Search

Decay rates of the positronium molecule ${\text{Ps}}_{2}$ into two photons and into an electron-positron pair are determined. Previous studies find that these rates are very different, $\mathrm{\ensuremath{\Gamma}}({\text{Ps}}_{2}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}})/\mathrm{\ensuremath{\Gamma}}({\text{Ps}}_{2}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})\ensuremath{\simeq}250$. This is puzzling since both processes have two-body final states and are of the same order in the fine-structure constant. We propose a simple calculational method and test it with the well-established decay of the positronium ion into an electron and a photon. We then employ it to correct predictions for both these ${\text{Ps}}_{2}$ decays. We find that previous studies overestimated the ${e}^{+}{e}^{\ensuremath{-}}$ channel and underestimated the $\ensuremath{\gamma}\ensuremath{\gamma}$ channel by factors of about 5.44 and 3.93, respectively. Our results give $\mathrm{\ensuremath{\Gamma}}({\text{Ps}}_{2}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}})/\mathrm{\ensuremath{\Gamma}}({\text{Ps}}_{2}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})\ensuremath{\simeq}11.7$.

An effective theory of the excited states of positronium is derived and some of its consequences are explored. At large physical separation, the binding of the electron and positron is assumed to be described completely by QED, whereas all short-ranged phenomena, including those within and beyond QED, can be accounted for with energy-dependent quantum defects. This theory has at least two practical applications. First, it provides an accurate and economical, yet largely QED-independent, means to fit the positronium spectrum in order to predict and compare the outcome of experiments. Second, matching the spectrum in this effective theory to that predicted by QED reveals nontrivial relationships that exist \emph{within} bound-state QED; some higher order contributions to the spectrum may be obtained from lower order contributions. These relations are verified up to order $m\alpha^{6}$, and predictions are made for the order $m\alpha^{7}$ and $m\alpha^{8}$ corrections. This theory and its extensions to other hydrogenic systems may provide a useful complement to bound-state QED., Comment: 7 pages, 3 figures. Phys Rev A accepted

Positron-impact cross sections of 1-1-${C}_{2}{H}_{2}{F}_{2}$ are reported in the energy range from 0.1 to 4000 eV. The multicenter Gaussian-type orbitals were expanded within a single determinant Hartree-Fock self-consistent field scheme to obtain molecular wave function. The elastic cross sections were computed within fixed nuclei approximation by invoking the single center expansion approach. The target interactions were described by their local nature. The direct ionization and electron excitation cross sections were estimated using the binary-encounter-Bethe model for positron and scaled-Born approximation, respectively. The experimental scattering data were corrected to account for the angular discrimination effects. The corrections enhanced experimental measurements substantially in the low-energy range. The elastic cross sections are in an excellent agreement with the forward angle corrected total cross sections below positronium formation. The cross sections obtained by summing the elastic and inelastic cross sections incoherently were in good agreement with the experimental results. An in-depth analysis of the angular limitation of spectrometers employed in measuring the positron impact scattering cross sections and corrections arising out of the polar molecule are an integral part of the study. Some key aspects of electron and positron scattering are also discussed.

We study realistic 3D laser cooling of positronium (Ps) in the presence of a magnetic field. Triplet and singlet states mixing due to the magnetic field, and dynamical Stark effect, generally produce higher annihilation rates than in the zero-field case. 3D cooling is efficient only at very low field $B\ensuremath{\lesssim}50\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ and at high field values $B⪆0.7\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Near $100\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$ long laser pulses, spectrally broad enough to cover most of the Ps Doppler profile and with energy in the $\mathrm{mJ}$ range, are required to cool Ps. Simulations based on full diagonalization of the Stark and Zeeman Hamiltonian and a kinetic Monte Carlo algorithm exactly solving the rate equations indicate that an efficient cooling (typically from $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ down to below $50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is possible even in a magnetic field. We also propose 3D moving molasses cooling that can produce a well-defined monochromatic Ps beam useful for applications.

