Showing papers on "Doppler broadening published in 1995"

Electromagnetically induced transparency in ladder-type inhomogeneously broadened media: Theory and experiment

Abstract: We develop a theory of electromagnetically induced transparency in a three-level, ladder-type Doppler-broadened medium, paying special attention to the case where the coupling and probe beams are counterpropagating and have similar frequencies, so as to reduce the total Doppler width of the two-photon process. The theory is easily generalized to deal with the \ensuremath{\Lambda} configuration, where the ideal arrangement involves two copropagating beams. We discuss different possible regimes, depending on the relative importance of the various broadening mechanisms, and identify ways to optimize the absorption-reduction effect. The theory is compared to the results of a recent experiment (on a ladder-type system), using the Rb D2 line, with generally very good agreement. The maximum absorption reduction observed (64.4%) appears to be mostly limited by the relatively large (\ensuremath{\sim}5 MHz) linewidth of the diode lasers used in our experiment.

Electromagnetically induced transparency in a three-level Λ-type system in rubidium atoms

Abstract: An electromagnetically induced transparency is observed at one arm of a three-level \ensuremath{\Lambda}-type system in a rubidium D1 line (794.8 nm) with an 85% reduction in absorption, when a pumping field is present at the other arm. This reduction in absorption for the weak probe field is due to the atomic coherence produced by the strong pumping field. This experiment is done in a Rb vapor cell at room temperature with cw diode lasers for both pumping and probe beams in a Doppler-free configuration. A simple theoretical treatment including Doppler broadening is in good qualitative agreement with the experimental measurement.

Identification of vacancy defects in compound semiconductors by core-electron annihilation: Application to InP

Abstract: We show that the Doppler broadening of positron annihilation radiation can be used in the identification of vacancy defects in compound semiconductors. Annihilation of trapped positrons with surrounding core electrons reveals chemical information that becomes visible when the experimental background is reduced by the coincidence technique. We also present a simple calculational scheme to predict the high-momentum part of the annihilation line. The utility of the method is demonstrated by providing results for vacancies in InP. In electron irradiated InP the isolated In and P vacancies are distinguished from each other by the magnitude of the core electron annihilation. In heavily Zn-doped InP we detect a native vacancy defect and identify it to a P vacancy decorated by Zn atoms.

Investigation of OH absorption cross sections of rotational transitions in the A 2 ∑ + , ( υ ′ = 0 ) ← X 2 Π , ( υ ″ = 0 ) band under atmospheric conditions: Implications for tropospheric long‐path absorption measurements

Abstract: The accuracy of tropospheric hydroxyl radical measurements by long-path absorption spectroscopy is ultimately limited by the uncertainty of the effective OH absorption cross sections. The latter were determined from calculated spectra for the Q1(2), Q1(3), and P1(1) rotational lines of the OH A2∑+, (υ′=0)←X2Π, (υ″=0) transition at 308 nm. The calculations took into account Doppler broadening, measured data of the collisional broadening of OH by air molecules, and instrumental line broadening effects. The calculated spectra were compared with OH absorption spectra measured in a flow reactor near room temperature at 1013 hPa. Excellent agreement between calculated and measured OH spectra was achieved using the collision-broadening parameters determined recently by Leonard (1990). The effective absorption cross sections at the line center (peak absorption cross sections) calculated for the Voigt line shape at 300 K and 1013hPa are: σeff(v˜0)[ Q1(2) ]=(1.67±0.1)×10-16 cm2, σeff(v˜0)[ Q1(3) ]=(1.4±0.1)×10-16 cm2, σeff(v˜0)[ P1(1) ]=(1.41±0.1)×10-16 cm2. The full width at half maximum of the spectral lines is 2.5 pm, and the shape factor a is 1.41. Calculations of the effective absorption cross section under different atmospheric conditions demonstrate a strong pressure dependence for all lines. At 2800 m altitude (720 hPa) the peak absorption cross section of the OH Voigt lines is about 35% higher than at sea level. The temperature dependence in an interval of ±20 K around room temperature (at 1013 hPa) is only small (±3% for the Q lines and ±6% for the P1(1) line). The estimated total uncertainty of the effective absorption cross section is 8%.

