1,147 research outputs found
Observation of the nuclear magnetic octupole moment of Yb from precise measurements of hyperfine structure in the state
We measure hyperfine structure in the metastable state of
Yb and extract the nuclear magnetic octupole moment. We populate the
state using dipole-allowed transitions through the and
states. We measure frequencies of hyperfine transitions of the line at 770 nm using a Rb-stabilized ring cavity resonator
with a precision of 200 kHz. Second-order corrections due to perturbations from
the nearby and states are below 30 kHz. We obtain the
hyperfine coefficients as: MHz, MHz, which
represent two orders-of-magnitude improvement in precision, and
MHz. From atomic structure calculations, we obtain the nuclear moments:
quadrupole b and octupole b\,.Comment: 5 pages, 1 figur
Atomic fountain of laser-cooled Yb atoms for precision measurements
We demonstrate launching of laser-cooled Yb atoms in a cold atomic fountain.
Atoms in a collimated thermal beam are first cooled and captured in a
magneto-optic trap (MOT) operating on the strongly-allowed transition at 399~nm (blue line). They are then transferred to a MOT
on the weakly-allowed transition at 556~nm (green
line). Cold atoms from the green MOT are launched against gravity at a velocity
of around 2.5~m/s using a pair of green beams. We trap more than atoms
in the blue MOT and transfer up to 70\% into the green MOT. The temperature for
the odd isotope, Yb, is 1~mK in the blue MOT, and reduces by a
factor of 40 in the green MOT.Comment: 6 pages, 7 figure
Primary Blast Traumatic Brain Injury in the Rat: Relating Diffusion Tensor Imaging and Behavior
The incidence of traumatic brain injury (TBI) among military personnel is at its highest point in U.S. history. Experimental animal models of blast have provided a wealth of insight into blast injury. The mechanisms of neurotrauma caused by blast, however, are still under debate. Specifically, it is unclear whether the blast shockwave in the absence of head motion is sufficient to induce brain trauma. In this study, the consequences of blast injury were investigated in a rat model of primary blast TBI. Animals were exposed to blast shockwaves with peak reflected overpressures of either 100 or 450 kPa (39 and 110 kPa incident pressure, respectively) and subsequently underwent a battery of behavioral tests. Diffusion tensor imaging (DTI), a promising method to detect blast injury in humans, was performed on fixed brains to detect and visualize the spatial dependence of blast injury. Blast TBI caused significant deficits in memory function as evidenced by the Morris Water Maze, but limited emotional deficits as evidenced by the Open Field Test and Elevated Plus Maze. Fractional anisotropy, a metric derived from DTI, revealed significant brain abnormalities in blast-exposed animals. A significant relationship between memory deficits and brain microstructure was evident in the hippocampus, consistent with its role in memory function. The results provide fundamental insight into the neurological consequences of blast TBI, including the evolution of injury during the sub-acute phase and the spatially dependent pattern of injury. The relationship between memory dysfunction and microstructural brain abnormalities may provide insight into the persistent cognitive difficulties experienced by soldiers exposed to blast neurotrauma and may be important to guide therapeutic and rehabilitative efforts
Ab initio Wannier-function-based correlated calculations of Born effective charges of crystalline LiO and LiCl
In this paper we have used our recently developed ab initio
Wannier-function-based methodology to perform extensive Hartree-Fock and
correlated calculations on LiO and LiCl to compute their Born effective
charges. Results thus obtained are in very good agreement with the experiments.
In particular, for the case of LiO, we resolve a controversy originating
in the experiment of Osaka and Shindo {[}Solid State Commun. 51 (1984) 421] who
had predicted the effective charge of Li ions to be in the range 0.58--0.61, a
value much smaller compared to its nominal value of unity, thereby, suggesting
that the bonding in the material could be partially covalent. We demonstrate
that effective charge computed by Osaka and Shindo is the Szigeti charge, and
once the Born charge is computed, it is in excellent agreement with our
computed value. Mulliken population analysis of LiO also confirms ionic
nature of the bonding in the substance.Comment: 11 pages, 1 figure. To appear in Phys. Rev. B (Feb 2008
Modeling Two-Phase Flow and Vapor Cycles Using the Generalized Fluid System Simulation Program
This work presents three new applications for the general purpose fluid network solver code GFSSP developed at NASA's Marshall Space Flight Center: (1) cooling tower, (2) vapor-compression refrigeration system, and (3) vapor-expansion power generation system. These systems are widely used across engineering disciplines in a variety of energy systems, and these models expand the capabilities and the use of GFSSP to include fluids and features that are not part of its present set of provided examples. GFSSP provides pressure, temperature, and species concentrations at designated locations, or nodes, within a fluid network based on a finite volume formulation of thermodynamics and conservation laws. This paper describes the theoretical basis for the construction of the models, their implementation in the current GFSSP modeling system, and a brief evaluation of the usefulness of the model results, as well as their applicability toward a broader spectrum of analytical problems in both university teaching and engineering research
Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study
Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which
has been studied during the last years for its efficient photo-luminescence. In
contrast, the molecular geometry, providing the basis for the electronic and
optical properties, has been hardly investigated. In this paper, we apply a
density-functional-theory based molecular-dynamics approach to reveal the
molecular structure of PDPA in detail. We find that oligomers of this material
are limited in length, being stable only up to eight repeat units, while the
polymer is energetically unfavorable. These facts, which are in excellent
agreement with experimental findings, are explained through a detailed analysis
of the bond lengths. A consequence of the latter is the appearance of
pronounced torsion angles of the phenyl rings with respect to the plane of the
polyene backbone, ranging from up to . We point out
that such large torsion angles do not destroy the conjugation of the
electrons from the backbone to the side phenyl rings, as is evident from the
electronic charge density.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
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