The authors use a previously developed free-electron-gas model to calculate cross sections for collisions of positronium with O_2 and CO_2 molecules. The results agree with previous experimental observations showing a similarity between electron and Ps scattering when the cross section is plotted as a function of projective velocity.

Low-energy properties of spin-polarized electron-helium $(2\phantom{\rule{0.16em}{0ex}}^{3}S)$ and spin-polarized positronium-helium $(2\phantom{\rule{0.16em}{0ex}}^{3}S)$ scattering are calculated using the modified confined variational method. To take the van der Waals interaction into consideration, the van der Waals coefficient between positronium and helium $(2\phantom{\rule{0.16em}{0ex}}^{3}S)$ is calculated using the explicitly correlated Gaussian basis. Compared with the ground state noble gases, the larger scattering length and zero pickoff annihilation rate indicates that $\mathrm{He}(2\phantom{\rule{0.16em}{0ex}}^{3}S)$ may be a more suitable cooling gas. In the energy range we considered, no similarity is found for the $S$-wave cross sections between these two scatterings. A Ramsauer-Townsend minimum is observed in spin-polarized electron-helium$(2\phantom{\rule{0.16em}{0ex}}^{3}S)$ scattering due to the large polarization of metastable helium. This may open a new way to find and study the Ramsauer-Townsend effect.

We report new measurements of the positronium (Ps) $2^{3}S_{1}\ensuremath{\rightarrow}2^{3}P_{J}$ fine-structure intervals, ${\ensuremath{\nu}}_{J}$ ($J=0,1,2$). In the experiments, Ps atoms, optically excited to the radiatively metastable $2^{3}S_{1}$ level, flew through microwave radiation fields tuned to drive transitions to the short-lived $2^{3}P_{J}$ levels, which were detected via the time spectrum of subsequent ground-state Ps annihilation radiation. Both the ${\ensuremath{\nu}}_{1}$ and ${\ensuremath{\nu}}_{2}$ line shapes were found to be asymmetric, which, in the absence of a complete line-shape model, prevents accurate determination of these fine-structure intervals. Conversely, the ${\ensuremath{\nu}}_{0}$ line shape did not exhibit any significant asymmetry; the observed interval, however, was found to disagree with QED theory by 4.2 standard deviations.

A simple locally accurate uniform approximation for the nodeless wavefunction is constructed for a {\it neutral} system of two Coulomb charges of different masses $(-q,m_1)$ and $(q,m_2)$ at rest in a constant uniform magnetic field for the states of positive and negative parity, ${(1s_0)}$ and ${(2p_0)}$, respectively. It is shown that by keeping the mass and charge of one of the bodies fixed, all systems with different second body masses are related. This allows one to consider the second body as infinitely-massive and to take such a system as basic. Three physical systems are considered in details: the Hydrogen atom with (in)-finitely massive proton (deuteron, triton) and the positronium atom $(-e,e)$. We derive the Riccati-Bloch and Generalized-Bloch equations, which describe the domains of small and large distances, respectively. Based on the interpolation of the small and large distance behavior of the logarithm of the wavefunction, a compact 10-parametric function is proposed. Taken as a variational trial function it provides accuracy of not less than 6 significant digits (s.d.) ($\lesssim 10^{-6}$ in relative deviation) for the total energy in the whole domain of considered magnetic fields $[0\,,\,10^4]$ a.u. and not less than 3 s.d. for the quadrupole moment $Q_{zz}$. In order to get reference points the Lagrange Mesh Method with 16K mesh points was used to get from 10 to 6 s.d. in energy from small to large magnetic fields. Based on the Riccati-Bloch equation the first 100 perturbative coefficients for the energy, in the form of rational numbers, are calculated and, using the Pad\'e-Borel re-summation procedure, the energy is found with not less than 10 s.d. at magnetic fields $\leq 1$\,a.u., Comment: post-publication version: 51 pages, 8 tables (one new), 5 figures, 3 appendices; several important typos corrected, extra table, clarifications added -- all is published in erratum: Phys.Rev.A 105, 049901 (2022)

