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Physics Faculty Research & Creative Works

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Articles 1711 - 1740 of 1762

Full-Text Articles in Physical Sciences and Mathematics

Absolute Excitation Cross Sections Of He+ In 20-100-Kev He+-He Collisions Using Energy-Loss Spectrometry, D. R. Schoonover, John T. Park Jan 1971

Absolute Excitation Cross Sections Of He+ In 20-100-Kev He+-He Collisions Using Energy-Loss Spectrometry, D. R. Schoonover, John T. Park

Physics Faculty Research & Creative Works

Application of positive-ion energy-loss spectrometry has been extended to include experimental determination of absolute excitation cross sections of ground-state helium ions. Helium ion-atom collisions were studied for impact energies ranging between 20-100 keV, in 10-keV intervals. The data were taken with an apparatus resolution between 0.6-0.8 eV full width at half-maximum (FWHM). Cross sections for transitions from ground state to the second and third principal quantum levels of the ion plotted as a function of impact energy were still rapidly increasing at 100 keV. The cross sections at this energy were (1.64 ± 0.28) x 10-18 cm2 for He+(1s122)→He+ (n=2) …

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Surface Plasmons And The Reflectivity Of N-Type Insb, William E. Anderson, Ralph William Alexander, Robert John Bell Jan 1971

Surface Plasmons And The Reflectivity Of N-Type Insb, William E. Anderson, Ralph William Alexander, Robert John Bell

Physics Faculty Research & Creative Works

The Reflectivity of N-Type InSb Has Been Measured in the Far Infrared. the Doping of the Samples Was Such that the Free-Carrier Plasma Frequency Was Near the LO Mode Frequency. the Results Suggest that Samples with a Sufficiently Thick Damage Layer Show Effects Due to Surface Plasmons. Use of a Simple Model Indicates that the Surface-Plasma Excitations Are Coupled to the Phonons. © 1971 the American Physical Society.

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Scattering Of Waves In Many Dimensions, I. Adawi Jan 1971

Scattering Of Waves In Many Dimensions, I. Adawi

Physics Faculty Research & Creative Works

Scattering by a spherical potential is discussed in all dimensions by one formulation using the partial-wave expansion method. The optical theorem relating the total scattering cross section σ to the forward scattering amplitude f(0) is derived.

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Experimental And Theoretical Study Of The Sign Preference In The Nucleation Of Water Vapor, Daniel R. White, James L. Kassner Jan 1971

Experimental And Theoretical Study Of The Sign Preference In The Nucleation Of Water Vapor, Daniel R. White, James L. Kassner

Physics Faculty Research & Creative Works

The nucleation of water vapor on ions in atmospheres of helium and argon was studied using an expansion type cloud chamber. Separation of the positive and negative ions was achieved so that the nucleation could be studied as a function of both the sign of the ionic charge and the supersaturation. A semiphenominological theory was developed as an extension of the classical liquid drop theory to include the effects of the ionic charge on the nucleation process. The theoretical model of the prenucleation embryo was assumed to possess an oriented dipole surface layer with the direction of orientation dependent on …

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Heisenberg Ferromagnet With Biquadratic Exchange, Harry A. Brown Jan 1971

Heisenberg Ferromagnet With Biquadratic Exchange, Harry A. Brown

Physics Faculty Research & Creative Works

The usual Heisenberg Hamiltonian with bilinear exchange -2JS→1•S→2 has been extended to include a biquadratic term -2αJ(S→1•S→2)2, with an adjustable parameter α. A method equivalent to constant coupling was employed to calculate the effect of the biquadratic exchange term on the Curie temperature, magnetization, susceptibility, specific heat, and entropy for lattices with spin-1 atoms. As α goes from 0 to 1, the Curie temperature falls by a factor 2 to 3, while the asymptotic Curie temperature is reduced by the factor 2. The magnetization rises much more rapidly below TC, and the specific heat has a peak and discontinuity several …

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Mössbauer Studies Of Bifeo₃-Pbtio₃ Perovskite-Type Solid Solutions, C. M. Yagnik, Robert Gerson, William Joseph James Jan 1971

