Showing papers on "Energy (signal processing) published in 1973"

Resonances in Electron Impact on Diatomic Molecules

Abstract: In this review we present the energies, configuration, and other properties of resonances (also called "compound states" and "temporary negative ions") in diatomic molecules. Much of the information is presented in the form of tables and energy level diagrams. Vibrational, rotational, and electronic excitation are discussed whenever these processes have given information on resonances; often these excitation processes proceed via resonances. The paper is divided according to molecular species (${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, CO, NO, ${\mathrm{O}}_{2}$), but the main conclusions are discussed by the nature of the processes involved.

Electronic Structure of AgIn Se 2 and CuIn Se 2

Abstract: We report electroreflectance and photoluminescence studies of the chalcopyrite compounds AgIn${\mathrm{Se}}_{2}$ and CuIn${\mathrm{Se}}_{2}$. Observation of photoluminescence at low temperatures at the same energy as the direct energy gaps located by electroreflectance measurements confirms that both compounds have direct band gaps. At 300 \ifmmode^\circ\else\textdegree\fi{}K, the values for the energy gaps are 1.24 and 0.96 eV, respectively. The spin-orbit splittings of the uppermost valence bands as observed in electroreflectance measurements are considerably less than expected for $p$ levels, a result which we attribute to \ensuremath{\sim} 17% hybridization of Ag $4d$ levels, and \ensuremath{\sim} 34% hybridization of Cu $3d$ levels, with the otherwise $p$-like valence bands. An ultraviolet electroreflectance structure observed in CuIn${\mathrm{Se}}_{2}$ may result from transitions from the $d$ levels themselves to the lowest conduction-band minimum. The crystal-field and spin-orbit parameters for the uppermost valence bands of CuIn${\mathrm{Se}}_{2}$ disagree with values found in a recent energy-band calculation ignoring $d$ bands, a calculation which also predicted that CuIn${\mathrm{Se}}_{2}$ has an indirect energy gap. We also observe an anomalous temperature dependence of the energy gap in AgIn${\mathrm{Se}}_{2}$. Whereas the energy gap in CdSe (the binary analog of AgIn${\mathrm{Se}}_{2}$) decreases by approximately 80 meV as the temperature increases from 77 to 300 \ifmmode^\circ\else\textdegree\fi{}K, the energy gap of AgIn${\mathrm{Se}}_{2}$ is independent of temperature over this range within experimental error (\ifmmode\pm\else\textpm\fi{} 5 meV).

Electronic Structure of Transition Metals. I. Quantum Defects and Model Potential

Abstract: Recent applications of the concept of quantum defects in setting up model pseudopotentials for simple or transition-metal ions presuppose that the atomic-spectroscopy data of such metals can be expressed in terms of certain quantum defects. Experience indicates, however, that the old quantum-defect idea applies only to group I and II metals at best, but not to metals of higher chemical valence ($z\ensuremath{\ge}3$, $z$ being the nominal valence given by the group number, $\mathrm{I}A$ or $B$, $\mathrm{II}A$ or $B$, etc. in the Periodic Table). In this paper, a generalized quantum-defect law applicable to elements having $z\ensuremath{\ge}2$ is deduced empirically from an extensive study of the spectroscopic data of the first six rows of the Periodic Table. The empirical law states that the energy levels ${E}_{nl}$ of a single electron in the field of the positive ions of elements having the same inert-gas core, e.g., the elements of the isoelectric sequence, ${\mathrm{Li}}^{+}$, ${\mathrm{Be}}^{2+}$,...,${F}^{7+}$, which are given (in the Heine-Abarenkov model-potential method) by the spectroscopic term value of the ion plus one electron, i.e., by the term values of Li, ${\mathrm{Be}}^{+}$,...,${\mathrm{F}}^{6+}$, by the term values of Li, ${\mathrm{Be}}^{+}$,..., ${\mathrm{F}}^{7+}$, obey the relation ${E}_{\mathrm{nl}}=\ensuremath{-}\frac{{z}^{2}}{{(n\ensuremath{-}{\ensuremath{\delta}}_{\mathrm{nl}})}^{2}}+{\ensuremath{\Delta}}_{\mathrm{nl}}$, $z\ensuremath{\ge}2$, for the same quantum defects (${\ensuremath{\delta}}_{\mathrm{nl}},{\ensuremath{\Delta}}_{\mathrm{nl}}$). The old quantum defect (${\ensuremath{\delta}}_{\mathrm{nl}}$) and the new quantum defect (${\ensuremath{\Delta}}_{\mathrm{nl}}$), equivalent to an "atomic core shift") thus represent the deviation of the atomic potential of a given inert-gas configuration from a Coulombic potential due to a nuclear charge $z|e|$. On the basis of this empirical law, the parameters of a transition-metal model potential of the Heine-Abarenkov type, adjusted to the energies ${\mathcal{E}}_{\mathrm{nl}}\ensuremath{\equiv}{E}_{\mathrm{nl}}\ensuremath{-}{\ensuremath{\Delta}}_{\mathrm{nl}}$, have been calculated for all 30 group-$B$ (excepting rare-earth) metals of the Periodic Table; and it has been found that the $l=2$ model potential parameter ${A}_{2}$ reflects the Ziman-Heine-Hubbard resonance model of $s\ensuremath{-}d$ hybridization through its strong energy dependence of the form ${(\mathcal{E}\ensuremath{-}{\mathcal{E}}_{3d})}^{\ensuremath{-}1}$ for the $3d$ series, and similarly for the $4d$ and $5d$ series. The application of the new model potential to the calculation of the various aspects of the electronic structure of solids will be presented in the next and subsequent papers of this series.

