Scis0000002's atoms [17 articles]

Abstract

We present a new approach to generating electrostatic potential (ESP) derived charges for molecules. Themajor strength of electrostatic potential derived charges is that they optimally reproduce the intermolecularinteraction properties of molecules with a simple two-body additive potential, provided, of course, that a suitablyaccurate level of quantum mechanical calculation is used to derive the E S P around the molecule. Previously,the major weaknesses of these charges have been that they were not easily transferable between commonfunctional groups in related molecules, they have ...

Abstract

The present work examines the conditioning of the least-squares matrix for obtaining potential derived charges and presents a modification of the CHELP method for fitting atomic charges to electrostatic potentials. Results from singular value decompositions (SVDs) of the least-squares matrices show that, in general, the least-squares matrix for this fitting problem will be rank deficient. Thus, statistically valid charges cannot be assigned to all the atoms in a given molecule. We find also that, contrary to popular notions, increasing the point ...

Abstract

Charge states of atoms can be investigated with scanning tunneling microscopy, but this method requires a conducting substrate. We investigated the charge-switching of individual adsorbed gold and silver atoms (adatoms) on ultrathin NaCl films on Cu(111) using a qPlus tuning fork atomic force microscope (AFM) operated at 5 kelvin with oscillation amplitudes in the subangstrom regime. Charging of a gold atom by one electron charge increases the force on the AFM tip by a few piconewtons. Moreover, the local contact potential ...

Abstract

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, ...

Abstract

There has been a great deal of debate surrounding the issue of whether it is possible for a single photon to exhibit nonlocality. A number of schemes have been proposed that claim to demonstrate this effect, but each has been met with significant opposition. The objections hinge largely on the fact that these schemes use unobservable initial states and so, it is claimed, they do not represent experiments that could actually be performed. Here we show how it is possible to overcome these objections by presenting an ...

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Between December 1992 and November 30, 1992, we refined our atom interferometer and started to perform experiments with spatially separated beams. The interferometer is now operating with smaller period gratings, providing greater beam separation. The experiments were performed with the aid of an interaction region that inserts a thin metal foil between the beams. This allowed us to manipulate the atomic wave function in only one arm of the interferometer. The key component of our interferometer is the set of three matched transmission diffraction gratings which we constructed at ...

Abstract

10.1126/science.1079430 ...

Abstract

We discuss the impact of thermally excited near fields on the coherent expansion of a condensate in a miniaturized electromagnetic trap. Monte Carlo simulations are compared with a kinetic two-component theory and indicate that atom interactions can slow down decoherence. This is explained by a simple theory in terms of the condensate dynamic structure factor. ...

Abstract

We study the suppression of noise-induced phase decoherence in a single atomic qubit by employing pulse sequences. The atomic qubit is composed of a single neutral atom in a far-detuned optical dipole trap and the phase decoherence may originate from the laser intensity and beam pointing fluctuations as well as magnetic field fluctuations. We show that suitable pulse sequences may prolongate the qubit coherence time substantially as comparing to the conventional spin echo pulse. ...

Abstract

Bragg diffraction of atoms at thick standing light waves requires that the wave-matching condition is fulfilled. This usually means that the atomic beam crosses the light wave exactly at the Bragg angle. Nevertheless, our experiments also demonstrate Bragg diffraction at detuned angles if the amplitude of the standing light wave is temporally modulated with an appropriate frequency. If, on the other hand, the phase of the light wave is modulated no diffraction is observed. Both modulation processes produce frequency sidebands which ...

Abstract

We study the possibility of using matter-wave interferometry techniques to build a gravitational wave detector. We derive the response function and find that it contains a term proportional to the derivative of the gravitational wave, a point which has been disputed recently. We then study in detail the sensitivity that can be reached by such a detector and find that, if it is operated near resonance, it can reach potentially interesting values in the high frequency regime. The correlation between two ...

Abstract

Using information theory, it is argued that from among possible definitions of what an atom is when it is in a molecule, a particular one merits special attention. Namely, it is the atom defined by the “stockholders partitioning” of a molecule invented by Hirshfeld [(1977) 44, 129]. The theoretical tool used is the minimum entropy deficiency principle (minimum missing information principle) of Kullback and Liebler [(1951) 22, 79]. A corresponding analysis is given of the problem of assessing similarity ...

Abstract

The relative proportions of the different chemical elements in the universe. In practice, the `cosmic' abundance figures that are usually quoted are derived from spectroscopic analysis of the Sun supplemented by chemical analyses of chondritic meteorites and terrestrial and lunar rocks. The abundance of a particular element is usually given as the ratio of the number of atoms of that element to t... ...

Abstract

One of the main ingredients in most quantum information protocols is a reliable source of two entangled systems. Such systems have been generated experimentally several years ago for light but has only in the past few years been demonstrated for atomic systems. None of these approaches however involve two atomic systems situated in separate environments. This is necessary for the creation of entanglement over arbitrary distances which is required for many quantum information protocols such as atomic teleportation. We present an experimental realization of such distant entanglement based on ...

Abstract

In this work the dynamical behavior of the center of mass of a two-level atom in sequences of multiple optical standing waves is explored. In a standing wave the atomic momentum distribution fans out. But with a suitably designed sequence of standing waves it is possible to refocus the momentum-space wave packet and bring the momentum distribution to the initial value, which may be regarded as an atom optical analog to spin or photon echo. If the total interaction time is fixed, the refocusing performance gets better as ...

Abstract

The nonlinear Schrödinger equation with Gaussian convolution kernel K2 induces the group SU3 with reference to the classification of the multiplet structure of the eigenstates. Such a field can be used to describe some atoms (where the outermost electrons are related tos-orbitals) as a self-interacting, extended particle with an internal structure. In the case of those atoms, where the valence electrons are described byp-orbitals, and almost all molecules the Gaussian kernel K2 has to be generalized by Hermite polynomials. By that, ...