Physics Commons^™

Surface Core-Level Shifts And Atomic Coordination At A Stepped W(110) Surface, D. Mark Riffe, B. Kim, J. L. Erskine, N. D. Shinn

All Physics Faculty Publications

Core-level 4f_7/2 photoemission spectra have been measured from a single, bifacial W crystal, which has both a flat W(110) and a vicinal, stepped W(110) [W(320)] surface. This procedure reduces uncertainties in the quantitative description of peaks in the spectra from W(320). Various analyses, including nonlinear least-squares curve fitting, show that the average surface core-level shift (SCS) for W(320) is only ∼-140 meV, compared to -310 meV for W(110) and that, at a maximum, only two of five terrace rows are isoelectronic to W(110) surface atoms. The absence of a large SCS for the step-edge atoms contradicts earlier interpretations of …

Time-Resolved Electron Temperature Measurement In A Highly-Excited Gold Target Usingfemtosecond Thermionic Emission, W. Y. Wang, D. Mark Riffe, Y. S. Lee, M. C. Downer

All Physics Faculty Publications

We report direct measurement of hot-electron temperatures and relaxation dynamics for peak electron temperatures between 3400 and 11 000 K utilizing two-pulse-correlation femtosecond (fs) thermionic emission. The fast relaxation times (<1.5 ps) are described by extending RT characterizations of the thermal conductivity, electron-phonon coupling, and electronic specific heat to these high electron temperatures.

Nanometer Scale Patterning And Oxidation Of Silicon Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, G. C. Abeln, T. -C. Shen, C. Wang, J. R. Tucker

T. -C. Shen

Nanoscale patterning of the Si(100)‐2×1 monohydride surface has been achieved by using an ultrahigh vacuum (UHV) scanning tunneling microscope(STM) to selectively desorb the hydrogen passivation. Hydrogen passivation on silicon represents one of the simplest possible resist systems for nanolithography experiments. After preparing high quality H‐passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(100)‐2×1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and the electron current for low …

Asymmetric Adsorbate And Substrate Interactions In Physisorbed Systems: N2 On Graphite And Dipolar Molecules On Ionic Substrates, Teresa Ellen Burns

All Graduate Theses and Dissertations, Spring 1920 to Summer 2023

Asymmetries in physisorbed systems give rise to interesting phases and phase transitions in two-dimensional (2D) monolayer and multilayer systems. The effects of asymmetric adsorbate and substrate interactions in monolayers of dipolar molecules on ionic substrates and N₂ on graphite are studied.

In the case of dipolar molecules on ionic substrates, 2D dielectric phase transitions using a modified Blume-Emery-Griffiths (BEG) model are determined theoretically. A dipole adsorbed vertically above a metal ion lattice site, and pointing up (down), is assigned a spin S=+1 (S=-1). An empty lattice site is assigned a spin S=0. Analytic solutions for both ferroelectrically and antiferroelectrically …

Convection In Chemical Waves, D. A. Vasquez, J. M. Littley, J. W. Wilder, Boyd F. Edwards

All Physics Faculty Publications

We present a theory for the vertical propagation of chemical waves near the onset of convection. Fluid motion, coupled to a standard reaction-diffusion mechanism for chemical wave propagation, determines the speed and shape of the reaction front in a two-dimensional slab. Our model is compared with experiments in capillary tubes. For tilted and horizontal tubes, fluid motion is always present with a corresponding increase in front speed.

A Monte Carlo Simulation Of Coulomb Collisions And Wave-Particle Interactions In Space Plasma At High Latitudes, Imad Ahmad Barghouthi

All Graduate Theses and Dissertations, Spring 1920 to Summer 2023

Four studies were considered to simulate the ion behavior in the auroral region and the polar wind.

In study I, a Monte Carlo simulation was used to investigate the behavior of O⁺ ions that are E x B-drifting through a background of neutral O, with the effect of O⁺(Coulomb) self-collisions included. Wide ranges of the ion-to-neutral density ratio n_i / n_n and electrostatic field E were considered in order to investigate the change of ion behavior with respect to the solar cycle and altitude. For low altitudes and/or solar minimum (n_i / …

A Study Of Magnetic Activity Effects On The Thermospheric Winds In The Low Latitude Ionosphere, Ricardo Cruz Davila

All Graduate Theses and Dissertations, Spring 1920 to Summer 2023

The purpose of this thesis is to examine the effects of magnetic activity on the low latitude F-region thermospheric winds. The F-region (120-1600 km) is a partially ionized medium where O⁺ and O are the major ion and neutral species, respectively. The thermospheric winds at these altitudes are driven primarily by pressure gradient forces resulting from the solar heating during the day and cooling at night. For this study, we use measured Fabry-Perot Interferometer (FPD winds at Arequipa (16.5°S, 71.5°W) and measured FPI and Incoherent Scatter Radar (ISR) winds at Arecibo (18.6°N, 66.8°W).

