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1989

All Physics Faculty Publications

Articles 1 - 13 of 13

Full-Text Articles in Physical Sciences and Mathematics

Different Core-Hole Lifetime And Screening In The Surface Of W(110), D. Mark Riffe, G. K. Wertheim, P. H. Citrin Oct 1989

Different Core-Hole Lifetime And Screening In The Surface Of W(110), D. Mark Riffe, G. K. Wertheim, P. H. Citrin

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High-resolution 4f photoemission spectra from clean W(110) show that the natural lifetime width and the (electron-hole)-pair singularity index are both larger in the first atomic layer than in the bulk. Phonon broadening for the surface and bulk components are smaller than theoretical estimates, and little excess broadening is detected in the surface layer. These findings are very different from the conventional picture of surface-atom core-level line shapes and have implications extending to other systems.

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Layer-By-Layer Growth Of Solid Argon Films On Graphite As Studied By Neutron Diffraction, J. Z. Larese, Q. M. Zhang, L. Passell, J. M. Hastings, John R. Dennison, H. Taub Sep 1989

Layer-By-Layer Growth Of Solid Argon Films On Graphite As Studied By Neutron Diffraction, J. Z. Larese, Q. M. Zhang, L. Passell, J. M. Hastings, John R. Dennison, H. Taub

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The layer-by-layer growth of solid argon films on graphite at T=10 K is studied using elastic neutron diffraction. The growth is characterized by individual layers with commensurate in-plane lattice constants. As the coverage is increased beyond two layers, evidence of the coexistence of ABC and ABA stacking is apparent, with the ABC sequence dominating as the film thickens. A continuous decrease in the Debye-Waller factor also occurs as the film thickness grows, indicating a crossover from two-dimensional to three-dimensional behavior. As the coverage is increased beyond about four nominal layers, there is evidence of bulk crystallite formation. The diffraction results …

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Multi-Layer Structure Of Nitrogen Adsorbed On Graphite, S. K. Wang, J. C. Newton, R. Wang, H. Taub, John R. Dennison, H. Shechter May 1989

Multi-Layer Structure Of Nitrogen Adsorbed On Graphite, S. K. Wang, J. C. Newton, R. Wang, H. Taub, John R. Dennison, H. Shechter

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Elastic neutron diffraction has been used to study the structure and layering of nitrogen films adsorbed on the (002) surfaces of an exfoliated graphite substrate. The neutron-diffraction pattern of the fully compressed monolayer at a coverage Θ=1.67 layers and a temperature ≲11 K which we reported earlier has been reanalyzed (unity coverage corresponds to a complete layer having the commensurate √3 × √3 structure). We now find it to be consistent with a four-sublattice pinwheel structure as well as the two-sublattice herringbone structure which we found previously. Below 11 K, we infer crystallization of the bilayer at Θ between 2.6 …

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Mapping The Wind In The Polar Thermosphere: A Case Study Within The Cedar Program, R W. Smith, J W. Meriwether, G Hernandez, R David, Vincent B. Wickwar, O De La Beaujardiere, T L. Killeen Mar 1989

Mapping The Wind In The Polar Thermosphere: A Case Study Within The Cedar Program, R W. Smith, J W. Meriwether, G Hernandez, R David, Vincent B. Wickwar, O De La Beaujardiere, T L. Killeen

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The thermosphere is that region of neutral atmosphere in which atmospheric constituents are gravitationally bound to the Earth but are barometrically distributed according to their molecular or atomic weights. Unlike the lower atmosphere, mixing processes a reweak, which allows each constituent gas to behave independently. The thermosphere begins at about 100-km-altitude and extends up to 500 km or beyond. The temperature increases with height throughout the layer, which is a stabilizing influence.

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Nature Of Carbon-Carbon Bonding In Evaporated And Ion-Sputtered (Diamondlike) Amorphous Carbon From (E, 2e) Spectroscopy, Chao Gao, Yun Yu Wang, A. L. Ritter, John R. Dennison Feb 1989

Nature Of Carbon-Carbon Bonding In Evaporated And Ion-Sputtered (Diamondlike) Amorphous Carbon From (E, 2e) Spectroscopy, Chao Gao, Yun Yu Wang, A. L. Ritter, John R. Dennison

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The local carbon-carbon bonding in evaporated (e-C) and ion-sputtered (i-C) amorphous carbon has been investigated by transmission electron-energy-loss spectroscopy (EELS) and by (e, 2e) spectroscopy. The EELS data indicate that the e-C is graphiclike and the i-C is diamondlike. The (e, 2e) data demonstrate that the concentration of sp2 bonding is ∼100% in both materials (0.85≤csp2≤1.0) for e-C and 0.75≤csp2≤1.0 for i-C). There is significant rehybridization of the π orbital in i-C.

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World Sheet Diffeomorphisms And The Canonical String, Charles G. Torre Jan 1989

World Sheet Diffeomorphisms And The Canonical String, Charles G. Torre

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The degree to which the phase space of a closed bosonic string carries a representation of the two‐dimensional diffeomorphism group Diff(M) is investigated. In particular, homomorphic mappings from the associated Lie algebra diff(M) into the Poisson algebra of functions on three natural phase spaces associated with the string are constructed. Two of these spaces are extended phase spaces based on the conformal and harmonic gauges, respectively. The third space is essentially the original phase space; the homomorphism in this case relies on the validity of the light‐cone gauge. Homomorphisms from diff(M) into the extended phase spaces of the Batalin–Fradkin–Vilkovisky formalism …

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Hamiltonian Formulation Of Induced Gravity In Two Dimensions, Charles G. Torre Jan 1989

