Showing papers on "Wavelength published in 1977"

Exchange-correlation energy of a metallic surface: Wave-vector analysis

Abstract: The exchange-correlation energy of a jellium metal surface is analyzed in terms of the wavelength of the fluctuations that contribute to it, using a three-dimensional scheme different from that used by other authors. It is shown that with this scheme there exists an exact limiting form at long wavelengths which includes all many-body correlations and which is independent of the surface density profile. The local-density approximation is formulated as a function of wavelength, and it is shown to be exact at short wavelength. The interpolation scheme between these limits, which was discussed previously, is formulated and checked more completely and used to calculate surface energies.

Large-scale heterogeneities in the lower mantle

Abstract: Coefficients of a spherical harmonic expansion, up to angular order 3, of velocity anomalies in five shells within the earth's mantle were obtained from an analysis of nearly 700,000 P wave travel time residuals. The results for depths less than 1100 km are unreliable; on the basis of tests and numerical experiments we infer that lateral heterogeneities in this depth interval are dominated by velocity perturbations of lateral dimensions less than 5000 km. Relatively large wavelength features were resolved below 1500-km depth; the average perturbation level increases in the lowermost mantle. The region between 1100 and 1500 km may represent a transition zone with respect to the dimensions of anomalies. We present statistical evidence for a negative correlation between the long wavelength gravity anomalies observed at the surface and those computed from velocity anomalies at depths greater than 1100 km under the assumption of proportionality between velocity and density perturbations. The proportionality coefficient Δυ/Δρ has been determined by using two different methods: the values are −4.45 and −6.02 (km/s)/(g/cm3). Only minor changes in the velocity distribution are needed to satisfy the long wavelength gravity field exactly. Possible origins of the correlation are (1) sinking of eclogite-rich material into the lower mantle from regions of lithospheric subduction, (2) chemical plumes of light high-velocity material originating near the core-mantle boundary, (3) temperature differences and perturbations of the core-mantle boundary and the earth's surface associated with mantle-wide convection, or (4) static chemical heterogeneities in a nonconvecting mantle. The first three alternatives, all involving flow in the lower mantle, may be complementary but act on a different time scale. There appears to be a westward or northwestward translation of some anomalies with respect to the pattern obtained for the innermost shell. In particular, the direction of translation of a large negative anomaly under the Pacific is in agreement with the sense of motion of the Pacific plate. We must caution the reader, however, that this is a highly speculative interpretation. If correct, it would favor the convective hypotheses.

Observation of Resonances in the Radiation Pressure on Dielectric Spheres

Abstract: We report an experimental check of the Mie-Debye theory for the variation of radiation pressure on dielectric spheres with wavelength and size using optical-levitation techniques. Sharp resonances are observed which are shown to be related to dielectric surface waves. They permit particle-size measurement to a precision of 1 part in ${10}^{5}$ to ${10}^{6}$.

Enhanced elastic modulus in composition‐modulated gold‐nickel and copper‐palladium foils

Abstract: The biaxial elastic modulus Y[111] has been measured by bulge testing in Au‐Ni and Cu‐Pd foils containing short‐wavelength one‐dimensional composition modulations produced by vapor deposition. As compared with homogeneous foils of the same average composition, the modulated foils exhibited an appreciable increase in modulus—from 0.21 to 0.46 TPa for Au‐Ni and from 0.27 to 1.31 TPa for Cu‐Pd. For the latter system, the increase was found to be proportional to the square of the amplitude of the modulation. The enhancement of the modulus decreased with increasing wavelength and for wavelengths greater than 3 nm the modulus was the same as that for homogeneous foils. It was also observed that the deformation was non‐Hookian; the slope of the stress‐strain curves decreased with increasing strain.

