Showing papers on "Positronium published in 1994"

Positron Annihilation in Chemistry

Abstract: 1Introduction- 11 Positron Annihilation in Chemistry- 12 History of Chemistry-Oriented Positron Annihilation Studies- 13 Review of Physics-Oriented Positron Research- 14 The Content and Intention of this Book- 2 Theory- 21 The Annihilation Probability- 22 Atomic Physics of Positronium (Ps)- 23 Application of the Theory- 3 Experimental Techniques- 31 Positron Lifetime (LT) Technique- 311 Elementary Description of a LT Spectrometer- 312 Special Problems of LT Measurements- 313 General Remarks on the Data Analysis- 314 Data Analysis of LT Spectra- 32 Angular Correlation (AC) Measurements- 321 Experimental- 322 Data Analysis of AC Spectra- 33 Doppler Broadening (DB) Measurements- 34 Miscellaneous Measurements- 4 Light Particle States in Insulating Matter- 41 General Remarks- 42 The Excess Electron- 43 The Positron States in Liquids- 44 The Ps States in Liquids- 45 Short Remarks on Light Particle States in Solids and Gases- 46 Measurable Properties of Light Particles- 5 The Positron Spur and Ps Formation in Pure Liquids- 51 The Positron Track and its Terminal Spur- 52 Homogeneous, Long Time Irradiation by Positron Slowing Down- 53 Determination of the Ps Yield- 54 Formation of Ps in the Positron Spur- 55 The Measured Ps Yields for Pure Liquids- 6 Solvated Positron Reactions Models of Reaction Kinetics- 61 Hydrated Positron Reactions with Halides- 62 Reaction Kinetics in Positron Annihilation- 63 Discussion of $$ {\text{e}}_{\text{aq}}^{\text{C}} $$ Reactions Low Concentrations- 64 Discussion of $$ {\text{e}}_{\text{aq}}^{\text{C}} $$ Reactions High Concentrations- 7 Positronium Formation in Solutions- 71 Special Cases of Ps Formation in Nonpolar Mixtures- 72 Theory of Ps Inhibition and Antiinhibition- 73 Empirical Expression for the Ps Yield- 74 Positronium Formation in Mixtures of Nonpolar Liquids- 75 Positronium Formation in Solutions of Polar Liquids- 76 Miscellaneous Ps Yield Effects- 77 The Older Ps Formation Models- 8 Positronium Reactions in Solutions- 81 Kinetic Equations of Positronium Reactions- 82 An Example of Ps Quenching and Inhibition- 83 Positronium Quenching by Mainly Organic Molecules in Solutions- 84 Paramagnetic Quenching of Positronium in Solutions- 85 Positronium Reactions with Ions in Aqueous Solutions- 9 Principles of Positron Annihilation in Molecular Solids- 91 Positronium Bloch Functions and Ps Trapping in Defects- 92 Positronium Trapping in Vacancies in Plastic Crystals- 93 Positronium Formation in Solid Aromatic Hydrocarbons- 94 Positronium in Some Other Molecular Crystals- 95 Slow-Positron Beam Studies of Solids- 10 Polymers and Miscellaneous Molecular Compounds- 101 Transition from Glassy to Normal Liquid State- 102 Positron Annihilation in Polymers- 103 Electric Field Dependence of the Ps Yield- 104 Liquid Crystals and Miscellaneous Liquids- 105 Miscellaneous Solids- 11 Conclusion- References- Index of Chemicals