Electromagnetic Instabilities, Filamentation, and Focusing of Relativistic Electron Beams

Abstract: We report nonlinear studies of the Weibel instability of a relativistic electron beam in a plasma. If ${n}_{b}\ensuremath{\ll}{n}_{p}$, the beam splits into self-pinched filaments at density ${n}_{p}$. These filaments then recombine into a single dense beam, from which the return current is expelled. The ratio of final magnetic to final streaming energy is $O(\frac{\ensuremath{ u}}{\ensuremath{\gamma})}$, and significant plasma heating occurs.

K-Vacancy Sharing in Near-Symmetric Heavy-Ion Collisions

Abstract: Relative production cross sections for beam (${Z}_{1}$) and target (${Z}_{2}$) $K$ vacancies in heavyion collisions are explained for situations where ${Z}_{1}\ensuremath{\simeq}{Z}_{2}$. Electron promotion creates a vacancy in the $2p\ensuremath{\sigma}$ molecular orbital. As the collision partners retreat, the vacancy has a probability $w({v}_{1}, {Z}_{1}, {Z}_{2})l~\frac{1}{2}$ to be transferred to the $1s$ level of the higher-$Z$ collision partner. With a charge-transfer theory of Demkov, a universal form for $w$ is obtained, which is in excellent agreement with all available data for Ni, Br, and I beams over extended target and energy ranges.

Optical Properies of Graphite

Abstract: A three-dimensional Fourier expansion has been developed to describe the dispersion relations $E(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}})$ for the $\ensuremath{\pi}$ bands of graphite throughout the Brillouin zone. The coefficients of the Fourier expansion are evaluated by a fit to the experimentally determined parameters of the Slonczewski-Weiss model, as well as to the optical data. Using this energy-band model, the interband contribution to the frequency-dependent dielectric constant has been calculated in the energy range $0.5l\ensuremath{\hbar}\ensuremath{\omega}l6$ eV for both $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$. For this calculation, a numerical-integration procedure has been developed to carry out the full-zone integrations. The results for the dielectric constant are primarily dependent on the values of the McClure parameters ${\ensuremath{\gamma}}_{0}$, ${\ensuremath{\gamma}}_{1}$, ${\ensuremath{\gamma}}_{3}$, and ${\ensuremath{\gamma}}_{4}$, which enter the Slonczewski-Weiss Hamiltonian for graphite.

Complete Hydrogen and Helium Particle Spectra from 30- to 60-MeV Proton Bombardment of Nuclei with A = 12 to 209 and Comparison with the Intranuclear Cascade Model