Steep gravity waves in water of arbitrary uniform depth

Abstract: Modern applications of water-wave studies, as well as some recent theoretical developments, have shown the need for a systematic and accurate calculation of the characteristics of steady, progressive gravity waves of finite amplitude in water of arbitrary uniform depth. In this paper the speed, momentum, energy and other integral properties are calculated accurately by means of series expansions in terms of a perturbation parameter whose range is known precisely and encompasses waves from the lowest to the highest possible. The series are extended to high order and summed with Pade approximants. For any given wavelength and depth it is found that the highest wave is not the fastest. Moreover the energy, momentum and their fluxes are found to be greatest for waves lower than the highest. This confirms and extends the results found previously for solitary and deep-water waves. By calculating the profile of deep-water waves we show that the profile of the almost-steepest wave, which has a sharp curvature at the crest, intersects that of a slightly less-steep wave near the crest and hence is lower over most of the wavelength. An integration along the wave profile cross-checks the Pade-approximant results and confirms the intermediate energy maximum. Values of the speed, energy and other integral properties are tabulated in the appendix for the complete range of wave steepnesses and for various ratios of depth to wavelength, from deep to very shallow water.

Infrared Astronomical Satellite

Abstract: The objective of the Infrared Astronomical Satellite is to produce an unbiased all-sky survey in the wavelength region between 8 and 120 microns. Using a 60 cm diameter helium cooled telescope and detector arrays which are essentially zodiacal light background photon noise limited, heretofore unprecedented sensitivity can be achieved. The optical design, the focal plane layout and expected performance of the current design concept are discussed.

Theory of Hot-Electron Spectra at High Laser Intensity

Abstract: A simple model involving resonant absorption in a self-consistent sharp density gradient is found to be in quantitative agreement with the suprathermal-electron spectrum from recent laser-plasma--interaction experiments and to explain qualitatively the scaling of that suprathermal-electron spectrum with laser power and wavelength.

Optical properties of rough surfaces: General theory and the small roughness limit

Abstract: The diffraction of electromagnetic waves from the rough surface of a material of finite permittivity is examined for the case where the wavelength of the incident radiation is comparable to the dimensions of the surface roughness. Two methods of calculation studied are the Rayleigh method and the extinction-theorem integral-equation method. The latter is shown to have a unique exact solution. This property is, in turn, used to show how the Rayleigh method can be modified to give convergent results. The extinction theorem is also used to reduce the Rayleigh equations to a simpler form. These reduced equations, which are extremely convenient to use in the case of small roughness, are applied in this case to find perturbative expressions for the reflected field and for the surface-plasmon dispersion relation.

Extraction of wave parameters from measured HF radar sea-echo Doppler spectra

Abstract: Computerized techniques for extracting rms wave height and dominant wave period from HF radar sea-echo Doppler spectra are derived. Earlier theoretical models for first- and second-order sea backscatter (derived elsewhere) are employed to obtain the simple, closed-form inversion equations giving these two radar-deduced quantities. The results are general in that no specific models for the radial or azimuthal form of the wave height directional spectrum need be assumed; the resulting formulas are only weakly dependent upon the radar/wind direction. Approximations made in the derivation are stated, and they are ultimately tested by comparing wave heights and periods extracted from the inversion model with the actual values input to the original equations for scatter. The derived relationships are then tested against some eighty hours of radar measurements taken at a variety of frequencies and buoy-measured sea conditions. Theoretically predicted and empirically determined correction factors for wave height and period are in agreement. Finally, the measurements show that when sea wave height is greater than one twentieth of the radar wavelength, wave height extraction (rms) errors are less than 23%, and wave period extraction errors less than 12%.

The Born approximation in the theory of the scattering of elastic waves by flaws

Abstract: We used the integral equation formulation of the scattering of elastic waves to generate an approximate solution analogous to the Born approximation in quantum mechanics. This solution is attractive because of the ease with which it may be applied to scatterers of complicated shapes. We investigated the validity of the approximation by comparing it with exact results for spherical scatterers. Our conclusion for voids in elastic media is that the approximation describes well the scattering when the wavelength of the incident wave is approximately an order of magnitude larger than the scatterer and when the scattering is viewed in the backscattered directions. For many applications this range of validity is experimentally accessible. For elastic inclusions, however, where the properties of defect and host differed by 20–40%, the Born approximation is surprisingly good for all angles and even at short wavelengths.