Cross-channel coupling in positron-atom scattering

Abstract: Coupled-state calculations including positronium channels are reported for positron scattering by atomic hydrogen, lithium and sodium. Integrated cross sections and total cross sections are presented for all three atoms. For lithium differential cross sections are also given. Throughout, comparison is made between results calculated with and without inclusion of the positronium channels. S-wave cross sections for positron scattering by atomic hydrogen in the Ps(1s, 2s, 2p)+H(1s, 2s, 2p) approximation show the high energy resonance first observed by Higgins and Burke in the coupled-static approximation. This resonance has now moved up to 51.05 eV and narrowed in width to 2.92 eV. Other pronounced structure is seen in the S-wave cross sections between 10 and 20 eV; it is tentatively suggested that this structure may be due to the formation of a temporary pseudo-molecular collision complex. Results calculated in the Ps(1s, 2s, $$\overline {3s} ,\overline {4s} $$ , 2p, $$\overline {3p} ,\overline {4p} ,\overline {3d} ,\overline {4d} $$ ,+H(1s, 2s, $$\overline {3s} ,\overline {4s} $$ , 2p, $$\overline {3p} ,\overline {4p} ,\overline {3d} ,\overline {4d} $$ approximation show convergence towards accurate values in the energy region below and in the Ore gap. Contrary to previous work on lithium using only an atomic basis, it is found that coupling to the 3d state of lithium is not so important when positronium channels are included; this is because a mixed basis of atom and positronium states gives a more rapidly convergent approximation than an expansion based on atom states alone. The threshold behaviour of the elastic cross section and the Ps(1s) formation cross section for lithium is investigated. Results in the Ps(1s, 2s, 2p)+Na(3s, 3p) approximation for sodium show good agreement with the total cross section measurements of Kwan et al.

Measurements of positronium formation cross sections for positron scattering by K, Na, and Ar atoms.

Abstract: The first measurements of positronium (Ps) formation cross sections (upper and lower limits) for positron scattering by K (1 to 100 eV) and Na (1 to 20 eV) are reported. They indicate significant Ps formation below 20 eV and, along with recent measurements and calculations of total cross sections, provide evidence that coupling effects between Ps formation and other scattering channels may be very important for energies below 10 eV.

Angular correlation studies of positron annihilation in the noble gases

Abstract: Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a02, where E is the Ps energy in eV.

Hyperspherical coupled-channel study of positronium formation

Abstract: Positronium formation in positron-hydrogen collisions is studied by the coupled-channel method based on the hyperspherical-coordinate representation The coupled equations are solved by the diabatic-by-sector method expanding the scattering wave function in terms of sector adiabatic states associated with H(n=1--3) and Ps(n=1,2) in the separated-atom limit In the energy range of the Ore gap, even the two-state expansion shows a good convergence, and both the formation and the elastic cross sections agree well with the existing elaborate variational calculations As the collision energy exceeds the Ore gap, the contribution from the level associated with H(n=2) becomes significant The present s-wave formation cross section does not show the resonance structures, which were predicted recently by some theoretical calculations, above the ionization threshold of the hydrogen atom

Threshold effects in positron scattering on noble gases.

Abstract: The energy dependence of the positronium (Ps) formation cross section for positrons colliding with all the stable inert atoms is analyzed near threshold. Using R-matrix and threshold theories, the corresponding variations with energy of the elastic-scattering and total cross sections are predicted and compared to experiment where possible. As the target atomic number increases from He to Xe, Wigner cusps are predicted to develop in the elastic cross section near the Ps formation threshold, reflecting the progressive increase of the interaction strength between the positron and the atoms.

Positronium lifetime in an ellipsoidal free-volume hole of polymers

Abstract: A new relationship between the ortho-positronium lifetime and the anisotropic dimension in an ellipsoidal free-volume hole is developed. The positronium wave function is solved in a model of the particle-in-an-ellipsoid and the lifetime is obtained by a numerical integration method. The obtained positronium lifetime in an ellipsoidal hole is found to be shorter than that in a sphere with the same volume. The positronium lifetime in a stretched PEEK semi-crystalline polymer is measured to demonstrate the application of this new relationship to probe the anisotropic hole properties in polymers.

Ab initio calculation of [OH - ;e + ] system with consideration of electron correlation effect

Abstract: Ab initio calculations are made to examine theoretically the possibility of stable existence of [OH−;e+] system. Diffuse functions are added to the conventional 6‐31G basis set, considering the wide spread of positron orbital. Moreover, the Mo/ller–Plesset perturbation of the second order is calculated to take the electron correlation into account. These two improvements are found to be very effective for the stable existence of the system. The positron affinity of OH− is computed to be 4.9 eV, and the binding energy of positronium to OH as 0.7 eV which is in good agreement with experimental estimate.

Mechanism for fragmentation of molecules by positron annihilation

Abstract: A mechanism is proposed for fragmentation resulting from positron-induced ionization of polyatomic molecules at energies below the threshold for producing positronium. The mechanism is supported by calculation, and by comparison with experimental data.

