These edge states show up for drops large enough to develop well defined surface and bulk regions together with a contact line, and they are structurally different from the well-known Andreev states that appear at the free surface and at the liquid-solid interface of films. We illustrate the one-body density of 3He in a drop with 1000 4He atoms, and show that for a sufficiently large number of impurities the density profiles spread beyond the edge, coating both the curved drop surface and its flat base and eventually isolating it from the substrate.

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PubMed2004

Absorption enhancement of an electric quadrupole transition of cesium atoms in an evanescent field.

We have observed for the first time reflection spectra of an electric quadrupole transition for the cesium atom (6 (2)S(1/2)-5 (2)D(5/2)) line at an angle of incidence from theta(c)-11.9 to theta(c)+107.5 mrad, where theta(c) is the critical angle for total reflection. From a comparison with the calculated absorption in the attenuated total reflection, the oscillator strengths for s and p polarizations were found to increase with an increase in the angle of incidence by a factor up to 1.5 at theta(c)+83.8 mrad and 2.4 at theta(c)+107.5 mrad, respectively, in the experiment. The dependences of the observed enhancement on the angle of incidence were in good agreement with the calculated ones for the oscillator strength of the quadrupole transition in the evanescent light.

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PubMed2005

Field emission patterns from first-principles electronic structures: application to pristine and cesium-doped carbon nanotubes.

A general approach is introduced to calculate field emission properties of any kind of nanostructure based on the first-principles local density of states (LDOS) and effective potentials. The experimental field emission spectroscopy images are explained as LDOS at the structure-vacuum barrier, weighted by the probability of electron tunneling. The method excellently reproduces the experimental field emission patterns of pristine capped carbon nanotubes. We show that cesium adsorbates even with a low doping ratio of one dopant per nanotube increase the emission current around 2.5 times, due to a generated dipole field.

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PubMed2006

Frequency shift of the cesium clock transition due to blackbody radiation.

We perform ab initio calculations of the frequency shift induced by a static electric field on the cesium clock hyperfine transition. The calculations are used to find the frequency shifts due to blackbody radiation. Our result [deltanu/E2=-2.26(2)x10(-10) Hz/(V/m)2] is in good agreement with early measurements and ab initio calculations performed in other groups. We present arguments against recent claims that the actual value of the effect might be smaller. The difference (approximately 10%) between ab initio and semiempirical calculations is due to the contribution of the continuum spectrum in the sum over intermediate states.

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PubMed2006

Dipole blockade at Förster resonances in high resolution laser excitation of Rydberg states of cesium atoms.

High resolution laser excitation of np Rydberg states of cesium atoms shows a dipole blockade at Förster resonances corresponding to the resonant dipole-dipole energy transfer of the np+np --> ns+(n+1)s reaction. The dipole-dipole interaction can be tuned on and off by the Stark effect, and such a process, observed for relatively low n(25-41), is promising for quantum gate devices. Both Penning ionization and saturation in the laser excitation can limit the range of observation of the dipole blockade.

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PubMed2003

Influence of a static magnetic field on the detected atomic velocity distribution in an optically pumped cesium beam frequency standard.

Irregularities of the microwave Ramsey patterns in an optically pumped cesium atomic beam frequency standards with a sharp angle incidence detecting laser beam have been diagnosed. They arise from a separated two-peak detected velocity distribution, which is caused by static magnetic fields. Experimentally measured results and theoretical analysis of this phenomenon are presented in this paper.

Letter↗
PubMed2006

To simulate blackbody radiation frequency shift in cesium fountain frequency standard with CO2 laser.

An experimental scheme to simulate the Blackbody radiation (BBR) frequency shift with carbon dioxide (CO2) laser in cesium fountain frequency standard is proposed. The alternative (ac) electric field from a CO2 laser with 10.6 microm wavelength causes the similar ac Stark shift of the clock transition frequency, but with much higher intensity than BBR power intensity at about room temperature. This scheme provides a simple, new method for measuring the BBR frequency shift in fountain clock. It provides the possibility to explain the difference of BBR shifts in Cs fountain clocks between two measurement methods.

Letter↗
PubMed2002

Closed-shell molecules that ionize more readily than cesium.

We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies.

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PubMed2002

Bose-Einstein condensation of cesium.

Bose-Einstein condensation of cesium atoms is achieved by evaporative cooling using optical trapping techniques. The ability to tune the interactions between the ultracold atoms by an external magnetic field is crucial to obtain the condensate and offers intriguing features for potential applications. We explore various regimes of condensate self-interaction (attractive, repulsive, and null interaction strength) and demonstrate properties of imploding, exploding, and non-interacting quantum matter.

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