Two-Photon Detachment Cross Section of the Positronium Negative Ion

The two-photon detachment cross section of positronium negative ion is calculated within the lowest-order perturbation theory for the final-state energies below the Ps(n = 2) threshold. A coupled-channel method with positronium orbital expansion is used to prepare the wavefunctions. The resonance structure below the threshold is clearly seen.     

Unified treatment of positron annihilation and positronium formation

The effects of the positron (e(+)) annihilation in e(+) scattering by the H atom are included directly in the Hamiltonian as an absorption potential, and hence the finite lifetimes Gamma(ns) of positronium (Ps) in states ns are automatically taken account of. The Schr?dinger equation is solved using the hyperspherical close-coupling method. The annihilation and Ps formation are shown to be inseparable near the Ps(1s) threshold E(th); Ps formation constitutes just an indirect pathway to annihilation. The annihilation cross section, which would diverge near E(th) if Gamma(1s) were infinite, connects smoothly across E(th) to the cross section for Ps formation, which is meaningful only at energies well above E(th).     

Dipole excitation of the positronium molecule ps2

The energy interval between the ground and the P-wave excited states of the recently discovered positronium molecule Ps2 is evaluated, including the relativistic and the leading logarithmic radiative corrections, E_{P}-E_{S}=0.181 586 7(8) a.u.. The P state, decaying usually via annihilation, is found to decay into the ground state by an electric dipole transition 19% of the time. Anticipated observation of this transition will provide insight into this exotic system.     

Decay of the positronium molecule into a positronium atom and photons

Decays of the positronium molecule Ps₂ into para- or orthopositronium Ps in the ground state and photons are investigated. The differential probabilities of the decays are determined. The total probabilitiesw _2γ ^(Ps2) andw _3γ ^(Ps2) of Ps₂ annihilation with the production of two and three photons and positronium are calculated to be $$w_{2\gamma }^{(P_{S_2 } )} = 1.6 \cdot 10^{10} \sec ^{ - 1} ,w_{3\gamma }^{(P_{S_2 } )} = 0.43 \cdot 10^8 \sec ^{ - 1} $$ . The curve of the angular correlation of the γ rays on the decay of Ps₂ into two photons and parapositronium is studied. The width of this curve is Δ¦P¦=0.128 a.u. (¦P¦ is the total photon momentum), which corresponds to a deviation of the emission angle of the γ rays from π: θ ? 0.934 mrad. The maximum in the distribution of the photons with respect to the momenta ¦P¦ in the center of mass of the annihilating pair is attained at ¦P¦=0.175 a.u. The calculations were made on an M-222 computer, and their accuracy is determined by the choice of the wave function of the positronium molecule and the accuracy in the computer calculation of the integrals.     

The role of positronium decoherence in positron annihilation in matter

A small difference between the energies of the para-positronium (p-Ps) and ortho-positronium (o-Ps) states suggests the possibility of the superposition of p-Ps and o-Ps during the formation of positronium (Ps) from pre-Ps, terminating its migration in the matter in a void. It is shown that such a superposition decoheres in the basis of p-Ps and o-Ps. The decoherence time scale estimated here motivates a correction in the precise analysis of the positron annihilation lifetime spectra. More generally, the superposited Ps state should contribute to the theory of the evolution of positronium in matter.     

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.     

A possible spectrum of positron states in porous silicon

A kinetic scheme of transformations and annihilation decays of positron and positronium states is considered in the bulk of a crystal in the crystal skeleton, and on the surface and in the bulk of pores of porous silicon. Formulas relating the intensities of temporal components of annihilation decay to the rates of decays and transformations of positron and positronium states in the pore volume are derived. The rate of interaction with the pore surface, accompanied by the formation of the surface state of a positronium atom, is estimated at 10⁷–10⁸ s^?1, while the estimated value for the pore radius is ≈2 nm.     

