Showing papers on "Doppler broadening published in 1978"

cw three-wave mixing in single-mode optical fibers

Abstract: Strong continuous three‐wave mixing of 514.5‐nm argon laser light in a single‐mode fiber is reported. The effect, due to the third‐order nonlinearity of silica, has been observed for light whose frequency spectrum consists of either a few discrete monochromatic frequency components separated by ∼1 GHz or a quasicontinuous distribution of frequencies having a spectral envelope ∼4 GHz wide. We show that the effect provides a simple and effective method for measuring the nonlinearity of silica. In the first manifestation of the effect, the nonlinearity mixes the frequency components to produce new frequencies. In the second, multiple mixing occurs that broadens the quasicontinuous spectrum. This manifestation of the effect is large; broadening by a factor of 4 has been observed with lower intensity levels than are required to produce stimulated Brillouin scattering in the same fiber. A theoretical model is presented to describe spectral broadening by three‐wave mixing for the case of small broadening. The effect of three‐wave mixing on the operation of continuous stimulated Brillouin and Raman oscillators is also discussed. Finally, it is noted that the presence of this effect may constrain the design of long‐haul single‐mode fiber optical communication trunks.

Collision Experiments with Laser Excited Atoms in Crossed Beams

Abstract: Publisher Summary This chapter discusses collision experiments using laser excited atoms in crossed beams. With the advent of tunable narrow-band lasers, especially cw dye lasers, the situation has changed and it has become clear that it should be possible to excite atoms optically within the scattering region of an otherwise conventional crossed-beam experiment. In this way a steady-state upper-state population could be reached that may be comparable to the ground-state population. When an atomic beam is excited, it is free of internal collisions and, for right angle intersection with the laser, free of Doppler broadening. Then the laser properties allow selecting the state into which the atom is excited. Specific fine- and hyperfine-structure states may be chosen as well as a particular combination of sub-states. The novel techniques allow preparing states with an angular momentum different from zero and to vary systematically the alignment and orientation of the resulting non-spherical interaction potentials. Frequency doubling of dye lasers can also widen the scale of possible applications. Very high powers are needed and one probably would have to use a pulsed laser. An alternative to frequency doubling is the direct two-photon excitation of atoms.

Radial dependence of solar wind properties deduced from Helios 1/2 and Pioneer 10/11 radio scattering observations

Abstract: Radio scattering measurements at 2.3 GHz, conducted over the heliocentric distance range of 1.7 to 180 solar radii, provide data for a study of the radial dependence of the solar wind characteristics. The data are obtained from the Helios 1 and 2 and the Pioneer 10 and 11 spacecraft. Spectral broadening measurements, together with angular broadening data for the spacecraft signal, indicate a value of 24 km/sec for the solar wind velocity at 1.7 solar radii. A velocity profile of the solar wind in the acceleration range is developed. In addition, phase scintillation measurements obtained suggest that the solar wind is slightly converging in the equatorial region between approximately 20 and 180 solar radii.

The spectral broadening of sound by turbulent shear layers. Part 2. The spectral broadening of sound and aircraft noise

Abstract: It has been observed experimentally by Candel, Julienne & Julliand (1975) that a monochromatic test tone generated by a source inside a jet is received outside as a broad frequency band of definite shape. This phenomenon of spectral broadening occurs during transmission through the shear layer, which generally has a randomly irregular and unsteady shape, contains in addition distributed turbulence, and separates the jet and the ambient medium. We show in the first place that, in the audible range of frequencies, neither the approximation which treats the shear layer as a scattering interface with a convected undulating shape nor the opposite, high frequency limit obtained by means of asymptotic estimation of integrals derived for the diffraction of rays in turbulence is sufficient to provide a satisfactory theory of the observations. The refraction integrals obtained in part 1 have to be evaluated exactly in order to account for the phenomenon of spectral broadening, the methods used possibly being of interest in other branches of wave theory. The formation of the transmitted spectrum from an incident tone can be illustrated by representing a simple shear layer as an array of elements each re-radiating energy received from the source with its own characteristic attenuation and frequency shift. A computer program is used to obtain spectra under conditions corresponding to the experiments of Candel, Guedel & Julienne (1975) and gives encouraging agreement with their measurements, which were made with high frequency sources immersed in low speed jets. The theory can also be applied to the prediction of spectra received at various angles to the axis of high subsonic jets, but depends on extrapolation when supersonic exhausts are considered. We conclude with an example of the possible relevance of spectral broadening as a means of reducing the noise disturbance from current jet-powered aircraft, such as Concorde.

