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Differential And Total Cross Sections For Ionization Of Helium And Hydrogen By Electrons, M. Eugene Rudd

M. Eugene Rudd Publications

A comprehensive semiempirical model for singly and doubly differential and total ionization cross sections for electron production by electron impact is presented. The model is discussed in relation to several theoretically required constraints. Available experimental data for hydrogen and helium targets are examined, and recommended values of the data are given for all ranges of primary and secondary energies and angles of ejection.

Ultrafast Electronic Disordering During Femtosecond Laser Melting Of Gaas, Peter N. Saeta, J.-K. Wang, Y. Siegal, N. Bloembergen, E. Mazur

All HMC Faculty Publications and Research

We have observed an ultrarapid electronic phase transformation to a centrosymmetric electronic state during laser excitation of GaAs with intense femtosecond pulses. Reflection second-harmonic intensity from the upper 90 atomic layers vanishes within 100 fs; reflectivity rises within 0.5 ps to a steady value characteristic of a metallic molten phase, long before phonon emission can heat the lattice to the melting temperature.

Frequency Shifts Of Molecules At Rough Metal Surfaces, M. H. Hider, P.T. Leung

Physics Faculty Publications and Presentations

The frequency shifts for dipolar transitions of molecules at rough metallic surfaces are studied in a phenomenological model following the approach of the authors' previous work [Phys. Rev. B 36, 4664 (1987)], with the surface roughness modeled by a shallow grating profile in most cases. The main findings here are as follows: (1) such surface-induced shifts are indeed observable for molecular frequencies away from the surface-plasmon resonance frequency of the metal; (2) the presence of surface roughness can either enhance or suppress the flat-surface-induced shifts, leading to extra morphology-dependent resonances originated from the radiative coupling between the molecular emission and …

X-Ray Emission From Slow Highly Charged Ar Ions Interacting With A Ge Surface, Michael Schulz, C. Lewis Cocke, Siegbert Hagmann, Martin P. Stockli, Horst Schmidt-Bocking

Physics Faculty Research & Creative Works

We have measured K x-ray spectra and yields from Ar¹⁷⁺ ions slowly approaching a single-crystal Ge surface. The yields were measured as a function of the projectile velocity component perpendicular to the surface. From the data a characteristic time of approximately 1 psec was extracted for the in-flight filling above the surface of the Ar K vacancy.

Measurement Of The Ratio Of Double-To-Single Photoionization Of Helium At 2.8 Kev Using Synchrotron Radiation, Jon C. Levin, Dennis W. Lindle, N. Keller, R. D. Miller, Y. Azuma, N. Berrah Mansour, H. G. Berry, Ivan A. Sellin

Chemistry and Biochemistry Faculty Research

We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He⁺⁺/He⁺=1.6±0.3% at hν=2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Åberg, who predict 1.7%.

Anisotropy Of Polarized X-Ray Emission From Molecules, S. H. Southworth, Dennis W. Lindle, R. Mayer, P. L. Cowan

Chemistry and Biochemistry Faculty Research

Strongly anisotropic, polarized Cl K-V x-ray emission from gas-phase CF₃Cl has been observed following resonant excitation with a linearly polarized x-ray beam. Distinctively different angular distributions are observed for x-ray emission involving molecular orbitals of different symmetries. A classical model of the x-ray absorption-emission process accurately describes the observed radiation patterns.

Inhibited Spontaneous Emission By Photonic Band Gaps In A Square Lattice Of Periodic Dielectric Medium, Abdullah Al-Ghamdi

Masters Theses

The concepts of band theory for electrons can be employed to describe the behavior of electromagnetic waves propagating in two-dimensional, periodic dielectric structures. These two-dimensional, periodic structures can produce photonic band gaps in which the spontaneous emission by atoms embedded in the structure is prohibited and as a result the performance of many semiconductor devices can be enhanced. The calculations are based on finding the eigenvalues of the algebraic equation for the frequencies of electromagnetic waves moving in a square lattice composed of square dielectric rods with different dielectric constants.

