Showing papers on "Doppler broadening published in 2000"

Observation of a transition state resonance in the integral cross section of the F+HD reaction

Abstract: We have studied the reaction F+HD at low collision energies using a combination of experimental and theoretical methods. Clear evidence for a reactive resonance is found in the integral cross section for the reactive channel F+HD→HF+D. Using a crossed molecular beam apparatus, the total reactive cross sections for the HF+D and DF+H channels were obtained in the collision energy range of 0.2–5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive steplike feature near 0.5 kcal/mol. Furthermore, the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feature is shown to arise from a reactive resonance localized near the transition state. Theoretical scattering calculations have been carried out using the Stark–Werner potential energy surface, which accurately ...

Positron confinement in ultrafine embedded particles: Quantum-dot-like state in an Fe-Cu alloy

Abstract: A quantum-dot-like positron state is demonstrated in the ``defect-free'' particles using coincidence Doppler broadening of positron annihilation radiation. The wave functions of all positrons are entirely confined spatially in all three directions within the embedded nanosize and subnanosize Cu particles in a dilute Fe-Cu alloy. With use of this probe, the ultrafine particles are revealed to have nearly the same electron momentum distribution as bulk Cu, to be free from Fe atoms and defects, three dimensional, and to have no open-volume defects at the interfaces which can trap the positron. These successes indicate that this positron state promises to be a powerful tool for the studies of mesoscopic systems in metals and semiconductors.

Chandra Discovers a Very High Density X-Ray Plasma on the O Star ζ Orionis

Abstract: We report on a Chandra line spectrum observation of the O supergiant ζ Orionis (O9.7 Ib). A 73.4 ks High-Energy Transmission Grating Spectrometer observation shows a wide range of ionization stages and line strengths over the wavelength range of 5-26 A. The observed emission lines indicate a range in temperature of 2-10 MK, which is consistent with earlier X-ray observations of ζ Ori. Many lines are spectrally resolved showing Doppler broadening of 900 ± 200 km s-1. The observed He-like ions (O VII, Ne IX, Mg XI, and Si XIII) provide information about the spatial distribution of the X-ray emission. Although the observations support a wind distribution of X-ray sources, we find three conflicting results. First, line diagnostics for Si XIII indicate that this line emission forms very close to the stellar surface, where the density is of order 1012 cm-3, but the velocity there is too small to produce the shock jump required for the observed ionization level. Second, the strong X-ray line profiles are symmetric and do not show any evidence of Doppler-blueshifted line centroids, which are expected to accompany an outwardly moving source in a high-density wind. Third, the observed velocity dispersions do not appear to correlate with the associated X-ray source radii velocities, contrary to expectations of wind-distributed source models. A composite source model involving wind shocks and some magnetic confinement of turbulent hot plasma in a highly nonsymmetric wind appears to be needed to explain the line diagnostic anomalies.

Chandra Discovers a Very High Density X-ray Plasma on the O-Star Zeta Orionis

Abstract: We report on a Chandra line spectrum observation of the O supergiant, Zeta Orionis (O9.7 Ib). A 73.4 ks HETGS observation shows a wide range of ionization stages and line strengths over the wavelength range of 5 to 26 A. The observed emission lines indicate a range in temperature of 2 to 10 MK which is consistent with earlier X-ray observations of Zeta Ori. Many lines are spectrally resolved showing Doppler broadening of 900 +/- 200 km/s. The observed He-like ions (O VII, Ne IX, Mg XI, and Si XIII) provide information about the spatial distribution of the X-ray emission. Although the observations support a wind distribution of X-ray sources, we find three conflicting results. First, line diagnostics for SIXIII indicate that this line emission forms very close to the stellar surface, where the density is of order 10^{12} cm^{-3}, but the velocity there is too small to produce the shock jump required for the observed ionization level. Second, the strong X-ray line profiles are symmetric and do not show any evidence of Doppler blue-shifted line centroids which are expected to accompany an outwardly moving source in a high density wind. Third, the observed velocity dispersions do not appear to correlate with the associated X-ray source radii velocities, contrary to expectations of wind distributed source models. A composite source model involving wind shocks and some magnetic confinement of turbulent hot plasma in a highly non-symmetric wind, appears to be needed to explain the line diagnostic anomalies.

