Spin-orbit quenching of positronium during atomic collisions

When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is ortho-Ps-->para-Ps conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.     

Investigations of positronium formation and destruction using 3gamma/2gammaannihilation-ratio measurements

Positronium (Ps) produced by 4 to 40 eV positrons colliding with Ne, Ar, Kr, CO2, and O2 is investigated by measuring the ratio of signals of two gamma rays in coincidence resulting from (a) three gamma annihilation of ortho-Ps and (b) two annihilation gamma rays due to para-Ps decay and destruction of ortho-Ps at an aluminum scattering cell surface. These ratios provide evidence that relates to the kinetic energy dependence of ortho-Ps interactions with an aluminum surface, the Ps formation potential at this surface, and the fact that Ps is being formed with inner orbital electrons for CO2 and O2.     

Measurements of positronium formation cross sections for positron-ar and-K scattering

We have made preliminary measurements of positronium (Ps) formation cross sections for 9 to 452 eV positrons scattered from Ar atoms and for 1 to 20 eV positrons scattered from K atoms. Our experimental approach involves setting lower and upper limits on Ps formation cross sections using a combination of (1) the detection of the coincidences of 511 keV annihilation gamma rays produced by the decay of para-Ps and by the interaction of ortho-Ps with the wall of the scattering cell in which the Ps is formed, and (2) the determination of scattering cross sections associated with the measured transmission of the positron beam through the gas in our scattering cell with the angular discrimination of our apparatus deliberately made as poor as possible. The constraints placed by these lower and upper limits are used to check for consistency with prior experimental and theoretical results for Ar and to provide the first measurements of Ps formation cross sections for K, which are compared with available theoretical results.     

Exchange collisions between positronium and Cu2+ ions

Summary The fate and properties of the ortho-Ps in aqueous solutions of CuSO₄ and CuCl₂ have been investigated through lifetime measurements and through a magnetic-quenching experiment. The obtained results are interpreted in terms of exchange collisions between Ps and unpaired electron. An upper limit is given for the chemical-quenching rate. Our measurements have shown that the presence of paramagnetic centres determines a strong shift of the levels of the Ps hyperfine structure. This work was entirely supported by CISM (Centro Interuniversitario di Struttura della Materia) of the Ministero della Publica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Mössbauer investigations on intramolecularly phosphoryl coordinated organotin compounds

Recently, high-quality positron annihilation lifetime measurements showed that four components of lifetime spectra for many liquids could be resolved. The well-known three components (para-Ps (positronium), free positron, and ortho-Ps) and a short-lived ortho-Ps component were obtained in many liquids. This short-lived ortho-Ps is probably caused by secondary reactions of ortho-Ps and other reactants, such as a positive ion, in the positron spur. We report the first results of measurements of the yield of this short-lived ortho-Ps component in the system of hexane and the electron scavengers CCl₄, CS₂, C₆F₆, and SF₆.Deceased.     

Interactions between positronium atoms in porous silica

Interactions between pairs of positronium (Ps) atoms confined in porous silica films have been directly observed for the first time. Because of selection rules, the nature of such interactions should depend on the structure of the porous medium: if a Ps surface state exists, dipositronium (Ps2) molecules may be created, and if there is a continuum of cavity energy levels, spin exchanging collisions may occur. Using two structurally different silica films, we have been able to isolate and study these two processes. Our data indicate that Ps2 formation occurs primarily via a Langmuir-Hinshelwood-type mechanism on the internal pore surfaces, with an interaction length of the order of 7 x 10(-8) cm, and that the effective cross section for nonthermalized Ps-Ps spin exchange quenching in porous silica is around 9 x 10(-15) cm2.     

Effect of interstitial impurities on positronium formed in voids of vanadium

Angular correlation of two-photon annihilation radiation (ACAR) measurements have been performed to study the effect of interstitial impurities (O, C and D) on positronium (Ps) formation in irradiation-induced voids of vanadium. It has been observed that Ps formation is sensitively affected by doping with the interstitial impurities, irradiation dose, irradiation temperature, and also by post-irradiation annealing. The Ps component intensity is found to be related to segregation of the interstitial impurities and provides a new experimental method to study void surfaces.     

