Barium isotopes in allende meteorite: evidence against an extinct superheavy element

Carbon and chromite fractions from the Allende meteorite that contain isotopically anomalous xenon-131 to xenon-136 (carbonaceous chondrite fission or CCF xenon) at up to 5 x 10(11) atoms per gram show no detectable isotopic anomalies in barium-130 to barium-138. This rules out the possibility that the CCF xenon was formed by in situ fission of an extinct superheavy element. Apparently the CCF xenon and its carbonaceous carrier are relics from stellar nucleosynthesis.     

Tunneling into a single magnetic atom: spectroscopic evidence of the kondo resonance

The Kondo effect arises from the quantum mechanical interplay between the electrons of a host metal and a magnetic impurity and is predicted to result in local charge and spin variations around the magnetic impurity. A cryogenic scanning tunneling microscope was used to spatially resolve the electronic properties of individual magnetic atoms displaying the Kondo effect. Spectroscopic measurements performed on individual cobalt atoms on the surface of gold show an energetically narrow feature that is identified as the Kondo resonance-the predicted response of a Kondo impurity. Unexpected structure in the Kondo resonance is shown to arise from quantum mechanical interference between the d orbital and conduction electron channels for an electron tunneling into a magnetic atom in a metallic host.     

Xenon as a complex ligand: the tetra xenono Gold(II) cation in AuXe(4)2+(Sb(2)F(11)-)(2)

The first metal-xenon compound with direct gold-xenon bonds is achieved by reduction of AuF(3) with elemental xenon. The square planar AuXe(4)2+ cation is established by a single-crystal structure determination, with a gold-xenon bond length of approximately 274 picometers. The bonding between gold and xenon is of the final sigma donor type, resulting in a charge of approximately 0.4 per xenon atom.     

A continuous source of Bose-Einstein condensed atoms

A continuous source of Bose-Einstein condensed sodium atoms was created by periodically replenishing a condensate held in an optical dipole trap with new condensates delivered using optical tweezers. The source contained more than 1 x 10(6) atoms at all times, raising the possibility of realizing a continuous atom laser.     

Localization of metastable atom beams with optical standing waves: nanolithography at the heisenberg limit

The spatially dependent de-excitation of a beam of metastable argon atoms, traveling through an optical standing wave, produced a periodic array of localized metastable atoms with position and momentum spreads approaching the limit stated by the Heisenberg uncertainty principle. Silicon and silicon dioxide substrates placed in the path of the atom beam were patterned by the metastable atoms. The de-excitation of metastable atoms upon collision with the surface promoted the deposition of a carbonaceous film from a vapor-phase hydrocarbon precursor. The resulting patterns were imaged both directly and after chemical etching. Thus, quantum-mechanical steady-state atom distributions can be used for sub-0.1-micrometer lithography.     

Fast atom bombardment mass spectrometry

Fast atom bombardment mass spectrometry has become a powerful structural tool since the first reports of its use in 1981. Samples are ionized in the condensed state, usually in a glycerol matrix, by bombarding the matrix with xenon or argon atoms with energies of 5000 to 10,000 electron volts. This yields both positive and negative secondary ions, which are sputtered from the surface. The technique has been used to detect inorganic ion clusters to mass 25,800 and biologically active peptides to mass 5700, and it gives molecular ions of such highly polar or labile organic compounds as glycosphingolipids and polyene antibiotics. It can be especially valuable in determining the sequences of amino acids in polypeptides.     

The atom-cavity microscope: single atoms bound in orbit by single photons

The motion of individual cesium atoms trapped inside an optical resonator is revealed with the atom-cavity microscope (ACM). A single atom moving within the resonator generates large variations in the transmission of a weak probe laser, which are recorded in real time. An inversion algorithm then allows individual atom trajectories to be reconstructed from the record of cavity transmission and reveals single atoms bound in orbit by the mechanical forces associated with single photons. In these initial experiments, the ACM yields 2-micrometer spatial resolution in a 10-microsecond time interval. Over the duration of the observation, the sensitivity is near the standard quantum limit for sensing the motion of a cesium atom.     