The confined variational method is used to study $S$-, $P$-, and $D$-wave positronium-hydrogen scattering with the scattering energy between 0.068 and 3.333 eV. Accurate phase shifts and $S$-wave scattering lengths are calculated and compared with other theoretical methods. Existing discrepancies for the $D$-wave phase shifts are resolved. According to collision momentum, total orbital angular momentum, and spin configuration, the distortion effects of positronium are quantitatively studied. Finally, the finite nuclear mass effects are investigated.

We study electron and positron exchange without a spin flip in a single collision between two polarized orthopositronium beams. We use angular-momentum coupling and density-matrix techniques to calculate the probabilities of obtaining only parapositronium, both para- and orthopositronium or only orthopositronium, after the collision. The probabilities are functions of the angle between the polarization vectors of the beams and scattering amplitudes labeled with total electron spin and total positron spin. The real parts of the scattering amplitudes and products of the polarization tensors for the beams are given in terms of the probabilities for certain angles and initial spin orientations. We show that quenching probabilities for different transitions and the spin-exchange quenching cross section depend on the angle, but the ratio of the total quenching probability to the total probability for positronium scattering does not.

Machine-learning techniques are used to check the theoretical and experimental predictions of positron binding to general molecules. The bound or unbound character of previous calculations for polar molecules are mostly confirmed. Binding for so far unexplored polar molecules is predicted. For apolar molecules, a formula for the binding energy in terms of isotropic polarizability and ionization potential is obtained, leading to unprecedented agreement with experiments as well as prediction of previously unidentified bound systems. The role of the ionization potential is suggested as a consequence of enhanced formation of virtual positronium at short distances.

A high-precision measurement of positronium’s fine structure delivers a puzzling discrepancy with predictions from quantum electrodynamics.

The projection method with explicitly correlated Gaussians is used to demonstrate the existence of two doubly excited states of the positronium molecule (${\mathrm{Ps}}_{2}$). One is below the $\mathrm{Ps}(n=2)\phantom{\rule{4pt}{0ex}}+\phantom{\rule{4pt}{0ex}}\mathrm{Ps}(n=2)$ threshold, with ${A}_{1}$ symmetry, and the other is below the $\mathrm{Ps}(n=2)+\mathrm{Ps}(n=3)$ threshold, with $E$ symmetry. These states exist as resonances in the Ps-Ps continuum. Moreover, the resonance positions and resonance widths of the two states are calculated using the complex rotation method with basis sets obtained via the orthogonalizing pseudoprojector method. The resonance positions obtained using the complex rotation method agree with the results of the orthogonalizing pseudoprojector method. We also investigate the various structural properties of these states as well as the decay probabilities of $2\ensuremath{\gamma}$ emission due to electron-positron annihilation.

High-precision quantum-mechanical calculations have been developed to investigate positronium (Ps) scattering. Positronium scattering experiments are a powerful tool to study positronium-matter interactions, but the theoretical description of these experiments needs better accuracy. We have developed an ab initio confined variational approach that can reach higher collision energy, includes higher orbital momenta and uses small confining radii. Calculation of the Ps--He momentum-transfer cross section shows that the experimental Doppler broadening spectroscopy results are questionable. The energy dependence of the pickoff annihilation rate is also calculated, demonstrating an important role of the so far neglected $P$-wave contribution.