Hamiltonian Formulation Of Induced Gravity In Two Dimensions, Charles G. Torre

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A Hamiltonian formulation of the theory of induced gravity in two dimensions is constructed. This formulation differs from previous efforts in that the formalism is covariant under all relevant transformation groups. In particular, spatial diffeomorphism covariance and/or invariance is manifest throughout; the phase space carries a representation of the Lie algebra of the spacetime diffeomorphism group; the group of conformal isometries is projectively represented on the phase space as a symmetry group. The key ingredient that allows covariance with respect to the above groups is the enlargement of the gravitational phase space by the inclusion of the cotangent bundle over …

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Global Scale, Physical Models Of The F Region Ionosphere, Jan Josef Sojka Jan 1989

Global Scale, Physical Models Of The F Region Ionosphere, Jan Josef Sojka

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During the last decade, ionospheric F region modeling has reached an accurate climatological level. We now have global computer models of the F region which simulate the interactions between physical processes in the ionosphere. Because of their complexity, these climatological models are confined to modern day supercomputers. This review focuses on the development and verification of these physical ionospheric models. Such models are distinct from local models, steady state models, and empirical models of the ionosphere, which are, by their conception, unable to represent physically the range of F region variability or storm dynamics. This review examines the limitations of …

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A Three-Dimensional Time-Dependent Model Of The Polar Wind, Robert W. Schunk, Jan Josef Sojka Jan 1989

A Three-Dimensional Time-Dependent Model Of The Polar Wind, Robert W. Schunk, Jan Josef Sojka

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A time-dependent global model of the polar wind was used to study transient polar wind perturbations during changing magnetospheric conditions. The model calculates three-dimensional distributions for the NO+, O2+, N2+, N+ and O+ densities and the ion and electron temperatures from diffusion and heat conduction equations at altitudes between 120 and 800 km. At altitudes above 500 km, the time-dependent nonlinear hydrodynamic equations for O+ and H+ are solved self-consistently with the ionospheric equations. The model takes account of supersonic ion outflow, shock formation, and ion energization during …

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Theoretical Study Of The Seasonal Behavior Of The Global Ionosphere At Solar Maximum, Jan Josef Sojka, Robert W. Schunk Jan 1989

Theoretical Study Of The Seasonal Behavior Of The Global Ionosphere At Solar Maximum, Jan Josef Sojka, Robert W. Schunk

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A time-dependent, three-dimensional, multi-ion numerical model of the global ionosphere was used to study the asymmetry in large-scale ionospheric features between the northern and southern hemispheres. The comparisons were done for June and December solstice conditions at solar maximum for quiet geomagnetic activity. Simple conditions and diurnally reproducible ionospheric features were established in order to elucidate the intrinsic hemispherical differences that are associated with the different displacements between the geomagnetic and geographic poles and the different atmospheric conditions. In comparing the ionospheric densities in the northern and southern hemispheres for a given season, we found the following: (1) The winter …

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Modeled F Region Response To Auroral Dynamics Based Upon Dynamics Explorer Auroral Observations, Jan Josef Sojka, Robert W. Schunk, J. D. Cravens, L. A. Frank, J. Sharber, J. D. Winningham Jan 1989

Modeled F Region Response To Auroral Dynamics Based Upon Dynamics Explorer Auroral Observations, Jan Josef Sojka, Robert W. Schunk, J. D. Cravens, L. A. Frank, J. Sharber, J. D. Winningham

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Auroral images from the Dynamics Explorer 1 (DE 1) scanning auroral imager have been combined with in situ auroral precipitation data from the DE 2 low altitude plasma instrument to form a time-dependent global auroral energy flux model. This model has both good time (12 min) and spatial (100 km) resolution compared to that currently available for global scale ionospheric and thermospheric modeling. The development and comparison of this model with others are discussed. Data from an aurorally active period, November 25, 1981, are presented and used as a case study for this model. Using a global ionospheric model, the …

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Ion Beam Mixing In Ag-Pd Alloys, J L. Klatt, R S. Averback, David Peak Jan 1989

Ion Beam Mixing In Ag-Pd Alloys, J L. Klatt, R S. Averback, David Peak

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Ion beam mixing during 750 keV Kr+ irradiation at 80 K was measured on a series of Ag‐Pd alloys using Au marker atoms. The mixing in pure Ag was the greatest and it decreased monotonically with increasing Pd content, being a factor of 10 higher in pure Ag than in pure Pd. This large difference in mixing cannot be explained by the difference in cohesion energy between Ag and Pd in the thermodynamic model of ion beam mixing proposed by Johnson et al. [W. L. Johnson, Y. T. Cheng, M. Van Rossum, and M‐A. Nicolet, Nucl. …

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Thermospheric Dynamics During September 18-19, 1984, 2, Validation Of The Ncar Thermospheric General Circulation Model, G Crowley, B A. Emery, R G. Roble, H C. Carlson Jr., J E. Salah, Vincent B. Wickwar, K L. Miller, W L. Oliver, R G. Burnside, F A. Marcos Jan 1989

Thermospheric Dynamics During September 18-19, 1984, 2, Validation Of The Ncar Thermospheric General Circulation Model, G Crowley, B A. Emery, R G. Roble, H C. Carlson Jr., J E. Salah, Vincent B. Wickwar, K L. Miller, W L. Oliver, R G. Burnside, F A. Marcos

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The validation of complex nonlinear numerical models such as the National Center for Atmospheric Research thermospheric general circulation model (NCAR TGCM) requires a detailed comparison of model predictions with data. The Equinox Transition Study (ETS) of September 17-24, 1984, provided a unique opportunity to address the verification of the NCAR TGCM, since unusually large quantities of high-quality thermospheric and ionospheric data were obtained during an intensive observation interval. In a companion paper (paper 1) by Crowley et al. (this issue) a simulation of the September 18-19 ETS interval was described. Using a novel approach to modeling, the TGCM inputs were …

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