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Study Of The Dependence Of The Electron Emission Spectra On The Projectile Charge In H⁺, He2++He Collisions, C. O. Reinhold, Ronald E. Olson

Physics Faculty Research & Creative Works

The classical trajectory Monte Carlo method (CTMC) has been used to calculate doubly differential ionized-electron cross sections for the impact of He2+ and H+ on He at collision energies of 50 and 100 keV amu-1. The results exhibit the capture to the continuum peak and agree in both shape and magnitude with experimental data. The dependence of the cross sections on the projectile charge ZP is analyzed and it is found to be very asymmetric with regard to the capture to the continuum peak. This behavior is in agreement with very recent experimental data. Large deviations from the Z2P scaling …

The Role Of Small-Impact-Parameter Electron-Loss Processes On Ion Temperatures Measured By Active-Beam Plasma Diagnostics, A. J.H. Donné, F. J. De Heer, Ronald E. Olson

Physics Faculty Research & Creative Works

The shape of the energy distribution of scattered particles in an active beam scattering experiment can be influenced considerably by the presence of highly charged impurity ions in the plasma. In this work it is shown theoretically that multiply charged impurities have only a small effect on the scattering distribution at low energies (/amu) and at high energies (>75 keV/amu). The effect of the impurities is the most pronounced for energies close to 50 keV/amu. The results of the calculations are in good agreement with experimental results at various energies. Extrapolation to higher beam energies leads to the conclusion …

Hr Aurigae: An Active Binary ?, Charles G. Loomis, Edward G. Schmidt

Edward Schmidt Publications

Photometric and spectral observations of the variable star HR Aurigae are presented. Based on these data we conclude that this star, previously regarded as a type II Cepheid, is, in fact, an active binary.

Time And The Physical Universe, Norman E. Ramsey

DSLS 1989-1990

(This information was taken from the Distinguished Scientist Lecture Series Program 1989-1990).

Norman F. Ramsey is the Higgins Professor ofPhysics at Harvard University. He has been a Harvard faculty member since 1947. Norman Ramsey received his A.B. and M.A. from Columbia University and degrees from Cambridge University. In1940 he received aPh.D. from Columbia University for molecular beam studies of rotational magnetic moments of molecules. He was awarded an Sc.D. by Cambridge University in1954 and by Oxford University in1973, as well as honorary D.Sc.'s from Case-Western Reserve University, Middlebury College, and Rockefeller University. After periods at the Carnegie Institution of Washington, …

Doubly Differential Detachment Cross Sections For 0.5-Mev H- On He Including Projectile Excitation To H(N=2), Chih-Ray Liu, Anthony F. Starace

Anthony F. Starace Publications

Detailed theoretical results are presented for the electron-detachment cross section, doubly differential in both the electron momentum and angle, for the process 0.5-MeV H^-+He→H(n=2)+ e^-+He*. As discussed briefly elsewhere [C. R. Liu and A. F. Starace, Phys. Rev. Lett. 62, 407 (1989)], the laboratory-frame doubly differential cross sections (DDCS’s) for electron detachment in the forward direction are shown to depend sensitively on the low-energy states of the H(n=2)- e^- three-body system. In particular, the angular dependence of characteristic cusp and shape resonance features is presented. We find that the projectile frame …

Deviations From Bulk Transport Measurements In Semi-Insulating Gaas, N. C. Halder, David C. Look

Physics Faculty Publications

Because of the high bulk resistivity of semi‐insulating GaAs, surface or near‐surface effects can change the apparent magnitudes of resistivity, mobility, and carrier concentration. We consider the following causes: (1) above‐surface conduction, such as that due to impurities in a porous oxide; (2) subsurface conduction, due to sawing and polishing damage; (3) tunneling conduction in surface states; and (4) changes in near‐surface conduction due to the modification of surface potential by surface states or absorbates. The most important of these effects appear to be subsurface damage and surface potential changes.

Dynamics Of Dislocation-Mediated Melting In A Two-Dimensional Lattice In The Presence Of An Oscillatory Applied Strain, Arnold J. Dahm, M. A. Stan, Rolfe G. Petschek

Faculty Scholarship

The temperature and frequency dependence of the power dissipated in a two-dimensional crystal subjected to an applied strain is shown to exhibit a distinct signature for dislocation-dissociation melting. The power absorption is analogous to the case of thin helium films subjected to an applied velocity field. Power loss by dislocation motion in the compressional mode is severely limited by the presence of nonequilibrium densities of interstitial and vacancy defects that are created or annihilated by dislocation climb. Tables of analogs between dislocations, vortices, and charges in two-dimensional systems are presented. The relation of the results to published experiments is discussed.

