Showing papers on "Doppler broadening published in 1972"

Effect of Voids on Angular Correlation of Positron Annihilation Photons in Molybdenum

Abstract: POSITRON annihilation investigations of defects in crystals have shown that for sufficiently high defect concentrations (typically above about 10−6) all positrons become trapped in the defects before annihilation, thus changing the characteristics of the annihilation process. For example, trapping of positrons may result in the increase in the positron lifetime, a narrowing of the 2-γ angular correlation distribution, and a reduction in the Doppler broadening of the annihilation line. Vacancies in metals1,2, deformation effects in metals3, and defects (F-centres or cation vacancies) in ionic crystals4 have all now been studied by this technique. The trapping of positronium (Ps) in defects in quartz5 and ice6 has also been investigated. Here we report the study of another crystal defect, the void, a small vacancy of diameter few tens of Angstroms. In practice voids may contain gas atoms and may be disturbed either in a random fashion or arranged in a macro lattice7.

Line broadening theory of asymmetric-top molecule

Abstract: The Anderson-Tsao-Curnutte's theory (1.2) of collision broadening has been applied to the asymmetric-top molecules. Theoretical formulations of broadening due to dipole-dipole, dipole-quadrupole, quadrupole-dipole, quadrupole-quadrupole and Debye induction force interactions are developed. Asymmetry of the electric charge distribution was taken into account in considering the quadrupole moment of the asymmetric-top molecule. It is shown that the shape of the electric-charge distribution affects the value of the line width when the quadrupole moment is large.

Collision broadening and shift in the resonance line of calcium

Abstract: The broadening and shift of the calcium resonance line, lambda 4227 angstroms, have been measured with high accuracy when the line is formed in absorption under pressures of He, Ne, Ar, Kr, and Xe up to one atmosphere. He and Ne cause symmetrical broadening over the entire pressure range in agreement with the predictions of the 'impact theory'. The onset of asymmetrical broadening indicating breakdown of the 'impact approximation', is noticed at pressures of Ar, Kr, and Xe close to one atmosphere. The peculiar way in which broadening and shift change with perturbing gas is discussed in terms of 6-12 and 6-8-12 interatomic potentials.

Hyperfine structure of excited states and quadrupole moment of Ne-21 using laser-induced line-narrowing techniques.

Abstract: Observation results are presented on the optical hyperfine structure in Ne-21 obtained with the aid of laser-induced line-narrowing techniques. The output from a long stabilized single-mode 1.15-micron He-Ne laser focused into an external sample cell containing Ne-21 was used in implementing these techniques. Their applicability is demonstrated for optical hyperfine structure observation in systems whose features are ordinarily masked by Doppler broadening.

Molecular Beam Stabilized Argon Laser

Abstract: The frequency of a 5145‐A argon‐ion laser has been locked to an absorption line in a molecular beam of I2. The drift of the laser frequency was less than 20 kHz or Δν/ν<3 × 10−11 for the duration of a run which lasted about 20 min. A molecular‐beam external reference is attractive because transitions observed in the isolated conditions in the beam do not suffer shifts due to collisions or collisional broadening and, if the molecular beam is excited orthogonally, Doppler broadening can be virtually eliminated.

Electron Correlations in the Unified Model for Stark Broadening

Abstract: An improved treatment of electron correlations is presented within the framework of a previously developed unified model for Stark broadening by plasma electrons. This treatment employs the method of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy in connection with a closure relation corresponding essentially to the exclusion of simultaneous strong collisions. A line-shape expression is derived for the case that the perturbing electron gas is in thermal equilibrium. This expression depends on the dynamic dielectric constant and on the two-electron correlation function. If the latter is approximated by the linearized Debye-H\"uckel expression (low-density case), and if the result is simplified for the vicinity of the line center by using second-order perturbation theory for the atom-perturber interaction, the results of earlier treatments of electron correlations are recovered.

High-Resolution Raman Spectroscopy of Gases with Laser Sources, V.* Use of the Single-Mode Argon Laser†

Abstract: A description is presented of an experimental method of obtaining Raman spectra of low-pressure gases under high resolution. The technique utilizes a single-mode argon laser, a multiple-pass Raman cell located inside the laser cavity, and baked Kodak IIIa-J photographic plates. A method is described which uses the single-mode Fabry–Perot etalon to change the wavelength of the exciting line so as to compensate for changes of the dispersion of the spectrograph brought about by variations of barometric pressure. Rotational Raman spectra of heavy polyatomic molecules can now be photographed at room temperature with a resolution determined by the pressure and Doppler broadening of spectrum lines.

Quantum theory of stark broadening by electrons.