One and two-photon detachment cross-sections of Ps-

We present detailed calculations for one- and two-photon above-threshold detachment (ATD) cross-sections of the negative positronium ion Ps ^- ( e ⁺ e ^- e ^- ), below the threshold of Ps(n = 2), using a configuration interaction (CI) method on a B splines basis. Both the one- and two-photon detachment cross-sections have a form similar to the corresponding spectra of the H^- ion, scaled accordingly. The peak value of the one-photon cross-section agrees very well with the calculations by Bathia and Drachman [1], while it differs from those by Igarashi et al. [2], which give a value of 15% lower. Two-photon detachment cross-sections are also reported. Received 24 January 2002 / Received in final form 9 April 2002 Published online 19 July 2002     

Defects in electron irradiated amorphous SiO2 probed by positron annihilation

Defects in 3-MeV electron irradiated amorphous (a-) SiO₂ were studied by the positron annihilation technique. A high formation probability of positronium (Ps) was found for un-irradiated a-SiO₂ specimens. These Ps atoms were considered to be trapped by vacancy-type defects. The formation probability of Ps was drastically decreased by the electron irradiation. This fact can be attributed to an introduction of vacancy-type defects such as oxygen vacancies by the electron irradiation and a resultant trapping of positrons by such defects.     

Positronium interactions with gases in pores of silicagels

Positronium interaction with O₂, NO, Cl₂ molecules in pores of silicagels has been investigated by the methods of lifetime and angular correlation. For silicagel with diameter of pores ≈100 Å it was obtained that the rate constant of positronium conversion on oxygen was (2.4±0.2)10⁷ sec^?1 atm^?1 and on nitrogen oxide (2.6±0.2)10⁷ sec^?1 atm^?1. The rate constant of the positronium chemical reaction with Cl₂ was found to be ≈10^?11 sec^?1 atm^?1. The results of investigation of positronium interactions with oxygen in the gas phase, organic liquids, zeolites, liquid argon, and nitrogen are explained in terms of the formation of an excited complex PsO ₂ ^* . The decay of this complex to Ps and O₂ makes possible positronium conversion, and stabilization of the complex leads to annihilation in the bound state (chemical reaction). Study of positronium interaction with O₂ in silicagels (diameters of pores 100, 30 and 16 Å) confirms the assumption of the complex formation. The rates of chemical reaction and of conversion become equal for pores of diameter of 20 Å. The estimated lifetime of the PsO ₂ ^* complex is τ _k ? 10^?12 sec.     

Doubly Excited <Superscript>1,3</Superscript><Emphasis Type="Italic">P</Emphasis><Superscript>e</Superscript> Resonance States of the Positronium Negative Ion with Coulomb and Screened Coulomb Potentials

We have investigated the doubly excited ^1,3 P ^e resonance states of positronium negative ion with Coulomb and screened Coulomb potentials using highly accurate correlated exponential wavefunctions. For Coulomb interaction, the stabilization and the complex-rotation methods are employed to extract resonance parameters (resonance positions and widths). We have obtained two ¹ P ^e resonances and three ³ P ^e resonances below the n = 3 Ps threshold. In addition to Feshbach resonances lying below n = 3 Ps threshold, we have calculated one ³ P ^e shape resonances lying above the Ps (n = 2) threshold. For screened Coulomb (Yukawa) interaction, we employ the stabilization method to extract resonance parameters as functions screening parameter. The resonance energies and widths for ^1,3 P ^e resonance states of Ps⁻ below the n = 3 Ps threshold for different screening parameters ranging from infinity (Coulomb case) to small values are reported, along with the Ps(3S) and Ps(3P) threshold energies. The screened Coulomb results for the ^1,3 P ^e resonance states are reported for the first time in the literature.     

To the application of the emission Mössbauer and positron annihilation spectroscopies for detection of carcinogens

Being the main cause of cancer, almost all chemical carcinogens are strong electrophiles, that is, they have a high affinity for the electron. We have shown that positron annihilation lifetime spectroscopy (PALS) is able to detect chemical carcinogens by their inhibition of positronium (Ps) formation in liquid media. Electrophilic carcinogens intercept thermalized track electrons, which are precursors of Ps, and as a result, when they are present Ps atom does not practically form. Available biophysical data seemingly indicate that frozen solutions model better an intracellular medium than the liquid ones. So it is reasonable to use emission Mössbauer spectroscopy (EMS) to detect chemical carcinogens, measuring the yield of ⁵⁷Fe²⁺ions formed in reactions of Auger electrons and other secondary electrons they produced with ⁵⁷Fe³⁺. These reactions are similar to the Ps formation process in the terminal part the positron track: e⁺+ e^? =>Ps. So EMS and PALS are complementary methods for detection of carcinogenic compounds.     