Iron K Emission Lines in the Energy Spectra of Low-Mass X-Ray Binaries Observed with ASCA

Abstract: ASCA archive data of 20 low-mass X-ray binaries (LMXBs) were analyzed to study the iron K emission lines. The LMXBs analyzed here include Z sources, atoll sources, dippers, bursters, and ADC (accretion disk corona) sources. We paid particular attention to reducing systematic errors for the estimation of line parameters, and the combination of SIS and GIS, which have different energy resolutions, was found to be very useful for this purpose. We detected significant iron lines from about one-half of the sources. The line center was 6.56 keV on average, and a finite width of ~0.5 keV (FWHM) was obtained from six sources; these properties may be common to LMXBs. On the other hand, equivalent width (EW) of the lines showed large scatter among the sources, from less than 10 to 170 eV. These line parameters do not show any correlation with the source categories or luminosities. The iron K lines from LMXBs are likely produced through the radiative recombination of photoionized plasma. We discuss the origin of the line width, which may result from the combination of the line blending, Doppler broadening, and Compton scattering.

High-Resolution Spectroscopy of the X-Ray-photoionized Wind in Cygnus X-3 with the Chandra High-Energy Transmission Grating Spectrometer.

Abstract: We present a preliminary analysis of the 1-10 keV spectrum of the massive X-ray binary Cygnus X-3, obtained with the high-energy transmission grating spectrometer on the Chandra X-Ray Observatory. The source reveals a richly detailed discrete emission spectrum, with clear signatures of photoionization-driven excitation. Among the spectroscopic novelties in the data are the first astrophysical detections of a number of He-like "triplets" (Si, S, Ar) with emission-line ratios characteristic of photoionization equilibrium, fully resolved narrow radiative recombination continua of Mg, Si, and S, the presence of the H-like Fe Balmer series, and a clear detection of an ~800 km s-1 large-scale velocity field as well as an ~1500 km s-1 FWHM Doppler broadening in the source. We briefly touch on the implications of these findings for the structure of the Wolf-Rayet wind.

Simulations of two-dimensional femtosecond infrared photon echoes of glycine dipeptide

Abstract: The multidimensional optical response of the amide I band of glycine dipeptide is calculated using a vibrational‐exciton model, treating each peptide bond as a localized anharmonic vibration. The 2D photon echo signal is obtained by solving the non-linear exciton equations. Comparison of different models of spectral broadening (homogeneous and diagonal and off-diagonal static disorder) shows completely different 2D signals even when the 1D infrared spectra are very similar. The phase of the 2D signal may be used to distinguish between overtone and collective types of two-exciton states. Vanishing of the 2D signal along certain directions can be attributed to the variation of the phase. Copyright ” 2000 John Wiley & Sons, Ltd.

Calculation of the Doppler broadening of the electron-positron annihilation radiation in defect-free bulk materials

Abstract: Results of a calculation of the Doppler broadening of the positron-electron annihilation radiation and positron lifetimes in a large number of elemental defect-free materials are presented. A simple scheme based on the method of superimposed atoms is used for these calculations. Calculated values of the Doppler broadening are compared with experimental data for a number of elemental materials, and qualitative agreement is obtained. These results provide a database which can be used for characterizing materials and identifying impurityvacancy complexes.

Observation of inhomogeneous spectral broadening of stimulated brillouin scattering in an optical fiber

Abstract: We report experimental results which show that the stimulated Brillouin scattering (SBS) spectrum in an optical fiber is inhomogeneous, exhibiting spectral broadening and hole burning under cw monochromatic laser excitation. This phenomenon arises from the waveguide interaction of the pump and Stokes signals and is a fundamental property of SBS in waveguiding systems due to their ability to confine a fan of radiation wave vector directions.

