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Articles 481 - 501 of 501
Full-Text Articles in Entire DC Network
Stopping Power Of Matter For Alpha Particles At Extreme Relativistic Energies, P.T. Leung, M. L. Rustgi, J. E. Turner
Stopping Power Of Matter For Alpha Particles At Extreme Relativistic Energies, P.T. Leung, M. L. Rustgi, J. E. Turner
Physics Faculty Publications and Presentations
The stopping power of matter for alpha particles at extreme relativistic energies has been calculated by incorporating the charge form factor. A table is presented for aluminum, copper, and lead. It is found that at the highest energies considered, inclusion of form factor reduces the mass stopping power by about 6-8% in comparison with that predicted by Bethe's relativistic formula.
Dielectric Saturation In Dipolar Fluids. I. The Single-Molecule Distribution Function, John D. Ramshaw
Dielectric Saturation In Dipolar Fluids. I. The Single-Molecule Distribution Function, John D. Ramshaw
Physics Faculty Publications and Presentations
The functional-derivative formalism is used to investigate the form of the equilibrium single-molecule distribution function n(1) in a finite fluid system of rigid polar molecules subjected to a strong external electric field that varies slowly with position. The investigation is based on the assumption that the long-range asymptotic behavior of the direct correlation function is independent of the external field, and is hence the same as in the unperturbed fluid. This assumption implies that n(1) has the form of a single-molecule Boltzmann factor in which the interaction energy is that of a deformable quasidipole with the local Lorentz electric field …
Debye–Hückel Theory For Particles Of Arbitrary Electrical Structure, John D. Ramshaw
Debye–Hückel Theory For Particles Of Arbitrary Electrical Structure, John D. Ramshaw
Physics Faculty Publications and Presentations
Classical linearized Debye–Hückel theory is formulated for a finite fluid system, of arbitrary shape, composed of rigid particles with arbitrary internal electrical structure. The multipole description is eschewed in favor of the more basic description of a particle in terms of its charge density function. This function is left arbitrary, so the particles may be charged or neutral, polar or nonpolar, etc. The theory implies that the direct correlation function c(12)=−v(12)/k T, where v(12) is the Coulomb interaction energy between the charge densities of particles 1 and 2. In the case of uncharged polar …
Partial Chemical Equilibrium In Fluid Dynamics, John D. Ramshaw
Partial Chemical Equilibrium In Fluid Dynamics, John D. Ramshaw
Physics Faculty Publications and Presentations
An analysis is given for the flow of a multicomponent fluid in which an arbitrary number of chemical reactions may occur, some of which are in equilibrium while the others proceed kinetically. The primitive equations describing this situation are inconvenient to use because the progress rates ω [subscript s] for the equilibrium reactions are determined implicitly by the associated equilibrium constraint conditions. Two alternative equivalent equation systems that are more pleasant to deal with are derived. In the first system, the ω [subscript s] are eliminated by replacing the transport equations for the chemical species involved in the equilibrium reactions …
Brownian Motion In A Flowing Fluid, John D. Ramshaw
Brownian Motion In A Flowing Fluid, John D. Ramshaw
Physics Faculty Publications and Presentations
A phenomenological theory is developed for Brownian motion in a flowing incompressible fluid. The Brownian particles are regarded as an ideal gas subject to a position- and time-dependent force field that represents interactions with the host fluid. This approach immediately leads to deterministic partial differential equations of motion for the Brownian particles. These equations are then examined in the limit of large friction, in which they imply an expression for the diffusional mass flux of Brownian particles. This expression is a sum of terms representing concentration, forced, thermal, and pressure diffusion. Comparisons are made with earlier work, and with the …
Comments On The Theory Of Dipolar Fluids, John D. Ramshaw
Comments On The Theory Of Dipolar Fluids, John D. Ramshaw
Physics Faculty Publications and Presentations
The theory of dipolar fluids posbulated by Chan, Mitchell and Ninham is commented upon.
