Variations In Spectral Shape Research Articles

Variations in Secondary Electron Emission from MgO Characterized by Secondary Electron Spectroscopy

The recent developments of the in-lens field emission scanning electron microscopes make it possible to image specimen surfaces with subnanometer resolution by collecting high resolution type I secondary electron (SE) signals. SE images with a resolutiori better than 1 nm have also been obtained in scanning transmission electron microscope (STEM) instruments. A problem related to the correct interpretation of these high resolution images is the lack of knowledge on the energies of the collected electrons contributing to the images. On the other hand secondary electron spectroscopy (SES) on clean surfaces has been developed for several years and biased SE imaging of several deposits on metallic and semiconductor crystal surfaces has been demonstrated. The combination of ultra-high resolution SE imaging with SES and Auger electron spectroscopy (AES) can be realized in pur UHV STEM instrument (MIDAS). The description of the performance of the MIDAS is reported elsewhere. We report here some recent results on SE imaging of MgO smoke crystals, in the MIDAS, with energy analysis of the collected secondary electrons.Strong variations of SE signals from different MgO crystals have been observed and the intensity of the collected secondary electrons from some MgO cubes increases under electron beam irradiation. Figure 1 shows a SE image of several MgO cubes, revealing that some of the crystals (indicated by A) have much higher intensity than others (indicated by B) and bright diffuse patches on the dark crystal can be seen (indicated by D). We have used SES to study the variations in SE emission under electron irradiation. The fact that the velocity distribution of the emitted secondary electrons may depend strongly on the surface morphology of the samples studied makes the interpretation of these SE spectra complicated. In order to avoid this problem and other possible complications arising from the detector geometry we acquired SE spectra from a fixed flat area of a single crystal MgO shown in figure 2. The sample was biased at -30 V and the resolution of the SE spectrum is about 1 eV. Figure 3a shows the SE spectra, taken from the exit surface of the crystal. Spectrum 1 was obtained from a poorly emitting MgO cube as that shown in figure 2 (all the spectra were obtained from the circled area). Spectra 2, 3 and 4 were obtained after about 10, 20 and 40 minutes electron irradiation respectively. In order to study spectral shape variations the spectra were normalized so that they all had the same peak intensity. The normalized data were shown in figure 3b.

Resonance of a fluid‐driven crack: Radiation properties and implications for the source of long‐period events and harmonic tremor

A dynamic source model is presented, in which a three‐dimensional crack containing a viscous compressible fluid is excited into resonance by an impulsive pressure transient applied over a small area ΔS of the crack surface. The crack excitation depends critically on two dimensionless parameters called the crack stiffness, C = (b/μ)(L/d), and viscous damping loss, F = (12ηL)/(ρƒd2α), where b is the bulk modulus, η is the viscosity, ρƒ is the density of the fluid, μ is the rigidity, α is the compressional velocity of the solid, L is the crack length, and d is the crack thickness. The first parameter characterizes the ability of the crack to vibrate and shapes the spectral signature of the source, and the second quantifies the effect of fluid viscosity on the duration of resonance. Resonance is sustained by a very slow wave trapped in the fluid‐filled crack. This guided wave, called the crack wave, is similar to the tube wave propagating in a fluid‐filled borehole; it is inversely dispersive, showing a phase velocity that decreases with increasing wavelength, and its wave speed is always lower than the acoustic velocity of the fluid, decreasing rapidly as the crack stiffness increases. The source spectrum shows many sharp peaks characterizing the individual modes of vibration of the crack; the variation of spectral shape, both in the number and width of peaks, is surprisingly complex, reflecting the interference between the lateral and longitudinal modes of resonance, as well as nodes for these modes. The far‐field spectrum is marked by narrow‐band dominant and subdominant peaks that reflect the interaction of the various source modes. The frequency of the dominant spectral peak radiated by the source is independent of the radiation direction. The frequency, bandwidth, and spacing of the resonant peaks are strongly dependent on the crack stiffness, larger values of the stiffness factor shifting these peaks to lower frequencies and decreasing their bandwidth. The excitation of a particular mode depends on the position of the trigger and on the extent of the crack surface affected by the pressure transient. Fluid viscosity decreases the amplitudes of the main spectral peaks, smears out the finer structure of the spectrum, and greatly reduces the duration of the radiated signal. The energy loss by radiation is stronger for high frequencies, producing a seismic signature that is marked by a high‐frequency content near the onset of the signal and dominated by a longer‐period component of much longer duration in the signal coda. Such signature is in harmony with those displayed by long‐period events observed on active volcanoes and in hydrofracture experiments. The very low velocity which is possible in a crack with high stiffness (C ≥ 100) also provides an attractive explanation for very long period tremor, such as type 2 tremor at Aso volcano, Japan, without the requirement of an unrealistically large magma container. The standing wave pattern set up on the crack surface by the sustained resonance in the fluid is observable in the near field of the crack, suggesting that the location and extent of the source may be estimated from the mapping of the pattern of nodes and antinodes seen in its vicinity. According to the model, the long‐period event and harmonic tremor share the same source but differ in the boundary conditions for fluid flow and in the triggering mechanism setting up the resonance of the source, the former being viewed as the impulse response of the tremor generating system and the latter representing the excitation due to more complex forcing functions.