Intrinsic spectral broadening (ISB) in ultrasound Doppler as a combination of transit time and local geometrical broadening

Abstract: Doppler signals collected with a focused transducer are known to be affected by the so-called intrinsic spectral broadening (ISB). This article aims to point out how ISB is, in general, related to both the limited lateral extent of a focused beam (leading to a finite transit time), and the presence of several local insonation angles around the beam axis, due to focusing and diffraction effects (local geometrical broadening). The influence of these two elementary spectral contributions on the whole ISB is shown by considering the Doppler signal as simultaneously modulated in amplitude and frequency, and applying well-known relationships employed in the communication field. Such an analysis reveals that transit time and local geometrical broadening are two different phenomena, whose simultaneous knowledge is necessary for correctly evaluating the overall ISB. Finally, thanks to a novel technique for separately measuring transit time and local geometrical broadening effects on transducers with markedly different focusing properties, more than 1000 experimental acquisitions show how a proper combination of such measured contributions gives an accurate ISB estimation, confirming the theoretical expectations.

Structure and rovibrational analysis of the [O2(1Δg)v=0]2←[O2(3Σg−)v=0]2 transition of the O2 dimer

Abstract: The rotationally resolved absorption spectrum of the O2 dimer involving the [O2(1Δg)v=0]2←[O2(3Σg−)v=0]2 transition has been recorded near 632.6 nm by continuous wave Cavity Ring Down Spectroscopy in a supersonic slit jet expansion of pure O2. A quadratic dependence of the absorption in the jet versus the stagnation pressure is observed. A rotational temperature of 12 K is derived from the (O2)2 rotational analysis. The high spectral resolution of the CW-CRDS measurements limited by the residual Doppler broadening in the jet and the low rotational temperature allow the first rotational analysis in this open-shell complex. The same spectrum was also recorded by Intracavity Laser Absorption Spectroscopy and the comparison of the performances of the two methods is discussed. Among more than 600 lines measured between 15 800 and 15 860 cm−1 from the CW-CRDS spectrum, 40 were assigned to the RP0, RQ0, and RR0 branches of two subbands associated with B1−←A1+ and A1+←B1− transitions between the ground and excite...

Speed-dependent line mixing profiles

Abstract: Spectral profiles of overlapped lines are given for speed-dependent broadenings, shifts and couplings. Doppler broadening and Dicke narrowing are incorporated for analysis of atmospheric spectra. Dicke narrowing is treated in the hard velocity-changing collision limit both uncorrelated and correlated with phase- or state-changing collisions.

Study of plasma fluctuations in the Tuman-3m tokamak using microwave reflectometry with an obliquely incident probing beam

Abstract: Plasma fluctuations in the Tuman-3M tokamak are studied experimentally by analyzing backscattered radiation for different angles of incidence of the probing beam from the normal to the cut-off surface. The poloidal rotation velocity of the plasma fluctuations is determined from the Doppler shift of the reflected radiation spectrum measured on the edge of the tokamak during the transition to the H-mode. It is shown that, before the transition to the H-mode, the rotation velocity can be estimated quantitatively from the spectral shift or from the rate at which the phase of the reflected signal grows. The experimental data obtained during the transition to the H-mode provide evidence for the onset of a sheared poloidal flow. The shear makes it difficult to correctly estimate the poloidal rotation velocity in the improved confinement regime. The main mechanisms responsible for the broadening of the backscattered radiation spectra are considered. The turbulent diffusion coefficients determined under the assumption that the spectral broadening is diffusive in character are found to be close to those determined from the charged-particle balance.

Broadening, shifting, and line asymmetries in the 2←0 band of CO and CO–N2: Experimental results and theoretical calculations

Abstract: We have measured the room temperature, widths, pressure shifts, and line asymmetry coefficients for many transitions of the first overtone band of CO and CO perturbed by N2. The broadening coefficients were obtained with an accuracy of about 1%. The pure CO profiles have been fitted by a Voigt profile while the CO–N2 spectral profiles have been fitted with a Lorentz and an empirical line shape model (HCv) that blends together a hard collision model and a speed-dependent Lorentz profile. In addition to the Voigt, Lorentz, and HCv models, we have added a dispersion profile to account for weak line mixing. The line broadening and shift coefficients are compared to semiclassical calculations employing a variety of intermolecular interactions. The line asymmetry results are compared to line mixing calculations based on the energy corrected sudden (ECS) model. The results indicate that effects other than line mixing also contribute to the measured line asymmetry.