Effects of the radiator motion in the classical and quantum-mechanical theories of collisional spectral-line broadening

Abstract: A kinetic equation is derived for the average velocity-dependent electric dipole moment of an atomic radiator in a perturber gas. This equation determines the spectral-line profile in the impact limit, combining Doppler and collision broadening, and it has the same structure in the case of a classical or a quantum-mechanical description. The kinetic equation is solved in several limiting cases, which generalize previous work. Quantities determining the profile are generalized complex loss factors, source factors and friction constants, which are expressed in terms of the exact quantum-mechanical scattering amplitudes. It is found that state-selecting collisions increase the collisional width, and decrease Dicke narrowing of the Doppler width. This type of collision is not properly accounted for in a classical theory.

Evaluation of the Analytical Capabilities of a dc Plasma-Echelle Spectrometer System:

Abstract: The analytical capabilities of a dc plasma used in combination with an echelle spectrometer have been evaluated. Measurements of the spectrometer resolution capabilities have demonstrated that they closely approximate the high values theoretically predicted but are ultimately limited by Doppler broadening effects occurring in the plasma. Investigations of possible ionization interference effects have shown that enhancements due to the addition of easily ionized elements are generally observed for elements having ionization potentials below approximately 8.5 eV; these may be compensated for by the addition of an excess of an easily ionized element. Evaluation of solute vaporization interference effects indicates that they also occur but can be negated by proper choice of operational conditions and/or the addition of releasing agents. Comparison of the detection capabilities of the system with those reported for other techniques shows that they are equivalent or superior to those characteristic of flame atomic absorption and often competitive with those obtained with the inductively coupled plasma, atomic emission system.

Theoretical analysis of spectral modulation behaviour of semiconductor injection lasers

Abstract: The influence of the spectral width on the modulation behaviour of semiconductor injection lasers has been investigated theoretically on the basis of multimode rate equations. Analytical expressions are derived yielding an effective rate of spontaneous emission for a suitable description of dynamic characteristics of injection lasers. This effective emission rate may drastically exceed the total rate of spontaneous emission into the lasing modes. In the case of large signal modulation it has been shown that a large modulation depth as required for PCM-transmission also yields a considerable spectral broadening.

Ion energy balance in Ohmically heated PLT discharges

Abstract: Radial profiles of the ion temperature have been measured on PLT by using neutron flux Doppler broadening of oxygen, carbon and helium line radiations, and charge exchange. The ion temperature ( 1.2 keV) is consistent with Hinton-Rosenbluth neoclassical heat conduction in the plateau regime as the dominant energy loss from the plasma with central ion energy confinement times 0.1 s.

Positron annihilation studies of quenched aluminium

Abstract: Positron annihilation Doppler broadening measurements were made on quenched aluminium single-crystal and polycrystal samples as a function of isochronal annealing. Changes in the annihilation lineshape due to clustering of vacancies were observed. The measurements show that clusters begin to form very early in stage III and that the degree of clustering depends strongly on the density of vacancy sinks in the samples.

Auger electron emission from target ions under heavy-ion impact after molecular dissociation

Abstract: The line shape of the (1s2s2p)4P (metastable) to (1s2)1S+e- Auger electron transition from heavy-ion impact is measured for targets containing carbon, nitrogen, oxygen and neon. The angular distribution of this transition is also measured for molecular gas targets containing carbon. Ar, Kr, Xe and U projectiles from the UNILAC were used with energy 1.4 MeV amu-1. It is found that the line shape depends strongly on the initial molecular target. Kinematic effects such as projectile-induced target recoil do not account well for these changes. A simple model is proposed that assumes the creation of a highly ionised molecule. The line shape is due to Doppler broadening from the velocity acquired by the dissociation of the molecule. Reasonable agreement between this Coulomb explosion model and the experimental results is found, allowing dissociation velocities to be calculated from the experimental line shapes.