Free electron laser

Abstract: The possibility of a new type of laser has been investigated by computer simulation using a fully relativistic electromagnetic particle code which has one spatial and three velocity dimensions. By passing a relativistic electron beam over a rippled static magnetic field, high frequency electromagnetic radiation is generated. If the ripple wavelength is λ0, the lasing wavelength is roughly λ0/2γ2. Thus, such a laser is continuously tunable by varying γ. It has been observed in simulation that as much as 35% of the beam energy can be converted into radiation, of which as much as nearly 90% can be in the most rapidly growing mode. A theory of the coupling between the negative energy plasma wave and the electromagnetic radiation by means of the rippled magnetic field is presented. Good agreements have been obtained between the simulation and the theory. The saturation mechanism is found to be the trapping of the beam by the unstable plasma wave. A theoretical estimate of the amount of energy that can be converted into radiation from the electron beam is also given.

Influence of the amplitude of a solid wavy wall on a turbulent flow. Part 1. Non-separated flows

Abstract: Measurements of the shear-stress variation along and the velocity profiles above a solid wavy wall bounding a turbulent flow are presented for waves with height-to-length ratios of 2a/λ = 0·0312 and 0·05. These are compared with previous measurements of the wall shear stress reported by Thorsness (1975) and by Morrisroe (1970) for 2a/λ = 0·012. The investigation covered a range of conditions from those for which a linear behaviour is observed to those for which a separated flow is just being initiated.Pressure measurements indicate a linear response in that the spatial variation is described quite well by a single harmonic with a wavelength equal to that of the surface. However, the variation of τw for waves with 2a/λ = 0·0312 and 0·05 can be more rapid on the leeward side of the wave. The degree of departure from a sinusoidal variation increases with increasing wave height and fluid velocity and, from the results reported in this paper, it is suggested that nonlinear behaviour will become evident when au*/v [ges ] 27.Many aspects of the flow for all three waves are described by a solution of the linear momentum equations previously presented by Thorsness (1975) and by Thorsness & Hanratty (1977). These include the phase and amplitude of the pressure profile and the first harmonic of the shear-stress profile and the velocity field outside the viscous wall region.These results suggest that up to separation the flow is approximated quite well by linear theory. Nonlinearities affect the flow only in a region very close to the wave surface and are manifested by the appearance of higher harmonics in the variation of τw.

Spatial Information Capacity of Compound Eyes

Abstract: The capacity of the compound eye to perceive its spatial environment is quantified by determining the number of different pictures that can be reconstructed by its array of retinula cells. We can then decide on the best compromise between an animal's capacity for fine detail and contrast sensitivity. The theory accounts for imperfect optics, photon noise, and angular motion limitations to acuity. 1. There is an optimum parameterp = D Δ φ, whereD is the facet diameter andΔ φ is the interommatidial angle, for each mean luminance, angular velocity and mean object contrast. We find that this value ofp is approximately that found by Snyder (1977) for threshold resolution of a sinusoidal grating at the ommatidial sampling frequency. 2. A diffraction limited eye (D Δ φ ≅λ/√¯3) is the optimum design only for those animals that are active in the brightest sunlight, and have a region of their eye that normally experiences low angular velocity, otherwise it is better to have a largerD Δ φ. λ is the wavelength of light in vacuum. 3. The design of the flyMusca is consistent with that of an animal with high angular velocity.

Optical attenuation in pure and doped fused silica in the ir wavelength region

Abstract: Infrared absorption spectra of GeO2‐, P2O5‐, and B2O3‐doped fused silica and nondoped fused silica have been obtained in the 3–25‐μm wavelength region. It is shown that the intrinsic infrared absorption loss of doped silica fiber increases by several orders of magnitude beyond 1.8–2.0 μm because of the overtone and combination tone vibrations of the SiO4 tetrahedron. The loss spectrum is affected by dopants, especially in the case of B2O3. The lower limit of attenuation in optical fiber is estimated to be about 0.2 dB/km at 1.6 μm.

Seismic body waves in anisotropic media: synthetic seismograms

Abstract: Summary. Synthetic seismograms and particle motion diagrams are computed for simple, layered Earth models containing an anisotropic layer. The presence of anisotropy couples the P, SV and SH wave motion so that P waves incident on the anisotropic layer from below produce P, SV and small-amplitude SH waves at the surface both the P velocity and the amplitudes of the converted phases vary with azimuth. Significant SH amplitudes may be generated even when the wavelength of the P wave is much greater than the thickness of the anisotropic layer. Incident SV or SH waves may each generate large amplitudes of both SV and SH motion. This strong coupling is largely independent of the degree of velocity anisotropy of the medium. The arrivals from short-period S waves exhibit S-wave splitting, but arrivals from longer period S waves superpose into a modified waveform. This strong coupling does not allow the arrival of separate phases with pure SV and SH polarization except along directions of symmetry where the motion decouples.