Existence Of The Dielectric Constant In Fluids Of Nonlinear Rigid Polar Molecules, John D. Ramshaw
Existence Of The Dielectric Constant In Fluids Of Nonlinear Rigid Polar Molecules, John D. Ramshaw
Physics Faculty Publications and Presentations
The existence of the dielectric constant epsilon is investigated for fluids composed of nonlinear rigid polar molecules. The investigation is performed using the functional-derivative approach previously employed to establish sufficient conditions for the existence of epsilon in fluids of linear (axially symmetric) molecules. It is shown that these same conditions are sufficient for nonlinear molecules of arbitrary symmetry. An expression for epsilon in terms of the direct correlation function emerges automatically from the development. This expression, which involves the inversion of a 3 x 3 matrix, is a slight generalization of one obtained earlier by Hoye and Stell using an …
Existence Of The Dielectric Constant In Rigid-Dipole Fluids: The Functional-Derivative Approach, John D. Ramshaw
Existence Of The Dielectric Constant In Rigid-Dipole Fluids: The Functional-Derivative Approach, John D. Ramshaw
Physics Faculty Publications and Presentations
In a previous article [J. Chem. Phys. 57, 2684 (1972)] sufficient conditions were established for the existence of the dielectric constant ɛ in rigid-dipole fluids. One of these conditions was an unrealistic restriction on the angular dependence of the direct correlation function c (12) at short range. Here it is shown that this restriction can be removed without altering the previous conclusions. Consequently, ɛ rigorously exists if c (12) depends only upon relative positions and orientations of molecules 1 and 2, and becomes asymptotic to -φd(12)/kT at long range, where φd(12) is the dipole-dipole potential. The development is based upon …
Debye–Hückel Theory For Rigid‐Dipole Fluids, John D. Ramshaw
Debye–Hückel Theory For Rigid‐Dipole Fluids, John D. Ramshaw
Physics Faculty Publications and Presentations
The dipolar analog of classical linearized Debye–Hückel theory is formulated for a finite fluid system of arbitrary shape composed of rigid polar molecules. In contrast to the ionic case, the dipolar Debye–Hückel (DDH) theory is nonunique due to an inherent arbitrariness in the choice of a local field E*. This nonuniqueness is expressed in terms of a parameter ϑ related to the ellipticity of the spheroidal cavity used to define E*. The theory then leads to an expression for the direct correlation function c (ϑ) as a function of ϑ. Only the short‐range part of c (ϑ) depends upon ϑ; …
On The Reduction Of Many-Body Dielectric Theories To The Onsager Equation, John D. Ramshaw
On The Reduction Of Many-Body Dielectric Theories To The Onsager Equation, John D. Ramshaw
Physics Faculty Publications and Presentations
An approximate theory for the dielectric constant ℇ of a dense polar fluid was derived by Ramshaw, Schaefer, Waugh, and Deutch (RSWD). In the present article, the RSWD theory is generalized and made rigorous by another method of derivation. The result is a rigorous expression for ℇ which differs from the RSWD expression by the presence of a fluctuation term. Both the rigorous expression and the RSWD expression are then specialized to the Onsager model. It is found that the rigorous expression for ℇ reduces to the Onsager equation, but that the RSWD expression does not because the fluctuation term …
Cryogenic Thin-Film Electron Emitters, Pavel Smejtek, David G. Onn, M. Silver
Cryogenic Thin-Film Electron Emitters, Pavel Smejtek, David G. Onn, M. Silver
Physics Faculty Publications and Presentations
Thin‐film electron emitters are described, which operate below 200°K and below a limiting critical applied voltage (νc) in a stable temperature‐independent regime. Current‐voltage characteristics and normal electron energy distributions are presented. Fabrication and operation criteria are outlined. Comparison with temperature‐dependent emitters is made, and possible conduction mechanisms discussed briefly.