Mössbauer Studies Of Bifeo₃-Pbtio₃ Perovskite-Type Solid Solutions, C. M. Yagnik, Robert Gerson, William Joseph James

Physics Faculty Research & Creative Works

The Mössbauer spectra of the ferroelectric solid solutions (BiFeO3)x(PbTiO)1-x have been studied as a function of temperature, mainly between room temperature and 600°C, using 57Fe in the solutions as an absorber. The quadrupole splitting increased and the isomer shift decreased between x = 0.05 and x = 0.70, a range in which the solutions are tetragonal and paramagnetic, and in which the unit cell volume increases with increasing x. The isomer shift, in particular, was primarily determined by the unit cell volume. Data are presented on the internal magnetic field of the antiferromagnetic …

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Doppler Shift In Frequency In The Transport Of Electromagnetic Waves Through An Underdense Plasma, Jerry Peacher, Kenneth M. Watson Oct 1970

Doppler Shift In Frequency In The Transport Of Electromagnetic Waves Through An Underdense Plasma, Jerry Peacher, Kenneth M. Watson

Physics Faculty Research & Creative Works

In an earlier publication, the validity of the radiation transport theory was studied for the calculation of multiple scattering of electromagnetic waves by a turbulent plasma. In the present paper, we extend the transport theory to include a description of the Doppler shift in frequency caused by electron motion.

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Differential Cross Section For Charged A₁ Photoproduction Using The Regge Exchange Formalism, Keiji Kikkawa, Barbara N. Hale Feb 1970

Differential Cross Section For Charged A₁ Photoproduction Using The Regge Exchange Formalism, Keiji Kikkawa, Barbara N. Hale

Physics Faculty Research & Creative Works

The Regge-pole formalism is applied to the calculation of the differential cross section for A1+ photo-production in the region |tmin| < -t < 10µ2. The p, A1, A1-daughter, A2, and π trajectory contributions are considered, and use is made of chiral dynamics to estimate the unknown coupling constants. We find that the π and the A2 trajectories provide the dominant contributions.

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Note On "Approximate Free Energies For Heisenberg Ferromagnets", J. H. Hemmann, Harry A. Brown Jan 1970

Note On "Approximate Free Energies For Heisenberg Ferromagnets", J. H. Hemmann, Harry A. Brown

Physics Faculty Research & Creative Works

No abstract provided.

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Cross Section For Excitation Of The Fourth Positive Band System In Carbon Monoxide By 20-120 Kev Protons, John T. Park, D. R. Schoonover, George W. York Jan 1970

Cross Section For Excitation Of The Fourth Positive Band System In Carbon Monoxide By 20-120 Kev Protons, John T. Park, D. R. Schoonover, George W. York

Physics Faculty Research & Creative Works

Excitation cross sections for the fourth positive band system in carbon monoxide have been determined from studies of the energy-loss spectra of 20-120-keV protons incident on gaseous CO targets. The energy-loss spectra had an energy resolution of about 2 eV. Prominent peaks in the spectra were observed at 8.5 and 13.8 eV. The first peak is believed to be due to excitation of the fourth positive band system of CO (X+1A 1, while the 13.8-eV peak covers a number of possible states, including the B+2 and the C+1 states. Changes in the slope of the ionization continuum were noted at …

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Theory Of Droplet Growth In Clouds. I. The Transient Stage Of The Boundary-Coupled Simultaneous Heat And Mass Transport In Cloud Formation, John C. Carstens, Joseph T. Zung Jan 1970

Theory Of Droplet Growth In Clouds. I. The Transient Stage Of The Boundary-Coupled Simultaneous Heat And Mass Transport In Cloud Formation, John C. Carstens, Joseph T. Zung

Physics Faculty Research & Creative Works

Two solutions to the system of equations describing the simultaneous heat and mass transport involved in the condensational growth of a droplet in a supersaturated atmosphere are presented. The first, valid for very short times, describes the transient stage of such growth; the second, valid for longer times, presupposes the establishment of a steady-state condition. The two are shown to be complementary for the cases examined. The equations examined satisfy the usual boundary conditions imposed on a drop in a concentric sphere as required by the cellular model for cloud formation. Hence our results can be immediately extended to the …