Bethe Stopping-Power Theory For Heavy-Target Atoms, P.T. Leung

Physics Faculty Publications and Presentations

The Bethe stopping-power theory is considered in the context of heavy target atoms in which the relativistic effects of the target electrons are partially accounted for within the semirelativistic and independent-particle description. Numerical results show that, for target elements with large atomic numbers, these relativistic effects can become comparable with other corrections such as the shell correction, the Barkas effect, and the Bloch term.

Critical Behavior Of A Frustrated Ising System, O. Heinonen, Rolfe G. Petschek

Faculty Scholarship

Finite-size-scaling analysis of a simulation of the critical behavior of a nonrandom three-dimensional frustrated system yields a tricritical-like behavior not predicted by a simple Landau-Ginzburg-Wilson theory. We argue that, because of the frustration, the free energy of many phases is close to that of the observed ordered phase and that a complete theory must take into account interactions between fluctuations of order parameters of all these phases. We suggest that these properties are generic to frustrated systems. Related transitions in the model are also discussed.

The Period-Decline-Rate Relation For Pulsating Stars, Edward G. Schmidt, Kathryn H. Wiese

Edward Schmidt Publications

The relationships between the periods and the rate of decline in V and R for pulsating stars are investigated. It is shown that these relationships are useful for making preliminary estimates of periods for stars with little data. These estimates can then be used to optimize times of further observations.

The Electrical Noise Of Carbon Fibers, R.O. Dillon

Roger Kirby Publications

The low-frequency excess electrical noise has been measured on carbon fibers with a wide range of crystalline perfection and corresponding electrical and mechanical properties. Fibers include those prepared from ex-PAN and ex-pitch polymers, and a catalytic-chemical vapor deposited filament. The extensional (Young's) moduli of these fibers varied from about 220 to 890 GPa (35–130 Msi), while the electrical resistivities varied from about 19 to 1 µΩ m. The low-frequency electrical noise of each fiber was found to be proportional to l² and to vary as 1/f^α, where f is the frequency and α is about 1.15. …

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

All Physics Faculty Publications

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.

Low-Lying Negative-Ion States Of Calcium, A.R. Johnston, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Studies of electron transmission through Ca vapor reveal two prominent resonances in the range 0–4 eV. The energies and widths of these temporary anion states suggest that they may be assigned to the strongly mixed (4s²3d) ²D and (4s4p²)²D states of Ca^-. This interpretation is supported by ab initio configuration-interaction calculations and is consistent with previously unassigned structure observed in the photodetachment cross section.

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

Aerospace, Physics, and Space Science Faculty Publications

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 …

Accurate Calibration Of The Four-Detector Photopolarimeter With Imperfect Polarizing Optical Elements, R. M.A. Azzam, Ali G. Lopez

Electrical Engineering Faculty Publications

The first three columns of the instrument matrix A of the four-detector photopolarimeter (FDP) are determined by Fourier analysis of the output current vector I(P) as a function of the azimuth angle P of the incident linearly polarized light. Therefore 12 of the 16 elements of A are measured free of the imperfections of the (absent) quarter-wave retarder (QWR). The effect of angular beam deviation by the polarizer is compensated for by taking the average, (1/2) [I(P) + I(P + 180°)], of the FDP output at 180°-apart, optically equivalent, angular positions of the polarizer. The remaining fourth column of A …

Analysis Of Experimental Nucleation Data For Silver And Sio Using Scaled Nucleation Theory, Barbara N. Hale, Paul R. Kemper, Joseph A. Nuth

Physics Faculty Research & Creative Works

The experimental vapor phase nucleation data of Nuth et al., for silver [J. A. Nuth, K. A. Donnelly, B. Donn, and L. U. Lilleleht, J. Chem. Phys. 77, 2639 (1982)] and SiO [J. A. Nuth and B. Donn, J. Chem. Phys. 85, 1116 (1986)] are reanalyzed using a scaled model for homogeneous nucleation [B. N. Hale, Phys. Rev. A 33, 4156 (1986)]. The approximation is made that the vapor pressure at the nucleation site is not diminished significantly from that at the source (crucible). It is found that the data for ln S have a temperature dependence consistent with the …