Electrodynamics of quasi-two-dimensional electrons

Abstract: For a system of electrons confined to a thin but finite layer, the nonlocal frequency-dependent dielectric tensor is constructed in the random-phase approximation and used in the Maxwell equations to study the electromagnetic properties. Retardation effects are negligible if the thickness of the electron layer is small compared with the wavelength of the electromagnetic wave.

Photoacoustic technique for determining optical absorption coefficients in solids.

Abstract: An investigation was made of a photoacoustic technique for determining the optical absorption coefficient in solids. A train of laser pulses was passed through the solid, and a piezoelectric transducer attached directly to the sample measured the amplitude of the elastic wave generated by the absorbed radiation. Calibration was performed at a wavelength of known absorption. The sensitivity of the technique was found to be limited to about 1 × 10−5 cm−1 in our samples due to radiation scattered onto the transducer, but the technique is capable of measuring absorption coefficients in the 10−6 cm−1 range using laser powers of about 1 W.

Diffusion lengths in solar cells from short‐circuit current measurements

Abstract: The minority‐carrier diffusion length in the base region of a silicon solar cell has been determined by measuring the short‐circuit current as a function of the wavelength of incident light. The incident light intensity required to produce a given short‐circuit current is a linear function of the reciprical absorption coefficient for each wavelength, and the extrapolation of this linear relation to zero intensity yields the diffusion length. This method is similar to the surface photovoltage and open‐circuit voltage methods; however, the accuracy for high light bias levels appears to be greatly improved.

Viscous Internal Gravity Waves and Low–Frequency Oscillations in the Tropics

Abstract: In this paper we deal with the interpretation of observed oscillations in the tropical troposphere and stratosphere within the framework of the equatorial wave theory. A difficulty with this problem arises when one compares the short vertical wavelength (or equivalent depth) predicted by the classical theory and the observed large vertical scales associated with the low Doppler-shifted frequencies of the tropospheric oscillations. In this analysis it is shown that the inclusion of simple linear damping, justified by budget studies which revealed the important role of cumulus momentum transport, has a strong influence at low frequencies on the forced equatorial waves and results in two types of dispersive relationships. The first type is characteristic of the regular internal gravity waves which have fast phase speeds and weak vertical attenuation. The second type is dominated by the viscous damping time scale and has slow phase speeds and strong vertical trapping. The theory predicts that the str...

Pyrromethene-BF2 complexes (4,4′-difluoro-4-bora-3a,4a-diaza-s-indacenes). Synthesis and luminescence properties

Abstract: BF2 complexes of alkylated pyrromethenes were prepared by reacting the corresponding pyrromethenes with BF3·OEt2 under basic conditions. A striking aspect of these complexes is a pronounced fluorescence in organic solvents, even at room temperature, with fluorescence quantum yields between 0.3-0.8. The efficiency and wavelength maximum of the fluorescence seems to be correlated with the substitution pattern at the 2 and 6 position. In none of these systems could phosphorescence be detected.

On optical dispersion in transparent molecular systems

Abstract: In a “crossed photon beams” experiment the action of light in a transparent medium is studied by Mossbauer resonance. Using cooled, temperature stabilized targets such as sodium nitroprusside, olivine and glasses, containing iron, no change of the hyperfine spectrum was observed by illumination with normal incoherent light of low intensity within the region of normal dispersion in agreement with the theory of optical dispersion. There was, however, no agreement with theoretical expectation when sodium nitroprusside (whose structure is, therefore, reviewed in detail under the aspects of dispersion) was irradiated by laser light with otherwise unchanged experimental conditions, as intensity and range of wavelengths. A new kind of molecular state, producable only by polarized light, was observed. Its radiative lifetime is practically infinite, but it is thermally instable at elevated temperatures. The Mossbauer spectrum reveals a quadrupole splittingΔE Q= (+)(2.7357±0.0040)mm/s and an isomer shiftδ=}-(0.183±0.006)mm/s (relativ to Fe(Pd)) at 100 K, markedly different from the normal groundstate. In the new state the axial symmetry, the sign of thez-component, and the three principal tensor axes of the electric field gradient at the Fe central atom coincide within the experimental limits of error with those of the groundstate.