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Energy-Loss Spectra And Collision Cross Sections For Impact Of 20-120-Kev Positive Ions On Molecular Nitrogen, F. D. Schowengerdt, John T. Park Jan 1970

Energy-Loss Spectra And Collision Cross Sections For Impact Of 20-120-Kev Positive Ions On Molecular Nitrogen, F. D. Schowengerdt, John T. Park

Physics Faculty Research & Creative Works

Inelastic energy-loss spectra induced by impact of H+, H2+, and Ar+ on N2 have been measured at incident ion energies of 20-120 keV, with an energy resolution of about 2 eV. Prominent peaks are observed at energy losses of 9.5 and 13.8 eV. The first of these is well resolved and is attributed to excitation of the Lyman-Birge-Hopfield (LBH) system of N2. The peak at 13.8 eV is believed due primarily to excitation of the bu1 band of the Worley-Jenkins series. The locations of these features on the energy-loss scale do not agree with results of the electron impact work …

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Measurements Of Excited-State Population Ratios Of Atomic Hydrogen Produced By Charge-Exchange Neutralization Of Energetic Proton Beams, Robert H. Mcfarland, Archer H. Futch Jan 1970

Measurements Of Excited-State Population Ratios Of Atomic Hydrogen Produced By Charge-Exchange Neutralization Of Energetic Proton Beams, Robert H. Mcfarland, Archer H. Futch

Physics Faculty Research & Creative Works

Excited-state population ratios for atomic hydrogen have been measured for energetic hydrogen ions neutralized by charge exchange with a number of different metallic vapors and permanent gases. Electric field ionization techniques were used to determine excitation ratios. Detection of the energetic ions and neutrals was accomplished using partially depleted surface-barrier detectors. Counting, using phase-detection techniques in which an add-subtract scaler was phased to the signal, provided an improved signal-to-noise ratio. Two methods were used to provide desired targets. Metallic vapor and permanent-gas thin targets of the order of 10-5 mTorr cm were produced as chopped neutral beams, using conventional crossed-beam …

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Ground-State Wave Function Of Shallow Donors In Uniaxially Stressed Silicon: Piezohyperfine Constants Determined By Electron-Nuclear Double Resonance, Edward B. (Boyd) Hale, Theodore G. Castner Jan 1970

Ground-State Wave Function Of Shallow Donors In Uniaxially Stressed Silicon: Piezohyperfine Constants Determined By Electron-Nuclear Double Resonance, Edward B. (Boyd) Hale, Theodore G. Castner

Physics Faculty Research & Creative Works

Stress-induced changes in the ENDOR spectra of a shallow donor electron interacting with various Si29 nuclei neighboring the donor have been experimentally and theoretically investigated. For each of the three measured donors - As, P, and Sb - the compressional, uniaxial stress was applied along the [001] axis and its magnitude corresponded to strains up to 10-3. To describe the observed linear and quadratic shifts and splitting's of the lines in an ENDOR shell, we have defined a set of piezo hyperfine constants. One piezo hyperfine constant was measured for each axis-class shell; three independent piezo hyperfine constants were measured …

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Entropy Of The Mixed State Of Some Low-Κ Type-Ii Superconductors, Don Carlos Hopkins, R. R. Rice, J. M. Carter, J. D. Hayes Dec 1969

Entropy Of The Mixed State Of Some Low-Κ Type-Ii Superconductors, Don Carlos Hopkins, R. R. Rice, J. M. Carter, J. D. Hayes

Physics Faculty Research & Creative Works

The entropy of the mixed state of type-II superconductors with low κ(∼1) is studied for the alloys Pb0.96Tl0.04 and In0.88Sn0.12. The current theories are reviewed for approximations valid near Hc1 and near Hc2. The experiments show for the most reversible sample (Pb0.96Tl0.04) that (SB)T of the mixed state depends only on temperature, and that (ST)B increases linearly with B from its value for the superconducting state until it drops discontinuously to its normal state value at Hc2. There is no abrupt increase in the specific …