Abstract: The one electron approximation is used to determine the exact form of the wings of a line broadened by electron-atom collisions. The validity of this model is examined. The profile being proportional to the square of the scalar product of scattered waves, the impact approximation is obtained by replacing the exact form of the scattered waves by their asymptotic form. In this case of hydrogen the quasistatic approximation is obtained by using the JWKB method together with the usual assumptions.

Motional broadening of the quasi-elastic peak in neutrons scattered from polymeric materials

Abstract: Doppler broadening studies of the quasi-elastic peak of cold neutrons scattered from poly-(dimethylsiloxanes) are reported. The Doppler broadening is observed in the low-molecular-weight liquids and persists in high-molecular-weight gums and even in the cross-linked rubbers. The self-diffusion constants determined for the low-molecular-weight liquids are larger than the corresponding value obtained from n.m.r. spin echo measurements. Since the latter technique undoubtedly measures the translational contribution to self-diffusion, it is concluded that the neutron-scattering result includes a major contribution from internal rotational motions of the polymer chain. The activation energy for the neutron diffusional process is 2.0 kcal mol–1 compared with 4.0 kcal mol–1 for the translational diffusion process. A similar Doppler broadening phenomenon has been observed in poly(propylene)oxide at elevated temperatures.

Study of spectral broadening in a filament of light

Abstract: We have measured with nanosecond laser pulses the spectral broadening of light emitted from a filament under controlled input conditions. Our results are in semiquantitative agreement with predictions from the moving‐focus model.

Contribution of Hyperfine Structure to the Measured Widths of Gold Hollow-Cathode Lines

Abstract: The effects of hyperfine structure on the measured widths of some gold hollow-cathode lines are considered. Correction factors are presented for the apparent Doppler temperatures derived from line-profile measurements of the gold 4793- and 3123-A lines and from line-absorption and total-absorption measurements on the gold 2428- and 2676-A resonance lines. These factors account for much of the difference between the Doppler temperatures of the gold lines and of the neutral-argon lines are reported by Kreye and Roesler. The effect of the large ejection velocities of the sputtered gold atoms on the effective temperature of the emitting vapor was investigated using Monte Carlo techniques; the results indicate that this source of Doppler broadening is insignificant at the argon pressures normally used in hollow-cathode lamps.

Pressure Broadening of the O 2 Microwave Spectrum

Abstract: A general expression for the calculation of pressure broadening for vibrational-rotational lines is derived in the strong-collision model. Classical trajectories and a peaking approximation are used to calculate a unitary scattering operator avoiding perturbation expansions, impact-parameter cutoffs, and straight paths. The latter approximations are not expected to be good when short-range potentials dominate the collision interaction. Use of intermolecular-potential parameters determined from thermodynamics and a simple theory of dispersion forces for ${\mathrm{O}}_{2}$ calculations gave excellent agreement with experimental data on the magnitude, temperature, and quantum-number dependence of the linewidth parameter.

Cavity Detuning and Multimode Operation of an Optically Pumped Gas Laser

Abstract: In an earlier paper, the problem of the optically pumped gas laser was treated within the formalism of a modified Boltzmann equation and the equation of radiative transfer. The laser cavity was tuned precisely to the center of the atomic lasing frequency and only one cavity mode was excited above threshold. These restrictions are now removed and the effects on the laser population inversion, gain, and power output are presented. In the limit that Doppler broadening dominates collisional broadening, it is found that the power output for the single-mode operation increases as the laser cavity is detuned, until an optimum occurs when the cavity detuning is equal to the collisional linewidth. In the limit that collisional broadening dominates Doppler effects, power output is independent of detuning, until the detuning approaches the collisional linewidth. For multimode operation, it is shown that power output increases approximately linearly with the number of modes in oscillation, provided that the Doppler width is much larger than the collisional width and mode interaction is not occurring. When collisional broadening dominates, no increase in total power output is achieved by allowing more than one mode to go into oscillation. Most of these results are well known, for low levels of lasing intensity, through the semiclassical analysis of Lamb. However, the formulation here provides a more quantitative evaluation of the relative roles of inhomogeneous and homogeneous broadening for all collisional linewidths and high lasing intensities.

Broadening and Shift of Atomic Lines Produced by Fast Electron Collisions

Abstract: In this paper general expressions for the width and shift of atomic lines produced by fast electron collisions are obtained. We use the recent quantum-mechanical theories developed to explain the pressure line shape in the impact approximation. Only isolated and Lorentzian lines are analyzed. The calculations are performed by expanding the scattering matrix up to the second Born approximation. The contributions of both elastic and inelastic virtual collisions are calculated. Degenerate and nondegenerate atomic states are considered.