Positronium formation for positron scattering from metastable hydrogen

Positronium(Ps) formation for positron impact on metastable hydrogen in 2s state has been studied by using the twochannel, two-center eikonal final state-continuum initial distorted wave(EFS-CDW) method. The differential, integrated,and total cross sections for Ps formation in different states have been calculated from each channel opening thresholds to high energy region. The results are compared with other theoretical calculations available in the literature. For Ps formation in s-state at intermediate and high energies, our results are in good agreement with the prediction of distorted wave theory.Those formed in p-states and the total Ps formation cross sections are reported for the first time. It is shown that the total Ps formation cross sections for positron scattering from H(2s) state are significantly larger at relatively low energies, while smaller at high energies, compared with those obtained from hydrogen in ground state.     

Cavity induced shift and narrowing of the positronium Lyman-α transition

We report experiments in which the line shape of the Lyman-alpha (1S-2P) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target. The energy interval ΔE for confined atoms was observed to be larger than that of free Ps by 1.26±0.06 meV. A configuration interaction calculation yields results that are consistent with our ～5 nm sample, and suggests that ΔE decreases dramatically for larger cavity diameters. The linewidth of the transition, (0.066±0.004) nm (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement. Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.     

Total Born cross sections for positronium and helium atom scattering

A thorough and systematic study has been performed for positronium (Ps) and helium (He) atom scattering using the first Born approximation. As a first step, the direct scattering has been studied. Here excitations of both the Ps atom and He atom, the ionization of the Ps atom with all the excitations of the He atom, the ionization of the He atom with all the excitations of the Ps atom and the ionizations of both the Ps and He atoms are taken into consideration. For the different states of the He atom, the wavefunctions suggested by Winter and Lin [Phys. Rev. A 12 (1975) 434] are used. For the ionization channels the continuum Coulomb wavefunctions have been taken for the ionized electrons.     

NOVIKOV SUPER-ALGEBRAS WITH ASSOCIATIVE NON-DEGENERATE SUPER-SYMMETRIC BILINEAR FORMS

Novikov super-algebras are related to quadratic conformal super-algebras which correspond to Hamiltonian pairs and play fundamental role in completely integrable systems. In this paper, we focus on quadratic Novikov super-algebras, which are Novikov super-algebras with associative non-degenerate super-symmetric bilinear forms. We show that quadratic Novikov super-algebras are associative and the associated Lie-super algebras of quadratic Novikov super-algebras are 2-step nilpotent. Moreover, we give some properties on quadratic Novikov super-algebras and classify the associated Lie-super algebras of quadratic Novikov super-algebras up to dimension 7.     

Positron states between positronium and “free positron” states in condensed matter

Positronium (Ps), the bound state of an electron and a positron, is formed in many insulating solids and liquids. Recently, several cases of a transition from a Ps state to a non-Ps state have been observed. This has been found for liquid CS₂ if the temperature is decreased from about –40 °C to the melting point, –110.8 °C. In some solid aromatic hydrocarbons, such as naphthalene, biphenyl and p-terphenyl, similar transitions occur upon decreasing the temperature. These transitions are unique in physics since Ps is the only bound state of two light particles in normal low-energy physics. Measurements of the positron lifetime, angular correlation of the annihilation radiation, Ps chemical reactions, and Zeeman effect of Ps give information on the transitions.Deceased.     

Temperature dependence of the narrow component for positronium in Ice

The detailed shape of the Ps momentum distribution in ice, as given by the 2-photon angular correlation, has been obtained for several temperatures between −40 and 0°C. The narrow component was analyzed in terms of 2 parts, one caused by delocalized and the other by localized Ps. The latter one is interpreted in terms of Ps trapped by thermally excited defects with a formation energy of about 0.4 eV. Paper D4 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Physisorption of positronium on quartz surfaces

Whether positronium (Ps) can be physisorbed on a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a Ps2 molecule on a material host. Experiments in the past have produced evidence for physisorbed Ps on a quartz surface, but firm theoretical support for such a conclusion was lacking. We present a first-principles density-functional calculation of the key parameters determining the interaction potential between Ps and an alpha-quartz surface. We show that there is indeed a bound state with an energy of 0.14 eV, a value which agrees very well with the experimental estimate of approximately 0.15 eV. Further, a brief energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of physisorbed atoms shows that the formation and desorption of a Ps2 molecule in that picture is consistent with the above results.     

