共查询到20条相似文献，搜索用时 46 毫秒
1.
Danzl JG Haller E Gustavsson M Mark MJ Hart R Bouloufa N Dulieu O Ritsch H Nägerl HC 《Science (New York, N.Y.)》2008,321(5892):1062-1066
Molecular cooling techniques face the hurdle of dissipating translational as well as internal energy in the presence of a rich electronic, vibrational, and rotational energy spectrum. In our experiment, we create a translationally ultracold, dense quantum gas of molecules bound by more than 1000 wave numbers in the electronic ground state. Specifically, we stimulate with 80% efficiency, a two-photon transfer of molecules associated on a Feshbach resonance from a Bose-Einstein condensate of cesium atoms. In the process, the initial loose, long-range electrostatic bond of the Feshbach molecule is coherently transformed into a tight chemical bond. We demonstrate coherence of the transfer in a Ramsey-type experiment and show that the molecular sample is not heated during the transfer. Our results show that the preparation of a quantum gas of molecules in specific rovibrational states is possible and that the creation of a Bose-Einstein condensate of molecules in their rovibronic ground state is within reach. 相似文献
2.
K Mauersberger B Erbacher D Krankowsky G nther J R Nickel 《Science (New York, N.Y.)》1999,283(5400):370-372
Six rate coefficients of relative ozone formation contradict the role of molecular symmetry in the process that results in the enrichment of heavy ozone isotopomers. The results show that collisions between light atoms, such as 16O, and heavy molecules, such as 34O2 and 36O2, have a rate coefficient advantage of about 25 and 50 percent, respectively, over collisions involving heavy atoms and light molecules. These results suggest that the observed isotope effect for each isotopomer may be caused by the preponderance of a single reaction channel and not through molecular symmetry selection. 相似文献
3.
On the basis of estimates of molecular damage caused by the observation process, it is concluded that molecular microscopy of biological molecules in which the individual atoms are resolved is impossible with an electron or x-ray microscope. Microscopes that use low-energy helium atoms or neutrons as illuminants may be capable of serving as ultimate biomolecular microscopes. 相似文献
4.
Noyori R 《Science (New York, N.Y.)》1990,248(4960):1194-1199
As molecular recognition continues to gain importance in the biological and physical sciences as well as in the technologies of molecular electronics and optics, so has the need for efficient syntheses of chiral molecules. Chemists are fulfilling this need through use of chiral organometallic molecules. These chiral metal complexes precisely discriminate between enantiotopic atoms, groups, or faces in achiral molecules and catalyze production of a broad array of natural or unnatural substances of excellent enantiomeric purity. Because of their ability to efficiently multiply chirality, even on an industrial level, these catalysts promise to exert a general impact on molecular science and engineering. 相似文献
5.
Rosei F Schunack M Jiang P Gourdon A Laegsgaard E Stensgaard I Joachim C Besenbacher F 《Science (New York, N.Y.)》2002,296(5566):328-331
The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule. 相似文献
6.
Nuclear magnetic resonance (NMR) spectroscopy continues to evolve as a primary technique in the study of solids. This review briefly describes some developments in modern NMR that demonstrate its exciting potential as an analytical tool in fields as diverse as physics, chemistry, biology, geology, and materials science. Topics covered include motional narrowing by sample reorientation, multiple-quantum and overtone spectroscopy, probing porous solids with guest atoms and molecules, two-dimensional NMR studies of chemical exchange and spin diffusion, experiments at extreme temperatures, NMR imaging of solid materials, and low-frequency and zero-field magnetic resonance. These developments permit increasingly complex structural and dynamical behavior to be probed at a molecular level and thus add to our understanding of macroscopic properties of materials. 相似文献
7.
Viteau M Chotia A Allegrini M Bouloufa N Dulieu O Comparat D Pillet P 《Science (New York, N.Y.)》2008,321(5886):232-234
The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, cesium molecules initially formed via photoassociation of cold cesium atoms are in several vibrational levels nu of the electronic ground state. We applied a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation/spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the nu = 0 level, preventing re-excitation from that state. We observed a fast and efficient accumulation ( approximately 70% of the initially detected molecules) in the lowest vibrational level, nu = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules. 相似文献
8.