Abstract: Differential cross sections for the production of proton, deuteron, triton, helium-3, and $\ensuremath{\alpha}$ particles from as many as 10 targets ($A=12\ensuremath{-}209$) were measured using 29-, 39-, and 62-MeV incident protons. The particles were detected, with $\ensuremath{\approx}0.2$-MeV [full width at half maximum (FWHM)] energy resolution for protons, over a secondary energy range of $\ensuremath{\approx}2\ensuremath{-}6 \mathrm{to} 62$ MeV in a total absorption telescope composed of three solid-state detectors. Representative results are shown for cross sections differential in energy and angle, as well as for angle-and energy-integrated cross sections. For incident 60-MeV protons the integral magnitude of the nonevaporation charged-particle production is found to be $\ensuremath{\sim}{10}^{2}{A}^{\frac{1}{3}}$ mb. Fewer protons but more complex particles were measured for carbon and oxygen targets than expected from an ${A}^{\frac{1}{3}}$ dependence for either component alone. The continuum cross sections for $z=1$ particles at a given angle (mb ${\mathrm{sr}}^{\ensuremath{-}1}$ Me${\mathrm{V}}^{\ensuremath{-}1}$) are nearly independent of incident energy when measured with incident protons in the 30- to 60- MeV energy range. Nonevaporation production of complex particles ($A\ensuremath{\ge}2$) is 25-40% of that for protons. The proton spectra have been compared with predictions from the intranuclear cascade model. Differential spectral predictions compare well with the measured spectra for angles in the range \ensuremath{\sim}25-60\ifmmode^\circ\else\textdegree\fi{}, and relatively poor predictions for small and large angles are more favorable when reflection and refraction by the potential well are included. Evidence is given that predictions for backward angles are greatly improved by allowing proton scattering from nucleon pairs within the model nucleus, but the $A$-dependent underprediction at extreme forward angles is not understood at all. The calculated angle-integrated spectra reproduce the measured spectral shape but consistently predict $\ensuremath{\approx}30%$ too few nonevaporation protons for targets with $A\ensuremath{\ge}27$.[NUCLEAR REACTIONS $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{27}\mathrm{Al}$, $^{54}\mathrm{Fe}$, $^{56}\mathrm{Fe}$, $^{60}\mathrm{Ni}$, $^{89}\mathrm{Y}$, $^{120}\mathrm{Sn}$, $^{197}\mathrm{Au}$, $^{209}\mathrm{Bi}$, ($p,{p}^{\ensuremath{'}}X$), ($p,dX$), ($p,tX$), ($p,^{3}\mathrm{He}X$), ($p,\ensuremath{\alpha}X$), $E=62,39,29$ MeV; semi; measured $\ensuremath{\sigma}(E;{E}_{{p}^{\ensuremath{'}}},{E}_{d},{E}_{{3}_{\mathrm{He}}},{E}_{\ensuremath{\alpha}},\ensuremath{\theta})$; deduced $\ensuremath{\sigma}(E)$. $2\ensuremath{\lesssim}{E}_{{p}^{\ensuremath{'}}},{E}_{d},{E}_{t},{E}_{{3}_{\mathrm{He}}},{E}_{\ensuremath{\alpha}}\ensuremath{\lesssim}70$ MeV. Comparisons with intranuclear cascade model.]

Band Structure of Mo S 2 and Nb S 2

Abstract: The simplified linear combination of muffin-tin orbitals method has been used to calculate the energy bands of Mo${\mathrm{S}}_{2}$ and Nb${\mathrm{S}}_{2}$. The calculated bands are in excellent agreement with photoemission data and provide a new interpretation of optical absorption spectra different from that of Wilson and Yoffe. The effect of the weak interlayer potential on the ${d}_{{z}^{2}}$ conduction band is shown explicitly.

Dispersion of Raman Cross Section in CdS and ZnO over a Wide Energy Range

Abstract: The dispersion of the Raman cross sections of several phonon modes in CdS and ZnO using a wide range of laser energies has been measured. The ${E}_{1}(\mathrm{LO})$ and multi-LO (2-LO, 3-LO and 4-LO) modes in CdS have been studied over the energy range extending from $0.45{E}_{\mathrm{ex}}$ (exciton energy) to $1.1{E}_{\mathrm{ex}}$. For the ${E}_{1}(\mathrm{LO})$ mode, both the "forbidden"\ifmmode\bullet\else\textbullet\fi{} ($\mathrm{zz}$) and allowed ($\mathrm{zx}$) geometries were employed. The dispersive behavior of the cross section in these two geometries is in agreement with theory for energies below ${E}_{\mathrm{ex}}$. The dispersion of multi-LO-phonon scattering is also presented. For ZnO, the dispersion of the cross section for the ${E}_{1}(\mathrm{LO})$, ${E}_{1}(\mathrm{TO})$, ${A}_{1}(\mathrm{TO})$, and ${E}_{2}$ modes was measured over the energy range of $(0.57\ensuremath{-}0.76){E}_{\mathrm{ex}}$. The behavior of the LO modes is expalined in terms of a cancellation between the deformation and Fr\"ohlich contributions to the LO scattering cross section. For the TO modes, resonant cancellation was not observed, in contrast to the results of recent studies of CdS and ZnS. A simple model is used to discuss these differences between the ZnO and CdS behavior. Finally, a resonant enhancement of the scattering from the 2-LO mode in ZnO when the scattered-photon energy was coincident with ${E}_{\mathrm{ex}}$ is reported.