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A Heavy-Ion Energy Loss Spectrometer, John T. Park, F. D. Schowengerdt Dec 1969

A Heavy-Ion Energy Loss Spectrometer, John T. Park, F. D. Schowengerdt

Physics Faculty Research & Creative Works

A high-resolution system for the measurement of the energy loss spectra of 20-250 keV heavy ions incident on gas targets is described. The high resolution is obtained by decelerating the ion beam to a low, well-defined energy before performing the energy analysis. The energy loss spectrum is determined by changing the accelerator energy to compensate for energy lost in collisions with the gas atoms. The system is arranged so that normal fluctuations in the accelerator's power supply do not affect the energy resolution. The resolving power of the system is 2.5x104. The primary proton beam has a detected WHM of …

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Some New Approaches To Shallow Impurity States, Wayne E. Tefft, Robert John Bell, Harold V. Romero Dec 1969

Some New Approaches To Shallow Impurity States, Wayne E. Tefft, Robert John Bell, Harold V. Romero

Physics Faculty Research & Creative Works

A variational calculation of shallow surface impurity states including image-charge and anisotropic mass effects was performed using two different sets of orthogonal functions. The first set was obtained by varying the Bohr radius in the hydrogenic functions, separately for each state. In the second set the Bohr radius was allowed to become anisotropic, and the elements were separately treated as variational parameters. In each case it was shown that with certain restrictions on the variational parameters the set remained complete and orthogonal. General expressions for the energy eigenvalues for both the bulk and surface problem obtained from the first set …

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Charge-Transfer Cross Sections For Negative Ions On Atomic And Molecular Targets, W. (William) R. Snow, R. D. Rundel, R. Geballe Dec 1969

Charge-Transfer Cross Sections For Negative Ions On Atomic And Molecular Targets, W. (William) R. Snow, R. D. Rundel, R. Geballe

Physics Faculty Research & Creative Works

Charge-transfer cross sections for H- on O, O2, and NO2; O- on H, O, H2, O2, and NO2; and C- on H and O have been measured over an energy range of 0.5 to 4 keV using a modulated crossed-beam apparatus. The experimental cross sections are compared with theories of resonant and non-resonant charge transfer based on a two-state approximation. For the reactions H-+O O-+H, the cross sections are found to be consistent with detailed balancing. The effect of electron detachment on charge-transfer …

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Formation Of Carbides On Tungsten Wires, W. J. Croft, Kaare J. Nygaard Dec 1969

Formation Of Carbides On Tungsten Wires, W. J. Croft, Kaare J. Nygaard

Physics Faculty Research & Creative Works

It has been found that layers of tungsten carbide form on tungsten wires processed under ultra-high vacuum. The carbide is formed from carbon impurities in the volume of material.

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Heavy-Particle Energy-Loss Spectrometry: Inelastic Cross Sections For Protons Incident Upon Helium, John T. Park, F. D. Schowengerdt Dec 1969

Heavy-Particle Energy-Loss Spectrometry: Inelastic Cross Sections For Protons Incident Upon Helium, John T. Park, F. D. Schowengerdt

Physics Faculty Research & Creative Works

A method is described whereby absolute cross sections are obtained from inelastic energy-loss spectra induced by ions incident upon gaseous targets at energies of 25-125 keV. Inelastic energy-loss spectra of helium for proton impact are presented. Apparent differential cross sections and absolute total cross sections are obtained from the spectra. Total cross sections for the sum of the 1S1→2S1 and 1S1→2P1 excitations are reported, along with estimates of the relative contributions of each. Total ionization cross sections, total cross sections for inelastic scattering, apparent energy distributions of ejected electrons, and partial ionic stopping powers are also reported. The excitation cross …

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Infrared Reflectivity Of Doped Insb And Cds, Thomas J. Mcmahon, Robert John Bell Dec 1969