Cossaro A Mazzarello R Rousseau R Casalis L Verdini A Kohlmeyer A Floreano L Scandolo S Morgante A Klein ML Scoles G 《Science (New York, N.Y.)》2008,321(5891):943-946
The structure of self-assembled monolayers (SAMs) of long-chain alkyl sulfides on gold(111) has been resolved by density functional theory-based molecular dynamics simulations and grazing incidence x-ray diffraction for hexanethiol and methylthiol. The analysis of molecular dynamics trajectories and the relative energies of possible SAM structures suggest a competition between SAM ordering, driven by the lateral van der Waals interaction between alkyl chains, and disordering of interfacial Au atoms, driven by the sulfur-gold interaction. We found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first gold surface layer contains gold atom vacancies (which are partially redistributed over different sites) as well as gold adatoms that are laterally bound to two sulfur atoms. 相似文献
9.
State-selected rubidium-87 molecules were created at rest in a dilute Bose-Einstein condensate of rubidium-87 atoms with coherent free-bound stimulated Raman transitions. The transition rate exhibited a resonance line shape with an extremely narrow width as small as 1.5 kilohertz. The precise shape and position of the resonance are sensitive to the mean-field interactions between the molecules and the atomic condensate. As a result, we were able to measure the molecule-condensate interactions. This method allows molecular binding energies to be determined with unprecedented accuracy and is of interest as a mechanism for the generation of a molecular Bose-Einstein condensate. 相似文献
10.
During the course of studying the dehydrogenation reactions of hydrocarbons by titanium atoms, ions, and clusters, an exceptionally stable and abundant cluster which contains 8 titaniums and 12 carbons was discovered. "Titration" reactions with ND(3) reveal the uptake of eight molecules, pointing to the fact that the titanium atoms are at exposed positions of similar coordination. A dodecahedral structure of T(h) point group symmetry is proposed to account for the unusual stability of this molecular cluster. The Ti(8)C(12)(+) dodecahedron has 12 pentagonal rings and each of the rings is formed by two titanium and three carbon atoms, where each titanium is bound to three carbons. Based on the model, it is expected that neutral Ti(8)C(12) would be a stable metallo-carbododecahedral molecule and may comprise one member of a new class of molecules, namely metallo-carbohedrenes. 相似文献
11.
Devries GA Brunnbauer M Hu Y Jackson AM Long B Neltner BT Uzun O Wunsch BH Stellacci F 《Science (New York, N.Y.)》2007,315(5810):358-361
Nanoparticles can be used as the building blocks for materials such as supracrystals or ionic liquids. However, they lack the ability to bond along specific directions as atoms and molecules do. We report a simple method to place target molecules specifically at two diametrically opposed positions in the molecular coating of metal nanoparticles. The approach is based on the functionalization of the polar singularities that must form when a curved surface is coated with ordered monolayers, such as a phase-separated mixture of ligands. The molecules placed at these polar defects have been used as chemical handles to form nanoparticle chains that in turn can generate self-standing films. 相似文献
12.
Saecker ME Govoni ST Kowalski DV King ME Nathanson GM 《Science (New York, N.Y.)》1991,252(5011):1421-1424
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. 相似文献
13.
In spite of their importance in fundamental and applied studies, the preparation of endohedral fullerenes has relied on difficult-to-control physical methods. We report a four-step organic reaction that completely closes a 13-membered ring orifice of an open-cage fullerene. This process can be used to synthesize a fullerene C60 encapsulating molecular hydrogen, which can be isolated as a pure product. This molecular surgical method should make possible the preparation of a series of C60 fullerenes, encapsulating either small atoms or molecules, that are not accessible by conventional physical methods. 相似文献
14.
Scanning tunneling microscopy and monoenergetic molecular beams have been used to obtain real-space atomic images of the competition between abstractive and dissociative chemisorption. The size distribution of Si-F adsorbates on the Si(111)-(7x7) surface was examined as a function of the incident translational energy of the F(2) molecules. For F(2) molecules with 0.03 electron volt of incident energy, the dominant adsorbate sites were isolated Si-F species. As an F(2) molecule with low translational energy collides with the surface, abstraction occurs and only one of the F atoms chemisorbs; the other is ejected into the gas phase. For F(2) molecules with 0.27 electron volt of incident energy, many adjacent Si-F adsorbates (dimer sites) were observed because F(2) molecules with high translational energy collide with the surface and chemisorb dissociatively so that both F atoms react to form adjacent Si-F adsorbates. For halogens with very high incident energy (0.5-electron volt Br(2)), dissociative chemisorption is the dominant adsorption mechanism and dimer sites account for nearly all adsorbates. 相似文献
15.