X-Ray Photoelectron Band Structure of Some Transition-Metal Compounds

Abstract: The valence-band structures of MnO, CoO, NiO, CuO, ${\mathrm{Cu}}_{2}$O, Ni${\mathrm{Cl}}_{2}$, NiS, and KNi${\mathrm{F}}_{3}$ have been obtained by x-ray photoelectron spectroscopy. Band widths and positions are obtained, and are compared with the results of band-structure calculations. Bandwidths for the oxides are in accord with augmented-plane-wave calculations. Bands positions are better described by shifting free-ion energy levels according to the Madelung energy. The structure of the $d$ band of NiO is compared with other experimental and theoretical results.

Energy Relaxation of Photoexcited Hot Electrons in GaAs

Abstract: The energy relaxation of an initially hot photoexcited free-electron population has been investigated in high-purity GaAs at lattice temperatures ${T}_{L}=2 \mathrm{to} 4$ K. The electron energy distribution function $f(E)$ was determined directly from the line shape of the conduction-band-acceptor luminescence. Two different experiments---transient and steady state---were performed: (i) the time development of $f(E)$ in the electron energy range $0\ensuremath{\le}E\ensuremath{\le}8$ meV was measured with subanosecond time resolution after excitation with short light pulses (\ensuremath{\sim} 0.2 nsec, $\ensuremath{\hbar}\ensuremath{\omega}=1.92$ eV), and (ii) the steady-state dependence of $f(E)$ on cw-excitation-light power was measured. The experimental results are compared with theoretical energy relaxation rates using the known standard electron-phonon scattering mechanisms. In the electron temperature range above \ensuremath{\sim} 16 K both experiments indicate higher energy-loss rates than expected theoretically. At lower electron temperatures good agreement with theory is obtained. It is shown that interelectronic collisions effectively randomize the electron energies during the relaxation process even for the low electron densities (${n}_{e}$ as low as 2 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) achieved in the experiments.

Pauli-Force Model Potential for Solids

Abstract: A model potential previously applied to molecules is adapted to the study of structural trends in solids. The potential, which has simple analytic one-electron eigenfunctions and eigenvalues, is based upon a Pauli-force concept and retains the salient features of ab initio pseudopotentials. The eigenvalues are of the quantum-defect form, so that treatment of the energy dependence of the potential parameter is particularly straightforward. A form factor is calculated in a local-screening approximation, and algebraic expressions for a core radius ${r}_{c}$ and the form-factor node ${q}_{0}$ are obtained. These expressions have transparent physical interpretations, and form the basis for a discussion of chemical trends in ${r}_{c}$ and ${q}_{0}$ in terms of a few simple parameters. The connection with well-known local model potentials is briefly explored.

Electron correlations and moment sum rules

Abstract: A theory of electron correlations based on a generalized random-phase approximation is presented An expression for the local-field correction is obtained using the third frequency moment of the spectral function of the electron-density response function The local field is a functional of the structure factor $S(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ which in turn is related to the imaginary part of the dielectric function via the fluctuation-dissipation theorem The equations are self-consistently solved to determined $S(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ as originally suggested by Singwi et al The pair-correlation function, compressibility, plasma dispersion, and correlation energy of electron liquid at metallic densities have been calculated