Positronium formation in SiO2 films grown on Si substrates studied by monoenergetic positron beams

Abstract: The annihilation characteristics of positrons in SiO2 films grown on Si substrates were studied by using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured as a function of incident positron energy for SiO2 (166 nm)/Si specimens fabricated by thermal oxidation. From the measurements, it was found that about 90% of positrons implanted into the SiO2 film annihilate from positronium (Ps) states. This fact was due to the trapping of positrons by open‐space defects and a resultant enhanced formation of Ps in such regions. For the SiO2 film grown at 650 °C, the lifetime of ortho‐Ps was found to be shorter than that in the film grown at 1000 °C. This result suggests that the volume of open‐space defects in the SiO2 film decreased with decreasing the growth rate of the SiO2 film.

Hyperspherical close-coupling calculation of positronium formation cross sections in positron-hydrogen scattering at low energies

Abstract: The hyperspherical close-coupling method is applied to calculate both the elastic and positronium formation cross sections for positron collisions with atomic hydrogen at low energies. By treating the hyperradius as a slow variable, the Schrodinger equation in the body frame at each fixed hyperradius is solved using the higher-order finite-element method and the resulting hyperradial equations are solved using the diabatic-by-sector method. The coupled hyperradial equations are integrated to a large hyperradius where the wavefunctions are matched to the known asymptotic solutions to extract the scattering matrix. Both the elastic and the positronium formation cross sections are calculated at energies below the H(n=2) excitation threshold for J=0, 1, 2 and 3. It is shown that the present hyperspherical close coupling method gives results comparable to those obtained by elaborate variational methods.

Projectile and electron spectra resulting from positron-argon collisions

Abstract: A three-body classical trajectory Monte-Carlo calculation is used to evaluate the differential cross sections for the ejected electron, as well as the projectile scattering for 50, 100 and 150 eV e++Ar0 electron removal collisions. In the ionized electron spectra both the electron capture to the continuum (ECC) and binary peak structures are observed. The theoretical ECC peak is diffuse and shifted to lower energies than expected from simple arguments. The calculated double differential cross sections for electron ejection have been compared to experiment. When the experimental apparatus function of Moxom et al. (1992) is used to convolute the theoretical spectra, good agreement with their data is found that confirms a sharp ECC structure is not present in this system. Moreover, comparisons made at non-zero angles show substantial agreement between theory and experiment. The scattered positrons agree well with measurements made at 3 degrees by Kover et al. (1993), but differ at larger angles. In addition, the differential cross section for positronium formation is presented; no evidence of an observable Thomas peak is present.

Calculation of the total and total ionization cross sections for positron scattering on atomic hydrogen.

Abstract: The total and total ionization cross sections for positron scattering on atomic hydrogen are calculated by applying the convergent-close-coupling method to the model where positronium-formation channels are omitted. This model accurately describes the physics of the scattering whenever the positronium formation cross section is negligible, in particular, above 100 eV for this system. The total ionization cross section results in this energy region are in excellent agreement with the recent measurements of Jones et al. [J. Phys. B 26, L483 (1993)], and so lie below the earlier measurements of Spicher [Phys. Rev. Lett. 64, 1019 (1990)], and the recent calculations of Acacia et al. [Phys. Rev. Lett. (to be published)]. The total cross section is in very good agreement with the recent measurements of Zhou et al. (unpublished) down to 30 eV.

The positron-hydrogen system in a six state model

Abstract: Close coupling calculations of positron-hydrogen scattering using a six state model (H(1s, 2s, 2p), Ps(1s, 2s, 2p)) are completed for incident energies from zero to 4.0 Rydberg. The calculations are very comprehensive and all cross sections and phaseshifts for the e+-H(1s) entrance channels have been computed. The cross sections for positronium formation in the Ps(1s), Ps(2s) and Ps(2p) levels underestimate the latest experimental values. Cross sections are also computed for the Ps(1s)-p entrance channel, and these to our knowledge represent the first detailed calculation of the Ps(1s)-p scattering. Particular attention is paid to the cross sections for electron transfer since these are relevant to proposed experiments to form antihydrogen in sufficient quantities to perform spectroscopic experiments. Resonances are identified at incident positron energies both below and above the ionization threshold.