Hot Electron Injection Into Dense Argon, Nitrogen, And Hydrogen, Pavel Smejtek, M. Silver, K. S. Dy, David G. Onn
Hot Electron Injection Into Dense Argon, Nitrogen, And Hydrogen, Pavel Smejtek, M. Silver, K. S. Dy, David G. Onn
Physics Faculty Publications and Presentations
Hot electrons have been injected into very dense argon, nitrogen, and hydrogen gases and liquids. The current‐voltage characteristics are experimentally determined for densities (N) of argon, nitrogen, and hydrogen ranging from about 10²⁰ to 10²² cm⁻³ and applied fields (E) ranging from about 10 to 10⁴ V cm⁻¹. The argon data show a square root E∕N dependence of the current. The nitrogen and hydrogen data show a complicated dependence of the current on E∕N due to the rapid thermalization in the region of the image potential of the injected electrons through inelastic collision processes not present in argon. A hydrodynamic‐two‐fluid …
Decay Of Multiple Spin Echoes In Dipolar Solids, C. H. Wang, John D. Ramshaw
Decay Of Multiple Spin Echoes In Dipolar Solids, C. H. Wang, John D. Ramshaw
Physics Faculty Publications and Presentations
In this paper we derive a general expression describing the evolution of the transverse nuclear-spin magnetization for the Ostroff-Waugh multiple-spin-echo experiment in dipolar solids. Our approach consists of expressing the formula for the magnetization at even echoes in a form resembling an ordinary time-correlation function, and then evaluating this quantity by means of Zwanzig's projection-operator technique. For long times, we show that under certain conditions the echo envelope decays exponentially, in agreement with experiment. A general expression is obtained for the time constant T* associated with the decay. This result may be used to generate an expansion of 1T …
Existence Of The Dielectric Constant In Rigid-Dipole Fluids: The Direct Correlation Function, John D. Ramshaw
Existence Of The Dielectric Constant In Rigid-Dipole Fluids: The Direct Correlation Function, John D. Ramshaw
Physics Faculty Publications and Presentations
The question of whether the dielectric constant ε exists (is well defined) for a finite fluid system of rigid dipolar molecules is reconsidered and reformulated. It is found that this question can most simply be expressed in terms of the behavior of the position‐ and orientation‐dependent direct correlation function c(r1,ω1; r2, ω2). It is shown that ε exists if c satisfies the following two conditions: (a) c~–φ/kT for |r1–r2|>σ, where φ is the dipole‐dipole potential and σ is a length which is large microscopically but small macroscopically. …
New Method For Generating Density Expansions, John D. Ramshaw
New Method For Generating Density Expansions, John D. Ramshaw
Physics Faculty Publications and Presentations
The calculus of finite differences is used to develop a new method for expressing the thermodynamic limit of a reasonably arbitrary statistical?mechanical average as a power series in the number density ?. The method is simple, straightforward, and purely analytic: it involves no intermediate expansion in powers of the activity and it avoids the use of graph theory. Moreover, the method is developed independently of the prescription for computing the statistical average, a fact which lends to the results an especially wide range of applicability. In particular, these results may be used in classical or quantum statistical mechanics, for intermolecular …
New Thin-Film Tunnel Triode Using Amorphous Semiconductors, Pavel Smejtek, R. F. Shaw, H. Fritzsche, M. Silver, S. Holmberg, S. R. Ovshinsky
New Thin-Film Tunnel Triode Using Amorphous Semiconductors, Pavel Smejtek, R. F. Shaw, H. Fritzsche, M. Silver, S. Holmberg, S. R. Ovshinsky
Physics Faculty Publications and Presentations
A new thin‐film tunnel triode is discussed which uses a p‐type amorphous film to achieve amplification of injected current from a tunnel cathode. It is not only the basis for a new semiconductor device but also suggests a novel method for measuring electrical properties of semiconductors.