How a quantum-dot laser turns on

Abstract: The turn-on dynamics of quantum-dot lasers is modeled theoretically. The impact of the—so far technologically inevitable—inhomogeneous broadening σ, the homogeneous broadening Γ, and the interlevel relaxation time on the relaxation oscillations (ROs) is investigated. When the inhomogeneous broadening is dominant (Γ≪σ), the individual nanostructures in the ensemble exhibit independent ROs. In the case of significant homogeneous (Γ⩾σ) broadening, the quantum-dot ensemble exhibits collective, synchronized ROs, leading to a stronger intensity modulation and a larger turn-on delay.

Protein dynamics at low temperatures

Abstract: The low temperature conformational dynamics of the heme type protein mesoporphyrin-IX-substituted horseradish peroxidase is investigated by spectral diffusion waiting time/aging experiments. Spectral diffusion broadening is governed by a power law in time. There is a small but significant aging effect. It is assumed that the conformational dynamics of the protein which leads to the spectral broadening of the burnt-in holes is governed by a diffusion type equation. In this case the shape of the spectral diffusion kernel is Gaussian. This model is contrasted with spectral diffusion phenomena as described by the TLS-model (TLS, two level system).

Sub-Doppler resolution limited Lamb-dip spectroscopy of NO with a quantum cascade distributed feedback laser.

Abstract: A quantum cascade distributed feedback laser operating at 5.2 microm is used to obtain sub-Doppler resolution limited saturation features in a Lamb-dip experiment on the R(13.5)1/2 and R(13.5)3/2 transitions of NO. The dips appear as transmission spikes with full widths of ~ 4.3 MHz. At this resolution the 73 MHz _-doubling of the R(13.5)3/2 line, which is normally obscured by the 130 MHz Doppler broadening, is easily resolved.

Spectroscopic measurements of the ion temperature profile in front of a limiter in TEXTOR-94

Abstract: Ion temperatures have been measured in front of a limiter in TEXTOR-94 under various plasma conditions by means of the Doppler broadening of the C5+ line (λ = 529 nm), which is excited by charge exchange processes with hydrogen (deuterium) atoms recycled at the limiter surface. The ion temperatures have been compared with electron temperatures measured by atomic beam techniques. It has been found that the ion temperatures are considerably larger than those of the electrons. In the scrape-off layer (SOL) the ratio between the ion and electron temperatures (Ti/Te) can reach values of up to three at low plasma densities, but this ratio decreases with increasing density. The SOL e-folding length of the ion temperature is much larger than that of the electron temperature. The influence of the test limiter position rL on the ion temperature measurements has been investigated but found to be of no significant importance.

Speed-dependent line profile: A test of a unified model from the Doppler to the collisional regime for molecule-molecule collisions

Abstract: A speed-dependent line profile combining soft and hard fully correlated Dicke-narrowing collisions was recently successfully tested on Ar-broadened H2 spectra in a wide density and temperature range. A further test for mixtures of H2 in nitrogen molecules (instead of Ar atoms) is presented. This test is also based on high resolution Raman investigation of the isotropic Q(1) line of H2 from low to high density at various temperatures. The same consistency of the speed-dependent line profile as for H2–Ar is obtained for H2–N2 through a remarkable agreement with all the data by using a unique set of four parameters (the collisional width and shift, the kinetic frequency, and a characteristic velocity memory parameter). The present study is a preliminary step for the hydrogen CARS thermometry in H2–air flames at high pressure.

The effects of numerical resolution on hydrodynamical surface convection simulations and spectral line formation

Abstract: The computationally demanding nature of radiative-hydrodynamical simulations of stellar surface convection warrants an investigation of the sensitivity of the convective structure and spectral synthesis to the numerical resolution and dimension of the simulations, which is presented here. With too coarse a resolution the predicted spectral lines tend to be too narrow, reflecting insufficient Doppler broadening from the convective motions, while at the currently highest affordable resolution the line shapes have converged essentially perfectly to the observed profiles. Similar conclusions are drawn from the line asymmetries and shifts. In terms of abundances, weak FeI and FeII lines show a very small dependence (~0.02 dex) while for intermediate strong lines with significant non-thermal broadening the sensitivity increases (~0.10 dex). Problems arise when using 2D convection simulations to describe an inherent 3D phenomenon, which translates to inaccurate atmospheric velocity fields and temperature and pressure structures. In 2D the theoretical line profiles tend to be too shallow and broad compared with the 3D calculations and observations, in particular for intermediate strong lines. In terms of abundances, the 2D results are systematically about 0.1 dex lower than for the 3D case for FeI lines. Furthermore, the predicted line asymmetries and shifts are much inferior in 2D. Given these shortcomings and computing time considerations it is better to use 3D simulations of even modest resolution than high-resolution 2D simulations.

