Showing papers on "Doppler broadening published in 1996"

Theoretical and experimental study of positron annihilation with core electrons in solids

Abstract: A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively. @S01631829~96!04327-5#

Demonstration and characterization of filtered Rayleigh scattering for planar velocity measurements

Abstract: Filtered Rayleigh scattering is an optical diagnostic technique that allows for simultaneous planar measurement of velocity, temperature, and pressure in unseeded flows. An overview of the major components of a filtered Rayleigh scattering system is presented. In particular, a detailed theoretical model is developed and discussed with associated model parameters and related uncertainties. Based on this model, results for two experimental conditions are presented: ambient room air and a Mach 2 freejet. These results include two-dimensional, spatially resolved measurements of velocity, temperature, and pressure derived from time-averaged spectra. ILTERED Rayleigh scattering (FRS), a recently developed flow diagnostic technique,1'2 achieves large suppression of background scattering allowing planar flowfield visualization and obtains quantitative measurements of velocity, temperature, and density in unseeded gaseous flows. This technique makes use of Rayleigh scattering from molecules in the flow and is driven by a high-power, narrow linewidth, tunable, injection seeded laser. When imaging the scattered light onto a charge-coupled device (CCD) camera, unwanted background scattering from stationary objects may be filtered out by tuning the frequency of the narrow linewidth laser to coincide with an atomic or molecular absorption line and by placing a cell containing the atomic or molecular species between the camera and the flow. This cell acts as a notch filter, absorbing all background scatter at the laser frequency. Scattered light that is Doppler shifted, however, passes through the filter and is imaged on the camera. Quantitative measure of flow properties is achieved by measuring the total intensity, Doppler shift, and spectral profile of the Rayleigh scattered light. The total intensity is directly proportional to density; the Doppler shift is directly proportional to velocity; and the spectral profile is a function of temperature and pressure. The scattering intensity, Doppler shift, and spectral profile are determined by passing the scattered light through the notch absorption filter and then by imaging it onto an intensified CCD camera. Because the filter absorbs light in a narrow frequency band, it converts the spectral information contained in the Doppler shift and Rayleigh profile into intensity information at the camera. By collecting data (camera pixel intensity) for varying conditions, v, T, and P may be determined. Previous work has concentrated on the use of this technique for background suppression when visualizing flows and for the measurement of velocity. The background suppression feature of FRS has been used to image flowfields that otherwise would be completely obscured by the strong scattering from wind-tunnel surfaces. The authors have used this technique to image the flowfield inside a Mach 3 inlet and to generate volumetric images of the crossing shocks and boundary layer.3 Elliott et al.4 have also used this technique to observe structures in compressible mixing layers. The use of FRS to measure velocity was initially demonstrated using scattering

Inhibition and Enhancement of Two Photon Absorption.

Abstract: The possibility of transparency against two photon absorption is predicted. Detailed absorption profiles under different conditions of the control laser are given. A novel explanation of the absorption minimum is given in terms of the two photon Fermi golden rule and the dressed states. Possibility of considerable enhancement of two photon absorption even in the presence of Doppler broadening is demonstrated.

Fast sampling algorithm for the simulation of photon compton scattering

Abstract: A simple algorithm for the simulation of Compton interactions of unpolarized photons is described. The energy and direction of the scattered photon, as well as the active atomic electron shell, are sampled from the double-differential cross section obtained by Ribberfors from the relativistic impulse approximation. The algorithm consistently accounts for Doppler broadening and electron binding effects. Simplifications of Ribberfors' formula, required for efficient random sampling, are discussed. The algorithm involves a combination of inverse transform, composition and rejection methods. A parameterization of the Compton profile is proposed from which the simulation of Compton events can be performed analytically in terms of a few parameters that characterize the target atom, namely shell ionization energies, occupation numbers and maximum values of the one-electron Compton profiles.