Coupled-channel calculations of positron-atom scattering at intermediate energies

We present the cross sections for positron collisions with the alkali atoms Li, Na and K in the close-coupling approximation within the framework of a single-active-electron model. Our target basis sets are represented by Li(2s, 3s, 2p, 3p), Na(3s, 4s, 3p, 4p), and K(4s, 5s, 4p, 5p) respectively. The effects of coupling to the lowest lying Ps channels have been investigated by augmenting the target basis sets with the Ps(1s, 2s, 2p) eigenstates. The calculations have been made for impact energies below 50 eV. The interaction between the positron and the alkali target atom is represented by a model potential obtained using a single-active-electron approximation. The effect on elastic scattering and excitation due to the coupling between the direct and the Ps formation channels is found to be of great significance at low impact energies. The case of K is of particular interest. The inclusion of the Ps channels quantitatively reproduces the maximum in the total cross section at about 6 eV as reported in a recent experiment by Parikh et al. We find that the calculated total cross sections for Na and K are in good agreement with the experimental data of Parikh et al. and Kwan et al. respectively in the energy range of 2–50 eV provided allowance is made for the incomplete elastic discrimination in the experiment. The Ps formation reduces the K(4s4p) cross section at all energies below 30 eV and a reduction is also seen in the elastic cross section for energies below about 7 eV. Similar effects are observed in the case of Li. In the case of Na for impact energies between 2 and 20 eV Ps formation significantly reduces the Na(3s3p) excitation cross section, while the elastic scattering cross section increases. The calculated total Ps formation cross sections for Na and K are in good agreement with the preliminary experimental data of Kwan, Stein and co-workers.     

Positronium diffusion in porous methylsilsesquioxanethin films

Depth profiled positronium (Ps) annihilation lifetime spectroscopy (PALS) is an extremely useful probe of the pore characteristics in nanoporous low-dielectric (low-k) constant thin films. PALS has also been considered as a potential probe to investigate diffusion barrier integrity and the structural changes of porous low-k films during their integration with Cu. Hence, it is essential to understand the diffusion behaviour of positronium/Cu atoms in the films. In this work, based on the fact that porous materials possess characteristics of statistical self-similarity, a fractal model, the Menger sponge model, has been applied to simulate the structure of a promising dielectric, porous methylsilsesquioxane (MSQ) films. The diffusion behaviour of Ps out of the fractal model and into the surrounding vacuum is studied by means of the diffusion equation and traditional advective-diffusive theory. Predictive results from our model show good agreement with measurement data.     

Positronium formation in Debye plasma

Positronium (Ps) formation during positron-hydrogen collisions in Debye plasma has been studied in detail by using the second-order distorted-wave approximation with the inclusion of the adiabatic dipole polarization potential. Reliable results have been reported at several incident positron energies in the range from the Ps formation threshold to 500 eV for different values of the Debye screening parameter μ. Resonances for S-, P- and D-wave partial Ps formation cross sections appear to exist for 0.03 ≤ μ ≤ 0.3. Interesting structures in the differential cross sections are displayed in the surface plots. It is reported here for the first time that there is a huge probability of Ps formation in dense plasma corresponding to Debye screening parameter μ ≥ 0.3.     

Excitation and ionization of positronium by 50–100 fs laser pulses

The interaction between Ps and a strong laser pulse of short duration is studied. Substantial population is deposited in excited states under few photon excitation. The possible usefulness as a source of excited Ps to spectroscopy and the formation of antihydrogen is discussed. Finally, photoelectron energy spectra are calculated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Transitive Novikov Algebras on Four-Dimensional Nilpotent Lie Algebras

Novikov algebras were introduced in connection with the Poisson brackets (of hydrodynamic type) and Hamiltonian operators in the formal variational calculus. The commutator of a Novikov algebra is a Lie algebra, and the radical of a finite-dimensional Novikov algebra is transitive. In this paper, we give a classification of transitive Novikov algebras on four-dimensional nilpotent Lie algebras based on Kim (1986, Journal of Differential Geometry 24, 373–394).     

Enhanced Ps-Ps interactions due to quantum confinement

Slow positrons implanted into a porous silica film may efficiently form positronium (Ps) atoms that diffuse through a network of interconnected pores. At high Ps densities, the long lifetime of ortho-positronium atoms is reduced due to Ps-Ps spin dependent interactions at a rate that implies an effective free-space scattering cross section, σ(e) = (3.4 ± 0.5) × 10(-14) cm(-2), at least 25 times larger than the theoretical value. This enhanced interaction rate may be explained if the quantum confinement of Ps results in interpore tunneling rates that depend critically on the distribution of pore sizes, so that rather than uniformly sampling the porous matrix Ps diffusion is limited to a small subset of the pores.