Crystal Field Splitting And Charge Flow In The Buckled-Dimer Reconstruction Of Si(100)—2× 1, G. K. Wertheim, D. Mark Riffe, J. E. Rowe, P. H. Citrin

All Physics Faculty Publications

The effect of the 2×1 reconstruction on the core-electron binding energies of the outermost Si(100) layers has been determined using high-resolution photoemission data. A previously unobserved 190-meV crystal-field splitting is resolved for the up-atoms of the asymmetric surface dimers, whose average core-level shift is -400 meV. The signal from the down-atoms is clearly identified and has a shift of +220 meV. These new findings indicate a charge flow of ∼0.05e from the subsurface to the surface layers, with a substantially larger difference of ∼0.34e between the up-atoms and down-atoms in the dimer.

Infrared Quenching And Thermal Recovery Of Thermally Stimulated Current Spectra In Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

Thermally stimulated current (TSC) spectra stimulated by infrared (hν≤1.12 eV) light at 90 K have been used to study the photoquenching and thermal recovery of several dominant TSC peaks in Ga‐rich, semi‐insulating GaAs. The filling‐pulse‐length dependence of the quenching for these traps, and the temperature dependence of their recovery are clearly identified with the ground‐to‐metastable state transition of EL2. The data are consistent with the traps having a direct association with EL2 or EL2∗ rather than an indirect association which could result from a change in the dominant‐free carriers as EL2 transforms to EL2∗. If so, they likely …

On Shock Capturing For Liquid And Gas Media, Tze Jang Chen

Mathematics & Statistics Theses & Dissertations

The numerical investigation of shock phenomena in gas or liquid media where a specifying relation for internal energy is absent poses special problems. Classically, for gas dynamics the usual procedure is to employ a splitting scheme to remove the source terms from the Euler equations, then up-wind biased shock capturing algorithms are built around the Riemann problem for the system which remains. However, in the case where the Euler equations are formulated in the term of total enthalpy, a technical difficulty associated with equation splitting forces a pressure time derivative to be treated as a source term. This makes it …

Dynamics Of The Nematic-Electroclinic Effect, Rajesh Ambigapathy, Rolfe G. Petschek, Charles Rosenblatt

Faculty Scholarship

The effective response time eff of the nematic-electroclinic effect was determined as a function of driving frequency and temperature. Near the nematicsmectic-A transition temperature TNSm-A, eff was found to be a function of driving frequency, indicating the existence of more than one physical process. Several degrees above TNSm-A, eff was found to be frequency independent up to 100 kHz. At these temperatures, moreover, the effective response times are quite small, of order 100 ns.

Computer Simulations Of Quasilattice Models For Novel Ferroelectric Liquid Crystals, Rolfe G. Petschek

Faculty Scholarship

It has been suggested that ferroelectric smectic phases may be formed by fraternal-twin liquid-crystal molecules, consisting of two very different mesogens bonded together by an appropriate spacer [R. G. Petschek and K. M. Wiefling, Phys. Rev. Lett. 59, 343 (1987)]. We discuss the range in which these ferroelectric phases appear in a Monte Carlo simulation of a simple computer model for such mesogens. The mesogens consist of two identical flexible segments that are bonded to two rigid segments with very different lengths which, in turn, are bonded together by a flexible segment. In the computer model these monomers can move …

On-Wafer Hall-Effect Measurement System, P. D. Mumford, David C. Look

Physics Faculty Publications

A novel system capable of making on‐wafer Hall‐effect measurements of a patterned wafer during the fabrication sequence has been developed. A flat, powerful rare‐earth magnet provides the magnetic field required. The wafer need only have van der Pauw patterns available for on‐wafer measurement capability. Measurement of room temperature Hall mobility can quickly and easily be obtained, making possible detailed study of carrier concentration and mobility variations during wafer fabrication.