The prospect of manipulating matter on the atomic scale has fascinated scientists for decades. This fascination may be motivated by scientific and technological opportunities, or from a curiosity about the consequences of being able to place atoms in a particular location. Advances in scanning tunneling microscopy have made this prospect a reality; single atoms can be placed at selected positions and structures can be built to a particular design atom-by-atom. Atoms and molecules may be manipulated in a variety of ways by using the interactions present in the tunnel junction of a scanning tunneling microscope. Some of these recent developments and some of the possible uses of atomic and molecular manipulation as a tool for science are discussed. 相似文献
16.
Boguslavskiy AE Mikosch J Gijsbertsen A Spanner M Patchkovskii S Gador N Vrakking MJ Stolow A 《Science (New York, N.Y.)》2012,335(6074):1336-1340
Subcycle strong-field ionization (SFI) underlies many emerging spectroscopic probes of atomic or molecular attosecond electronic dynamics. Extending methods such as attosecond high harmonic generation spectroscopy to complex polyatomic molecules requires an understanding of multielectronic excitations, already hinted at by theoretical modeling of experiments on atoms, diatomics, and triatomics. Here, we present a direct method which, independent of theory, experimentally probes the participation of multiple electronic continua in the SFI dynamics of polyatomic molecules. We use saturated (n-butane) and unsaturated (1,3-butadiene) linear hydrocarbons to show how subcycle SFI of polyatomics can be directly resolved into its distinct electronic-continuum channels by above-threshold ionization photoelectron spectroscopy. Our approach makes use of photoelectron-photofragment coincidences, suiting broad classes of polyatomic molecules. 相似文献
17.
The atomic electron orbitals that underlie molecular bonding originate from the central Coulomb potential of the atomic core. We used scanning tunneling microscopy and density functional theory to explore the relation between the nearly spherical shape and unoccupied electronic structure of buckminsterfullerene (C60) molecules adsorbed on copper surfaces. Besides the known pi* antibonding molecular orbitals of the carbon-atom framework, above 3.5 electron volts we found atomlike orbitals bound to the core of the hollow C60 cage. These "superatom" states hybridize like the s and p orbitals of hydrogen and alkali atoms into diatomic molecule-like dimers and free-electron bands of one-dimensional wires and two-dimensional quantum wells in C60 aggregates. We attribute the superatom states to the central potential binding an electron to its screening charge, a property expected for hollow-shell molecules derived from layered materials. 相似文献
18.
A general method of manipulating adsorbed atoms and molecules on room-temperature surfaces with the use of a scanning tunneling microscope is described. By applying an appropriate voltage pulse between the sample and probe tip, adsorbed atoms can be induced to diffuse into the region beneath the tip. The field-induced diffusion occurs preferentially toward the tip during the voltage pulse because of the local potential energy gradient arising from the interaction of the adsorbate dipole moment with the electric field gradient at the surface. Depending upon the surface and pulse parameters, cesium (Cs) structures from one nanometer to a few tens of nanometers across have been created in this way on the (110) surfaces of gallium arsenide (GaAs) and indium antimonide (InSb), including structures that do not naturally occur. 相似文献
19.
This review puts into perspective the present state and prospects for controlling quantum phenomena in atoms and molecules. The topics considered include the nature of physical and chemical control objectives, the development of possible quantum control rules of thumb, the theoretical design of controls and their laboratory realization, quantum learning and feedback control in the laboratory, bulk media influences, and the ability to utilize coherent quantum manipulation as a means for extracting microscopic information. The preview of the field presented here suggests that important advances in the control of molecules and the capability of learning about molecular interactions may be reached through the application of emerging theoretical concepts and laboratory technologies. 相似文献
20.
Wiberg KB Hadad CM Breneman CM Laidig KE Murcko MA Lepage TJ 《Science (New York, N.Y.)》1991,252(5010):1266-1272
The properties of a molecule are determined by the distribution of its electrons. This distribution can be described by the charge density, which is readily obtained from the wave functions derived by ab initio molecular orbital calculations. The charge density may be analyzed in a number of different fashions to give information about the effects of substituents, structural changes, and electronic excitation on the properties of molecules; one common procedure makes use of projection density or charge difference plots. Charge density also may be partitioned among atoms, and by numerical integration over appropriate volume elements one may obtain atomic charges, dipoles, kinetic energies, and other properties of the atoms in a molecule. Many chemical phenomena have been analyzed in terms of charge densities. 相似文献