Photoabsorption and Charge Oscillation of the Thomas—Fermi Atom

Abstract: The complications of calculating the photoabsorption cross section of atoms for frequencies in the far ultraviolet and soft x-ray regions motivate an analysis of this problem on the basis of the Bloch semiclassical model of hydrodynamic charge oscillation in the neutral Thomas-Fermi atom. The hydrodynamic modes of oscillation of the neutral Thomas-Fermi atom have a continuous frequency spectrum. The resulting photoabsorption cross section is a continuous function of frequency which scales with atomic number $Z$ and in this sense is a universal cross section, approximately applicable to all heavy atoms. Numerical solutions of the normal mode functions of dipole charge oscillation are used to calculate the photoabsorption cross section in a range of photon energies $0.816 \mathrm{Z}\mathrm{eV}l\ensuremath{\Elzxh}\ensuremath{\omega}l272 \mathrm{Z}\mathrm{eV}$, the range where the model is expected to be most realistic. In this range the semiclassical hydrodynamic cross section agrees with experimental data for the noble gases as well as could be expected for a cross section applicable to all atoms. The model cross section, extended to zero and infinite frequencies by analytical calculation, checks the sum rule to within 2%; but gives a value $I=4.95$ $Z$ eV for the logarithmic mean excitation energy of stopping power formulas. The unrealistically low value of $\frac{I}{Z}$ results because the Thomas-Fermi atom exaggerates the number of electrons which absorb at low frequencies. Use of the hydrodynamic cross section in the approximate range of validity of this model $\ensuremath{\Elzxh}\ensuremath{\omega}g0.816$ Z eV gives $I=12.4{e}^{\ensuremath{\beta}}$ Z eV, where $\ensuremath{\beta}$ depends upon the oscillator strength below $\ensuremath{\Elzxh}\ensuremath{\omega}=0.816$ Z eV which cannot be satisfactorily determined from the Bloch model. Used within its limitations the Bloch model of hydrodynamic oscillation in the statistical atom provides a useful method of estimating photoabsorption cross sections and could possibly be applied to other atomic processes.

Electronic Properties of Complex Crystalline and Amorphous Phases of Ge and Si. II. Band Structure and Optical Properties

Abstract: We present calculations of the band structures and the imaginary part of the dielectric function ${\ensuremath{\epsilon}}_{2}$ as a function of energy for Ge and Si in the diamond, wurtzite, Si-III (BC-8) and Ge-III (ST-12) structures using the empirical pseudopotential method. In particular we have obtained the symmetries of wave functions along important symmetry directions and identified the major contributions to the optical structure. A further study is made into the optical properties of amorphous Ge and Si using our short-range-disorder model. We find that, unlike long-range-disorder models, short-range disorder can explain both the amorphous density of states and the amorphous ${\ensuremath{\epsilon}}_{2}$. In particular we find that the ${\ensuremath{\epsilon}}_{2}$ spectrum has the same form as an averaged matrix element as a function of frequency.

Static wall coercive force in ferromagnetic thin films

Abstract: The static wall coercive force of thin ferromagnetic films has been calculated from a model of conservative wall energy. The spatial dependence of the wall energy γ w is assumed to be given by the mean value of the local anisotropy energy, averaged over a coupling volume of the wall. The result of the calculation shows that H_{w} \propto \frac{S}{M_{s}} (\frac{D}{W})^{1/2} \frac{1}{L} where the structure constant S includes the local anisotropy, D is the mean diameter of a region with constant anisotropy (for instance crystallites), W is the wall width; and L is the coupling length parallel to the wall.

Erratum: Spin-wave analysis of the quadratic-layer antiferromagnets K 2 NiF 4 , K 2 MnF 4 , and Rb 2 MnF 4

Abstract: In this and the following two papers, low-temperature spin-wave properties of quadratic-layer antiferromagnets having the ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$ structure are reported and analyzed in detail. Here we present the results of a least-squares adjustment of spin-wave theory to the temperature variation of the sublattice magnetization in the compounds ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$, ${\mathrm{K}}_{2}$Mn${\mathrm{F}}_{4}$, and ${\mathrm{Rb}}_{2}$Mn${\mathrm{F}}_{4}$, as reflected by $^{19}\mathrm{F}$ NMR frequency measurements in zero field. Lowest-order temperature-dependent and temperature-independent corrections to simple spin-wave theory, as formulated by Oguchi, are included in the analysis. The free parameters of the fits are taken to be the exchange coupling, the zero-temperature spin-wave gap energy, and the zero-temperature $^{19}\mathrm{F}$ NMR frequency. Our conclusions are as follows. Spin-wave theory accounts for the sublattice magnetization of these compounds up to somewhat less than one-half the N\'eel temperature, with the temperature-dependent corrections yielding less than 20% improvement in the range of fit for the ${\mathrm{Mn}}^{2+}$ compounds and a negligible improvement for ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$. The breakdown of spin-wave theory is clearly not ascribable to spin-wave interaction effects and is apparently caused by excitations of a fundamentally different nature. Exchange values obtained are in excellent agreement with data from neutron and susceptibility measurements. The "effective" spin-wave-energy-gap values obtained give some evidence for interplanar exchange coupling between second-neighbor planes, yielding upper limits for such coupling of a few parts in 1${0}^{4}$ of the primary exchange. Earlier conclusions regarding the large zero-point spin reduction in ${\mathrm{K}}_{2}$Ni${\mathrm{F}}_{4}$ are refined here, giving a result slightly larger than but within error limits of the spin-wave-theory value (17.7%).