The spectral shape and variability of the blazar 3C 446

The radio to X-ray spectrum and variability of 3C 446 are discussed, and it is shown that the entire infrared-ultraviolet wavelength region exhibits strongly correlated variations, with the infrared and optical fluxes varying together on a time-scale of weeks or less. The variations in spectral shape which occur in the submillimeter to optical wavelength region are examined; it is proposed that continuous injection or reacceleration of the emitting electrons takes place during the observed flare events, and that the changes in spectral shape reflect the different effects that radiative energy losses have on the energy distribution of the electrons during and after such periods of injection or reacceleration. An overall brightening of the source observed throughout the millimeter to optical wavelength region between late 1968 and late 1983 is attributed to an increase in a slowly varying 'quiescent' emission component.

High-energy electrons in the nonuniform galactic disk model

The nonuniform galactic disk model, which we previously proposed to explain the unexpectedly large flux of interstellar antiprotons, is also able to account in a natural way for the spectral shape variation of high-energy cosmic-ray electrons. A comparison of the model prediction with the measured electron data suggests that only about 5% of the observed protons are of local origin and that the electron intensity inside a spiral arm is much lower than that outside it. The model further predicts that the electron energy spectrum should flatten again at very high energies.

Viscosity-induced emission anomalies in 1,2-diarylethylenes and in distyryl-benzenes and -naphthalenes

trans-Stilbenes and a variety of compounds related to trans-1-styrylnaphthalene exhibit normal fluorescence behaviour in fluid solutions. At high viscosities (>105 cP) of the medium, anomalous phenomena start to appear: the spectra change their shape and vary with the excitation wavelength, particularly with excitation in the red tail of the absorption spectrum. The extent of these anomalies increases with the viscosity and with the size of the aryl group. Another anomaly, observed in 1,2-di-(9- anthryl)ethylenes, is a viscosity-controlled. very pronounced enhancement of fluorescence yields. Our working hypothesis is based on the restriction, at high viscosities, of the rate of torsional relaxation, so that emission is mainly from molecules still having the geometry of the absorbing state. Irradiation at very low energies excites selectively those ground-state molecules having a geometry similar to that of the relaxed excited state. In the dianthrylethylenes, the rate of non-radiative decay processes S1→S0 associated with torsional relaxation is reduced at high viscosities, causing the observed strong enhancement of fluorescence yields, in addition to the variation of spectral shape with the temperature and the excitation wavelength.

