Doppler-broadened Lineshapes Research Articles

Doppler-broadened line shapes of atomic hydrogen in a parallel-plate radio frequency discharge

Doppler-shifted atomic hydrogen emission (Balmer α) is observed from a low-frequency rf discharge through molecular hydrogen by collecting the light through a hole in one of the electrodes. Doppler shifts as large as 0.7 nm, corresponding to an energy of 540 eV or 85% of the peak applied voltage, are observed when ions are accelerated by the sheath electric field. The mechanisms for hot atom production are discussed in terms of both gas-phase and surface ion-impact phenomena. Hot atoms are produced via gas-phase ion-atom and ion-molecule collisions, as a result of simultaneous neutralization and reflection of ions at the electrode surface, and/or by sputtering of adsorbed hydrogen. As much as 30% of the atomic hydrogen emission is substantially Doppler shifted, indicating that most of the atomic emission in the sheaths is actually produced by ion impact and not by electron impact.

Low Temperature Dependence of Positron Annihilation in Quenched Aluminum

Doppler-broadened lineshapes of positron annihilation radiation in an aluminum single crystal after quenching have been measured as a function of temperature between 4.2 and 200 K. The peak parameter for positron annihilation in the trapped state at vacancies showed no significant temperature dependence below 80 K, while above 100 K, it showed an almost linear temperature dependence with a temperature coefficient about twice that found for positron annihilation in the free state. A low concentration of vacancies yielded a remarkable increase in the peak parameter as the temperature decreased below 80 K. The results showed a strong temperature dependence of the trapping rate of positrons at vacancies in the low temperature range.

Positron annihilation study of boron-doped Ni 3Al

Positron-lifetime and Doppler-broadening measurements were performed at room temperature on well-annealed polycrystalline Ni 75− x Al 25+ x alloys ( x = −1, 0, and + 1) containing 0, 100, and 500 wt. ppm boron. The large grain size ( ≈ 300–500μm) in these alloys rendered the grain-boundary contribution negligible in the positron annihilation process. Systematic dependences of the positron lifetimes and the Doppler-broadening lineshape parameter were observed as a function of both the aluminum and boron contents of the alloys. Positron trapping by defects was found in both the x = 0 and x = + 1 alloys, but was most pronounced in the nominally-stoichiometric alloys containing boron. No positron trapping was evident in the x = −1 alloys with or without boron. A model is suggested for the relationship between these results and the problem of ductilizing poly-crystalline Ni 3Al by boron doping.

Temperature dependence of Doppler-broadened lineshapes of positron annihilation in deformed aluminium

Doppler-broadened lineshapes of positron annihilation in deformed aluminium samples were measured in terms of the peak parameter S in the temperature range between 4.2 and 273K. A remarkable increase in S was seen at temperatures below about 100K as the temperature decreased. The results were interpreted in terms of positron trapping in a dislocation as a shallow line trap accompanied by point-like defects (e.g., jogs) as a deep trap. It was found that the binding energy between a positron and a dislocation line is about 0.005 eV.

Temperature dependence of the Doppler-broadened lineshape of positron annihilation in α-Ti

Doppler-broadened lineshapes of positron annihilation in pure alpha -Ti were measured in the temperature range between 293 and 1143K. The peak parameter S indicated significant effects from positron interactions with equilibrium vacancies above 970K. A vacancy formation enthalpy of 1.27+or-0.05 eV was obtained from the temperature where the curve of S against temperature deviates from the linear increase due to thermal expansion. A long exposure of the specimen to high temperatures, however, yielded a predominantly linear increase of S with temperature, as reported by Hood et al. (1976, 1978, 1979), and effects due to thermally created vacancies were smeared.