Damage-depth profiling of an ion-irradiated polymer by monoenergetic positron beams.

Abstract: Poly(aryl-ether-ether ketone) (PEEK) films irradiated with 1-MeV and 2-MeV ${\mathrm{O}}^{+}$ ions were exposed to positron beams to measure the positron annihilation Doppler broadening as a function of the positron energy. The annihilation lines recorded at relatively low positron energies were found to become broader with increasing irradiation dose, suggesting that positronium (Ps) formation is inhibited in the damaged regions. The positron data were compared with the results of dynamic hardness and electron-spin-resonance measurements. The slow-positron Doppler broadening technique is found to be a useful means for damage-depth profiling of Ps-forming polymers.

Plasma Spectroscopy: The Influence of Microwave and Laser Fields

Abstract: 1. Introduction.- 2. Analytical Methods for the Calculation of Quasienergy States (QS) of Quantum Systems.- 2.1 Interaction of Quantum Systems with a Nonstationary Field.- 2.2 Perturbation Theory for QSs of Degenerate Quantum Systems.- 2.2.1 Calculation of the QSs as a Stationary Problem.- 2.2.2 Perturbation Theory.- 2.3 High-Frequency or Very Intense Nonstationary Fields.- 2.3.1 Calculation of the QSs as a Stationary Problem.- 2.3.2 Perturbation Theory.- 2.3.3 Generalizations.- 3. Action of One-Dimensional Quasimonochromatic Electric Fields (QEF) on Coulomb Emitters.- 3.1 Splitting of Hydrogen-like Spectral Lines in a Single-Mode QEF.- 3.1.1 Analytical Investigation.- 3.1.2 Numerical Calculations. Oscillatory Behavior of Halfwidths and Intensities.- 3.1.3 Doppler Broadening. Formal Analogy with Thomson Scattering in the Presence of QEFs.- 3.2 Splitting of Hydrogen-like Spectral Lines in a Multimode QEF.- 3.2.1 Analytical Investigation for the Number of Modes Approaching Infinity, and Equal to 2.- 3.2.2 Numerical Calculations.- 3.3 Impact Broadening of Hydrogen-like Spectral Lines.- 3.3.1 Modifications of Impact Broadening Due to QEF.- 3.3.2 Modification of QEF-induced Line Splitting Due to Impact Broadening.- 3.4 Frequency-integrated Radiative Characteristics of Hydrogen-like Emitters Interacting with a Resonant Laser Field and a Low-Frequency QEF.- 3.4.1 Resonant Multiquantum Interaction.- 3.4.2 Non-Degenerate Case.- 3.4.3 Degenerate Case.- 3.4.4 Applications of the Results.- 4. Action of Multidimensional Dynamic Electric Fields on Coulomb Emitters.- 4.1 Splitting of Hydrogen-like Spectral Lines in a Plane Polarized QEF.- 4.1.1 Analytical Results for a Circularly Polarized Field.- 4.1.2 Multiquantum Dynamic Resonance in an Elliptically Polarized Field.- 4.1.3 Elliptically Polarized Fields in the High-Frequency Limit.- 4.1.4 Measurements of Elliptically Polarized Field Parameters.- 4.1.5 Analytical Investigation of Two-Dimensional Multimode QEFs.- 4.2 Joint Action of QEF and Quasistatic EF on Hydrogen-like Spectral Lines.- 4.2.1 Dynamic Resonance.- 4.2.2 Hydrogen-like Lines at a Multiquantum Dynamic Resonance and Away from the Resonance.- 4.2.3 The L? Spectral Line with Detuning from Resonance.- 4.2.4 Dips in Hydrogen Spectral Lines Resulting from the Resonance Effects.- 4.2.5 Intra-Stark Spectroscopy. Diagnostic Recommendations.- 4.3 Hydrogen-like Spectral Lines in a High-Frequency or Strong QEF with a Quasistatic EF.- 4.3.1 Calculation of Quasienergy States.- 4.3.2 Calculations of the L?, L? and H? Line Profiles.- 4.3.3 Further Generalizations for the Action of a Static Magnetic Field.