Evidence for channel-coupling effects in positron scattering by sodium and potassium atoms

Abstract: In this letter we present measurements of total cross sections for 1-10 eV positrons scattered from sodium (Na) atoms. Comparisons of these measurements and other recent measurements of total and positronium (Ps) formation cross sections for positron-Na and potassium (K) scattering by our group with the close-coupling approximation calculations by Hewitt et al. (1993) provide rather clear evidence that coupling effects between Ps formation and other scattering channels are playing a very important role in positron scattering by Na and K.

Close coupling theory of positron scattering from alkali atoms

Abstract: The close coupling equatious for positron-alkali atom scattering are written as a set of coupled momentum-space Lippmann-Schwinger equations. The alkali atom is represented by a frozen-core model based upon the Hartree-Fock approximation. The interaction between the positronium and the residual ion is modified by the inclusion of a core potential. Similarly, a core term is present in the interaction describing the rearrangement process. Close coupling calculations of positron scattering from sodium are performed in a model containing multiple sodium (3s, 3p, 4s, 3d, 4p) and positronium (Is, 2s, 2p) states. Cross sections are reported for an energy range from threshold to 50·eV; the total cross sections are in agreement with experimental data.

On the one-photon annihilation of the Ps− ion

Abstract: With the consideration of all the Feynman diagrams of the lowest order and electron spin correlation in the initial state, the one-photon decay rate of the Ps- ion in its ground state is found to be 3.9*10-2 s-1. Comparison is made with the previous calculations of other authors.

The effect of virtual positronium formation on positron-atom scattering

Abstract: A new method is proposed to calculate the contribution of virtual Ps formation to elastic positron-atom scattering. The calculations were performed using the many-body perturbation theory approach. It is shown that for the scattering of positrons by hydrogen and helium atoms, the virtual Ps formation contribution is about 30% and 20% of the total correlation potential, respectively. Good agreement was obtained with the results of precise variational calculations and with experimental data.

Internal energy deposition into molecules upon positron-electron annihilation

Abstract: The dissociative ionization of organic molecules, induced by positrons having energies below the positronium threshold (0.5--3 eV), has been studied. Cross sections for the ionization-fragmentation processes increase as positron energies decrease. The data reveal that the energy required for dissociative ionization is supplied by the annihilation process, and that fragmentation occurs by breakage of C-C single bonds in preference to double and triple bonds.

Differential cross sections for elastic scattering and positronium formation for positron collisions with Ne, Ar, Kr and Xe

Abstract: Differential cross sections for elastic scattering and Ps(1s) formation are calculated in the truncated coupled-static (TCS) approximation. Comparison is made with the elastic scattering measurements of Dou et al. on Ar and Kr. There is no support from the TCS approximation for the view that structure seen in the experimental results for Ar in the energy range 55–60 eV may be due to a resonance associated with coupling to the positronium formation channel. Rather, we believe that Dou et al. are right in correlating this feature with the maximum in the ionization cross section. In the experiment on Kr structures are observed at 25 and 200 eV. It is tentatively suggested that the structure at 200 eV may be the resonance seen in the TCS calculation at 100 eV, but modified by polarization and absorption effects. It is also suggested that the feature at 25 eV could be associated with coupling to excited states of positronium. The TCS results for the Xe target predict some very pronounced behaviour which would be worth experimental investigation. TCS differential cross sections for Ps(1s) formation by capture of an electron from the outer p-shell of the atom are presented for impact energies up to 75 eV. A noticeable property of these cross sections is that they do not usually peak at the forward direction; this is consistent with an experimental observation by Laricchia et al. on He and Ar targets. The importance of also looking at electron capture from inner shells is emphasized and illustrated by the cross section for electron capture from the 3s shell of Ar.

Hyperspherical coupled-channel calculation for antihydrogen formation in antiproton-positronium collisions

Abstract: Antihydrogen formation cross sections are calculated for low-energy collisions of antiproton with positronium by the hyperspherical coupled-channel method. The scattering wavefunctions are expanded in terms of a large number of sector-adiabatic states associated with the n=1-4 states of antihydrogen and n=1-3 states of positronium in the asymptotic separated-atom region. The present formation cross sections differ considerably from those of the unitarized Born approximation reported by Mitroy and Stelbovics (1994). The disagreement becomes larger with decreasing collision energy owing to the different energy dependence of the cross sections. The authors have confirmed that the formation cross sections from the 2s and 2p states are larger than that from the ground state by one order of magnitude.