Recently, Belle Collaboration reported a new exotic state $Y(4620)$ with mass at 4625.9 MeV in the positronium annihilation process. Inspired by experiment, we study the tetraquark system $c\bar{s}s\bar{c}$ with quantum numbers $J^{P}=1^{-}$ in the framework of chiral constituent quark model with the help of Gaussian expansion method. Two structures, diquark-antidiquark and meson-meson, with all possible color and spin configurations are considered. The result shows that no bound state can be formed. To investigate the possible resonance states, the real scaling method is employed. Several resonance states with energies 4354, 4408, 4469, 4497 and 4531 MeV, are proposed. Taking into account the errors in calculating the $q\bar{q}$ mesons, the system errors in the calculation of four-quark system are around 60$\sim 100$ MeV. The resonance with energy 4531 MeV is possible the candidate of the newly found state $Y(4620)$., Comment: 6 pages, 3 figures

The energies and wave functions of the two-body Dirac equation for positronium are compared with those of the Pauli approximation and the Bethe-Salpeter equation. The unusual behavior of the ground-state wave function of the Dirac equation is explained in terms of anomalous bound-state solutions., Comment: 42 pages, 5 figures

We report a theoretical calculation of the relativistic corrections to the binding energy of positronic alkali-metal atoms. The ground state for the positronic alkali-metal atom is a loosely bound state with the structure of an alkali-metal ion surrounded by a positronium cloud. The correlation between the valence electron and positron in the field of a residual core ion is taken into account using a three-body model. Relativistic corrections to the binding energy for positronic alkali-metal atoms are evaluated based on the Breit-Pauli perturbation theory up to the second order of the fine structure constant. The relativistic corrections caused by the interaction between the valence electron and core ion was evaluated from the decomposed expectation values of the perturbation Hamiltonian. We found that the importance of relativistic corrections to the binding energy, namely the ratio of the net relativistic correction in the total binding energy, is remarkably enhanced in the loosely bound states of positronic alkali-metal atoms, where the charge of the valence electron is screened by the positron as positronium in the outermost region, while the electron is released from the positron near the nucleus. The results imply that positronic atoms with a dominant positronium can serve as an apt testing ground for relativistic quantum mechanics.

Cross sections for positron scattering from molecular nitrogen are reported for energies below the positronium formation threshold. The elastic and state-to-state rotational and vibrational cross sections were obtained within the close-coupling formalism, using a three-dimensional potential energy surface for the interaction of the system. The inelastic cross sections was involved in a kinetic model to describe recent experimental results about positron cooling in a buffer gas of molecular nitrogen. Unlike a previous model [M. R. Natisin et al., J. Phys. B 47, 225209 (2014)], our prediction attributes a leading role to the $0\ensuremath{\rightarrow}1$ vibrational excitation of ${\mathrm{N}}_{2}$.

We investigate experimentally the possibility of enhancing the production of 2S3 positronium atoms by driving the 1S3–3P3 and 3P3–2S3 transitions, overcoming the natural branching ratio limitation of spontaneous decay from 3P3 to 2S3. The decay of 3P3 positronium atoms toward the 2S3 level has been efficiently stimulated by a 1312.2 nm broadband IR laser pulse. The dependence of the stimulating transition efficiency on the intensity of the IR pulse has been measured to find the optimal enhancement conditions. A maximum relative increase of ×(3.1±1.0) in the 2S3 production efficiency, with respect to the case in which only spontaneous decay is present, was obtained.

A hollow spherical bubble containing thousands of spin-aligned triplet positronium (Ps) atoms in superfluid liquid $^{4}\mathrm{He}$ would be stable against breakup into smaller bubbles, and the Ps would form a Bose-Einstein condensate (BEC) with a number density of $\ensuremath{\sim}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and a BEC critical temperature ${T}_{c}\ensuremath{\approx}300$ K. Estimates suggest that one could make such bubbles in the laboratory containing ${10}^{5}$ Ps atoms using presently known methods.