Twin matter waves for interferometry beyond the classical limit

Interferometers with atomic ensembles are an integral part of modern precision metrology. However, these interferometers are fundamentally restricted by the shot noise limit, which can only be overcome by creating quantum entanglement among the atoms. We used spin dynamics in Bose-Einstein condensates to create large ensembles of up to 10(4) pair-correlated atoms with an interferometric sensitivity -1.61(-1.1)(+0.98) decibels beyond the shot noise limit. Our proof-of-principle results point the way toward a new generation of atom interferometers.     

Synthesis and Structure of an Iron(III) Sulfide-Ferritin Bioinorganic Nanocomposite

Amorphous iron sulfide minerals containing either 500 or 3000 iron atoms in each cluster have been synthesized in situ within the nanodimensional cavity of horse spleen ferritin. Iron-57 M?ssbauer spectroscopy indicated that most of the iron atoms in the 3000-iron atom cores are trivalent, whereas in the 500-iron atom clusters, approximately 50 percent of the iron atoms are Fe(III), with the remaining atoms having an effective oxidation state of about +2.5. Iron K-edge extended x-ray absorption fine structure data for the 500-iron atom nanocomposite are consistent with a disordered array of edge-shared FeS(4) tetrahedra, connected by Fe(S)(2)Fe bridges with bond lengths similar to those of the cubane-type motif of iron-sulfur clusters. The approach used here for the controlled synthesis of bioinorganic nanocomposites could be useful for the nanoscale engineering of dispersed materials with biocompatible and bioactive properties.     

Atomic resolution imaging of adsorbates on metal surfaces in air: iodine adsorption on pt(111)

The adsorption of iodine on platinum single crystals was studied with the scanning tunneling microscope (STM) to define the limits of resolution that can be obtained while imaging in air and to set a target resolution for STM imaging of metal surfaces immersed in an electrochemical cell. Two iodine adlattice unit cells of slightly different iodine packing density were clearly imaged: ( radical7 x radical7) R19.1 degrees -I, surface coverage ?(I) = 3/7; and (3 x 3)-I, ?(I) = 4/9. The three iodine atoms in the ( radical7 x radical7) unit cell form a regular hexagonal lattice interatomic distance d(I) = 0.424 nanometer, with two atoms adsorbed in threefold hollow sites and one atom adsorbed at an atop site. The (3 x 3) unit cell showed two different packing arrangements of the four iodine atoms exit. In one of the (3 x 3) structures, the iodine atoms pack to form a hexagonal lattice, d(I) = 0.417 nanometer, with three of the iodine atoms at twofold adsorption sites and one atom at an atop site. Another packing arrangement of iodine into the (3 x 3) unit cell was imaged in which the iodine atoms are not arranged symmetrically.     

Molecular imaging using a targeted magnetic resonance hyperpolarized biosensor

A magnetic resonance approach is presented that enables high-sensitivity, high-contrast molecular imaging by exploiting xenon biosensors. These sensors link xenon atoms to specific biomolecular targets, coupling the high sensitivity of hyperpolarized nuclei with the specificity of biochemical interactions. We demonstrated spatial resolution of a specific target protein in vitro at micromolar concentration, with a readout scheme that reduces the required acquisition time by >3300-fold relative to direct detection. This technique uses the signal of free hyperpolarized xenon to dramatically amplify the sensor signal via chemical exchange saturation transfer (CEST). Because it is approximately 10,000 times more sensitive than previous CEST methods and other molecular magnetic resonance imaging techniques, it marks a critical step toward the application of xenon biosensors as selective contrast agents in biomedical applications.     

Cooling Bose-Einstein condensates below 500 picokelvin

Spin-polarized gaseous Bose-Einstein condensates were confined by a combination of gravitational and magnetic forces. The partially condensed atomic vapors were adiabatically decompressed by weakening the gravito-magnetic trap to a mean frequency of 1hertz, then evaporatively reduced in size to 2500 atoms. This lowered the peak condensate density to 5 x 10(10) atoms per cubic centimeter and cooled the entire cloud in all three dimensions to a kinetic temperature of 450 +/- 80 picokelvin. Such spin-polarized, dilute, and ultracold gases are important for spectroscopy, metrology, and atom optics.     