Defect identification using the core-electron contribution in Doppler-broadening spectroscopy of positron-annihilation radiation.

Abstract: Reduction of background using a coincidence-detection system in Doppler-broadening spectroscopy of positron-annihilation radiation allows us to examine the contribution of high-momentum core electrons. The contribution is used as a fingerprint to identify chemical variations at a defect site. The technique is applied to study a variety of open volume defects in Si, including decorated vacancies associated with doping. \textcopyright{} 1996 The American Physical Society.

Analysis of positron beam data by the combined use of the shape- and wing-parameters

Abstract: An improved approach is presented for the analysis of positron beam Doppler broadening data. Instead of analyzing the energy‐dependent shape parameter, the so‐called S(E) data, we combined the shape S(E) and wing W(E) data by plotting them as a trajectory in the S–W plane, using the implantation energy as a running parameter. It is shown that this plot is of particular interest for the qualitative interpretation of the data. Furthermore, it allows the independent determination of the characteristic shape and wing parameters of the different positron trapping layers without the use of a numerical simulation and fitting program. The method and its advantages and limitations are illustrated for three cases: a silicon sample implanted with helium, a metal–oxide–silicon system subjected to a bias voltage and a bare oxide layer on silicon.

X-ray reflection in Galactic Black Hole Candidates: Smeared edge profiles and resonant Auger destruction

Abstract: We consider the spectra of Thomson-thick, geometrically-thin accretion discs around Galactic black hole candidates in the reflection model and compute their iron K edges and iron K$\alpha$ lines. We compare the smeared iron K edge profiles that we compute with observation and find them to be a satisfactory description of the data. We find that a combination of Doppler broadening and resonant Auger destruction of line photons can make iron K$\alpha$ lines very difficult to detect in highly ionized inclined discs. We detail the physics of resonant Auger destruction at the level it is currently understood and point out its implications.

Spectroscopic temperature measurements in a microwave discharge

Abstract: We report temperature measurements in a low-pressure hydrogen microwave plasma. Translational temperatures both of H and of (using Doppler broadening), as well as the rotational temperature of , are simultaneously determined. It is first shown that the rotational temperature of the excited state is not in equilibrium with the translational temperatures of the neutral particles. Then, using a high-resolution Fourier transform spectrometer, we show that the H atom kinetic temperature is higher than the one. This result is interpreted in considering the mechanisms of relaxation of the hot H atoms, produced by electron impact dissociation of , in the molecules and on the tube walls.

Doppler Broadening and Magnetic Field Effects on the Balmer Lines Emitted at the Edge of a Tokamak Plasma

Abstract: Profiles of the Balmer lines D α (H α ), D β (H β ) and D γ (H γ ) have been measured in the scrape-off layer and within the edge of the TEXTOR (upgrade) plasma, under Ohmic conditions and with neutral-beam injection. Each line profile shows a strong Zeeman effect in the vicinity of line centre, and a marked central dip when mainly the σ components are observed. The line core evidently originates from cold atoms in the edge plasma, excited in the course of molecular dissociation, while the broad pedestal on which the core rests is radiated by excited atoms produced through charge-exchange recombination of deuterons (protons), transported outwards from the much hotter plasma interior, and by atoms heated directly by collisions with the deuterons (protons). Core temperatures of about 0.5 eV and less are obtained from line profile analysis.

Sub-Doppler resolution in inhomogeneously broadened media using intense control fields.

Abstract: We propose a scheme for obtaining sub-Doppler resolution for one transition of an inhomogeneously broadened, three-level atomic system, by using an intense control field at the other transition. Analytical and numerical calculations are presented to delineate the mechanism responsible for this sub-Doppler resolution, and quantify the extent to which Doppler broadening can be reduced. \textcopyright{} 1996 The American Physical Society.

