89 research outputs found
The twisted arc model for chiral molecules
We present a simple model for chiral molecules which yields the
frequency-dependent multipole-multipole polarizabilities required for
calculating the dispersive van der Waals constants in a simple, closed, and
consistent form. The model consists of a single effective charge confined to a
one-dimensional wire formed by two circular arcs, which are twisted with
respect to each other by an angle characterizing the chirality. The resulting
polarizabilities show a simple functional dependence on the model parameters,
and they serve to mimic the chiral properties of small molecules, such as H2S2,
in a reasonably realistic way.Comment: 21 pages, 4 figure
Using atomic interference to probe atom-surface interaction
We show that atomic interference in the reflection from two suitably
polarized evanescent waves is sensitive to retardation effects in the
atom-surface interaction for specific experimental parameters. We study the
limit of short and long atomic de Broglie wavelength. The former case is
analyzed in the semiclassical approximation (Landau-Zener model). The latter
represents a quantum regime and is analyzed by solving numerically the
associated coupled Schroedinger equations. We consider a specific experimental
scheme and show the results for rubidium (short wavelength) and the much
lighter meta-stable helium atom (long wavelength). The merits of each case are
then discussed.Comment: 11 pages, including 6 figures, submitted to Phys. Rev. A, RevTeX
sourc
Unified Treatment of Asymptotic van der Waals Forces
In a framework for long-range density-functional theory we present a unified
full-field treatment of the asymptotic van der Waals interaction for atoms,
molecules, surfaces, and other objects. The only input needed consists of the
electron densities of the interacting fragments and the static polarizability
or the static image plane, which can be easily evaluated in a ground-state
density-functional calculation for each fragment. Results for separated atoms,
molecules, and for atoms/molecules outside surfaces are in agreement with those
of other, more elaborate, calculations.Comment: 6 pages, 5 figure
The reactions of active nitrogen with molecular oxygen, atomic oxygen and hydrogen.
For the reaction of nitrogen atoms with molecular oxygen, the nitrogen and oxygen atom concentrations were estimated, at different temperatures and pressures, by addition of excess N0 and measurement of N2O and NO produced. The rate constant was given by 2.3 x 10^12 exp(-5,900/RT) cc mole^-1 sec^-1. The maximum production of oxygen atoms was found to correspond to the maximum amount of HCN produced in the active nitrogen-ethylene reaction. A rate constant of 1.83(+- 0.2) x 10^15 cc^2 mole^-2 sec^-1 was determined for the reaction of nitrogen atoms with oxygen atoms, in the absence of molecular oxygen, at pressures of 3, 3.5 and 4 mm, in an unheated reaction tube. The nitrogen atom consumption was estimated by titrating active nitrogen with NO at different positions along the reaction tube. Using the same titration method, the rate constant for the decay of nitrogen atoms, in an unheated vessel, was found to have the value of 5.2 x 10^15 cc^2 mole^-2 sec^-1, with y= 7.5 (-+ 0.6) x 10^-5, in the pressure range from 0.5 to 4 mm. Under similar conditions, and by similar methods, a value of 4.87(+- 0.8) x 10^14 cc^2 mole^-2 sec^-l was obtained for the rate constant of the reaction between nitrogen atoms and hydrogen atoms, over the pressure range from 2.5 mm to 4.5 mm
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