Giant magnetic anisotropy of single cobalt atoms and nanoparticles

The isotropic magnetic moment of a free atom is shown to develop giant magnetic anisotropy energy due to symmetry reduction at an atomically ordered surface. Single cobalt atoms deposited onto platinum (111) are found to have a magnetic anisotropy energy of 9 millielectron volts per atom arising from the combination of unquenched orbital moments (1.1 Bohr magnetons) and strong spin-orbit coupling induced by the platinum substrate. By assembling cobalt nanoparticles containing up to 40 atoms, the magnetic anisotropy energy is further shown to be dependent on single-atom coordination changes. These results confirm theoretical predictions and are of fundamental value to understanding how magnetic anisotropy develops in finite-sized magnetic particles.     

孤立原子和双模腔内原子之间的纠缠突然死亡研究

通过计算并发度和线性熵研究了初始处于纠缠态的两个两能级原子与双模场相互作用系统的纠缠动力学特性,讨论了原子初始纠缠度和腔场初始纠缠度对并发度的影响。结果表明,两原子之间的纠缠出现纠缠突然死亡(ESD)现象,纠缠死亡持续的时间长度和原子初始的纠缠度无关,然而依赖于腔场初始纠缠度;在整个时间演化过程中,两原子和腔场之间一直保持着纠缠状态。     

Controlled atomic doping of a single C60 molecule

We report a method for controllably attaching an arbitrary number of charge dopant atoms directly to a single, isolated molecule. Charge-donating K atoms adsorbed on a silver surface were reversibly attached to a C60 molecule by moving it over K atoms with a scanning tunneling microscope tip. Spectroscopic measurements reveal that each attached K atom donates a constant amount of charge (approximately 0.6 electron charge) to the C60 host, thereby enabling its molecular electronic structure to be precisely and reversibly tuned.     

Photoinduced Chemical Dynamics of High-Spin Alkali Trimers

Nanometer-sized helium droplets, each containing about 10(4) helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.     

Metastable oxygen emission bands

Recombination of ground-state oxygen atoms populates six different bound electronic states of molecular oxygen. Of the six optical transitions expected between the three upper states at 4 to 4.5 electron volts and the two lowest states, five have been observed in the afterglow of a conventional helium-oxygen microwave discharge in both 16O(2) and (18)O(2), three of them for the first time in gas-phase spectra. Generation of these emissions from oxygen atoms in a system free of molecular oxygen establishes that atom recombination is the production mechanism.     

Molecular beam scattering from liquid surfaces

By means of controlled collisions of atoms and molecules with liquid surfaces, molecular beam experiments can be used to probe how gases stick to, rebound from, and exchange energy with molecules in the liquid phase. This report describes measurements of energy exchange in collisions between gases (neon, xenon, and sulfur hexafluoride) and polyatomic liquids (squalane and perfluoropolyether). Energy transfer depends critically on liquid composition and is more efficient for the hydrocarbon than for the perfluorinated ether.     

Spin conservation accounts for aluminum cluster anion reactivity pattern with O2

The reactivity pattern of small (approximately 10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters AlnH-, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd-n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.     

Nitrogenase MoFe-protein at 1.16 A resolution: a central ligand in the FeMo-cofactor

A high-resolution crystallographic analysis of the nitrogenase MoFe-protein reveals a previously unrecognized ligand coordinated to six iron atoms in the center of the catalytically essential FeMo-cofactor. The electron density for this ligand is masked in structures with resolutions lower than 1.55 angstroms, owing to Fourier series termination ripples from the surrounding iron and sulfur atoms in the cofactor. The central atom completes an approximate tetrahedral coordination for the six iron atoms, instead of the trigonal coordination proposed on the basis of lower resolution structures. The crystallographic refinement at 1.16 angstrom resolution is consistent with this newly detected component being a light element, most plausibly nitrogen. The presence of a nitrogen atom in the cofactor would have important implications for the mechanism of dinitrogen reduction by nitrogenase.