Thermal equilibrium measurement of the Doppler-broadened lineshapes of positron annihilation in dilute aluminium-based alloys

Measurements have been made on the Doppler-broadened lineshapes of annihilation radiation in Al-0.1 at.% Ag, Al-0.1 at.% Ge and Al-0.1 at.% Si alloys as a function of temperature from 290 to 890K. The temperature dependence of the parameter S was analysed according to the positron trapping model based on Lomer's expression for the vacancy concentration in a dilute alloy. A least-squares analysis shows that the vacancy formation enthalpy in aluminium is (0.70+or-0.02) eV, and that the binding enthalpies between a vacancy and a solute atom are (0.00+0.02), (0.03+or-0.03) and (0.07+or-0.03) eV for the solutes Ag, Ge and Si, respectively. Furthermore, the specific trapping rates for vacancy-solute-atom pairs were estimated to be (2.6+or-0.6)*1013 s-1, 5*1013 s-1 and (9+or-2)*1013 s-1 for the solutes Ag, Ge and Si, respectively, using recent data for the monovacancy formation entropy and vacancy-solute binding entropies obtained from quenching experiments. These values are much smaller than that for monovacancies, 4*1014 s-1.

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2γ annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 A. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.

Elastic and inelastic scattering of 120 MeV 18O from 208Pb and the spin alignment of the 2 + state of 18O

The elastic and inelastic scattering of 18O ions at 120 MeV from a target of 208Pb have been studied. Cross sections for excitation of the 2 + state at 1.982 MeV in 18O and of the 3 − state at 2.61 MeV in 208Pb were measured. In addition, the populations of the m-substates for the 18O excitation were deduced from the Doppler-broadened line shapes. The data were subjected to a coupled-channels analysis using either Woods-Saxon or folding-model potentials. In addition, the 18O excitation was found to be described very well by use of a semi-microscopic model. The analyses consistently indicated the presence of a positive static hexadecapole moment of several e · fm 4 for the 2 + state of 18O. The m-substate population distributions were found to be better fitted if a vector spin-orbit coupling was introduced for the 2 + state of 18O with a sign opposite to that for the nucleon-nucleus case.

Vacancy in Graphite: Positron Srudies

The positron techniques of lifetime and Doppler-broadening were applied to vacancies in graphite. Vacancies were introduced into highly-oriented pyrolytic graphite by irradiation with 2 MeV electrons and fission reactor neutrons. The lifetime of positrons trapped at the vacancies was determined as 245±5 ps. Lifetime studies revealed two distinct vacancy annealing stages at 600–900°C and at 1300–1500°C. The Doppler-broadened lineshapes of the annihilation gamma-ray were taken in the directions perpendicular and parallel to the c -axis of the graphite lattice. Momentum distribution for an electron-positron pair at a vacancy was deduced and compared with the results of calculation based on the defect molecule model in the framework of LCAO-MO theory.

Collective Rotation of 238U at High Spins

An experimental study of the actinide nucleus 238U was performed by Coulomb excitation with 208Pb projectiles. The E2-transition matrix elements in the ground state band were measured up to spin 28 by comparing experimental Coulomb excitation yields to semiclassical calculations and by analysing Doppler broadened line shapes measured in coincidence with a gamma-ray sum spectrometer. The E2-metrix elements agree within their errors with the rigid rotor predictions. Neither an increase of the collective transition strength at high spin due to centrifugal stretching as predicted by the Rotation Vibration Model nor a cutoff in collectively as postulated by the Interacting Boson Model is observed. The excitation energies in the ground state band show the presence of an aligned angular momentum of 5ℏ at spin 30 which can be explained by a crossing of an aligned band at ℏω ≈ 250 keV. The octupole band shows a constant angular momentum alignment of 3ℏ (above spin 7) and no indication of a bandcrossing.

Lifetime of the 1.04 MeV state inF18, measured through the inverse reactionHe3(O16,p)F18

The mean lifetime of the 1.04 MeV (${J}^{\ensuremath{\pi}}={0}^{+},T=1$) state in $^{18}\mathrm{F}$ has been measured to be 2.7\ifmmode\pm\else\textpm\fi{}0.7 fs using the inverse reaction $^{3}\mathrm{He}(^{16}\mathrm{O},p)^{18}\mathrm{F}$ and the Doppler-shift attenuation method. Targets of $^{3}\mathrm{He}$ implanted into Al, Zr, Nb, and Au foils were employed in the measurements. The centroid shifts and Doppler-broadened line shapes observed at 0\ifmmode^\circ\else\textdegree\fi{} were used in obtaining the lifetime value. The lifetime of the 1.04 MeV state is compared with theoretical predictions.NUCLEAR REACTIONS $^{3}\mathrm{He}(^{16}\mathrm{O},p)^{18}\mathrm{F}$, $E=15.9$ MeV; measured Doppler-shift attenuation, $p\ensuremath{\gamma}$ coin. $^{18}\mathrm{F}$ 1.04 MeV level deduced $\ensuremath{\tau}$.