Comment On Electron Scattering In The Image Potential Well, Pavel Smejtek, M. Silver
Comment On Electron Scattering In The Image Potential Well, Pavel Smejtek, M. Silver
Physics Faculty Publications and Presentations
Comments are made on the model of electron injection into SiO₂ proposed by Berglund and Powell. Their assumptions on electron scattering, disregarding the change of the escape cone with the distance from the emitter, lead to serious underestimation of the injected current. Two alternative models of electron injection, based solely on elastic scattering are discussed and do not predict the experimental results. We suggest that observed field dependence of the injected current into SiO₂ indicates that energy relaxation associated with the injected electrons is responsible for the voltage dependence of the current.
On The Molecular Theory Of Dielectric Polarization In Rigid‐Dipole Fluids, John D. Ramshaw
On The Molecular Theory Of Dielectric Polarization In Rigid‐Dipole Fluids, John D. Ramshaw
Physics Faculty Publications and Presentations
A molecular theory is developed for the polarization P(r) produced by a weak position‐dependent external electric field E0(r) in a finite fluid system, of arbitrary shape, composed of rigid polar molecules. The theory differs from earlier work in that no assumption is made concerning the form of the electrostatic constitutive relation. [The usual assumption in this regard is that P(r) = (ε–1) E(r) / 4π, where E(r) is the total Maxwell electric field. The “dielectric constant” ε is well defined only if the relation between P(r) and E(r) …
Dielectric Polarization And Alignment And The Structure Of Polar Fluids, John D. Ramshaw, D. W. Schaefer, John S. Waugh, J. M. Deutsch
Dielectric Polarization And Alignment And The Structure Of Polar Fluids, John D. Ramshaw, D. W. Schaefer, John S. Waugh, J. M. Deutsch
Physics Faculty Publications and Presentations
An analysis is made of the information about the structure of dense polar fluids which resides in the dielectric constant, the Kerr constant, and the nuclear magnetic resonance (NMR) quadratic electric field effect. The inadequacy of the "local-field" model for liquids is discussed. The existence of a nonzero molecular hyperpolarizability is shown to destroy an equivalence which would otherwise exist between the Kerr and NMR experiments, and can easily account for apparent discrepancies between the reported Kerr and NMR data for nitrobenzene and nitromethane. A method is presented for removing dielectric boundary effects from statistical averages, so that the averages …
Hot Electron Injection Into Liquid Argon From A Tunnel Cathode, Pavel Smejtek, David G. Onn, M. Silver, P. Kumbhare
Hot Electron Injection Into Liquid Argon From A Tunnel Cathode, Pavel Smejtek, David G. Onn, M. Silver, P. Kumbhare
Physics Faculty Publications and Presentations
Hot electrons from a tunnel cathode have been injected into liquid argon (99.998% pure) at 87°K. The current vs voltage characteristics indicate that the injected hot electrons thermalize very slowly, losing their energy only by elastic scattering processes and finally by capture by the dilute impurities. The deduced thermalization time and distance are very long compared with that in helium, where bubble formation is responsible for energy loss.
Steady-State And Transient Currents In Organic Liquids By Injection From A Tunnel Cathode, Pavel Smejtek, David G. Onn, M. Silver
Steady-State And Transient Currents In Organic Liquids By Injection From A Tunnel Cathode, Pavel Smejtek, David G. Onn, M. Silver
Physics Faculty Publications and Presentations
Experimental data are presented on the currents induced in organic liquids by injection from a tunnel cathode. The injection level was varied over a wide range resulting in almost no space‐charge limitation to almost complete space‐charge limitation. Results were different from that usually observed in solids, in that at low fields, the steady‐state current was proportional to V², while at high fields the current was proportional to V. By proper choice of electrode spacing and applied voltage, space‐charge‐limited current transients as low as 10⁻¹¹ A∕cm² and 5 sec transit times were observed. A smooth transition between the electrode‐limited and the …