In the present work we study the applicability of the binary-encounter-dipole (BED) model and its simpler version the binary-encounter-Bethe model for positron-impact direct ionization. We show that ignoring the exchange and interference effects in the theory of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] yields simple analytical formulas with no fitting parameters that can be used to estimate cross sections over a wide energy range except near-threshold region. To correct this deficiency we combine BED theory with the Wannier-type threshold law derived by Klar [J. Phys. B 14, 4165 (1981)] for positron-impact ionization. We show that such combination is necessary in order to predict cross sections at low positron energies where strong polarization-correlation effects are present during collision and the positronium formation is a dominant scattering process. The present theory is tested for a wide range of targets including helium (He), atomic hydrogen (H), neon (Ne), argon (Ar), molecular hydrogen (${\mathrm{H}}_{2}$), nitrogen (${\mathrm{N}}_{2}$), oxygen (${\mathrm{O}}_{2}$), carbon monoxide (CO), carbon dioxide (${\mathrm{CO}}_{2}$), and methane (${\mathrm{CH}}_{4}$). An extensive comparison with available experiments and theories is done.

The confined variational method is applied to investigate the low-energy elastic scattering of orthopositronium from ${\text{H}}_{2}$ by first-principles quantum mechanics. Describing the correlations with explicitly correlated Gaussians, we obtain accurate $s$-wave phase shifts and pickoff annihilation parameters for different incident momenta. By a least-squares fit of the data to the effective-range theory, we determine the $s$-wave scattering length ${A}_{S}=2.02{a}_{0}$ and the zero-energy value of the pickoff annihilation parameter, ${}^{1}{Z}_{\text{eff}}=0.1839$. The obtained ${}^{1}{Z}_{\text{eff}}$ agrees well with the precise experimental value of 0.186(1) [G. L. Wright et al., J. Phys. B 16, 4065 (1983)] and the obtained ${A}_{S}$ agrees well with the value of $2.1(2){a}_{0}$ estimated from the average experimental momentum-transfer cross section for positronium energy below 0.3 eV [F. Saito et al., J. Phys. B 36, 4191 (2003)].

We report the results of experiments in which positronium (Ps) atoms, optically excited to Rydberg-Stark states with principal quantum numbers ranging from $n=13$ to 19, were transported along the axis of a multiring electrode structure. By applying alternate positive and negative potentials to the ring electrodes, inhomogeneous electric fields suitable for guiding low-field-seeking atoms along the guide axis were generated. The multiring configuration used has the advantage that once the atoms are confined within it appropriate time-varying fields can be generated for deceleration and trapping. However, in this type of structure the possibility of nonadiabatic transitions of the fast (100 km/s) Ps atoms to unconfined high-field-seeking states exists. We show that for typical guiding fields this is not a significant loss mechanism and that efficient Ps transport can be achieved. Our data are in accordance with a Landau-Zener analysis of adiabatic transport through the field minima and Monte Carlo simulations that take into account Ps velocity distributions, electric dipole moments, and lifetimes, as well as the electric-field distributions in the guide.

In this paper, the nonrelativistic asymptotic behavior of the transition $q\overline{q}\ensuremath{\rightarrow}2g\ensuremath{\rightarrow}q\overline{q}$ in the $^{1}{S}_{0}$ channel is discussed. Different from the usual calculation which expands the physical amplitude around the threshold of a quark and antiquark pair, we take the quarks and antiquarks off shell and expand the expression on the three-momentum of the quarks and antiquarks. We calculate the results to order 6. The imaginary part of the results after applying the on-shell conditions can reproduce the nonrelativistic QCD (NRQCD) results in the leading order of ${\ensuremath{\alpha}}_{s}$. The real part of the results can be used to estimate the mass shift of the $^{1}{S}_{0}$ heavy quark and antiquark system due to the $2g$ annihilation effect. The results can also be used to estimate the energy shifts of positronium due to two-photon annihilation.