Some Theoretical and Experimental Observations of the Wave Structure of Falling Liquid Films

Abstract: The linear stability problem associated with a vertical liquid film flowing under the action of gravity has been solved in terms of a numerical solution of the Orr-Sommerfeld equation. Results have been obtained for both the temporal and spatial representations of growing waves and for a wide range of the physical parameters char­ acterizing this type of flow. Experimental values of the wavelength and wave velocity have been determined for water films and the results are in reasonably good agreement with the theory. Calculated values for the wave­ length, wave velocity, and growth rate of the most unstable wave indicate only a small difference between the temporal and spatial formulations for water films.

A perturbation analysis of the attenuation and dispersion of surface waves

Abstract: A perturbation formulation of the equations of linear piezoelectricity is obtained using a Green’s function approach. Although the resulting equation for the first perturbation of the eigenvalue strictly holds for real perturbations of real eigenvalues only, it is formally extended to the case of purely imaginary perturbations of real eigenvalues. The extended equation is applied in the calculation of the attenuation of surface waves due to the finite electrical conductivity of thin metal films plated on the surface and air loading. The influence of the viscosity of the air is included in the air‐loading analysis, and the calculated attenuation increases accordingly. Since the metal films are thin compared with a wavelength, an approximate thin‐plate conductivity equation is employed in the determination of the attenuation due to the electrical conductivity of the films. The resulting attenuation is obtained over a very large range of values of sheet conductivity. This is accomplished by using the equatio...

Nitrous oxide ultraviolet absorption spectrum at stratospheric temperatures

Abstract: The absorption spectrum of nitrous oxide (N2O) has been determined at five temperatures from 194 to 302 K and over the wavelength range 173-240 nm. The absorption cross sections as a function of wavelength and temperature are expressed by a nine-term polynomial in a form useful for atmospheric models. The high resolution structure between 173 and 190 nm has been observed more clearly than by previous studies.

Gas breakdown initiated by laser radiation interaction with aerosols and solid surfaces

Abstract: The threshold for generation of high‐density plasma caused by the interaction of laser radiation with solids is examined theoretically. The developed semiquantitative model is applicable for predicting the threshold for generating laser‐supported absorption waves, absorbing plasma produced on surfaces with high‐intensity laser radiation, and also for predicting the threshold of aerosol‐induced gas breakdown, commonly referred to as dirty air breakdown. In developing the model, the vapor density generated by laser interaction with the solid is calculated and then the intensity required to cascade ionize the vapor to full ionization is calculated. Analytic expressions are derived for the breakdown intensity which show the parametric dependence of the threshold. Since the vapor density is a function of laser intensity, and characteristics of the solid, different parametric dependence of the threshold on wavelength, pulse length, and particle characteristics is predicted compared to those determined by previous theoretical models of gas breakdown. Comparison of the threshold predicted by the model agree within a factor of 2 of experimental data for a wide range of experimental conditions including laser pulse duration from 10 nsec to cw, particle sizes from 0.1 μ to laser spot sizes of 1 cm, and wavelength scaling of 1.06–10.6 μ wavelength. Based on the good agreement between theoretical predictions and experimental data, the model can be used to predict thresholds for plasma production for a wide variety of atmospheric and solid‐surface conditions.

Observation of laser-induced inelastic collisions

Abstract: We describe two new experiments which demonstrate laser-induced inelastic collisions. Energy was stored in the Sr $5p^{1}P^{o}$ state and selectively transferred both to the Ca $4{p}^{2}^{1}S$ and $5d^{1}D$ states. The collision cross sections maximized at the interatomic wavelengths of 4977 and 4711 \AA{}, respectively, and had a half-power width of about 14 ${\mathrm{cm}}^{\ensuremath{-}1}$.