Previous wind studies at Arequipa and …

Surface Core-Level Shifts Of Ta(110) And W(110) Vs. Alkali-Atom Coverage: Implications For Thealkali-Substrate Interaction, A. B. Andrews, D. Mark Riffe, G. K. Wertheim

All Physics Faculty Publications

The change in the average surface-atom core-level shift (SCS) produced by submonolayer coverages of alkali adsorbates is quite small for both Ta(110) and W(110), less than 45 meV in magnitude. The small change of the measured SCS’s as a function of coverage decisively supports the covalent-bonding picture of alkali adsorption proposed by Ishida and Terakura.

Nature Of The Charge Localized Between Alkali Adatoms And Metal Substrates, G. K. Wertheim, D. Mark Riffe, P. H. Citrin

All Physics Faculty Publications

Two previously unappreciated features in photoemission spectra from alkali atoms adsorbed on W(110), namely, the sign of the alkali-induced surface-atom core-level shift of the substrate at low coverage and the very large alkali shallow core-hole lifetime width at all coverages, show that the alkali-substrate interaction is not well described by a transfer of alkali charge. Instead, both features point to the formation of a charge cloud between the alkali adatom and substrate that is derived largely from alkali valence states.

Anharmonic Surface Vibrations In Photoemission From Alkali Metals, G. K. Wertheim, D. Mark Riffe, P. H. Citrin

All Physics Faculty Publications

The phonon widths of outermost core-electron photoemission spectra from (110)-oriented overlayers of Na, K, and Rb metals show the expected Debye behavior for the bulk atoms, but significant deviations for the surface atoms. The data indicate a softening of the surface vibrational mode above 200 K. This effect, which is weak in Na but strong in K and Rb, demonstrates that the vibrational mode normal to the surface is anharmonic.

Comparison Between Calculatedand Observed F-Region Density Profiles At Jicamarca, Peru, A. J. Preble, D. N. Anderson, Bela G. Fejer, P. H. Doherty

Bela G. Fejer

Electron density profiles and isodensity contours derived from Jicamarca incoherent scatter radar observations in Peru for October 1–2, 1970, are compared in detail with results from the Phillips Laboratory global theoretical ionospheric model. This model solves the ion continuity equation for O+ concentration through production, loss, and transport of ionization. The primary factor controlling the peak plasma density at Jicamarca is the vertical E×B drift, which drives the ionization upward during the day and downward at night. When we use the measured drift in the model, we achieve excellent results with the measured electron density profiles. We illustrate the sensitivity …

Nanoscale Patterning And Oxidation Of H-Passivated Si(100)-2x1 Surfaces With An Ultrahigh Vacuum Scanning Tunneling Microscope, J. W. Lyding, T. -C. Shen, J. S. Hubaceck, J. R. Tucker, G. C. Abeln

T. -C. Shen

Nanoscale patterning of the hydrogen terminated Si(100)‐2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope.Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H‐passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.

Ionospheric Response To The Sustained High Geomagnetic Activity During The March '89 Great Storm, Jan Josef Sojka, Robert W. Schunk, W. F. Denig

All Physics Faculty Publications

A simulation was conducted to model the high-latitude ionospheric response to the sustained level of high geomagnetic activity for the great magnetic storm period of March 13-14, 1989. The geomagnetic and solar activity indices and the DMSP F8 and F9 satellite data for particle precipitation and high-latitude convection were used as inputs to a time-dependent ionospheric model (TDIM). The results of the TDIM were compared to both DMSP plasma density data and ground-based total electron content (TEC) measurements for the great storm period as well as with earlier storm observations. The comparisons showed that the overall structure of the high-latitude …

Efficient Timestamp Input And Output, Curtis Dyreson, Richard T. Snodgrass

Curtis Dyreson

In this paper we provide efficient algorithms for converting between the internal form of a timestamp, and various external forms, principally character strings specifying Gregorian dates. We give several algorithms that explore a range of time and space tradeoffs. Unlike previous algorithms that explore a range of time and space tradeoffs. Unlike previous algorithms, those discussed here have a constant time cost over a greatly extended range of timestamp values. These algorithms are especially useful in operating systems and in database management systems.