Wave spectra and statistics off Godavari during September-October 1980

An analysis of sixty one wave records collected using a shipborne wave recorder (SBWR) showed wide variations in spectral shape. Some of the spectra are relatively narrow and peaked, some are proad with small peaks and some have multiple peaks. Half spectral peak bandwidth (BW) gives a more meaningful value to explain the spectral shape compared to spectral width parameter, ( Es from spectra and Ez from zero-upcrossing method) and spectral peakedness factor (Qp). The average zero-upcrossing period, Tz varies from 8.1 to 12.9 sec and compares well with the significant wave period, Ts. Tz and Ts are found to be better estimates compared to other average periods. During the period of observation (September -October) since calm weather prevailed (storms were absent in the Bay of Bengal) the significant wave heights are centred around 1 m. Es and Ez vary between 0.53 and 0.82 and between 0.52 and 0.80 respectively. BW varies from 0.02 to 0.07. The primary peaks are observed between 8 and 17 sec. The present study fairly describes the average condition of the sea surface ID the absence of any storm.

A nondestructive method for predicting laser emission wavelength from photocurrent spectra of GaAlAs double heterostructure wafers

Using nondestructive pressure contacts, a GaAlAs double heterostructure (DH) wafer may be operated as a solar cell. When the spectrum of the photocurrent is observed, the band edge of the active layer is clearly identifiable. A model of photocurrent in a GaAlAs DH wafer shows that a reliable measurement of the active layer band edge can be made in spite of substantial variations in spectral shape that can be caused by variations in the GaAs cap layer thickness. Using this method, the emission wavelength of a laser can be predicted to ±62 Å with 90 percent confidence, making photocurrent spectroscopy a useful tool for characterizing wafers.

Theory of the pressure–strain rate. Part 2. Diagonal elements

A theoretical calculation is made of (the diagonal elements of) pressure-strain-rate calculation ρ0−1〈p[∇u + (∇u)T]〉 for a simple turbulent shear flow. This calculation parallels a previous calculation of the off-diagonal element. The calculation is described as follows. (1) Beginning with the Navier-Stokes equation, an expression for the (diagonal) pressure-strain-rate term is derived analytically in terms of measurable quantities (velocity spectra) - this derivation makes use of a cumulant discard. (2) It is proved that, to lowest order in the spectral anisotropy, the diagonal pressure-strain-rate term is linearly proportional to the diagonal Reynolds-stress elements. (3) A formula is derived for the proportionality constants (Rotta constants) in terms of arbitrary spectra. (4) This formula is used to calculate theoretically the numerical value of Rotta's constant Cii for models of velocity spectra (the variation of Cii with variations of spectral shapes and of Reynolds number are also determined). (5) Deficiencies and limitations of Rotta's model are identified and discussed.It is found that Rotta's expression for 2ρ0−1〈p∂ui/∂i〉 is only valid for special spectra. Surprisingly large deviations of Rotta's expression from theory are found for a more complex spectra thought to be typical of simple shear flow. In addition, it is found that Cxz is intrinsically and quantitatively different from Cii because the latter depends importantly on the large-wavenumber part of the spectrum (the inertial subrange) whereas the former does not. The numerical ratio Czz/Cxz is calculated theoretically and shown to be about 2 for the zero-moment model. It is concluded that a linear term in the stress anisotropy as proposed by Rotta does not always exist. The deviation of Rotta's model from theory is understood by distinguishing between the spectral anisotropy and the stress anisotropy.For the zero-moment spectral model, where the Rotta relation is valid, it is found that Cii varies significantly with large Reynolds number but is rather insensitive to the large-wavelength part of the spectrum.

A spectral theory for circular seismic sources; simple estimates of source dimension, dynamic stress drop, and radiated seismic energy

abstract The far-field body wave radiation from a class of circular rupture models is investigated as a function of takeoff angle, rupture velocity, and stopping behavior. In particular, the variation of spectral shape, pulse shape, and energy flux over the focal sphere is quantified. These results provide two new methods for estimating the source dimension, the first through the inversion of a characteristic frequency, and the second using the rise time of the displacement pulse shape. The class of kinematic rupture models which has been constructed also allows a direct estimate of the dynamic stress drop using the velocity pulse shape: as an example, this technique is applied to an SMA1 recording of a high stress drop Aleutian earthquake, Finally, the introduction of a new spectral parameter, the integral of the square of the ground velocity, is shown to considerably simplify calculation of the total radiated energy. Together with a re-evaluation of corner frequency and spectral falloff, these new results constitute a spectral theory for circular seismic sources which includes directivity and is valid for a range of subsonic rupture velocities.