Infrared Reflectivity Of Doped Insb And Cds, Thomas J. Mcmahon, Robert John Bell

Physics Faculty Research & Creative Works

The coupling of free carriers and lattice-vibrational modes in CdS and InSb was investigated by using reflectance measurements. When the free-carrier concentration is such that the longitudinal-optical-mode frequency is at or near the plasma frequency, the longitudinal mode takes on a mixed character and is shifted in a manner similar to that described by Varga in 1965, while the transverse mode remains fixed. The electron effective masses in such a situation can be determined with the lattice effects excluded. In the case of CdS, the polaron mass and additional absorption in the plasma mode above the longitudinal-optical phonon frequency must …

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Mössbauer Effect In The Ferroelectric Pbtio₃-Bifeo₃ Solid Solutions, C. M. Yagnik, James P. Canner, Robert Gerson, William Joseph James Nov 1969

Mössbauer Effect In The Ferroelectric Pbtio₃-Bifeo₃ Solid Solutions, C. M. Yagnik, James P. Canner, Robert Gerson, William Joseph James

Physics Faculty Research & Creative Works

The Mössbauer spectra of the system (PbTiO3)0.95(BiFeO3)0.05 have been studied from room temperature to 585°C. From the isomer-shift of 0.491±0.02 mmsec-1 at room temperature, the ionicity is found to be 60% ± 20%. A discontinuity in the plot of isomer-shift versus temperature at Tc is observed. The quadrupole splitting at room temperature is 0.295 ± 0.02 mm/sec-1. A dip in the Mössbauer fraction is observed at the Curie temperature.

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Consistency And Cluster Size In The Effective Field Theories Of Ferromagnetism, J. G. Chervenak, Harry A. Brown Jan 1969

Consistency And Cluster Size In The Effective Field Theories Of Ferromagnetism, J. G. Chervenak, Harry A. Brown

Physics Faculty Research & Creative Works

No abstract provided.

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Mixed Magnetic Systems: A Model With Application To (1-X)Cdcr2s4 · Xcdcr2se4, Harry A. Brown Jan 1969

Mixed Magnetic Systems: A Model With Application To (1-X)Cdcr2s4 · Xcdcr2se4, Harry A. Brown

Physics Faculty Research & Creative Works

The Curie temperatures and asymptotic Curie temperatures are calculated for mixed magnetic systems by a semi-classical Bethe-Peierls-Weiss cluster approach. The systems are typified by (1-x) CdCr2Se4 · xCdCr2Se4 in which the substitution changes the anions but not the magnetic cations. It is found that next-nearest-neighbor interactions are needed to match the data but that, with them, excellent agreement can be obtained for a reasonably unique choice of the exchange integrals. © 1969.

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A Determination Of The Speed Of Light By The Phase-Shift Method, John Rogers, Ronnie Mcmillan, Robert Pickett, Richard A. Anderson Jan 1969

A Determination Of The Speed Of Light By The Phase-Shift Method, John Rogers, Ronnie Mcmillan, Robert Pickett, Richard A. Anderson

Physics Faculty Research & Creative Works

A low-frequency, phase-shift method for the measurement of the speed of light has been developed. This technique gives results commensurate with other advanced laboratory methods. One advantage of this technique is that the apparatus is of reasonable size and most of the circuitry involves widely known amateur radio techniques. Furthermore, our use of a low modulating frequency permits use of the solid-state, electro-optical light shutter. This eliminates the rather dangerous liquid Kerr cell, thus making our apparatus more acceptable to application in the undergraduate advanced laboratory. From a pedagogical point of view, the student is allowed to use and become …

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Semiempirical Determination Of The Hydrogen Bond Energy For Water Clusters In The Vapor Phase. I. General Theory And Application To The Dimer, Richard W. Bolandee, James L. Kassner, Joseph T. Zung Jan 1969

Semiempirical Determination Of The Hydrogen Bond Energy For Water Clusters In The Vapor Phase. I. General Theory And Application To The Dimer, Richard W. Bolandee, James L. Kassner, Joseph T. Zung