Modelling Sun-Aligned Polar Cap Arcs, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, Lie Zhu

All Physics Faculty Publications

We present results of a new model of the time-dependent ionospheric response to a generalized steady state Sun-aligned (SA) arc structure. The thermal and plasma structure of a “prototype” arc is compared to the general features of observed SA arcs. We find that the general features of electron density, N_e , electron temperature, T_e , and ion temperature, T_i , are determined by the distribution of the particle precipitation and E × B convection associated with the SA arc. The model results are extended to predict the possible variation of N_e, T_e , and …

Ionospheric Response To Traveling Convection Twin Vortices, Robert W. Schunk, Lie Zhu, Jan Josef Sojka

All Physics Faculty Publications

Traveling convection twin vortices have been observed for several years. At ionospheric altitudes, the twin vortices correspond to spatially localized, transient structures embedded in a large‐scale background convection pattern. The convection vortices are typically observed in the morning and evening regions. They are aligned predominantly in the east‐west direction and have a horizontal extent of from 500–1000 km. Associated with the twin vortices are enhanced electric fields, particle precipitation, and an upward/downward field‐aligned current pair. Once formed, the twin vortex structures propagate in the tailward direction at speeds of several km/s, but they weaken as they propagate and only last …

Magnetospheric Shortcomings In Ionospheric-Magnetospheric Coupling: An Ionospheric Perspective, Jan Josef Sojka

All Physics Faculty Publications

The ionosphere, on a global scale, is reasonably well understood from a climatology perspective. However, the storm dynamics of the ionosphere are not fully understood. This partly arises from the complex response function of the Thermosphere-Ionosphere(T-I) system but also from the uncertainty in the space and time dynamics of the magnetospheric inputs to the ionosphere. In the context of M-I coupling, the ionosphere responds to magnetospheric electrodynamic forcing by altering the conductivity in the ionosphere and by plasma transport. Phenomenologically,we understand how to let the ionospheric conductivity evolve in response to local precipitation and how to transport plasma in the …

Patches In The Polar Ionosphere: Ut And Seasonal Dependence, Jan Josef Sojka, M. D. Bowline, Robert W. Schunk

All Physics Faculty Publications

The seasonal and UT dependencies of patches in the polar ionosphere are simulated using the Utah State University time dependent ionospheric model (TDIM). Patch formation is achieved by changing the plasma convection pattern in response to temporal changes in the interplanetary magnetic field (IMF) B_y component during periods of southward IMF. This mechanism redirects the plasma flow from the dayside high-density region, which is the source of the tongue of ionization (TOI) density feature, through the throat and leads to patches, rather than a continuous TOI. The model predicts that the patches are absent at winter solstice (northern hemisphere) …

Effect Of High Latitude Ionospheric Convection On Sun-Aligned Polar Caps, Jan Josef Sojka, Lie Zhu, D. J. Crain, Robert W. Schunk

All Physics Faculty Publications

A coupled magnetospheric-ionospheric (M-I) MHD model has been used to simulate the formation of Sun-aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fields. The formation process involves launching an Alfvén shear wave from the magnetosphere to the ionosphere where the ionospheric conductance can react self-consistently to changes in the upward currents. We assume that the initial Alfvén shear wave is the result of solar wind-magnetosphere interactions. The simulations show how the E region density is affected by the changes in the electron precipitation that are associated with the upward currents. These changes …

Model Study Of Multiple Polar Cap Arcs: Occurrence And Spacing, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain

All Physics Faculty Publications

A new scenario for the formation of multiple polar cap arcs is proposed based on the results from a time‐dependent electrodynamic model of polar cap arcs developed by Zhu et al. [1993]. The results suggest that the appearance of multiple polar cap arcs may not be due to multiple structures in the magnetospheric source region, but instead, may primarily be determined by the coupled magnetosphere‐ionosphere system in which the ionosphere plays an active role. It was found that with the same magnetospheric driver, a strong ionospheric background convection and an ionospheric background Hall conductance in the range of from 0.5 …

Theoretical Study Of Polar Cap Arcs: Time-Dependent Model And Its Applications, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain

All Physics Faculty Publications

A time-dependent theoretical model of polar cap arcs developed during the Coupling, Energetics, and Dynamics of Atmospheric Regions/High-Latitude Plasma Structures (CEDAR/HLPS) campaigns in the past two years is briefly described. In the model the electrodynamics of the polar cap arcs are treated self-consistently in the frame of the coupled magnetosphere-ionosphere system. The preliminary simulation results of the temporal evolution and spatial structure of the polar cap arcs for both winter and summer conditions are presented. The model can be used to conduct both the model-observation study of specific features of the polar cap arcs and the quantitative theoretical study of …

Photoemission Measurement Of Equilibrium Segregation At Gesi Surfaces, J. E. Rowe, D. Mark Riffe, G. K. Wertheim, J. C. Bean

All Physics Faculty Publications

Photoemission spectroscopy is used to demonstrate that Ge segregates to the first atomic layer of Ge_0.5Si_0.5(100)2×1 and that the second layer is predominantly Si. Comparison of the resolved signals from the dimer atoms of the reconstructed (100)2×1 surfaces of Ge, Si, and equiatomic Ge‐Si alloy shows that the surface layer of the alloy is extremely Ge rich and the second layer is occupied mainly by Si atoms. This result is in good agreement with theoretical predictions.