Power broadening and Doppler effects of coherent dark resonances in Rb

Abstract: Using a phase-locked laser pair we have observed dark resonances with linewidths below 30 Hz in a rubidium cell filled with neon as buffer gas. A model allowing for pressure broadening correctly reproduces the dependence of the width on the laser intensity. Consideration of velocity changing collisions reveals the absence of Doppler effects in the position and width of the dark resonance at high buffer-gas pressure.

Spectroscopic investigations of a dielectric-surface-discharge plasma source

Abstract: Spectroscopic investigations of the properties of a plasma produced by a flashboard plasma source, commonly used in pulsed plasma experiments, are presented. The plasma is used to prefill a planar 0.4 μs conduction time plasma opening switch (POS). A novel gas-doping technique and a secondary surface flashover plasma source are used to locally dope the plasma with gaseous and solid materials, respectively, allowing for spatially resolved measurements. The electron density, temperature, and plasma composition are determined from spectral line intensities and line profiles. Detailed collisional-radiative modeling is used to analyze the observed line intensities. The propagation velocity and divergence angle of various ions are determined from time-of-flight measurements and Doppler broadening of spectral lines, respectively. This allows for distinguishing the secondary plasma ejected from the POS electrodes from the plasma of the flashboard source.

Analytic formula for radiation trapping with partial frequency redistribution and foreign gas broadening

Abstract: A recently developed analytic formula for radiation trapping in cylindrical geometry with partial frequency redistribution and with a spectral lineshape dominated by almost any combination of natural (radiative) broadening, resonance collisional broadening, and Doppler broadening is generalized to include foreign or buffer gas broadening. The effect of foreign gas broadening is incorporated by rescaling the radius and absorbing gas density. In some cases rescaling results in an unphysically small radius. The analytic formula reported here, which is accurate to zero radius, is an improved version of the earlier formula. The accuracy of the improved formula is verified with 198 new simulations, in addition to the original 294 simulations, all using a highly realistic Monte Carlo code.

First direct observation of coulomb explosion during the formation of exotic atoms

Abstract: A Doppler broadening of x-ray transitions from pionic nitrogen and muonic oxygen, which is attributed to Coulomb explosion of the molecules, has been observed by using a crystal spectrometer. Large linewidths indicate fast ionization of the molecules and a charge of (3-4)e for the accelerated fragments.

Spectral Broadening of Femtosecond Laser Pulses in Fibers with a Photonic-Crystal Cladding

Abstract: Changes in the spectra of femtosecond laser pulses propagating through fibers with a cladding having the structure of a two-dimensional photonic crystal are experimentally investigated. It is demonstrated that the waveguide properties of defect modes of photonic-crystal fibers provide an opportunity to considerably increase the efficiency of spectral broadening of short laser pulses as compared with conventional fibers.

Numerical analysis of electron cyclotron current drive with suppressed Doppler broadening

Abstract: An electron cyclotron current drive by the fundamental O-mode is numerically analysed in the off-axis region of a reactor-grade tokamak. By scanning the launching direction in both the toroidal and poloidal directions, the optimum direction of beam injection is obtained to drive a current with a maximum value of I/ρw at the desired magnetic surface, where I is the total driven current and ρw is the radial width of the driven current. When the current is driven at the position where a ray trajectory becomes tangent to the magnetic surface (referred to as the `tangential resonance'), Doppler broadening of the current profile is significantly reduced and I/ρw reaches a maximum value. In the case of tangential resonance, the driven current width is mainly determined by the effects of the beam divergence and finite spatial width of the beam at the launching position. The width of the driven current is kept in a range of 2-5% of the minor radius under the assumption of a beam divergence with a full angle of 2°. The dependence of the driven current on both the location of beam injection and the wave frequency is also examined.