Physics Faculty Research & Creative Works

A simple semiempirical method is given for determining the hydrogen bond energy for water clusters in the vapor phase. This method is based on a general statistical-mechanical theory of clustering. The partition function for a system of clusters is used to determine the equilibrium distribution of clusters. In conjunction with available thermodynamic and spectroscopic data, the cluster equilibrium constants can be used to calculate the cluster potential energy and the hydrogen bond energy. Results for the water dimer agree quite well with other reported values obtained either by quantum-mechanical calculations or approximate thermodynamic estimates. A correct temperature dependence of the …

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Rotational Excitation Of Polar Molecules By Electrons, Marvin H. Mittleman, Jerry Peacher, Balazs F. Rozsnyai Dec 1968

Rotational Excitation Of Polar Molecules By Electrons, Marvin H. Mittleman, Jerry Peacher, Balazs F. Rozsnyai

Physics Faculty Research & Creative Works

Rotational excitation of polar molecules is calculated in the approximation that the electron transit time is short compared with rotational periods with the result of an E-1 behavior of the cross section. Diffusion cross sections are calculated for Δl=0,1,2. Significant corrections to the Born approximation are obtained for large dipole moments. The range (in energy) of applicability of the result is discussed in terms of the energy dependence of the corrections, and a novel energy dependence of these corrections is encountered and explained.

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Measurement Of Positron Annihilation Line Shapes With A Ge(Li) Detector, H. (Henry) P. Hotz, J. M. Mathiesen, J. P. Hurley Dec 1968

Measurement Of Positron Annihilation Line Shapes With A Ge(Li) Detector, H. (Henry) P. Hotz, J. M. Mathiesen, J. P. Hurley

Physics Faculty Research & Creative Works

We observed that the annihilation radiation photopeak in a Ge(Li) detector is considerably broader than that of a γ ray of the same energy. It seems reasonable to assume that the increased width is the result of the Doppler broadening of the annihilation photopeak, i.e., the longitudinal Doppler shift of the radiations is measured, while the transverse shift is measured in the usual angular-correlation experiments. By using a computer stripping program to remove the distortion produced by the finite energy resolution of our detector, we obtain momentum distributions in agreement with those which have been published. Only one detector is …

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Longitudinal Optical Phonon-Plasmon Coupling In Cds, Robert John Bell, Thomas J. Mcmahon, Donald G. Rathbun Dec 1968

Longitudinal Optical Phonon-Plasmon Coupling In Cds, Robert John Bell, Thomas J. Mcmahon, Donald G. Rathbun

Physics Faculty Research & Creative Works

The longitudinal optical phonon-plasmon interaction theory of Varga is shown to be applicable to Gadoped CdS through the region of the first reflectivity minumum (2.5 to about 30 μ). The theoretical reflectivity vs wavelength for several concentrations of Ga-doped CdS are compared with experiment. The agreement between theory and experiment is particularly good for high dopant concentrations. A plot of the wavelength at minimum reflectivity vs impurity concentrations shows that the Varga theory does well in explaining the data. When phonon damping is introduced into the Varga dielectric function, the reflectivity over the whole region of resonance is more realistic. …

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Modified Phase Representation And Effects Of Inelasticity In N/D Calculation Of P-Wave Pion-Pion Scattering, Barbara N. Hale, Arnold Tubis Oct 1968

Modified Phase Representation And Effects Of Inelasticity In N/D Calculation Of P-Wave Pion-Pion Scattering, Barbara N. Hale, Arnold Tubis

Physics Faculty Research & Creative Works

An N/D formalism based on a modified phase representation is used to study the effects of inelasticity on the ρ-wave pion-pion amplitude. The effects of high-energy inelasticity are introduced in terms of the assumed behavior of the high-energy phase (not phase shift) of the partial-wave amplitude. Using a ρ-exchange input force with the experimental ρ mass and a ρ width of about 100 MeV, and the assumption that the average phase is (1/2)π, for total c.m. energies greater than about 8Mπ, we find that there is no appreciable reduction in the width of the calculated ρ-wave resonance. We …

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