- 5. Action of a One-Dimensional QEF on Non-Coulomb Emitters.- 5.1 Satellites of Dipole-Forbidden Spectral Lines Caused by a Nonresonant Action of QEFs (Three-Level Scheme).- 5.1.1 Dirac Perturbation Theory. Baranger-Mozer Method for Measurements of QEF Parameters.- 5.1.2 Adiabatic Theory of Satellites and Quasilocal Method for Measurements of QEF Parameters.- 5.1.3 Polarization of Satellites.- 5.1.4 Strong Asymmetry of Satellite Distribution in Very Intense QEFs.- 5.1.5 Modification of Helium-like Ion Satellites Caused by Mixing of Singlet and Triplet Terms.- 5.1.6 Satellites in a Stochastic QEF.- 5.2 Satellites of Dipole-Forbidden Spectral Lines in Resonant QEFs. Three-Level Scheme.- 5.2.1 Multiquantum Resonance in a Two-Level Subsystem.- 5.2.2 Spectrum of Spontaneous Transitions to a Third Level in a Multiquantum Resonance.- 5.3 Satellites of Dipole-Forbidden Spectral Lines in More Complicated (Four-Level) Systems.- 5.3.1 QSs of a Three-Level Subsystem in a High-Frequency or Intense Field.- 5.3.2 Radiative Transitions from the States 2P1/2, 2S1/2, 2P3/2 of a Hydrogen-like Ion in a High-Frequency or Intense Field.- 5.4 Electron Oscillatory Shift in Plasmas Interacting with a Powerful Coherent Radiation.- 5.4.1 Calculation in the Rectilinear Trajectories Approximation.- 5.4.2 Calculations Including Curved Trajectories.- 5.4.3 Discussion.- 5.5 Action of QEFs on Diatomic Polar Molecules.- 5.5.1 Satellites in Vibrational-Rotational Spectra.- 5.5.2 Ultra-sensitive Laser Induced Fluorescence Measurements of Weak QEFs in Low-Temperature Plasmas.- 5.6 Frequency-Integrated Radiative Characteristics of Non-Coloumb Emitters Interacting with a Resonant Laser Field and Low-Frequency QEF.- 6. Non-Coulomb Emitters Under Multidimensional Dynamic EFs (Elliptically Polarized QEFs Quasistatic EF plus QEF).- 6.1 Satellites of Dipole-Forbidden Spectral Lines Caused by an Elliptically Polarized QEF.- 6.1.1 Three-Level Scheme, Nonresonant QEF.- 6.1.2 Three-Level Scheme, Resonant QEF.- 6.1.3 Four-Level Scheme.- 6.2 Joint Action of QEF and Intraplasmic Quasistatic EF on Non-Coulomb Emitters.- 6.2.1 Strong Influence of a Quasistatic EF on Satellites of Dipole-Forbidden Lines.- 6.2.2 Intra-Stark Spectroscopy.- 6.2.3 Joint Action of a Quasistatic EF and a High-Frequency QEF on a Hydrogen-like Ion. Fine Structure and Lamb Shift. Local Measurements of Amplitude Angular Distributions of Low-Frequency Plasma Turbulence.- 6.3 Shift of Spectral Lines of Diatomic Polar Molecules in an Elliptically Polarized QEF.- 7. Applications of the Theory to Experimental Plasma Diagnostics.- 7.1 Preliminary Remarks.- 7.2 QEFs in ?-Pinches.- 7.2.1 QEFs Under a Magnetic Field Annihilation.- 7.2.2 QEFs Under a Rapid Compression of a ?-Pinch Plasma.- 7.3 QEFs in a Z-Pinch.- 7.4 New Features of Intra-Stark Spectroscopy Caused by a High Density of Plasmas.- 7.5 QEFs in Tokamaks.- 7.5.1 Intense EFs in the Edge Plasma of the T-10 Tokamak.- 7.5.2 Novel Spectroscopic Diagnostics of EFs in Tokamaks.- 7.6 QEFs in Plasmas Interacting with a Strong Microwave Field.- 7.6.1 Technique Utilizing Hydrogen or Deuterium Lines.- 7.6.2 Quasilocal Measurements Technique Utilizing Lines of Non-Coulomb Emitters.- 7.6.3 Techniques of Local Laser Fluorescence Diagnostics.- Appendices.- D Generalized QSs of a Hydrogen Atom in a Bichromatic EF.- E Influence of Bound Electrons on the Frequency and Damping of Langmuir Oscillations.- References.

