426 research outputs found
Transition from molecular complex to quantum solvation in OCS(He)_N
We present quantum calculations of the rotational energy levels and
spectroscopic rotational constants of the linear OCS molecule in variable size
clusters of 4He, using spectral evolution quantum Monte Carlo methods that
allow excited states to be accessed without nodal constraints. The rotational
constants of OCS are found to decrease monotonically from the gas phase value
as the number of helium atoms increases to N=6, after which the average
constant increases to saturation at a value in excellent agreement with
experimental measurements made on significantly larger clusters (N>1000). The
minimum is shown to indicate a transition from a molecular complex to a quantum
solvated molecule, with the former characterized by floppy but near rigid
behavior, while the latter is characterized by non-zero permutation exchanges
and a smaller extent of rigid coupling.Comment: 4 pages, 1 figure, submitted to Phys. Rev. Lett. on July 18,200
Dynamics of liquid He-4 in confined geometries from Time-Dependent Density Functional calculations
We present numerical results obtained from Time-Dependent Density Functional
calculations of the dynamics of liquid He-4 in different environments
characterized by geometrical confinement. The time-dependent density profile
and velocity field of He-4 are obtained by means of direct numerical
integration of the non-linear Schrodinger equation associated with a
phenomenological energy functional which describes accurately both the static
and dynamic properties of bulk liquid He-4. Our implementation allows for a
general solution in 3-D (i.e. no symmetries are assumed in order to simplify
the calculations). We apply our method to study the real-time dynamics of pure
and alkali-doped clusters, of a monolayer film on a weakly attractive surface
and a nano-droplet spreading on a solid surface.Comment: q 1 tex file + 9 Ps figure
Density of Superfluid Helium Droplets
The classical integral cross sections of large superfluid 4He_N droplets and
the number of atoms in the droplets (N=10^3-10^4) have been measured in
molecular beam scattering experiments. These measurements are found to be in
good agreement with the cross sections predicted from density functional
calculations of the radial density distributions with a 10-90 % surface
thickness of 5.7\AA. By using a simple model for the density profile of the
droplets a thickness of about 6-8\AA is extracted directly from the data.Comment: 27 pages, REVTeX, 5 postscript figure
High-quality variational wave functions for small 4He clusters
We report a variational calculation of ground state energies and radii for
4He_N droplets (3 \leq N \leq 40), using the atom-atom interaction HFD-B(HE).
The trial wave function has a simple structure, combining two- and three-body
correlation functions coming from a translationally invariant
configuration-interaction description, and Jastrow-type short-range
correlations. The calculated ground state energies differ by around 2% from the
diffusion Monte Carlo results.Comment: 5 pages, 1 ps figure, REVTeX, submitted to Phys. Rev.
Pinning of quantized vortices in helium drop by dopant atoms and molecules
Using a density functional method, we investigate the properties of liquid
4He droplets doped with atoms (Ne and Xe) and molecules (SF_6 and HCN). We
consider the case of droplets having a quantized vortex pinned to the dopant. A
liquid drop formula is proposed that accurately describes the total energy of
the complex and allows one to extrapolate the density functional results to
large N. For a given impurity, we find that the formation of a
dopant+vortex+4He_N complex is energetically favored below a critical size
N_cr. Our result support the possibility to observe quantized vortices in
helium droplets by means of spectroscopic techniques.Comment: Typeset using Revtex, 3 pages and 5 figures (4 Postscript, 1 jpeg
Observation of Large Differences in the Diffraction of Normal- and Para-H2 from LiF(001)
Theoretical Chemistr
Hybrid Sample-based Surface Rendering
The performance of rasterization-based rendering on current GPUs strongly depends on the abilities to avoid overdraw and to prevent rendering triangles smaller than the pixel size. Otherwise, the rates at which highresolution polygon models can be displayed are affected significantly. Instead of trying to build these abilities into the rasterization-based rendering pipeline, we propose an alternative rendering pipeline implementation that uses rasterization and ray-casting in every frame simultaneously to determine eye-ray intersections. To make ray-casting competitive with rasterization, we introduce a memory-efficient sample-based data structure which gives rise to an efficient ray traversal procedure. In combination with a regular model subdivision, the most optimal rendering technique can be selected at run-time for each part. For very large triangle meshes our method can outperform pure rasterization and requires a considerably smaller memory budget on the GPU. Since the proposed data structure can be constructed from any renderable surface representation, it can also be used to efficiently render isosurfaces in scalar volume fields. The compactness of the data structure allows rendering from GPU memory when alternative techniques already require exhaustive paging
A Bright, Slow Cryogenic Molecular Beam Source for Free Radicals
We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam
source capable of producing bright beams of free radicals and refractory
species. Details of the beam properties (brightness, forward velocity
distribution, transverse velocity spread, rotational and vibrational
temperatures) are measured under varying conditions for the molecular species
SrF. Under typical conditions we produce a beam of brightness 1.2 x 10^11
molecules/sr/pulse in the rovibrational ground state, with 140 m/s forward
velocity and a rotational temperature of approximately 1 K. This source
compares favorably to other methods for producing beams of free radicals and
refractory species for many types of experiments. We provide details of
construction that may be helpful for others attempting to use this method.Comment: 15 pages, 14 figure
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