Thermal and Electric Field Broadening in Electro-Optical Effect

Abstract: The effect of broadening on electro-optical spectra is investigated. Two broadening factors due to thermal and electric field effects are taken into account as the Lorentzian convolutions in the electro-optical functions. The calculated electro-optical functions enable us to compare quantitatively with the spectra measured in the electro-optical experiments. The variations of the amplitudes and oscillation periods in the field induced changes of the dielectric constants, Δe1 and Δe2, with the amount of broadening factors are presented for a series of certain realistic parameters.

On the Stark broadening of Lyman-β

Abstract: Classical-path calculations for the Stark broadening of the Lyman-β line of hydrogen are presented using a procedure outlined in a previous paper. All multipoles in the perturber (free electron)—atom interaction are included and time ordering of the operators in the S-matrices is retained. Comparison is made with previous calculations. In particular, the profiles given here have very nearly the same half widths as the profiles of Kepple and Greim but show a significant increase in the intensity at the line center.

Method for measuring doppler shifts in arc-heated flows.

Abstract: A novel method of determining both the Doppler and Stark shifts in a single measurement of spectral lines emitted by the arc-heated flow from a plasma jet has been successfully demonstrated. The method uses a spherical mirror arranged with its optical axis coincident with the optical axis of a Fabry-Perot interferometer and with its center of curvature at the center line of the flow. The common optical axis lies at an angle to the flow. With this system, both red- and blue-shifted line profiles are recorded in the same spectral scan. If conditions are such that the red- and blue-shifted profiles are not resolvable, the blue-shifted component is chopped so that the recorded signal consists of the envelopes of both the red-shifted profile and the superimposed red- and blue-shifted profiles. The wavelength difference between the blue- and red-shifted line profiles is exactly twice the Doppler shift integrated along a line of sight through the flow and is independent of a Stark shift. The Stark shift is given by the wavelength difference between the absolute line center and the midpoint of the red- and blue-shifted lines. Abel inversion of integrated line shift data has yielded radial velocity profiles to an accuracy of ±3% in a supersonic, arc-heated argon flow.

Abundances in late-type giants.

Abstract: The narrow-band photometry of Spinrad and Taylor (1969) is analyzed by the synthetic spectra technique. Surface gravities, Doppler broadening velocities, effective temperatures, and metal abundances are determined for 131 K giants. Evolutionary tracks and gravities determined from the photometry are used to give space positions, space velocities, and absolute magnitudes of the 131 stars. We conclude that a class of stars exists with metals and/or nitrogen significantly enhanced with respect to the sun; further that these stars have average space motions similar to those of moderately old Population I stars. The Doppler broadening velocity is found to be a strong function of temperature for K giants, and in some cases, possibly a function of surface gravity.

Introduction to Saturated Optical Absorption

Abstract: Saturated absorption techniques are being used increasingly often to measure optical resonances having such narrow bandwidths that they would otherwise be obscured by Doppler broadening. The principle underlying these techniques is reviewed using four simple thought or "gedanken" experiments. A rudimentary explanation is based on Bennett's " hole-burning" picture. Two configurations of typical saturated absorption experiments are described. One illustrates absorption outside and the other inside of a laser cavity. The intracavity technique is recommended for weak absorbers and when especially simple compact apparatus is needed. For most other experiments, the gaseous absorber is better located outside the laser cavity.

Excitation Transfer Process in Self-Broadening

Abstract: The effect of the excitation transfer process on Doppler broadening is investigated in a gas of identical radiators. This effect can be described by a linear transport equation, which also includes the self-broadening. The resulting line shape exhibits shift, asymmetry, and some narrowing. The basic equations can be easily generalized to include the usual collisional narrowing.

A symmetrical laser Doppler velocity meter and its application to turbulence characterization

Abstract: A symmetrical method of optical heterodyning of the Doppler shifted scattered laser radiation developed for velocity measurements with a minimal instrumental spectral broadening and a high signal-to-noise ratio. The method employs two laser beams incident on the moving scatterer and does not use any reference beam for heterodyning. The Doppler signal frequency is independent of the scattering angle and the signal possesses no receiving aperture broadening. Optical alignment is simple. Typical values of the instrumental spectral broadening were approximately 0.8 percent of the center frequency of the Doppler signal, and the signal-to-noise ratio was approximately 25 dB, obtained from an air flow system using submicron dioctylphthalate scattering aerosol. Experimental and theoretical studies were made on the characteristics of the Doppler signal and the effect of system parameters in turbulent flow measurement. The optimization process involved in the beam optics and in the use of a spatial filter is described. For localized flow measurement in any direction of the three-dimensional orthogonal coordinates, the system, using uncorrected optical components, had a sensing volume which can be described by a sensitive length of 600 microns and a diameter of 100 microns.