Improved blood velocity estimation using the maximum Doppler frequency

Abstract: In vessels whose diameter is smaller than the length of the range cell or measurement volume, the maximum blood velocity is often calculated from the maximum frequency of the Doppler spectrum, using the classical Doppler equation. It is shown that the accuracy of this procedure is significantly improved at large beam-to-flow angles, if a correction for transit time broadening is made. This finding is based on the demonstration that the maximum frequency of the Doppler spectrum depends only on the maximum velocity passing through the measurement volume, but in a manner which is a function both of the Doppler shift frequency as well as the transit time broadening associated with the passage of scatterers through the beam width.

Correction of intrinsic spectral broadening errors in doppler peak velocity measurements made with phased sector and linear array transducers

Abstract: Commercial duplex ultrasound systems primarily measure peak velocity of blood flow to provide important information in diagnosing vascular disease. However, due to errors caused by intrinsic spectral broadening (ISB), the accuracy decreases as the Doppler angle increases. In general, vascular technologists and surgeons keep the Doppler angle under 60°. Even at 60°, ISB can produce an overestimation of peak velocity as high as 40%. This would indicate the necessity of reducing the Doppler angle even lower than 60°. Since most vessels in the body run parallel with the skin surface and criteria requiring a Doppler angle of say 45° or less would be difficult to achieve. Using the transverse Dooppler equation as a correction for ISB, in conjunction with the classical Doppler equation, accurate peak velocity measurements were obtained at Doppler angles as high as 80°. Corrections were made using both phased sector and linear array transducters for steered and unsteered beams. However, for beams steered at large angles, 20° or more, corresponding to Doppler angles of

Effects of velocity changing collisions on line shapes of HF in Ar

Abstract: The generalized Hess method (GHM) gives a line shape expression which is formally equivalent to the Rautian–Sobel’man hard collision model of Dicke narrowing, but differs radically in the definition of one of the relaxation terms. The relaxation term leading to pressure broadening is the same, but the term leading to Dicke narrowing and ultimately to Doppler line shapes at zero density differs in certain important respects: (1) in GHM it is a weighted sum of the pressure broadening coefficient and an optical diffusion coefficient and (2) there is no sharp distinction between ‘‘velocity changing’’ and ‘‘phase changing’’ collisions. The Dicke narrowing term should thus be understood as including both collision types irretrievably intermixed, with GHM providing a prescription for both relaxation terms. Applied to HF v=0→1, j→j±1 absorption spectra in a bath of Ar and using an accurate interaction potential obtained from spectra of the van der Waals complex and essentially exact close coupling scattering S ma...