Line shape analysis of Doppler broadened frequency‐modulated line spectra

Abstract: We present a method for recovering Doppler broadened absorption line shapes from frequency modulated (FM) line spectra. The method of analysis is calibrated and demonstrated with thermalized CN radicals produced by photodissociation of cyanogen (NCCN), probed on the A–X system near 800 nm with a frequency modulated Ti: sapphire ring laser. Nonthermal, Doppler broadened lines from translationally nascent photofragments can also be recovered by direct transformations of experimental FM line profiles acquired with a time resolution exceeding 100 ns. The superior signal‐to‐noise afforded by FM spectroscopy, relative to other direct absorption methods, should encourage the application of transient FM spectroscopy to problems in photoinitiated reaction dynamics.

Temperature field measurements in a sooting flame by filtered rayleigh scattering (FRS)

Abstract: Two-dimensional temperature measurements were performed in a sooting, methane-air flame by a modification of the conventional Rayleigh scattering technique. This new technique, filtered Rayleigh scattering (FRS), uses a molecular absorption filter in the detection path to spectrally separate part of the gas-phase Rayleigh scattering from the elastic scattering contributions of soot particles. It also suppresses laser-induced glare and background scattered light from walls and windows. Thus particle and background, unaffected, gas-phase Rayleigh scattering can be detected. This has been done in a weakly sooting flame where the temperature field information was obtained using a calibration of the FRS setup in a nonsooting flame by conventional Rayleigh scattering. This was necessary because the FRS gas scattering intensity is no longer a simple function of temperature and the known scattering cross sections of the individual gas species present. It also depends on the spectral broadening of the Rayleigh line relative to the incident radiation. Two theoretical models for the calculation of the dependence of the RRS signal on the experimental conditions, for the kinetic and the hydrodynamic regime, are presented in order to directly calculate gas-phase temperatures from FRS measurements without the need for calibration. For the present flames, the broadening mechanism is in an intermediate regime.

A diode laser absorption study on a 100 MHz argon ICP

Abstract: Diode laser absorption on the - argon transition is used to measure heavy particle temperatures in a 100 MHz argon inductively coupled plasma. Radial profiles of this temperature are obtained from the Gaussian part of the absorption profile with an accuracy of about 500 K, for four different input powers and at two different heights. The integrated profile is used to calculate the 4s level density and to trace the ionizing and recombining plasma parts. The measurements also show that the method of attributing the Lorentzian width only to Stark broadening for calculating electron densities is not correct for this argon transition in atmospheric plasmas. A second broadening process with Lorentzian shape, Van der Waals broadening, has to be taken into account. Under the measured conditions at the hottest positions in the plasma about 50% of the Lorentz component is due to Van der Waals broadening and this increases to almost 100% at the edges of the plasma.

Carrier wave shocking of femtosecond optical pulses.

Abstract: Numerical integration of Maxwell's equations for propagation of a femtosecond pulse in a medium with linear Lorentz response and a Kerr nonlinearity shows shock formation on the underlying carrier wave prior to the envelope shock. The carrier shock is characterized by the appearance of a strong third harmonic pulse, whereas the envelope shock appears later as spectral broadening and modulation of the fundamental and higher harmonic spectral features.

Role of inhomogeneous broadening in lasing without inversion in ladder systems.

Abstract: We study the effect of Doppler broadening on the inversionless gain that can be realized in a ladder configuration. The gain is calculated when the strong coherent pump and the weak probe are either copropagating or counterpropagating. The results indicate that the counterpropagating situation is the optimal one for obtaining maximum amplification, since for identical Doppler broadening, the counterpropagating geometry yields higher amplification than the copropagating geometry. The effect of Doppler broadening on electromagnetically induced transparency in the same atomic system is also briefly discussed. \textcopyright{} 1996 The American Physical Society.

