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Full-Text Articles in Physics

Fundamental Limits To Imaging Resolution For Focused Ion Beams, Jon Orloff, L. W. Swanson, Mark Utlaut Nov 1996

Fundamental Limits To Imaging Resolution For Focused Ion Beams, Jon Orloff, L. W. Swanson, Mark Utlaut

Physics Faculty Publications and Presentations

This article investigates the limitations on the formation of focused ion beam images from secondary electrons. We use the notion of the information content of an image to account for the effects of resolution, contrast, and signal‐to‐noise ratio and show that there is a competition between the rate at which small features are sputtered away by the primary beam and the rate of collection of secondary electrons. We find that for small features, sputtering is the limit to imaging resolution, and that for extended small features (e.g., layered structures), rearrangement, redeposition, and differential sputtering rates may limit ...


Rubidium Atomic Funnel, T. B. Swanson, N. J. Silva, Shannon K. Mayer, J. J. Maki, D. H. Mcintyre Sep 1996

Rubidium Atomic Funnel, T. B. Swanson, N. J. Silva, Shannon K. Mayer, J. J. Maki, D. H. Mcintyre

Physics Faculty Publications and Presentations

A low-velocity beam of rubidium atoms is produced from a two-dimensional magneto-optic trap or atomic funnel. Atoms from a thermal beam are slowed by chirped laser cooling and then loaded into the funnel. The cold atoms are ejected by moving molasses formed with frequency-shifted laser beams. The resultant atomic beam has a controllable velocity in the range of 3 to 10 m/s, a temperature of 500 μK, and a flux of 1010 atoms/s.


Independent Center, Independent Electron Approximation For Dynamics Of Molecules And Clusters, James H. Mcguire, Jack C. Straton, J. Wang, Y. D. Wang, O. L. Weaver, S. E. Corchs, R. D. Rivarola Aug 1996

Independent Center, Independent Electron Approximation For Dynamics Of Molecules And Clusters, James H. Mcguire, Jack C. Straton, J. Wang, Y. D. Wang, O. L. Weaver, S. E. Corchs, R. D. Rivarola

Physics Faculty Publications and Presentations

A formalism is developed for evaluating probabilities and cross sections for multiple-electron transitions in scattering of molecules and clusters by charged collision partners. First, the molecule is divided into subclusters each made up of identical centers (atoms). Within each subcluster coherent scattering from identical centers may lead to observable phase terms and a geometrical structure factor. Then, using a mean field approximation to describe the interactions between centers we obtain {ital A}{sub {ital I}}{approximately}{summation}{sub {ital k}}{product}{sub {ital ke}}{sup {ital i}{delta}{sup {ital k}}{sub {ital I}}}{ital A}{sub {ital Ik}}. Second ...


Phonon Amplification Using Evaporation And Adsorption Of Helium, Tamar More, J. S. Adams, S. R. Bandler, S. M. Brouër, R. E. Lanou, H. J. Maris, G. M. Seidel Jul 1996

Phonon Amplification Using Evaporation And Adsorption Of Helium, Tamar More, J. S. Adams, S. R. Bandler, S. M. Brouër, R. E. Lanou, H. J. Maris, G. M. Seidel

Physics Faculty Publications and Presentations

We report the results of experiments designed to investigate the feasibility of amplifying a phonon signal using the evaporation of helium from a superfluid film and its subsequent readsorption onto a helium-free surface. We envision a multistage amplifier in which helium is evaporated from a wafer with a helium film only on one side and then adsorbed onto the film-free surface of a similar wafer. The phonons created by the adsorption reach the film on the opposite side of the wafer and potentially desorb more helium than was evaporated by the first wafer. The amplification would come from the high ...


Multicomponent Diffusion In Two-Temperature Magnetohydrodynamics, John D. Ramshaw, C. H. Chang Jun 1996

Multicomponent Diffusion In Two-Temperature Magnetohydrodynamics, John D. Ramshaw, C. H. Chang

Physics Faculty Publications and Presentations

A recent hydrodynamic theory of multicomponent diffusion in multitemperature gas mixtures [J. D. Ramshaw, J. Non-Equilib. Thermodyn. 18, 121 (1993)] is generalized to include the velocity-dependent Lorentz force on charged species in a magnetic field B. This generalization is used to extend a previous treatment of ambipolar diffusion in two-temperature multicomponent plasmas [J. D. Ramshaw and C. H. Chang, Plasma Chem. Plasma Process. 13, 489 (1993)] to situations in which B and the electrical current density are nonzero. General expressions are thereby derived for the species diffusion fluxes, including thermal diffusion, in both single- and two-temperature multicomponent magnetohydrodynamics (MHD). It ...


Surface-Enhanced Raman Scattering At Elevated Temperatures, P.T. Leung, M. H. Hider, Erik J. Sánchez May 1996

Surface-Enhanced Raman Scattering At Elevated Temperatures, P.T. Leung, M. H. Hider, Erik J. Sánchez

Physics Faculty Publications and Presentations

The effect on surface-enhanced Raman scattering due to the increase in temperature of the metallic substrate is studied via a simple model for physisorbed molecules. Surface roughness is represented by a spherical or spheroidal island, and the temperature effects on the surface plasmon are accounted for via a slightly modified Ujihara model. It is found that the enhancement ratio in general decreases as substrate temperature increases. For noble metals like silver, this temperature effect is particularly pronounced at scattering frequencies close to that of the surface-plasmon resonance; and for frequencies well below the surface plasmon frequency, the enhancement ratio is ...


Friction-Weighted Self-Consistent Effective Binary Diffusion Approximation, John D. Ramshaw, C. H. Chang Jan 1996

Friction-Weighted Self-Consistent Effective Binary Diffusion Approximation, John D. Ramshaw, C. H. Chang

Physics Faculty Publications and Presentations

The self-consistent effective binary diffusion (SCEBD) approximation for multicomponent diffusion in gas mixtures is reconsidered and reformulated. The new formulation is based on the fact that a suitable rearrangement of the Stefan-Maxwell equations provides an exact expression for the complementary mean velocity ai for species i as a weighted average of the velocities of all the other species. The coefficients in ai are normalized friction coefficients which are simply related to the true binary diffusion coefficients. A simple factorized bilinear approximation to the friction coefficients then yields approximate species diffusion fluxes identical in form to those of a ...


Simple Approximation For Thermal Diffusion In Ionized Gas Mixtures, John D. Ramshaw Jan 1996

Simple Approximation For Thermal Diffusion In Ionized Gas Mixtures, John D. Ramshaw

Physics Faculty Publications and Presentations

A simple approximation for thermal diffusion in gas mixtures was recently proposed [1]. This approximation was based upon relations valid for rigid spheres. It is therefore appropriate for molecules with steep repulsive potentials, but not for ionized species interacting via the Coulomb potential. Here we formulate an analogous approximation for ionized species and free electrons. The resulting thermal diffusion coefficients differ in sign from those for hard molecules.