Line profile of H Lyman α from dissociative excitation of H2 with application to Jupiter

Abstract: Observations of the H Lyman(alpha) (Ly-alpha) emission from Jupiter have shown pronounced emissions, exceeding solar fluorescence, in the polar aurora and equatorial "bulge" regions. The H Ly-alpha line profiles from these regions are broader than expected, indicating high-energy processes producing fast atoms as determined from the observed Doppler broadening. Toward understanding that process a high-resolution ultraviolet (UV) spectrometer was employed for the first measurement of the H Ly-alpha emission Doppler profile from dissociative excitation of H2 by electron impact. Analysis of the deconvolved line profile reveals the existence of a narrow central peak of 40 +/- 4 mA full width at half maximum and a broad pedestal base about 240 mA wide. Two distinct dissociation mechanisms account for this Doppler structure. Slow H(2p) atoms characterized by a distribution function with peak energy near 80 meV produce the peak profile, which is nearly independent of the electron impact energy. Slow H(2p) atoms arise from direct dissociation and predissociation of singly excited states which have a dissociation limit of 14.68 eV. The wings of H Ly-alpha arise from dissociative excitation of a series of doubly excited states which cross the Franck-Condon region between 23 and 40 eV. The profile of the wings is dependent on the electron impact energy, and the distribution function of fast H(2p) atoms is therefore dependent on the electron impact energy. The fast atom kinetic energy distribution at 100 eV electron impact energy spans the energy range from 1 to 10 eV with a peak near 4 eV. For impact energies above 23 eV the fast atoms contribute to a slightly asymmetric structure of the line profile. The absolute cross sections of the H Ly-alpha line peak and wings were measured over the range from 0 to 200 eV. Analytic model coefficients are given for the measured cross sections which can be applied to planetary atmosphere auroral and dayglow calculations. The dissociative excitation process, while one contributing process, appears insufficient by itself to explain the line broadening observed at Jupiter.

Peak velocity overestimation and linear-array spectral Doppler.

Abstract: Ultrasound instruments are used to evaluate blood flow velocities in the human body. Most clinical instruments perform velocity calculations based on the Doppler principle and measure the frequency shift of a reflected ultrasound beam. Doppler-only instruments use single-frequency, single-crystal transducers. Linear- and annular-array multiple-crystal transducers are used for duplex scanning (simultaneous B-mode image and Doppler). Clinical interpretation relies primarily on determination of peak velocities or frequency shifts as identified by the Doppler spectrum. Understanding of the validity of these measurements is important for instruments in clinical use. The present study examined the accuracy with which several ultrasound instruments could estimate velocities based on the identification of the peak of the Doppler spectrum, across a range of different angles of insonation, on a Doppler string phantom. The string was running in a water tank at constant speeds of 50, 100, and 150 cm/sec and also in a sine wave pattern at 100- or 150-cm/sec amplitude. Angles of insonation were 30, 45, 60, and 70 degrees. The single-frequency, single-crystal transducers (PC Dop 842, 2-MHz pulsed-wave, 4-MHz continuous-wave) provided acceptably accurate velocity estimates at all tested velocities independent of the angle of insonation. All duplex Doppler instruments with linear-array transducers (Philips P700, 5.0-MHz; Hewlett-Packard Sonos 1000, 7.5-MHz; ATL Ultramark 9 HDI, 7.5-MHz) exhibited a consistent overestimation of the true flow velocity due to increasing intrinsic spectral broadening with increasing angle of insonation.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectroscopic investigation of fast (ns) magnetic field penetration in a plasma

Abstract: The time‐dependent magnetic field spatial distribution in a coaxial positive‐polarity plasma opening switch (POS) carrying a current ≂135 kA during ≂100 ns, was investigated by two methods. In the first, ionic line emission was observed simultaneously for two polarizations to yield the Doppler and Zeeman contributions to the line profiles. In the second method, the axial velocity distribution of ions was determined, giving the magnetic field through the ion equation of motion. This method requires knowledge of the electron density, here obtained from the observed particle ionization times. To this end, a lower bound for the electron kinetic energy was determined using various line intensities and time‐dependent collisional‐radiative calculations. An important necessity for POS studies is the locality of all measurements in r, z, and θ. This was achieved by using laser evaporation to seed the plasma nonperturbingly with the species desired for the various measurements. The Zeeman splitting and the ion motion showed magnetic field penetration through the 3.5 cm long plasma at a velocity ≂108 cm/s. The current density was found to be relatively high at the load‐side edge of the switch plasma. It is suggested that this may cause plasma acceleration into the vacuum section toward the load, which is supported by charge‐collector measurements. The fast magnetic field penetration agrees with estimates based on the Hall‐field mechanism.