Channeled-substrate-planar structure distributed-feedback semiconductor lasers

Abstract: Distributed‐feedback Ga1−xAlxAs lasers with a channeled‐substrate‐planar structure are fabricated. They operate in single transverse and longitudinal modes. No spectral broadening or excess noise is observed under high‐frequency modulation as a result of mode stabilization.

Collision broadening and shifts in the spectra of neutral and singly ionised calcium

Abstract: Measurements are reported of the broadening and shifts, due to collision with helium and argon, of the neutral calcium line 4227 AA and the Ca+ resonance lines 3968 and 3933 AA (the solar H and K lines) and the associated lines 8542 and 8662 AA. The lines are obtained in emission from a flow lamp and the profiles recorded from a scanning Fabry-Perot interferometer are analysed numerically. The very nearly equal broadenings of the H and K lines and the corresponding infrared lines indicate that the broadening in both multiplets is essentially determined by the common upper configuration. The observed broadening is between 1.3 and 2 times the predictions of van der Waals theory. Comparison with high-temperature shock-tube measurements indicate that the broadening varies as T0.4+or-0.02 for helium and T0.25+or-0.07 for argon.

Stark profile measurements on Lyman series lines of Al xiii

Abstract: Single shot photographic spectra of hydrogen‐like aluminum Lyman series lines are obtained from a low inductance vacuum spark. Corresponding pinhole pictures yield information on the source size. The observed line profiles are found to increase in width along the series and the higher series members Lγ and Lδ can be matched with convolved Stark and Gaussian profiles. The Gaussian profile includes the effects of source size, thermal Doppler broadening, and crystal resolution. The space and time‐averaged electron density determined from the profile analysis is 2.5×1021–1×1022 cm−3.

Phase-changing broadening of the laser line of Xe I at λ=3.51μm

Abstract: Phase-changing broadening constants of the line of Xe I at lambda =3.51 mu m are obtained by analysing Voigt profiles recorded by using a linear-absorption laser experiment. He, Ne, Ar, Kr and Xe perturbers are considered. Measurements made with He and Xe perturbers differ significantly from those obtained by use of saturated-absorption techniques. A variation of the Doppler width is observed at low pressures of Xe, probably caused by non-resonant excitation transfers from highly populated levels of Xe towards the upper level of the transition.

Some effects of temperature on positron annihilation characteristics in metals

Abstract: Measurements of positron lifetime changes from 100K to the onset of trapping at vacancies in Al, Cu, Ag and Au are presented. The extracted temperature coefficients of positron lifetimes increase with temperature so that they become greater than theoretical predictions based on expansion of a static lattice. In addition, a simple representation of a one-dimensional momentum distribution is used to establish a relationship between changes in positron annihilation rates with temperature and the corresponding changes in shape parameters of the momentum distributions observed in angular correlation or Doppler broadening measurements, permitting a comparison of theoretical predictions of the temperature dependence of annihilation rates with various experimental determinations of the temperature coefficients of the momentum shape parameters.

Ion fluctuations and velocity distribution in the presence of ion cyclotron waves

Abstract: Ion density fluctuations and the ion velocity distribution function in the presence of the current‐driven electrostatic ion cyclotron instability are determined using resonance fluorescence of the ions. The optical line intensity modulation shows that the ion density modulation can be as large as 90%. From the Doppler broadening of the lines it is found that the distribution function of ions heated by the unstable ion cyclotron waves is Maxwellian with an uncertainty of 5%.

Positron annihilation in the alkali halides

Abstract: Position lifetime spectra and Doppler broadening spectra have been measured for 17 alkali halides. Three significant lifetime components are found in almost all cases. The results are interpreted in terms of a model in which a fraction of the positrons form positronium and the remainder are initially in a state from which either annihilation or transitions may occur. The annihilation rate from the initial state is shown to depend on the distance separating negative ions, which suggests that the positron is in a bound state.Deconvoluted Doppler broadening data indicate that at least two momentum components are present.

Neutron resonance spectroscopy: Ta

Abstract: Results of high resolution neutron time of flight spectroscopy on natural tantalum (99.988% /sup 181/Ta, I/sup ..pi../+) are presented for l = 0 resonances to 2025 eV. Values of E and gGAMMA/sup 0//sub n/ are presented for 431 resonances, in addition to the three resonances below 15 eV. A Bayes analysis suggests that no p wave levels have been included. The analysis used a multilevel Breit-Wigner shape fitting to the measured transmission curves for three different sample thicknesses. Doppler broadening and potential-resonance interference effects were included, plus some simulation of the experimental resolution effects. The analysis gives = (4.17 +- 0.04) eV for the mean level spacing, and 10/sup 4/S/sub 0/ = (1.68 +- 0.11) for the l = 0 strength function.