DSAM lifetime measurements in 26Mg with high-velocity recoils

Mean lifetimes of eight levels in 26Mg have been measured using the 4He( 23Na, p) 26Mg inverse reaction. The lifetimes, determined by fitting Doppler-broadened line shapes observed at 0° to the beam, are ( E x in MeV, J π τ in ps): 1.809, 2 +, 0.653 ± 0.039; 2.938, 2 +, 0.204 ± 0.012; 3.941, 3 +, 1.020 ± 0.126; 4.318, 4 +, 0.392 ± 0.023; 4.332, 2 +, 0.029 ± 0.004; 4.834, 2 +, 0.041 ± 0.008; 4.900, 4 + 0.042 ± 0.008; 5.291, 2 +, < 0.015. The lifetime for the 4.900 MeV, 4 + level, combined with accurate branching ratios for its decay to the 1.809, 2 + level, indicates that it is the 4 + member of the ground-state rotational band.

Structural relaxation in amorphous Fe40Ni40P14B6alloy studied by positron annihilation

The Doppler-broadened line shape of 511 keV gamma rays emitted from annihilating positron-electron pairs, the positron lifetime and electrical resistivity of the amorphous Fe40Ni40P14B6 alloy have been measured in the 0-500 degrees C temperature range using isochronal annealing. The W parameter of the Doppler-broadened line shape, related to core-electron annihilation, increased in two stages by annealing below the crystallisation temperature. These increases appear to be due to the loss of excess free volume in the as-quenched amorphous alloy.

Observations of the prevacancy temperature dependence of positron annihilation in copper

The temperature dependence of the Doppler-broadening lineshape parameter has been studied for Cu in the temperature range 30-1040 degrees C, with special attention given to the prevacancy region (T<500 degrees C). Effects attributable to positron trapping were observed in the prevacancy region for samples annealed at temperatures T<or=950 degrees C. Measurements at T>500 degrees C allowed a quantitative comparison to be made between the prevacancy traps and thermally generated vacancies. This comparison permitted a determination of both the fraction of positrons trapped in the prevacancy region and the lineshape parameter characteristic of annihilation in the prevacancy traps. It was found that the fraction of positrons trapped in the prevacancy temperature region depends upon the thermal and mechanical history of the sample, and possibly also on its impurity content. In addition, two possible explanations of the prevacancy trapping are discussed.

Positron diffusion in metals and trapping at grain boundaries

Abstract Positron trapping has been observed in fine-grained specimens of Zn-22 wt. % A1 by measurements of the positron lifetime and Doppler-broadened lineshape. The trapping is attributed to large-angle grain boundaries. A plot of the trapping rate versus the inverse grain size squared is approximately linear, as predicted by a motion-limited trapping model. Under the assumption of motion-limited trapping, a value of 0.6 cm2 s−1 is obtained for the positron diffusion constant in this alloy at 293 K, with a corresponding mean diffusion distance of 0.14pm. The specific trapping rate μ at the boundaries of grains with sides 1 μm is 2 × 1012 s−1(at 293 K). A decomposition of the lifetime spectra indicated a trapped-positron lifetime of about 250 ps, implying that the local electron density at the trapping sites is comparable to that at Vacancies. No additional positron trapping was observed in a sample treated to yield a high density of Zn precipitates in the Al-rich phase, indicating that these precipitate...