Comparison Of Deep Centers In Semiinsulating Liquid-Encapsulated Czochralski And Vertical-Gradient Freeze Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

Three‐inch, semi‐insulating (SI) GaAs, grown by the vertical gradient freeze (VGF) technique, has been studied by IR absorption, temperature‐dependent dark current and Hall‐effect, thermally stimulated current (TSC), and photoinduced current transient spectroscopy and has been compared with undoped, SI GaAs, both As‐rich and Ga‐rich, grown by the high‐pressure liquid‐encapsulated Czochralski method. The results clearly indicate that (1) the VGF GaAs contains less EL2, which suggests a less As‐rich crystal stoichiometry; (2) in some VGF samples activation energies of 0.43 or 0.46 eV are deduced from temperature‐dependent carrier concentration or resistivity measurements, respectively, and (3) VGF samples often show a thermal …

Double Ionization Of Helium By Intermediate To High Velocity He+ Projectiles, Jun Forest

Masters Theses

Ionization of helium target atoms by He⁺ projectiles colliding with helium at energies of 0.125 to 3.0 MeV/u is investigated. Cross sections for single and double ionization, and ratios o f double-to-single ionization are determined for each outgoing projectile charge state, and are compared to previous studies. For the lowest energies investigated, the ratios are consistent with the two-step mechanism in which the projectile interacts separately with each target electron. At the highest energies, the ratios reach nearly constant values indicating approach to the high velocity limit. At these high energies, however, the ratios are all higher than the …

A Valence Quark Model, Tony A. Forest

Masters Theses

A quark model of the baryons has been developed for calculating properties of nucleons which can be useful in nuclear interactions. A Hamiltonian, motivated by QCD, was diagonalized in a harmonic oscillator basis. In addition to predicting the proton charge radius and Delta-Nucleon mass difference, this work also yields a value of the axial vector coupling constant which compares well with experiment. Electric and magnetic form factors represent the major success of this model. The electromagnetic transitions predicted by this model compete well with other models but are not able to reproduce experimental results completely. The advantage of this model …

Resonant Electron Transfer And K-Shell Excitation Of FQ+ (Q=6,8) In Collisions With Neutral He And H2 Targets, Konstantinos E. Zaharakis

Masters Theses

Measurements of resonant transfer and excitation (RTE) were conducted for 16.5-38 MeV F⁶⁺ (Li-like) ions colliding with H₂. In the RTE process electron capture and projectile excitation take place simultaneously due to the electronelectron interaction. In previous work, for the same collision system, at the Oak Ridge National Laboratory (Oak Ridge, TN) , the magnitude of the measured RTE cross sections was found to be nearly a factor of two smaller than theory. In order to investigate RTE in F⁶⁺ H₂ collisions more completely, we undertook measurements at Western Michigan University using the EN tandem …

Fock-Tani Hamiltonian For Reactions Involving Two-Electron Atoms, Jack C. Straton

Physics Faculty Publications and Presentations

The Fock-Tani Hamiltonian is found for scattering processes involving up to two ions and two electrons. Possible bound-state species include one or two electrons bound on an ion fixed at the origin, and a one-electron projectile atom. A diagrammatic technique is illustrated that simplifies the algebra of the transformation. Coulomb- or plane-wave states are automatically generated by the same asymptotic Hamiltonian for all arrangement channels.

Abnormal Behaviour Of Zero Degree Δ-Electron Emission On The Projectile Ionic Charge, O. Jagutzkit, S. Hagtnanni, H. Schmidt-Bockingt, Ronald E. Olson, D. R. Schultz, R. Dornert, R. Kocht, A. Skutlartz, A. González, T. B. Quinterosi, C. Kelbcht, P. Richardt

Physics Faculty Research & Creative Works

The 0° δ-electron emission was investigated by an electron-projectile coincidence technique as a function of incoming and outgoing projectile charge state for 0.53 MeV u^-1Cu^q+ on He. The electron emission spectra vary strongly with initial and final projectile charge state. For pure ionization channels the cross sections follow for low electron energy the q²-scaling law whereas in the binary encounter regime the scaling is reversed. Ctmc calculations are in fair agreement with the experimental data. © 1991 IOP Publishing Ltd.