Absorption and emission

Abstract: This chapter focuses on the absorption and emission processes in atoms, for which it is permissible to replace the totality of atomic states by just two discrete states between which the transition occurs. Such a simplification is possible if the energy separation between the two states of an atom correspond to the photon energy whereas all other levels are spaced so that there are no energy differences close to the the photon energy. In the semiclassical theory, the spontaneous part of the emission probability emerges after an attempt to reconcile the Planck's radiation law with thermodynamic requirements. However, the quantum–mechanical treatment of the radiation field gives the spontaneous emission automatically. The quantum–mechanical oscillator strength for an atomic transition corresponds to the number of classical oscillators that would emit or absorb the same amount of radiation. Collisions between atoms may have the effect of interrupting the wave train of the radiation emitted by an atom. In the case of quantum mechanics, an excited atom cannot only relax to the ground state by spontaneous emission but may also release its excitation energy to another atom in the course of an impact. The lifetime is therefore shortened and may be completely dominated by the time between collisions. The random motions of the atoms in a gas give rise to the Doppler Effect. The profile due to Doppler broadening is Gaussian because of the Maxwellian distribution of velocities. Doppler and collision broadening are independent.

Suppression of optical feedback effects on saturated absorption signals by phase modulation of the reflected light

Abstract: In high-resolution spectroscopy, the Doppler broadening is suppressed through the use of two counter-propagating beams so that one of the beams is obviously reflected back into the laser cavity. Conventional optical isolators are efficient enough to reduce the coupling but some intensity fluctuations always remain due to residual feedback. A simple technique is described which does not suppress the residual optic feedback but completely cancels its effect on line shapes and signals.

Excitation of transitions between atomic or molecular energy levels by monochromatic laser radiation - II. Excitation under conditions of strictly inhomogeneous line broadening and mixed homogeneous and inhomogeneous broadening

Abstract: The development with time of the excitation of a transition between two atomic or molecular energy levels under the influence of monochromatic laser radiation is examined under conditions of strictly inhomogeneous line broadening due to such causes as doppler shift arising from translational velocity. The ratio of the number of molecules, N 2 , in an excited state to the total number, N , is calculated for various ratios of the intensity parameter β ═ E 0 μ / ħ to the half line-width ∆ , where E 0 is the amplitude of the electric field in the incident radiation and is the dipole moment matrix element for the transition. Excitation functions obtained in a previous paper (I) for various values of the ratio γ/β , where γ is the half line-width in the absence of inhomo­geneous broadening, are used to obtain the variation of N 2 / N with time under conditions of mixed broadening for various values of the ratio γ/∆ and γ/β , when the exciting radiation is in exact resonance with the central frequency of the transition.

The X-ray diffraction line broadening due to the diffractometer condition as a function of 2θ

Abstract: The line profile due to diffractometer instrumental conditions was obtained from the measured line profile of a standard specimen by eliminating the spectral broadening, applying a model for the wavelength distribution. Under common experimental conditions, it was found that for 2 theta >50 degrees the spectral broadening is dominant in the line profile, and that for the larger 2 theta values for small values of the harmonic number the Fourier coefficients corresponding to the broadening caused by the instrumental conditions may be assumed to be independent of 2 theta .

Positron annihilation in 60Co irradiated alkali halides

Abstract: A systematic investigation of 17 gamma-irradiated alkali halides using positron lifetime and Doppler broadening techniques has been performed. For eight of the alkali halides (NaCl, NaBr, KCl, KBr, KI, RbCl, RbBr, and RbI), a new lifetime with values between 1.0 and 1.3 ns is attributed to positrons trapped in F centres. Despite large F centre concentrations in LiF, LiCl, LiBr, and NaF, no such lifetime component is observed in these samples. Doppler broadening measurements show that in nearly all cases the momentum distribution becomes narrower upon irradiation. The experimental data are discussed in view of a recent theoretical treatment of the positron – F centre problem.