Transition Probabilities and Differential Cross Sections for Vibrational Excitation in Collisions of H + with H 2 , HD, and D 2

Abstract: The angle and energy dependence of the differential cross sections are reported for resolved vibrational transitions in low-energy collisions of ${\mathrm{H}}^{+}$ with ${\mathrm{H}}_{2}$, HD, and ${\mathrm{D}}_{2}$. The specific transition probabilities for the different isotopic molecules scale in the order ${\mathrm{D}}_{2}$ g HD g ${\mathrm{H}}_{2}$ at the same initial kinetic energy and scattering angle, but when compared at the same reduced kinetic energy, ${E}_{r}=\frac{E}{\ensuremath{\hbar}\ensuremath{\omega}}$, are approximately equal, suggesting that the interaction leading to vibrational excitation in this system is primarily the dilution of the molecular bond as the proton passes. We discuss the results in terms of a semiclassical model for vibrational excitation based on an oscillator driven by a time-dependent force.

Energy Bands in Paramagnetic Chromium.

Abstract: The results of a self-consistent tight-binding calculation of the band structure of paramagnetic chromium are reported. The basis set consisted of atomic wave functions for the $1s$, $2s$, $3s$, $4s$, $2p$, $3p$, and $4p$ states expressed as linear combinations of Gaussian-type orbitals (GTO) and five individual GTO's for each $3d$ state. The exchange potential was calculated according to the $X\ensuremath{\alpha}$ method with $\ensuremath{\alpha}=\frac{2}{3}$. The initial Coulomb potential was constructed from the superposed charge densities of neutral chromium atoms in a $3{d}^{5}4{s}^{1}$ configuration. Eleven iterations were required to determine a self-consistent potential. The charge density was sampled at 55 inequivalent points in 1/48 of the Brillouin zone. Energy bands were calculated with the use of the self-consistent potential at 819 points in 1/48 of the zone. The density of states was calculated according to the Gilat-Raubenheimer method. Cross sections of the Fermi surface were obtained in several symmetry planes. The x-ray from factor was determined from the self-consistent wave functions.

Optical Measurements of Surface Plasmons in Gold

Abstract: The absorption associated with surface plasmons at a gold-air interface is measured using an evanescent wave coupler. The measurements cover the energy range 1.8-3 eV. Scans made for different values of surface wave vector in the range 1.2 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$-2.7 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ give the dispersion of the surface plasmon both for the gold-air interface and for a gold-silica interface, and also yield the optical constants of gold. Good agreement is obtained with lower-energy measurements of the dispersion by Ritchie et al. An analysis of the range of surface plasmons is presented using the present data and the Drude approximation for extrapolation into the infrared where the range increases to several centimeters.

Associative Detachment of O − with CO, H 2 , and O 2

Abstract: The associative detachment reactions of ${\mathrm{O}}^{\ensuremath{-}}$ with CO, ${\mathrm{H}}_{2}$, and ${\mathrm{O}}_{2}$ are studied experimentally using an ion beam (0-10 eV) impinging on a gas-filled chamber. The resultant electrons are energy analyzed using retarding potentials. At low ion energy, cross sections for total electron detachment for the reactions of ${\mathrm{O}}^{\ensuremath{-}}$ with CO and ${\mathrm{H}}_{2}$ vary as the inverse of the relative velocity. At higher energies, the cross sections show a broad peak. We interpret this shape in terms of the potential-energy curves of the compound states. The energy distributions for the electrons resulting from the reactions of ${\mathrm{O}}^{\ensuremath{-}}$ with CO and ${\mathrm{H}}_{2}$ exhibit peaks at low energy corresponding to the excitation of high vibrational and rotational states of the resultant triatomic molecules (C${\mathrm{O}}_{2}$, ${\mathrm{H}}_{2}$O). Progressions observed in these distributions have spacings corresponding to the preferential excitation of the bending modes of the triatomic molecules. The cross section for total electron detachment for the reaction of ${\mathrm{O}}^{\ensuremath{-}}$ with ${\mathrm{O}}_{2}$ exhibits a threshold near 0.7 eV and rises linearly at higher energy.