Measurement of ultra-fast γ-ray transitions from heavy-ion compound-nucleus reactions

Abstract: A method for correcting target-thickness-induced Doppler broadening in gamma-ray spectra from weakly populated, very short-lived (femtosecond) states is presented. The method is illustrated by an a ...

Ultrasonic doppler measurement of blood flow velocities by array transducers

Abstract: A Doppler signal processor is provided for an ultrasonic diagnostic imaging system which identifies the peak and mean flow velocities by Doppler interrogation of a sample volume within the body. The effects of spectral broadening resulting from the use of an array transducer are compensated by producing a peak velocity value which is a function of the dimension of the array aperture and its relation to the location of the sample volume. Mean velocity values calculated from received Doppler echoes are combined with an array distortion function to produce accurate mean velocity values. The entire spectrum may be corrected by deconvolving the received signal spectrum with an array distortion function. Alternatively the effects of spectral broadening are compensated by use of Doppler reference signals for each element of the array which are a function of the position of the individual elements in the array aperture.

Spectroscopic investigations of the plasma behavior in a plasma opening switch experiment

Abstract: The electron density, the electron kinetic energy, the particle motion, and electric fields in a coaxial positive‐polarity plasma opening switch (POS) were studied using spectroscopic diagnostics. A gaseous source that injects the plasma radially outward from inside the inner POS electrode was developed. The plasma was locally seeded with various species, desired for the various measurements allowing for axial, radial, and azimuthal resolutions both prior to and during the 180 ns long current pulse. The electron density was determined from particle ionization times and the electron energy from line intensities and time dependent collisional‐radiative calculations. Fluctuating electric fields were studied from Stark broadening. The ion velocity distributions were obtained from emission‐line Doppler broadenings and shifts. The early ion motion, the relatively low ion velocities and the nearly linear velocity dependence on the ion charge‐to‐mass ratio, leads to the conclusion that the magnetic field penetrat...

Derivation of gain spectra of laser diodes from spontaneous emission measurements

Abstract: The use of detailed balance relationships between absorption and emission to obtain a gain spectrum from a spontaneous emission spectrum is a relatively easy method of measuring the gain from a semiconductor laser. It has been shown that the usual theoretical gain and emission spectra do not satisfy these relationships casting doubt on the validity of the procedure. We show that this arises from the incorrect implementation of spectral broadening that leads to a situation where the carriers and photons are no longer in thermal equilibrium and the relationship between absorption and gain in the calculations breaks down. We show that with a more exact implementation of spectral broadening thermal equilibrium is preserved and that the relationship between absorption and emission remains valid. We discuss the accuracy of the measurements required to obtain a correct gain spectra. >

Measurement of the temperature of cold highly charged ions produced in an electron beam ion trap

Abstract: The temperature of highly charged titanium ions produced and trapped in an electron beam ion trap was determined by precisely measuring the broadening of K-shell emission lines in Ti19+ caused by the thermal Doppler motion. While interacting with a 3330 V electron beam the measured temperature of the titanium ions ranges from about 700 eV when the ions are confined in a deep trap to about 70 eV for ions in a shallow trap. The latter value represents the lowest temperature at which the K-shell X-ray emission from highly charged titanium ions has ever been recorded.

