1,696 research outputs found
Moment distributiuons of clusters and molecules in the adiabatic rotor model
We present a Fortran program to compute the distribution of dipole moments of
free particles for use in analyzing molecular beams experiments that measure
moments by deflection in an inhomogeneous field. The theory is the same for
magnetic and electric dipole moments, and is based on a thermal ensemble of
classical particles that are free to rotate and that have moment vectors
aligned along a principal axis of rotation. The theory has two parameters, the
ratio of the magnetic (or electric) dipole energy to the thermal energy, and
the ratio of moments of inertia of the rotor.Comment: 3 pages with 2 figure
Optical response of small carbon clusters
We apply the time-dependent local density approximation (TDLDA) to calculate
dipole excitations in small carbon clusters. A strong low-frequency mode is
found which agrees well with observation for clusters C_n with n in the range
7-15. The size dependence of the mode may be understood simply as the classical
resonance of electrons in a conducting needle. For a ring geometry, the lowest
collective mode occurs at about twice the frequency of the collective mode in
the linear chain, and this may also be understood in simple terms.Comment: 19 pages, Latex(Revtex), and 7 figures Postscript; to be published in
Zeit. Phys. D; contact is [email protected]
Time-Dependent Local Density Approximation for Collective Excitations of Atomic Clusters
We discuss the calculation of collective excitations in atomic clusters using
the time-dependent local density approximation. In principle there are many
formulations of the TDLDA, but we have found that a particularly efficient
method for large clusters is to use a coordinate space mesh and the algorithms
for the operators and the evolution equations that had been developed for the
nuclear time-dependent Hartree-Fock theory. The TDLDA works remarkably well to
describe the strong excitations in alkali metal clusters and in carbon
clusters. We show as an example the benzene molecule, which has two strong
features in its spectrum. The systematics of the linear carbon chains is well
reproduced, and may be understood in rather simple terms.Comment: 12 pages in Postscrip
Application of time-dependent density-functional theory to electron-ion couplng in ethylene
To examine the applicability of the time-dependent density-functional theory
(TDDFT) for treating the electron-nucleus coupling in excited states, we
calculate the strength distribution associated with the pi-pi* transition in
ethylene. The observed optical transition strength at 7-8.5 eV region shows a
complex structure arising from coupling to C-C stretch motion, to torsional
motion, and to Rydberg excitations. The mean energy of the observed peak is
reproduced to about 0.2 eV accuracy by the TDDFT in the local density
approximation (LDA). The reflection approximation is used to calculate the peak
broadening. Roughly half of the broadening can be attributed to the fluctuation
in the C-C coordinate. The asymmetry in the line shape is also qualitatively
reproduced by the C-C coordinate fluctuation. We find, in agreement with other
theoretical studies, that the torsional motion is responsible for the
progression of weak transition strength extending from the peak down to about 6
eV. The LDA reproduces the strength in this region to about factor of 3. We
conclude that the TDDFT is rather promising for calculating the electron
nucleus coupling at short times.Comment: 14 pages and 4 figures: an error is corrected in Table
Nuclear Breakup of Borromean Nuclei
We study the eikonal model for the nuclear-induced breakup of Borromean
nuclei, using Li11 and He6 as examples. The full eikonal model is difficult to
realize because of six-dimensional integrals, but a number of simplifying
approximations are found to be accurate. The integrated diffractive and
one-nucleon stripping cross sections are rather insensitive to the
neutron-neutron correlation, but the two-nucleon stripping does show some
dependence on the correlation. The distribution of excitation energy in the
neutron-core final state in one-neutron stripping reactions is quite sensitive
to the shell structure of the halo wave function. Experimental data favor
models with comparable amounts of s- and p-wave in the Li11 halo.Comment: 34 pages REVTeX, 14 postscript figures. Small changes in comparison
with experimen
Independent Orbiter Assessment (IOA): Assessment of the electrical power generation/fuel cell powerplant subsystem FMEA/CIL
Results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA effort first completed an analysis of the Electrical Power Generation/Fuel Cell Powerplant (EPG/FCP) hardware, generating draft failure modes and potential critical items. To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. The IOA results were then compared to the proposed Post 51-L NASA FMEA/CIL baseline. A resolution of each discrepancy from the comparison was provided through additional analysis as required. This report documents the results of that comparison for the Orbiter EPG/FCP hardware
Independent Orbiter Assessment (IOA): Analysis of the electrical power generation/fuel cell powerplant subsystem
Results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA approach features a top-down analysis of the hardware to determine failure modes, criticality, and potential critical items. To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. This report documents the independent analysis results corresponding to the Orbiter Electrical Power Generation (EPG)/Fuel Cell Powerplant (FCP) hardware. The EPG/FCP hardware is required for performing functions of electrical power generation and product water distribution in the Orbiter. Specifically, the EPG/FCP hardware consists of the following divisions: (1) Power Section Assembly (PSA); (2) Reactant Control Subsystem (RCS); (3) Thermal Control Subsystem (TCS); and (4) Water Removal Subsystem (WRS). The IOA analysis process utilized available EPG/FCP hardware drawings and schematics for defining hardware assemblies, components, and hardware items. Each level of hardware was evaluated and analyzed for possible failure modes and effects. Criticality was assigned based upon the severity of the effect for each failure mode
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