Reduced transition probabilities and energies of high-spin states inDy162

Multiple Coulomb excitation and Doppler-broadened lineshape analyses have been applied to the nucleus $^{162}\mathrm{Dy}$, allowing ground-band energies through the ${18}^{+}$, lifetimes through the ${14}^{+}$, and reduced transition probabilities through the ${18}^{+}$ state to be determined. Energy levels show smooth departure from the rotational model with no backbending, while the $B(E2)$ values are consistent with the rotational model through spin 18. Comparison is made to similar results for high spins in other good rotors. The results strengthen our conclusions reached in previous papers: The compression of energy levels observed at high spins in good rotors is largely due to pairing changes, which have only a small effect on transition probabilities.NUCLEAR REACTIONS $^{162}\mathrm{Dy}$($^{136}\mathrm{Xe}$, $^{136}\mathrm{Xe}$\ensuremath{'}), ${E}_{\mathrm{lab}}=547, 612$ MeV, measured $\ensuremath{\tau}$, ${E}_{\ensuremath{\gamma}}$; deduced $B(E2)$.

Positron annihilation in gold between 27K and 592K

The temperature dependences of the Doppler-broadening lineshape parameter W and the positron lifetime have been measured for positron annihilation taking place in Au in the prevacancy temperature range 27K to 592K. The results are consistent with a linear temperature dependence of these annihilation parameters. It is suggested that this observation represents correctly the intrinsic behaviour of the positron in the Au lattice. The results are compared with earlier work over similar temperature ranges, which has shown additional contributions to these annihilation characteristics.

Quantum theory of line shapes in coherent Raman spectroscopy of gases and liquids

The fully quantum mechanical theory of coherent Raman scattering in gases and liquids is developed. In this theory, the electromagnetic radiation is described as a quantized field, and the quantum mechanical transition amplitude for the scattering process is calculated. This is formally different from the usual semiclassical theory in which the radiation field is characterized as a classical electromagnetic wave and nonlinear susceptibilities are used to describe the interaction of radiation and matter. The Doppler broadened line shape for the ideal gas is calculated and found to be non-Gaussian with a width that is 1.2 times larger than the Doppler width for spontaneous Raman scattering in the forward direction. This differs from a previously published result. The rotational–vibrational band shape is shown to be related to a correlation function for rotational–vibrational motion. This correlation function is the same as that describing spontaneous Raman scattering, but the relationship of the correlation function to the spectrum is different. In the absence of correlations between rotation and vibration the spectrum can be related to a Laplace transform (with an imaginary argument) of a rotational correlation function. The line shape is calculated for two special cases, free rotational motion and Brownian rotational motion.

Low Energy Stopping Powers Determined by Time of Flight Techniques

Stopping powers have been measured for all ions of 6<Zl<20 at velocities of 0.82 and 1.0 V₀ (V₀ &#x02261; &#x0391;c = 0.219 cm ns^-1) in five materials C, Al, Ni, Ag and Au with additional data in some cases to a velocity of ~ 2 V₀. This paper will briefly describe the experimental method and give an overview of the results. More detailed descriptions with tables of experimental stopping powers and comparisons with other work will be published elsewhere. The topics discussed in this paper are the effects of multiple scattering in low energy measurements, Z₂ dependence of the Z₁ oscillation amplitude, and the velocity dependence of low energy stopping powers and its implications for the analysis of nuclear lifetime measurements based on the Doppler broadened line shape technique applied to low recoil velocity data.

Lifetime of the 6+ member of the ground-state band in 22Ne

Mean lifetimes of levels in 22Ne have been measured by the Doppler-shift attenuation method using the inverse reaction 4He(19F, pγ)22Ne. The Doppler-broadened lineshapes observed at 0° were analyzed using scaled experimental stopping powers and give the following results for each of the following levels (energy in keV and lifetime in fs): 4457 ( < 15), 5147 (1300 ± 500), 5522 (27 ± 4), 5641 ( < 20), 6311 (70 ± 10), 6345 ( < 15), 6636 ( < 20) and 6817 ( < 15). The lifetime of the 6311 keV 6+ level is particularly interesting for testing model predictions. The large basis shell model calculations of Preedom and Wildenthal are in good agreement with the experimental B(E2) values for the 2+, 4+ and 6+ members of the ground-state band.