Optical astronomical spectroscopy

Abstract: Preface Atomic Processes INTRODUCTION TO SPECTROSCOPY Historical background Types of spectroscopy THE PHYSICS OF ATOMS AND MOLECULES Sommerfeld's refinements Space quantization Atomic structure Other viewpoints Quantum/wave mechanics ATOMIC ENERGY LEVELS Multiple electron atoms ^IL-S coupling Space quantization Term formation Closed shells and subshells Notation Hyperfine structure TRANSITIONS Basics Selection rules Transition probabilities Absorption and emission coefficients Ionization and recombination X-ray spectra SPECTRA OF MOLECULES Introduction Rotational transitions Vibrational transitions Electronic transitions RADIATION IN THE PRESENCE OF FIELDS Zeeman effect Stark effect Free-free radiation Synchrotron and gyrosynchrotron radiation Cerenkov radiation The Faraday effect SPECTROSCOPY OF SOLID MATERIALS ASTRONOMICAL SPECTROSCOPIC TECHNIQUES OPTICAL SPECTROSCOPES Introduction Diffraction gratings Grating problems Diffraction grating spectroscopes Prisms Prism spectroscopes Fourier transform spectroscope (Michelson interferometer) Fabry-P^D'erot etalons SPECIALIZED OPTICAL SPECTROSCOPIC TECHNIQUES FOR ASTRONOMY Introduction Detectors Guiding Widening Image dissectors Dekkers Long slit spectroscopy Comparison spectra Flexure Temperature Exposures EXAMPLES OF OPTICAL SPECTROSCOPES Introduction A small basic spectroscope A conventional Cassegrain spectroscope Transmission grating spectroscopes Coud^D'e spectroscopes A GRISM-based spectroscope Multi-object spectroscopes An echelle grating spectroscope Infrared spectroscopes Spacecraft-borne spectroscopes Fabry-P^D'erot spectroscopes Fourier transform spectroscopes Objective prism spectroscopes The future Spectroscopy of astronomical sources SPECTRAL CLASSIFICATION Spectral class The non-core classes The luminosity class RADIAL VELOCITIES Introduction Traditional approach to radial velocity determination Objective prism spectra Machine-readable spectra Griffin's method Line identification SPECTROPHOTOMETRY Introduction Spectral calibration Photometric calibration Line strengths Line broadening Pressure broadening Doppler broadening Magnetic fields Other effects STARS Introduction Distance Temperature Element abundances Variable stars The Sun PLANETS AND OTHER MINOR BODIES OF THE SOLAR SYSTEM Introduction Planetary atmospheres and comets Planetary surfaces Rotation NEBULAE AND THE INTERSTELLAR MEDIUM Nebulae The interstellar medium EXTRA-GALACTIC OBJECTS Introduction Distances Spectra Ages Sizes Background radiation APPENDICES Conversion formulae Term formation of equivalent electrons Bibliography Constants Index

Doppler broadening and collisional relaxation effects in a lasing-without-inversion experiment

Abstract: In the early experiments on optical pumping and coherent population trapping (``dark resonances'') increasing buffer gas pressure enhances the effect. In our recent lasing-without-inversion experiments, based on population trapping, we find the opposite behavior, namely, decreasing atomic coherence with increasing buffer gas pressure. We provide a theoretical explanation of this result in agreement with experiment.

A Monte Carlo code for nuclear astrophysics experiments

Abstract: A Monte Carlo code, suited for nuclear astrophysics experiments, is described. The code has been developed in the frame of the LUNA pilot project at the Laboratori Nazionali del Gran Sasso. An accurate evaluation of ion energy and angular straggling, and Doppler broadening has been implemented, which are important at subCoulomb energies. The considered effects are compared with experimental data.