Population Of Highly Excited Intermediate Resonance States By Electron Transfer And Excitation, Reinhold S. Schuch, Edson L B Justiniano, Michael Schulz, Sheldon Datz, Peter F. Dittner, J. P. Giese, Herbert F. Krause, Harald Schone, Charles Randy Vane, Stephen M. Shafroth

Physics Faculty Research & Creative Works

Coincidences between two sulfur K x rays were detected from collisions of hydrogenlike S ions with H₂ gas in the projectile energy range between 150 and 225 MeV. These K x rays are emitted in the decay of doubly excited states formed in the collisions via transfer and excitation. The excitation function for two coincident Kβ transitions peaks at about 175 MeV, slightly above the expected KMM resonance energy for resonant transfer and excitation (RTE). This demonstrates the occurrence of ΔN≥2 transitions (i.e., KMM and higher resonances) in the RTE process. The cross sections for the population of the …

Quenching And Rotational Energy Transfer In Molecular Bromine: Br2(B), Dean A. Massman

Theses and Dissertations

This study investigates the rotational energy transfer processes of optically excited bromine molecules in the presence of several collision partners. Resolved laser induced fluorescence spectra of collisionally populated rotational states were analyzed by means of Stern Volmer analysis. Total rotational removal rate coefficients and state-to-state rotational removal rate coefficients for the v'=11,J'=35 level of Br₂(B) were obtained for collisions with Br₂(X), He, Ar and Xe. Rotational removal rate coefficients were also obtained for the v'11, J'=26 and 47; v'=14, J'=36 states of Br₂(B) with collisions with Ar. Total rotational removal rate for Br_{2 …}

Aerosol Particle Bounding In Cascade Impactors, Bisma Husen

Opportunities for Undergraduate Research Experience Program (OURE)

Particle deposition characteristics were observed from the aerosol sampling results of a UNICO cascade impactor. A simple model of particle motion combined with Hiemenz flow of stagnation flow on a flat plate were developed and discussed.

Both results and analysis were compared to recent models on particle bouncing errors or secondary deposition which included vortices formation around the edge of the slot, back spin (Magnus effect), and hydrodynamic conditions of the particles. The analysis was limited to a laminar inviscid and incompressible case. The main obstacle in the observation was the fact that impactor nozzles were unsymmetrical to a considerable …

Fourier Transform Infrared (Ftir) Spectroscopy Analysis Of Combustion Aerosol, Gary Jacquin

Opportunities for Undergraduate Research Experience Program (OURE)

In conjunction with my co-op program at McDonnell-Douglas, my current OURE project is the analysis of combustion aerosols using Fourier Transform Infrared (FTIR) Spectroscopy. Aerosol science is a very important field as it is concerned with atmospheric chemistry, atmospheric physics, climate, public health, and environmental issues.

In the Cloud and Aerosol Sciences Laboratory (CASL) at UMR, combustion aerosols are generated by an in-house designed torch or burner. The fuels, JP4 and JP5, are provided by McDonnell-Douglas. Some other commercial fuels are also used. The primary objective of my OURE project was to generate and collect combustion aerosols for chemical analysis. …

Nonlinear Dynamics And The Lorenz Water Wheel, Richard Margis

Opportunities for Undergraduate Research Experience Program (OURE)

Undoubtedly one of the most exciting frontiers in physics today is the study of nonlinear systems, more popularly called chaos. Chaos is the key to many time-dependent processes in as varied fields as meteorology (such as weather modeling), mechanics (the three-body problem in gravitational fields), and economics (prediction of trends in the stock market).

Process For Scanning Photographic Slides Of Laser Light Scattering With A Transmission Densitometer, Don Newburry

Opportunities for Undergraduate Research Experience Program (OURE)

Non-symmetric variations in photographic slides of polarized laser light scattering experimental results are difficult to see. This paper explains a process developed for measuring the light density at points throughout the slide area using a transmission densitometer, so an accurate description of the laser light scattering may be obtained. A slide was tested using the described process and the results are presented. The results turned out well, but there were some errors introduced by the instability of the densitometer and by human error in reading the chart recorder output.