Quantum Transport In The Presence Of Random Traps, John Wm Edwards, Paul Ernest Parris

Physics Faculty Research & Creative Works

We calculate the asymptotic decay of a quantum particle moving in a d-dimensional medium doped with randomly placed trapping impurities, focusing on contributions from slowly decaying long-wavelength modes centered in large compact regions devoid of traps. By averaging the decay over the statistical distribution associated with these regions we find that the survival probability, P(t) ~exp(-At^d/(d+3)), decays more slowly in any dimension than for diffusive transport.

Domain Decomposition For Multigrid, Finite Volume Flow Solvers, Victor Robert Lessard

Mechanical & Aerospace Engineering Theses & Dissertations

Computing the flow fields about three-dimensional complex configurations accurately becomes a difficult task, if it is attempted to generate a single, body fitted grid with proper clustering. The domain decomposition methods, which divide the computational domain into less complex subdomains, are extensively used to decrease the grid generation workload. A domain decomposition technique also allows the use of different solution methods for different subdomains. The objective of this work is to develop a domain decomposition method via overlapping/embedding the component grids, which is to be used by upwind, multigrid, finite volume solution algorithms. A computer code, given the name MaGGiE, …

Angular And Energy Distributions Of Electrons From 7.5- 150-Kev Proton Collisions With Oxygen And Carbon Dioxide, Wen-Qin Cheng, M. Eugene Rudd, Ying-Yuan Hsu

M. Eugene Rudd Publications

Cross sections for the ejection of electrons, differential in the angle and energy of emission, were measured for proton collisions with two molecular gases, oxygen and carbon dioxide, over the primary energy range of 7.5-150 keV and an angular range of 10° to 160°. The energy distributions, obtained by integration over the angle, were fitted by an analytical model. A discrepancy in the angular distributions compared to those of Gibson and Reid [J. Phys. E 17, 1227 (1984); J. Phys. B 19, 3265 (1986); Radiat. Res. 112, 418 (1987); Australian Atomic Energy Commission Report No. AAEC/E659, 1987 (unpublished)] is discussed. …

Application Of Shape Sensitivity Analysis For Computation Of Transient Free-Surface Flows, Frank Nicolas Michel

Mechanical & Aerospace Engineering Theses & Dissertations

Shape sensitivity analysis is introduced into a numerical scheme simulating a transient free-surface flow and applied to the case of a fluid advancing through a bed of cylinders. For this incompressible Newtonian flow, viscous forces are dominant. A penalty function finite element formulation is used to solve the problem. Coordinates of nodes on the moving boundary are chosen as design variables and an analytical formulation of the gradient of the finite element algebraic system, with respect to these variables, is derived.

In a numerical example, the results obtained with and without sensitivity analysis are compared. The efficiency and accuracy of …

Navier-Stokes Analysis Of Cold Gas Scramjet Nozzle-Afterbody Flowfields, Walter C. Engelund

Mechanical & Aerospace Engineering Theses & Dissertations

The recent renewal of interest in hypersonic aerodynamics, brought about largely in part due to the development of the National Aerospace Plane (NASP), is resulting in an increased reliance on computational fluid dynamics for flowfield data in this flight regime. The design of the nozzle-afterbody section for a hypersonic transport such as the NASP is currently underway, and is being conducted using both computational fluid dynamics and cold, non-reacting, simulant gas experimental models. In this study, a computational analysis is conducted for the flow through a scramjet nozzle-afterbody test section.

Computations have been performed for a cold gas simulation of …

Radiative Kaon Capture At Rest In Hydrogen, William J. Fickinger, D. K. Robinson

Faculty Scholarship

The photon spectrum from K- stopping in liquid hydrogen has been measured with a high-resolution (1.5% FWHM at 300 MeV) NaI(Tl) detector. The branching ratios for K-p→Λγ (Eγ=281.4 MeV) and K-p→Σ0γ (Eγ=219.5 MEV) were obtained. The results are RΛγ=(0.86±0.07-0.08+0.1)×10-3 and RΣ0γ=(1.44±0.20-0.10±0.12)×10-3 where the first error is statistical and the second systematic. These results are not in good agreement with published predictions or with previous Λγ measurements.