Origin of the Doppler ultrasound spectrum from blood

Abstract: Backscattering of an incident ultrasound beam by blood gives rise to a Doppler spectrum whose characteristics are affected not only by the velocity distribution, but also by certain basic aspects of the red blood cells (RBC's) behavior. Starting from fundamental assumptions, an explicit expression is derived for the Doppler spectrum in terms of the variance in the scatterer number density and the backscattering cross section. This shows that the Doppler power at a given frequency is weighted by the backscattering cross section and the manner in which the RBC's are packed (packing factor). Since spatial variations in the flow field can result in changes in the variance and backscattering cross section, the mean Doppler frequency will not necessarily be proportional to the mean flow through the sample volume. Experimental results for two different flow fields are used to illustrate these effects. The implications of these findings are discussed in relation to volumetric flow estimation and the power mode display used in some color Doppler flow imaging systems.

Analysis of simulated and measured pulse shapes of closed-ended HPGe detectors

Abstract: A program for simulating pulse shapes of HPGe detectors has been developed taking into account the closed-ended geometry as well as multiple interactions of γ-rays. Good agreement between simulated and experimentally determined pulse shapes is achieved. From a detailed investigation of risetime distributions a method to determine the radial coordinate of the first point of interaction has been substantiated. For a large Ge detector with 70 mm diameter a radial resolution of 4 to 8 mm has been obtained. This method, combined with segmentation of the detector, reduces the Doppler broadening of γ-lines significantly.

Characterization of Blood Flow Turbulence With Pulsed-Wave and Power Doppler Ultrasound Imaging

Abstract: Blood turbulence downstream of a concentric 86 percent area reduction stenosis was characterized using absolute and relative Doppler spectral broadening measurements, relative Doppler velocity fluctuation, and Doppler backscattered power. Bidimensional mappings of each Doppler index were obtained using a 10 MHz pulsed-wave Doppler system. Calf red cells suspended in a saline solution were used to scatter ultrasound and were circulated in an in vitro steady flow loop model. Results showed that the absolute spectral broadening was not a good index of turbulence because it was strongly affected by the deceleration of the jet and by the shear layer between the jet and the recirculation zones. Relative Doppler spectral broadening (absolute broadening divided by the frequency shift), velocity fluctuation, and Doppler power indices provided consistent mapping of the centerline axial variation of turbulence evaluated by hot-film anemometry. The best agreement between the hot-film and Doppler ultrasound methods was however obtained with the Doppler back-scattered power. The most consistent bidimensional mapping of the flow characteristics downstream of the stenosis was also observed with the Doppler power index. The relative broadening and the velocity fluctuation produced artifacts in the shear layer and in the recirculation zones. Power Doppler imaging is a new emerging technique that may provide reliable in vivo characterization of blood flow turbulence.

Measurements of density fluctuations by modulation spectroscopy

Abstract: Theoretical and experimental results describing the application of wavelength modulation spectroscopy to density fluctuations are given. The effects of concurrent amplitude modulation, which often occurs when such experiments are performed with diode lasers are accounted for. It is shown that the characteristics of the signal magnitude at line center as a function of density include the expected increase with density, n, in the Doppler regime. In the effective collision broadened regime the signal magnitude at line center falls approximately as n−N, where N is the order of the harmonic at which detection is performed. This result incorporates the well‐known result for direct absorption (N=0) in which the signal magnitude at line center stays constant while the linewidth increases in the collision broadened regime. It is shown that the sensitivity to density fluctuations measured by wavelength modulation spectroscopy around any ambient value depends strongly on the order of the detection harmonic employed,...

Comparison of Doppler signal analysis techniques for velocity waveform, turbulence and vortex measurement: A simulation study

Abstract: Simulated time-varying Doppler signals incorporating bandwidth, power variation and vortex simulation have been used to compare a number of signal analysis techniques with a view to optimising the accuracy of convective velocity waveform, spectral broadening and vortex signal estimation. The shorttime Fourier transform (STFT), the autoregressive (AR) modified covariance estimator, the time-frequency pseudo-Wigner-Ville and Choi-Williams distributions and a partial stationarising algorithm were investigated for a range of some analysis parameters (such as window duration, AR model order). It was found that all methods could estimate the convective velocity waveform well and all the nonclassical methods were an improvement over the STFT for bandwidth estimation with the stationarising method giving the lowest error. For vortex measurement, using parameters that were optimum for mean frequency and bandwidth estimation, the stationarising, modified covariance, pseudo-Wigner-Ville with a 10-ms window and Choi-Williams methods gave improved performances compared with the STFT.