Positron-hydrogen scattering at low energies.

Abstract: Calculations of positron-hydrogen scattering using the close-coupling approach are reported for incident positron energies below the ionization threshold. The channel space includes nine physical hydrogen and positronium states and, in addition, eleven hydrogen and positronium pseudostates. The accuracy of our model has been validated by computations of elastic phase shifts below the positronium threshold, and elastic and positronium-formation cross sections in the Ore gap. Calculations are performed for sufficient partial waves to obtain converged elastic, positronium, and total cross sections.

Formation of antihydrogen from antiproton collisions with positronium

Abstract: The cross section for antihydrogen formation in the n=1, 2 and 3 levels from antiproton-positronium collisions is computed in the unitarized Born approximation (UBA). Six physical states of antihydrogen (1s, 2s, 2p, 3s, 3p, 3d) and three physical states (1s, 2s, 2p) of positronium are included in the UBA basis. The cross section for antihydrogen formation from excited positronium targets is found to be much larger than that from a ground state positronium target at sufficiently low incident energies. This has implications for increasing the production of antihydrogen for planned high resolution spectroscopic experiments.

Spectroscopy of excited state positronium

Abstract: Electric dipole transitions in the microwave range have been induced between the fine-structure levels of positronium in the excited staten=2. As an indication of the transitions, we used the increase in Lyman-α radiation when the metastable 23S1-level is depopulated. The results for the transitions 23S1→23P0,1,2 areΝ 0=18499.65±1.20±4.00 MHz,Ν 1=13012.42 ±0.65±1.54 MHz andΝ 2=8624.38±0.54±1.40 MHz. The first error is statistical and the second systematic. The precision of the present measurement has improved by a factor of 3, compared to previous data. Recent bound state QED-calculations have been extended to the orderR t8 α 4lnα −1. The not yet completely calculated orderR t8 α 4 is estimated to contribute less than 1 MHz. Our experimental results are in good agreement with theory. By applying a weak magnetic field, we were able to observe the transition 23S1→21P1 which is strictly forbidden byC-invariance in zero field. Our result, corrected for Zeeman- and motional Starkeffect, isΝ 3=11180.0±5.0±4.0 MHz. An upper limit for theC-violating matrix element of $$|\langle 2^1 P_1 |H_ ot\subset |2^3 P_1 \rangle |< 76$$ MHz could be deduced. Our experiment used moderated slow positrons from the bremsstrahlung and pair production of a pulsed electron linear accelerator (TEPOS facility at the university of Giessen).

Positronium Formation by Positron Impact on Atoms at Intermediate Energies

Abstract: Publisher Summary This chapter discusses positronium (Ps) formation at intermediate energies from the threshold for ionization up to energies of a few hundred electron volts, where the effective coupling between the rearrangement and the direct channels becomes very small. In the interval above the ionization threshold, and extending to at least 100 eV, the Ps formation cross section is comparable to, or in some cases larger than, that for excitation; however, at high energies, the ratio of the formation cross section to the total inelastic cross section decreases rapidly. In the intermediate energy range up to energies of several hundred electron volts, the use of coupled channel models is feasible in principle and can be expected to provide accurate cross sections, including the formation cross section. At higher energies, although most approximations lead to similar values for the ground-state capture cross section the differential cross section is very sensitive to the details of the approximation employed and in contrast to the comparatively well-understood situation in charge exchange, there are still large gaps in the understanding of high-energy Ps formation.

Resonance structure in the positron-hydrogen system above the ionization threshold

Abstract: Positron scattering from atomic hydrogen is analysed above the ionization threshold and resonances for coupled-channels calculations comprising bases of up to three physical states of hydrogen H(1s,2s,2p) and three physical states of positronium Ps(1s,2s,2p) are determined. The calculations have been performed over the entire energy range from the ionization threshold up to a maximum incident positron energy of 4 Ryd. Additional resonances appear as the models increase in complexity. For the six-state model the authors report the identification of one new resonance for the J=0 partial wave, four for J=1, one for J=2 and one for J=3. No resonances are found in the higher partial waves. The positions and widths of the resonances are given.