Frontiers Of Material Research, Mildred S. Dresselhaus

DSLS 1989-1990

(This information was taken from the Distinguished Scientist Lecture Series Program 1989-1990).

Dr. Dresselhaus is currently Institute Professor at the Massachusetts Institute of Technology. She was formerly the holder of the Abby Rockefeller Mauze Chair in Electrical Engineering and in Physics at MIT. She is also affiliated with the Center for materials and Engineering, and with the Francis Bitter National Magnet Laboratory at MIT where some of the experimental work of her group is carried out. Dr. Dresselhaus holds professorships in MIT's Department of Electrical Engineering and Computer Science and the Department of Physics.

Dr. Dresselhaus was born in Brooklyn …

Hall-Effect Depletion Corrections In Ion-Implanted Samples: Si29 In Gaas, David C. Look

Physics Faculty Publications

The sheet free‐carrier concentration in a thin, conducting layer on an insulating substrate is lower than the net, sheet‐dopant concentration because of free‐carrier depletion in the surface and interface regions. Here we develop an algorithm to give the true, net sheet‐donor concentration from the measured sheet‐Hall concentration under the assumption of a Gaussian donor profile, which is usually sufficiently accurate for ion‐implanted samples. Correction curves are generated for Si²⁹ ions implanted into GaAs at energies of 60, 100, 130, 150, and 200 keV, and at doses of 1×10¹¹–2×10¹⁴ ions/cm². Also, the …

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

All Physics Faculty Publications

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 …

Silicon Crystallite Formation In Ion-Implanted Quartz, U. B. Ramabadran, H. E. Jackson, Gary C. Farlow

Physics Faculty Publications

Rapid thermally annealed silicon‐implanted x‐cut α‐quartz samples have been examined by Rutherford backscattering and Raman microprobe spectroscopy. The data indicate that the silicon has diffused at 1200 °C to form a buried layer of crystallites of size 1–10 μm. The crystallites are preferentially oriented and under substantial stress.

Post-Prior Symmetrical First-Order T Matrix For Charge Transfer, Jack C. Straton, M. D. Girardeau

Physics Faculty Publications and Presentations

The Fock-Tani Hamiltonian is found for systems containing two protons and one electron. It is shown that a post-prior symmetrical T-matrix element for a⁺ + (b⁺ c^-)→(a⁺ c^-)+b⁺ may be found from that of the simpler proton-proton-electron system if a and b are treated as isospin projections of a single type of nucleon. The Coulomb-exchange contribution to the inelastic (isospin flip) scattering of this system gives a first-order T matrix that is completely symmetrical with respect to post and prior interactions and orthogonalizations, a symmetry of the exact …

Recoil Distributions In Particle Transfer, James H. Mcguire, Jack C. Straton, W. J. Axmann, T. Ishihara, E. Horsdal

Physics Faculty Publications and Presentations

Classical Thomas peaks in various fast second-order particle transfer processes are quantum mechanically broadened by energy nonconservation in the intermediate states of collision. This quantum broadening is considered in observable velocity distributions of recoil particles.

Full Optical Potential For The Electron-Hydrogen Entrance Channel, Igor Bray, Don H. Madison, Ian E. Mccarthy

Physics Faculty Research & Creative Works

Differential and total elastic and total reaction cross sections are calculated for electron-hydrogen scattering at 30, 100, and 400 eV using an ab initio optical potential that treats bound and continuum nonelastic channels in the distorted-wave Born approximation. Multichannel and partial wave expansions are carried out to numerical convergence. Convergence criteria and quadratures for the continuum-energy integration are chosen for 1% overall accuracy. Results are compared with experiment and less detailed calculations.

One-Dimensional Quantum Transport In The Presence Of Traps, Paul Ernest Parris

Physics Faculty Research & Creative Works

A study is made of the survival probability P(t) for a quantum particle moving coherently on an ordered one-dimensional chain containing randomly placed irreversible traps in concentration q. We consider two separate models of the trapping process, focusing on the long-time limit of each. In the first model, intended to describe substitutional traps, the trapping impurities act as disruptive absorbing barriers which prevent further motion along the chain. For this model it is shown that -ln[P(t)]1/4t^1/4. The second model is intended to describe interstitial trapping impurities which do not disrupt the transport. It is argued for this case …