Theory of Hot Electrons in Strong Magnetic Fields. I

Abstract: Hot electron transport properties of semiconductors in strong magnetic fields have been investigated theoretically in the case of transverse electric and magnetic fields with the principal objective of elucidating the experiments on n -InSb at low temperatures. The model takes account of quantized subband structures and related periodic change of scattering rates with the energy of the electrons. The energy distribution function at high electric fields has a sharp cutoff at the optical phonon energy and deviates strongly from the Maxwellian shape. The average energy and conductivity of the electrons at high electric fields vary periodically with magnetic field and have extrema at the magnetophonon resonance condition: \(\hbar\omega_{\text{op}}{=}n\hbar\omega_{\text{c}}(n{=}1,2,3,\dots)\). The average energy is maximum and the transverse resistance is minimum at the resonance.

Relativistic electron beam heating of a fully ionized plasma

Abstract: Relativistic electron beams with $0.4l\frac{\ensuremath{ u}}{\ensuremath{\gamma}}l2.2$ have been used in this experiment to heat a high-density [$n\ensuremath{\sim}(5\ensuremath{-}8)\ifmmode\times\else\texttimes\fi{}{10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$] fully ionized hydrogen plasma immersed in a magnetic field. The resulting heating levels are strongly dependent on the effective diode impedance $Z=\frac{V}{I}$ and, correspondingly, the $\frac{\ensuremath{ u}}{\ensuremath{\gamma}}$ of the beam. Measurement of the total plasma energy from the plasma diamagnetism and the ion energy distribution from analyses of charge-exchange neutral atoms are used to determine scaling laws for the beam-plasma interaction.

Theory of Ortho-Para Conversion and its Effect on the NMR Spectrum of Ordered Solid Ortho-Hydrogen

Abstract: The NMR spectrum of a system of nuclear spins in thermal equilibrium is directly proportional to $\frac{\ensuremath{\mu}{H}_{0}}{kT}$, where $\ensuremath{\mu}{H}_{0}$ is the difference in Zeeman energy between adjacent nuclear magnetic states and $T$ is the temperature. The metastable system, solid ortho-hydrogen, is far from thermal equilibrium because of the large rotational energy of the ($J=1$) molecules. Thus the populations of the three magnetic states of the ($I=1$) total-nuclear-spin wave functions are affected not only by the magnetic field and the temperature but also by the rate of ortho-para conversion from each of the three states. In this paper we calculate the difference $D$ between the ortho-para conversion rates from the ${m}_{I}=0$ and the ${m}_{I}=\ifmmode\pm\else\textpm\fi{}1$ states for a crystal of ortho-hydrogen in the ordered state. It is found that $D$ depends on ($3{cos}^{2}\ensuremath{\beta}\ensuremath{-}1$), where $\ensuremath{\beta}$ is the angle between the magnetic field and the symmetry axis of the molecular wave function. We then compute the steady-state populations of the nuclear-spin states as a function of $\frac{\ensuremath{\mu}{H}_{0}}{kT}$, $D$, and the nuclear spin-lattice relaxation time ${T}_{1}$. These are used to calculate the shape of the NMR spectrum of a powder sample for values of ${T}_{1}$ which are appropriate to the ordered state. The result is that the usual Pake line shape is distorted by an enhancement which is linear in frequency shift and proportional to ${T}_{1}D$. An expression is also derived for the average ortho-para conversion rate as a function of molar volume and the Debye energy which shows that the conversion rate, which we have calculated for the two-phonon process, is negligible below 20 ${\mathrm{cm}}^{3}$/mole. By contrast, experiments show that at this molar volume the rate increases sharply with $\frac{1}{V}$. Our conclusion is that the increasing rate is due to a one-phonon process which is only effective for $V$ less than about 22 ${\mathrm{cm}}^{3}$/mole.

Observation and Interpretation of Angular Oscillations in 48 Ca( 14 N, 13 C) 49 Sc

Abstract: We describe a detailed and successful experimental search for fine angular oscillations in the reaction $^{48}\mathrm{Ca}$($^{14}\mathrm{N}$, $^{13}\mathrm{C}$)$^{49}\mathrm{Sc}$ at a laboratory energy of 50 MeV. Also presented is a qualitative, analytic description of the reaction extracted from the distorted-wave Born-approximation partial-wave analysis in which the universality of the oscillations is clearly indicated.