Positronium in the $2^3S$ metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns) making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, $2^3S$ positronium is produced - in absence of electric field - via spontaneous radiative decay from the $3^3P$ level populated with a 205nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full-width at half maximum, different velocity populations of a positronium cloud emitted from a nanochannelled positron/positronium converter were selected by delaying the excitation pulse with respect to the production instant. $ 2^3S $ positronium atoms with velocity tuned between $ 7 \cdot 10^4 $ m/s and $ 10 \cdot 10^4 $ m/s were thus produced. Depending on the selected velocity, a $2^3S$ production effciency ranging from $\sim 0.8 \%$ to $\sim 1.7%$, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the $3^3P$-$2^3S$ spontaneous decay of $(9.7 \pm 2.7) \% $. The present velocity selection technique could allow to produce an almost monochromatic beam of $\sim 1 \cdot 10^3 $ $2^3S$ atoms with a velocity spread $ < 10^4 $ m/s and an angular divergence of $\sim$ 50 mrad.

We report experiments in which positronium (Ps) atoms, optically excited to Rydberg states with principal quantum numbers $n$ in the range 18--25, were selectively ionized by both static and pulsed electric fields. The experiments were modeled using Monte Carlo simulations that include tunnel ionization rates calculated for hydrogen and scaled by the Ps reduced mass. Our measurements exhibit a small disagreement with the calculated tunnel ionization rates. Despite this we show that the electric fields in which different Ps states are ionized are sufficiently separated to allow selective field-ionization methods to be used in typical experimental conditions.

Positronium collisions with rare-gas atoms are treated using the free-electron-gas approximation for exchange and correlation potential. The results confirm the absence of the Ramsauer-Townsend minimum in elastic scattering cross sections, but show lower cross sections in the lower-energy region when compared to previous pseudopotential calculations. This is explained by a more attractive ab initio correlation potential as compared to the previously used empirical potential. The results in the thermal-energy region agree very well with most swarm measurements for all rare-gas atoms. At higher energies, the results are compared with beam experiments and agreement for heavier rare-gas atoms Ar, Kr, and Xe is found to be very good. For He and Ne, some discrepancies with beam measurements are observed. This is explained by a poorer performance of the free-electron-gas potentials, based on the statistical Thomas-Fermi model, for systems with fewer electrons.

In the Milky Way galaxy, positrons, which are responsible for the diffuse 511 keV gamma ray emission observed by space-based gamma ray observatories, are thought to annihilate predominantly through charge exchange interactions with neutral hydrogen. These charge exchange interactions can only take place if positrons have energies greater than 6.8 eV, the minimum energy required to liberate the electron bound to the hydrogen atom and then form positronium, a short-lived bound state composed of a positron-electron pair. Here we demonstrate the importance of positron interactions with neutral alkali metals in the warm interstellar medium (ISM). Positrons may undergo charge exchange with these atoms at any energy. In particular, we show that including positron interactions with sodium at solar abundance in the warm ISM can significantly reduce the annihilation timescale of positrons with energies below 6.8 eV by at least an order of magnitude. We show that including these interactions in our understanding of positron annihilation in the Milky Way rules out the idea that the number of positrons in the Galactic ISM could be maintained in steady state by injection events occurring at a typical periodicity $g\mathrm{Myr}$.

Producing positronium (Ps) in the metastable 23S state is of interest for various applications in fundamental physics. We report here on an experiment in which Ps atoms are produced in this long-lived state by spontaneous radiative decay of Ps excited to the 33P level manifold. The Ps cloud excitation is obtained with a UV laser pulse in an experimental vacuum chamber in presence of guiding magnetic field of 25mT and an average electric field of 300Vcm−1. The evidence of the 23S state production is obtained to the 3.6σ level of statistical significance using a novel analysis technique of the single-shot positronium annihilation lifetime spectra. The dynamic of the Ps population on the involved levels has been studied with a rate equation model.

This corrects the article DOI: 10.1103/PhysRevLett.104.083401.