Doppler‐limited spectroscopy at cryogenic temperatures: Application of collisional cooling

Abstract: A coolable gas cell for the measurement of mid‐infrared spectra of gases at cryogenic temperatures as low as 77 K is described. The design of the cell allows it to be operated in collisional cooling (or diffusive trapping) mode to generate significant nonequilibrium pressures of monomer vapor for gases which would normally have vanishingly small vapor pressure at low temperatures. Fourier transform infrared spectra of carbon dioxide (CO2), nitrogen dioxide (NO2), and methyl silane (CH3SiH3) vapors cooled by collisions with helium gas at 77 K are reported. Sufficiently high concentrations of monomer gas may be produced at low total pressure conditions (<10 h Pa) to allow spectra to be recorded at the Doppler resolution limit. The resulting spectroscopic simplification indicates that efficient collisional cooling of the vibrational, as well as rotational, degrees of freedom occurs.

Study of various types of diamonds by measurements of double crystal x‐ray diffraction and positron annihilation

Abstract: Annihilation characteristics of positrons in various types of diamonds were studied by measurements of two‐dimensional angular correlation of positron annihilation, those of Doppler Broadening profiles and those of lifetime spectra. From the measurements, it was found that there is a small amount of defects in type Ib synthesized diamond and the positron lifetime is 115 ps. On the other hand, it was found that positronium is formed in natural diamonds. Results of the double x‐ray measurements suggested that IIa, IIb, and Ia type natural diamonds also may have empty space between crystallites or grain boundaries.

Applications of solitons in transmission systems employing high launch powers

Abstract: A method for launching a high-power signal into anomalous-dispersion fiber without suffering excess spectral broadening and waveform distortion. A high-power signal is launched as very short optical solitons, typically with a pulse width of approximately 1 to 10 ps. The pulse width of the solitons will broaden adiabatically (or nearly adiabatically) as they propagate down the anomalous-dispersion fiber as the average power of the soliton decreases. At the same time, the optical spectrum will narrow. After some distance of travel, when the width of the pulses have broadened to a suitable duration and the average power has decreased to a point where penalties from nonlinearities are insignificant, the pulses enter a significantly linear propagation regime with first order dispersion near zero. A dispersion-shifted fiber may be used or any additional dispersion can be compensated at the receiver. Consequently, no further significant broadening of the spectrum occurs when the signal is received at the receiver. Additionally, the initial wide spectrum of the solitons will result in a high stimulated Brillouin scattering (SBS) threshold reducing the need for artificial broadening of the spectrum.

Degenerate four-wave-mixing line shapes of hydroxyl at high pump intensities.

Abstract: The degenerate four-wave-mixing spectral profile of the R1(9) transition in the A2∑+ → X2Π(0, 0) band of OH has been measured for various combinations of saturating pump beams. With increasing pump-beam intensity the degenerate four-wave-mixing line shape changes dramatically near line center. In phase-conjugate geometry, three distinct spectral line shapes were observed for the cases of (1) equally intense pump beams, (2) a strong forward pump and a weak backward pump, and (3) a weak forward pump and an intense backward pump. A significant saturation dip appears in the spectrum near line center for case (3). The measured spectra have been modeled by the use of nonperturbative numerical solutions of the density matrix equations, and agreement between the calculations and the experimental results is excellent. The differences in the saturated line shapes for cases (2) and (3) are explored theoretically, and the calculated results are compared with previous theoretical work [Bloch and Ducloy, J. Opt. Soc. Am. 73, 635 (1985)] in which the line shapes were calculated in the limit of infinite Doppler broadening.

Positron Methods for the Study of Defects in Bulk Materials

Abstract: The basic principles of positron annihilation physics are briefly discussed and the three most important experimental techniques used for bulk studies are described (i.e. positron lifetime, angular correlation, Doppler broadening). Several examples of the use of the positron methods are discussed for metals, ceramics and molecular materials, which illustrate the sensitivity of the positron annihilation techniques to vacancy type defects. For example it is shown how information can be obtained about vacancy formation energies, vacancy migration and clustering, vacancy-impurity interactions, densities of rare gasses in bubbles in metals, and about free volume in molecular materials