Synchrotron-Radiation-Induced Deposition Of Boron And Boron Carbide Films From Boranes And Carboranes: Decaborane, F. Keith Perkins, R.A. Rosenberg, Sunwoo Lee, Peter A. Dowben

Peter Dowben Publications

Boron has been deposited successfully on Si(111) from the synchrotron-radiation-induced decomposition of decaborane (14), i.e., B₁₀H₁₄. The rate of deposition is limited by the adsorption rate of decaborane (14) on the surface. In addition there is some indication that there is an activation barrier to dissociative adsorption. The synchrotron-radiation- induced growth rate of boron thin films from decaborane (14) is linear with coverage for a large range of thickness, suggesting a constant sticking coefficient for decaborane adsorption at room temperature.

Molecular-Orbital Studies Via Satellite-Free X-Ray Fluorescence: Cl-K Absorption And K–Valence-Level Emission Spectra Of Chlorofluoromethanes, Rupert C. Perera, P. L. Cowan, Dennis W. Lindle, R. E. Lavilla, T. Jach, R. D. Deslattes

Chemistry and Biochemistry Faculty Research

X-ray absorption and emission measurements in the vicinity of the chlorine K edge of the three chlorofluoromethanes have been made using monochromatic synchrotron radiation as the source of excitation. By selectively tuning the incident radiation to just above the Cl 1s single-electron ionization threshold for each molecule, less complex x-ray-emission spectra are obtained. This reduction in complexity is attributed to the elimination of multielectron transitions in the Cl K shell, which commonly produce satellite features in x-ray emission. The resulting "satellite-free" x-ray-emission spectra exhibit peaks due only to electrons in valence molecular orbitals filling a single Cl 1s vacancy. These …

Biological Effectiveness Of High-Energy Protons: Target Fragmentation, Francis A. Cucinotta, Robert Katz, John W. Wilson, Lawrence W. Townsend, Judy Shinn, Ferenc Hajnal

Robert Katz Publications

High-energy protons traversing tissue produce local sources of high-linear-energy-transfer (LET) ions through nuclear fragmentation. We examine the contribution of these target fragments to the biological effectiveness of high-energy protons using the cellular track model. The effects of secondary ions are treated in terms of the production collision density using energy-dependent parameters from a high-energy fragmentation model. Calculations for mammalian cell cultures show that at high dose, at which intertrack effects become important, protons deliver damage similar to that produced by γ rays, and with fragmentation the relative biological effectiveness (RBE) of protons increases moderately from unity. At low dose, where …

Phase Changes In Plasma‐Sprayed Zinc‐Nickel Ferrite, T. A. Dooling, Desmond C. Cook

Physics Faculty Publications

The magnetic and crystalline properties of a plasma‐sprayed coating of the microwave absorbing zinc‐nickel ferrite, (Zn_0.41Ni_0.59 )Fe₂O₄ have been studied at 300 K using the Mössbauer effect and x‐ray diffraction (XRD). Both techniques indicate that the plasma‐sprayed material separated into two phases. The first is a nickel‐rich phase with magnetic fields of 47.5 and 44.5 T at the tetrahedral and octahedral sites, respectively. The second is a zinc‐rich phase, zinc meta‐ferrite, having a small magnetic field of 4.2 T. The quadrupole splitting of this second phase is ΔE=0.34 mm s⁻¹, …

Degnerate Four-Wave Mixing In Rhodamine Doped Epoxy Waveguides, B. Rossi, Hugh Byrne, W. Blau

Articles

Self-diffraction from laser-induced gratings is observed in a 13 pm-thick film of rhodamine I3 doped epoxy. The decay of the grating is measured to be 2.05 ns. Degenerate forward four-wave mixing in a guided geometry is observed by coupling out all beams after nronagation over 1 mm. At 595 nm the third-order nonlinear susceptibility of the film is measured to be 1.5~10-‘~ m2Ve2.