1,150 research outputs found
The Role of the Gouy Phase in the Coherent Phase Control of the Photoionization and Photodissociation of Vinyl Chloride
We demonstrate theoretically and experimentally that the Gouy phase of a
focused laser beam may be used to control the photo-induced reactions of a
polyatomic molecule. Quantum mechanical interference between one- and
three-photon excitation of vinyl chloride produces a small phase lag between
the dissociation and ionization channels on the axis of the molecular beam.
Away from the axis, the Gouy phase introduces a much larger phase lag that
agrees quantitatively with theory without any adjustable parameters.Comment: 4 pages, 4 figure
Ion-Size Effect at the Surface of a Silica Hydrosol
The author used synchrotron x-ray reflectivity to study the ion-size effect
for alkali ions (Na, K, Rb, and Cs), with densities as high as
m, suspended above the surface of a
colloidal solution of silica nanoparticles in the field generated by the
surface electric-double layer. According to the data, large alkali ions
preferentially accumulate at the sol's surface replacing smaller ions, a
finding that qualitatively agrees with the dependence of the Kharkats-Ulstrup
single-ion electrostatic free energy on the ion's radius.Comment: 17 pages, 4 figure
Radius of a Photon Beam with Orbital Angular Momentum
We analyze the transverse structure of the Gouy phase shift in light beams
carrying orbital angular momentum and show that the Gouy radius
characterizing the transverse structure grows as with the
nodal number and photon angular momentum number . The Gouy radius is
shown to be closely related to the root-mean-square radius of the beam, and the
divergence of the radius away from the focal plane is determined. Finally, we
analyze the rotation of the Poynting vector in the context of the Gouy radius.Comment: 11 page
Heterodyne non-demolition measurements on cold atomic samples: towards the preparation of non-classical states for atom interferometry
We report on a novel experiment to generate non-classical atomic states via
quantum non-demolition (QND) measurements on cold atomic samples prepared in a
high finesse ring cavity. The heterodyne technique developed for the QND
detection exhibits an optical shot-noise limited behavior for local oscillator
optical power of a few hundred \muW, and a detection bandwidth of several GHz.
This detection tool is used in single pass to follow non destructively the
internal state evolution of an atomic sample when subjected to Rabi
oscillations or a spin-echo interferometric sequence.Comment: 23 page
Sum rules of codon usage probabilities
In the crystal basis model of the genetic code, it is deduced that the sum of
usage probabilities of the codons with C and A in the third position for the
quartets and/or sextets is independent of the biological species for
vertebrates. A comparison with experimental data shows that the prediction is
satisfied within about 5 %.Comment: 7 page
Incorporation of excluded volume correlations into Poisson-Boltzmann theory
We investigate the effect of excluded volume interactions on the electrolyte
distribution around a charged macroion. First, we introduce a criterion for
determining when hard-core effects should be taken into account beyond standard
mean field Poisson-Boltzmann (PB) theory. Next, we demonstrate that several
commonly proposed local density functional approaches for excluded volume
interactions cannot be used for this purpose. Instead, we employ a non-local
excess free energy by using a simple constant weight approach. We compare the
ion distribution and osmotic pressure predicted by this theory with Monte Carlo
simulations. They agree very well for weakly developed correlations and give
the correct layering effect for stronger ones. In all investigated cases our
simple weighted density theory yields more realistic results than the standard
PB approach, whereas all local density theories do not improve on the PB
density profiles but on the contrary, deviate even more from the simulation
results.Comment: 23 pages, 7 figures, 1 tabl
Description beyond the mean field approximation of an electrolyte confined between two planar metallic electrodes
We study an electrolyte confined in a slab of width composed of two
grounded metallic parallel electrodes. We develop a description of this system
in a low coupling regime beyond the mean field (Poisson--Boltzmann)
approximation. There are two ways to model the metallic boundaries: as ideal
conductors in which the electric potential is zero and it does not fluctuate,
or as good conductors in which the average electric potential is zero but the
thermal fluctuations of the potential are not zero. This latter model is more
realistic. For the ideal conductor model we find that the disjoining pressure
is positive behaves as for large separations with a prefactor that is
universal, i.e. independent of the microscopic constitution of the system. For
the good conductor boundaries the disjoining pressure is negative and it has an
exponential decay for large . We also compute the density and electric
potential profiles inside the electrolyte. These are the same in both models.
If the electrolyte is charge asymmetric we find that the system is not locally
neutral and that a non-zero potential difference builds up between any
electrode and the interior of the system although both electrodes are grounded.Comment: 16 pages, 5 figures, added a new appendix B and a discussion on ideal
conductors vs. good conductor
X-ray study of the electric double layer at the n-hexane/nanocolloidal silica interface
The spatial structure of the transition region between an insulator and an
electrolyte solution was studied with x-ray scattering.The electron density
profile across the n-hexane/silica sol interface (solutions with 5-nm, 7-nm,
and 12-nm colloidal particles) agrees with the theory of the electrical double
layer and shows separation of positive and negative charges. The interface
consists of three layers, i.e., a compact layer of Na+, a loose monolayer of
nanocolloidal particles as part of a thick diffuse layer, and a low-density
layer sandwiched between them. Its structure is described by a model in which
the potential gradient at the interface reflects the difference in the
potentials of "image forces" between the cationic Na+ and anionic nanoparticles
and the specific adsorption of surface charge. The density of water in the
large electric field (1-10 GV/m) of the transition region and the layering of
silica in the diffuse layer is discussed.Comment: 9 pages, 9 figure
Ion-ion correlations: an improved one-component plasma correction
Based on a Debye-Hueckel approach to the one-component plasma we propose a
new free energy for incorporating ionic correlations into Poisson-Boltzmann
like theories. Its derivation employs the exclusion of the charged background
in the vicinity of the central ion, thereby yielding a thermodynamically stable
free energy density, applicable within a local density approximation. This is
an improvement over the existing Debye-Hueckel plus hole theory, which in this
situation suffers from a "structuring catastrophe". For the simple example of a
strongly charged stiff rod surrounded by its counterions we demonstrate that
the Poisson-Boltzmann free energy functional augmented by our new correction
accounts for the correlations present in this system when compared to molecular
dynamics simulations.Comment: 5 pages, 2 figures, revtex styl
X-ray Near Field Speckle: Implementation and Critical Analysis
We have implemented the newly-introduced, coherence-based technique of x-ray
near-field speckle (XNFS) at 8-ID-I at the Advanced Photon Source. In the near
field regime of high-brilliance synchrotron x-rays scattered from a sample of
interest, it turns out, that, when the scattered radiation and the main beam
both impinge upon an x-ray area detector, the measured intensity shows
low-contrast speckles, resulting from interference between the incident and
scattered beams. We built a micrometer-resolution XNFS detector with a high
numerical aperture microscope objective and demonstrate its capability for
studying static structures and dynamics at longer length scales than
traditional far field x-ray scattering techniques. Specifically, we
characterized the structure and dynamics of dilute silica and polystyrene
colloidal samples. Our study reveals certain limitations of the XNFS technique,
which we discuss.Comment: 53 pages, 16 figure
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