Calculations of Coulomb systems with optimization of nonlinear parameters

Precise variational calculations of the energy and diverse physical characteristics are performed for the ground state of a fully nonadiabatic four-particle system—the positronium molecule e⁺e^?e⁺e^?. The nonlinear exponential parameters of correlated Gaussian basis functions dependent on all interparticle separations are systematically optimized. The dependence of the calculated energy values and the expectation values of interparticle separations (as well as their degrees and Dirac delta functions of interparticle separations) on the basis size N is examined. For N=200, the ground-state energy of the positronium molecule is found to be E=?0.5160028 au. For an extended basis set with N=4700 containing 200 basis functions with the optimized parameters and 4500 additional functions with random values of nonlinear parameters, E=?.5160036 au. At present, this result is the most accurate variational energy value of the positronium molecule, and it demonstrates the high efficiency of optimization of the nonlinear parameters in calculations of atomic-molecular systems. The lifetime of the positronium molecule e⁺e^?e⁺e^? with respect to electron-positron annihilation is calculated to be τ=2.26×10^?10 s.     

Complex-Scaling Calculations for Doubly Excited Resonances in Ps? Interacting with Screened Coulomb (Yukawa) Potentials

We have investigated the effect of screened Coulomb potentials on the high-lying doubly excited resonance states of the positronium negative ion in the framework of complex-scaling method. Highly correlated wave functions in Hylleraas coordinates are used. The resonance parameters below the Ps (2s²S) and Ps (3s²S) thresholds, for various screening parameters, are reported.     

Properties of the Positronium Negative Ion Interacting with Exponential Cosine-Screened Coulomb Potentials

The positronium negative ion (Ps ⁻) in exponential cosine-screened Coulomb potentials (ECSCP) has been theoretically investigated. Using highly correlated wave functions with 525 terms we report binding energies, annihilation rates and several other parameters for various values of the screening parameter. Our reported results for the unscreened case are in nice agreement with some of the most accurate results in the literature. Further we make a comparative study of the properties of Ps ⁻ in ECSCP with the corresponding properties in screened Coulomb potential (SCP). To the best of our knowledge, such a study on Ps ⁻ in ECSCP is carried out for the first time in the literature.     

A new view on positronium in polymers

A new model is presented describing the annihilation of positronium in polymers. Ps is formed in the bulk and is trapped into the free volume holes with epithermal initial energy where it thermalizes towards its ground state. This model was verified by interpreting lifetime spectra in an amorphous cross-linked polyurethane.     

Hydrogen-Antihydrogen Molecule and Its Properties

The metastability of the four-body system is discussed from the point of view of radiative rearrangement collisions of the hydrogen and antihydrogen. Such collisions lead to the formation of an intermediate molecular state which further decays into positronium and protonium or is destroyed via annihilation. We present calculations of the lifetime of the system and the branching ratios for Coulombic decay and annihilation.     

New approach for numerical solution of configuration-space Faddeev equations

A new computational scheme for solving the bound state configuration-space Faddeev equations is applied. The scheme is based on the spline-approximation and the adiabatic limit of Faddeev equations. An ordering of variables being in agreement with the limit was chosen. As a result the matrix of the eigenvalue problem has a sparse block structure. Calculations of the bound states of µpp, µdd, µtt mesic molecules and ¯pdd, ¯ptt antiprotonic ones, were performed. To check the method, calculations of the binding energies for such systems as the positronium ion Ps_–,³H and³He were carried out. The results are compared with the best results of other authors.     

Magnetic quenching of positronium in semiconductors

The positronium-determined characteristics of electron-positron annihilation in semiconductors in the presence of an external magnetic field are calculated. It is assumed that the positronium atoms can be in one of two states, delocalized or localized (at defects), and undergo not only pick-off annihilation but also orthoparaconversion (³S₁ ¹S₀) on free carriers. Numerical calculations were made for a field H=15 kG. The field has very different effects on localized and delocalized positronium, and in the first case the effect depends strongly on the nature of the defect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.11, pp. 75–78, November, 1980.     

Depth-dependent positronium formation in γ-irradiated polymers after 30-month aging

Depth-dependent positronium (Ps) formation and annihilation study of Co-60 γ-irradiated polyethylene and polytetrafluorethylene were performed. After 30-month aging, no free radicals effect on Ps formation was detected for the studied polymers. Efficient trapping of back-diffusion positrons was observed in heavily irradiated samples, where Ps formation probability near polymer surface and in the bulk turns to be equivalent to each other. Results shown that positron annihilation Doppler broadening in polytetrafluorethylene is prevailingly influenced by high momentum electrons of fluorine atoms instead of Ps intensity.