Stark broadening corrections to laser-induced fluorescence temperature measurements in a hydrogen arcjet plume

Abstract: Laser-induced fluorescence of the H(α) transition of atomic hydrogen has previously been performed in the plume of a hydrogen arcjet thruster. Measurements of plasma velocity and temperature, based on the Doppler shift and broadening of the H(α) line shape, were previously published [Appl. Opt. 32, 6117 (1993)]. In that paper the Stark broadening of the H(α) transition was estimated from static-ion calculations performed in the early 1970's and found to be negligible in comparison with the Doppler broadening. However, more recent dynamic-ion calculations have shown the Stark broadening to be considerably larger than was previously assumed, resulting in inaccurate temperature measurements. We present a reanalysis of the fluorescence data, taking into account the improved Stark broadening calculations. The correct atomic hydrogen translation temperature and electron number density are obtained from the Doppler and Stark broadening components of the measured line shape. The results indicate a substantial drop in temperature from those previously reported.

Effect of Doppler broadening on quantitative concentration measurements with degenerate four-wave mixing spectroscopy

Abstract: The effect of Doppler broadening on degenerate four-wave mixing (DFWM) signal intensities in the regime of high pump and probe laser intensities is investigated theoretically. DFWM reflectivities are calculated by solving the time-dependent density-matrix equations for a two-level system interacting with three laser fields. The density-matrix equations are integrated directly in the time domain on a grid of spatial locations along the phase-matching axis; the DFWM signal level is then calculated by summation of the polarization contribution (with the appropriate phase factor) from each of the spatial grid points. For the case in which the Doppler and the collisional linewidths are comparable, the DFWM reflectivity is found to be inversely proportional to the factor 1 + (bΔωD/ΔωC)2, where ΔωD is the Doppler width, ΔωC is the collisional width, and b is weakly dependent on the pump and the probe laser powers. We developed an analytical expression for the reflectivity of a line that is both collision and Doppler broadened by dividing the widely used Abrams and Lind expression for homogeneous reflectivity Rhom by the factor 1 + (bΔωD/ΔωC)2. This modified reflectivity expression is found to give accurate results for the DFWM reflectivity over a wide range of values for the ratio of Doppler to collisional width. With this modified Abrams–Lind expression, strategies for quantitative DFWM concentration measurements in flames and plasmas are proposed and analyzed. We conclude that, by selection of the appropriate rotational transition, a DFWM reflectivity that is directly proportional to the square of the total species number density can be obtained over a wide range of temperature for constant-laser-intensity spatial profile mapping in flames.

Hydrogen and helium broadening and pressure induced line shifts of 13CH4 in the ν4 band

Abstract: Hydrogen and helium pressure induced broadening and line shifts of the ν4 band of isotopic methane (13CH4) have been measured at room temperature for many lines in the P, Q and R branches. The measurements were obtained using the 1 m Fourier Transform Spectrometer at the KPNO McMath Solar Telescope with a spectral resolution of 0.0065 cm−1. Data were obtained for 266 absorption lines for a range of J values up to J = 16; the pressure broadening results are compared to similar measurements made for the common isotope 12CH4 and a small, but consistent variation is noted.

A possible difficulty with a nanoflare-heated corona

Abstract: Spectral line profiles associated with the nanoflare model of coronal heating are computed. It is shown that the reconnection jets associated with this heating model lead to Doppler shifts or line broadening of coronal spectral lines in excess of those observed. Some possible resolutions of the problem are suggested.