Stable free radical formation in the binary powders of silicates and organic compounds.

Stable free radical formation was observed in the binary powders of silicates and general organic compounds by the use of ESR at room temperature in the air. A large amount of free radicals was formed, if an OH or COOH group containing organic molecule was used. Free radicals were stable not only in the air but also in the suspension of benzene. However, they disappeared immediately upon contact with proton donating liquid molecules. Increase in spectral intensities with time and variation of spectral shapes with moisture were observed.

Odd-even effects in pre-equilibrium (p,n) reactions

Measurements of the (p, n) continuum spectra for targets of /sup 103/Rh, /sup 104/,/sup 105/,/sup 106/,/sup 108/,/sup 110/Pd and /sup 107/,/sup 109/Ag at proton energies of 18, 22, and 25 MeV are compared with the predictions of the hybrid model for pre-equilibrium reactions. The systematics of the odd-even differences in spectral shape are found to be much clearer than in a previous measurement at lower energy with targets near a closed shell. Most of the variation in spectral shape between even- and odd-A targets is attributed to energy shifts produced by pairing effects. Comparisons between the various even-A or odd-A targets show little change in shape and magnitude of the highest energy portion of the measured (p, n) spectra over this range in A; this is consistent with the predictions of the model. The density of one-proton one-neutron-hole states at low excitations is calculated from a single-particle basis; the calculated values show more structure than is implied by the data, but do have the correct energy dependence when averaged over a wide enough interval. (AIP)

Spectra and Dosimetry of Neutrons from Various Sources Shielded by Iron

Spectra, calculated by Monte Carlo, are presented for neutrons transmitted through various thicknesses of iron from sources of fission neutrons, H$sub 2$O-moderated fission neutrons, D$sub 2$O-moderated fission neutrons, and 14.7-MeV neutrons. The sources were located at the center of iron spheres or were in the form of beams incident normally on slabs. Variations in spectral shapes are discussed from the viewpoint of effects on the response of neutron dosimeters. Effective cross sections (sigma-bar) for threshold detectors $sup 103$Rh, $sup 115$In, $sup 32$S, $sup 237$Np, $sup 232$Th, and $sup 238$U were obtained by averaging the cross sections over the calculated spectra. Average kerma (K-bar), dose equivalent, and maximum dose per n/cm$sup 2$ were also calculated. Curves of sigma-bar/K-bar show the sensitivity of these detectors to spectral changes and permit the computation of correction factors to dosimeter readings. 15 figures. (auth)

Oblique VHF radar spectral studies of the equatorial electrojet

A new narrow-beam antenna at the Jicamarca Observatory permits oblique (zenith angle, 25°) radar spectral studies of the electrojet with an altitude resolution down to 1.1 km. Only daytime observations are possible presently, however. The general altitude variations in spectral shape observed are consistent with linear instability theory, if the effect of recombination is included. The height at which the mean Doppler shift of the echo maximizes, however, is about 4 km higher than one would expect on the basis of electrojet models. An increase in the assumed collision frequency would remove the discrepancy. Other data presented strongly suggest that at least some of the variations observed in the type 1 echo phase velocity are due to changes in the temperature of the E region.

Aftershock P-wave spectra and dynamic features of the aleutian islands earthquake sequence of February 4, 1965

The aftershock sequence starting February 4, 1965, in the Aleutian Islands region is studied with regard to its development in time and space. Energy release characteristics are evaluated and the possibility of a regular oscillation of seismic activity between the two extremes of the region is examined. No such oscillation is found to take place. Short-period P-wave spectra of the aftershocks are studied in relation to epicentral positions and characteristics of the aftershock process. The variation in spectral shape is considerable. Indications are found of the influence of aftershock location on spectra. The spectral effects of regional stress relaxation appear to be less than those of other causes.