Communicating over power lines

Abstract: An audiofrequency signal is applied to a power amplifier having an output impedance of considerably less than an ohm that is coupled through a network comprising at least a capacitor that presents a low impedance to this audiofrequency signal and much higher impedance to energy at the 60 Hz power frequency to a power line to provide an amplified signal that is transmitted to a receiver separated from the transmitter by at least one distribution transformer and having a high pass filter for rejecting the 60 Hz power frequency and a number of harmonics thereof while transmitting the audiofrequency signal. The system is especially advantageous for using ordinary power lines for transmitting information in connection with automatic meter reading.

Low-Lying Energy Spectrum of a One-Dimensional Disordered System

Abstract: The energy spectrum of a particle in the presence of random fixed finite-ranged repulsive potentials is investigated at low energy. The resulting asymptotically exact density of states is derived. It exhibits an exponential dependence on energy. This result is shown to be equivalent, for the case of $\ensuremath{\delta}$-function potentials, to that of Frisch and Lloyd (1960) in the limit of low energy.

Lepton pair production from two-photon processes

Abstract: An exact expression for the lepton-pair mass spectrum for an ${\ensuremath{\alpha}}^{4}$ two-photon process in lepton-lepton, lepton-hadron, and hadron-hadron scattering processes is derived. This result is applied to muon pair production in proton-proton scattering to show that such a process is an important background to the ${\ensuremath{\alpha}}^{2}$ one-photon process in certain energy ranges and can become physically significant by itself at very high energies. The general physical significance of such a two-photon process in hadron-hadron scattering is discussed, and comparison of our exact expression with some approximation schemes is made. The main differences between this work and earlier papers on the subject are that (1) exact calculations are given and (2) the inelastic contributions are included.

Relativistic Energy Band Structure and Properties of γ-Uranium

Abstract: The electronic band structure of the high-temperature phase of uranium has been determined by means of the symmetized relativistic augmented-plane-wave method. Six different crystal potentials (three atomic starting configurations, $5{f}^{4}7{s}^{2}$, $5{f}^{3}6{d}^{1}7{s}^{2}$, and $5{f}^{2}6{d}^{2}7{s}^{2}$, each taken together with $\ensuremath{\alpha}=\frac{2}{3}$ and $\ensuremath{\alpha}=1$ approximations for exchange) were employed in the warped-muffin-tin approximation. The relativistic effects are found to be very important and result in $5f$ bands which overlap and hybridize strongly with the very broad "$7s\ensuremath{-}p$" and broad $6d$ bands (which in the absence of the $5f$ states are found to be those typical of a high-atomic-number transition metal). The nonrelativistic energy bands are found to be incorrect in many ways. A calculated density of states shows considerable structure reflecting the $s\ensuremath{-}d\ensuremath{-}f$ hybridization and a relatively high density of states (1.45 states per atom eV) at the Fermi energy. The Fermi surface is found to be complicated and to consist of two hole and one electron surfaces.

Passive identification system

Abstract: A transceiver-responder identification system which utilizes a swept frequency spectrum signal in the microwave frequency range. The energy generated by the transceiver portion is transmitted in a predetermined number of discrete frequency steps within a frequency octave. This swept spectrum signal is received by the responder portion, which doubles the stepped frequencies comprising the swept spectrum signal, and then routes the signal through a selective filter arrangement comprising a transmission line bounded by a series of directional ring filters, each filter being tuned to one of the frequencies in the swept spectrum signal. If the ring filter corresponding to a specific frequency is physically complete, the signal energy present at that frequency will be coupled into the ring and subsequently dissipated in a load. If the ring filter corresponding to a specific frequency is not physically complete, the signal energy at that frequency continues along the transmission line. This remaining signal energy is then retransmitted to the transceiver portion. By arranging complete and incomplete rings in a unique pattern for each responder in the system, each responder will then transmit a signal unique to the animal or article bearing the responder, which signal is decoded by the transceiver and coverted to a corresponding digital word comprising ones and zeros. This digital word provides a common format for article identification purposes. The ones and zeros correspond to the presence or absence of return signal energy at the various frequencies originally generated and transmitted to the responder.