Positronium formation at Si(111) and Si(001) surfaces has been investigated by changing the doping level systematically over the range 300--1000 K. The temperature dependence of the positronium fraction varied with the doping condition, and there were practically no differences between the two surface orientations. In heavily doped $n$-type Si $(n\ensuremath{\gtrsim}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$, the positronium fraction (${I}_{\mathrm{Ps}}$) increased above 700 K and reached more than 95% at 1000 K. In undoped and lightly doped Si ($n$, $p\ensuremath{\lesssim}{10}^{15}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$), ${I}_{\mathrm{Ps}}$ decreased from 300 to 500 K and increased above 700 K. In heavily doped $p$-type Si ($p\ensuremath{\gtrsim}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$), ${I}_{\mathrm{Ps}}$ increased in two steps: one at 500--600 K and one above 700 K. Overall, the positronium fraction increased with the amount of $n$-type doping. These phenomena were found to be dominated by two kinds of positronium with energies of 0.6--1.5 eV and 0.1--0.2 eV, which were attributed to the work-function mechanism and the surface-positron-mediated process, respectively, with contributions from conduction electrons. The positron work function was estimated to be positive. This agrees with first-principles calculation. The positive positron work function implies that the formation of excitonic electron-positron bound states begins in the bulk subsurface region and transits to the final positronium state in the vacuum.

Exchange and correlations play a particularly important role in positronium (Ps) collisions with atoms and molecules, since the static potential for Ps interaction with a neutral system is zero. Theoretical description of both effects is a very challenging task. In the present work we use the free-electron-gas model to describe exchange and correlations in Ps collisions with molecules similar to the approach widely used in the theory of electron-molecule collisions. The results for exchange and correlation energies are presented as functions of the Fermi momentum of the electron gas and the Ps incident energy. Using the Thomas-Fermi model, these functions can be converted into exchange and correlation potentials for Ps interaction with molecules as functions of the distance between the projectile and the target.

Positronium (Ps) collisions with molecular hydrogen are investigated theoretically. Elastic and Ps ionization cross sections are calculated. For elastic scattering the pseudopotential method, previously developed for rare-gas atoms, is applied. Ps ionization cross sections are calculated using the binary-encounter approximation. The results agree with swarm measurements at low collision energies and with beam measurements at higher energies. The total Ps-${\mathrm{H}}_{2}$ cross section when plotted as a function of collision velocity is close to the ${e}^{\ensuremath{-}}\text{\ensuremath{-}}{\mathrm{H}}_{2}$ cross section at velocities above the Ps ionization threshold, confirming earlier observations [Brawley et al., Science 330, 789 (2010)]. However, below the threshold the two sets of cross sections are different because of the different nature of the long-range interaction between the projectile and the target, the polarization interaction in the case of ${e}^{\ensuremath{-}}\text{\ensuremath{-}}{\mathrm{H}}_{2}$ collisions and the van der Waals interaction in the case of Ps-${\mathrm{H}}_{2}$ collisions.

In spin-polarized surface positronium annihilation measurements, the spin polarizations of graphene and $h$-BN on Co(0001) were higher than those on Ni(111), while no significant differences were seen between graphene and $h$-BN on the same metal. The obtained spin polarizations agreed with those expected from first-principles calculations considering the positron wave function and the electron density of states from the first surface layer to the vacuum region. The higher spin polarizations of graphene and $h$-BN on Co(0001) as compared to Ni(111) simply reflect the spin polarizations of these metals. The comparable spin polarizations of graphene and $h$-BN on the same metal are attributed to the creation of similar electronic states due to the strong influence of the metals: the Dirac cone of graphene and the band gap of $h$-BN disappear as a consequence of $d\ensuremath{-}\ensuremath{\pi}$ hybridization.

We propose a method to convert measured positronium annihilation rates in gaseous xenon into total and differential cross sections of positronium-xenon collisions in an ultralow-energy region of less than 80 meV where their experimental determinations as functions of the positronium kinetic energy are extremely difficult. This method makes it possible to determine not only the $s$-wave collisional parameters but also the $p$-wave and $d$-wave parameters. We have found a small positive value of the scattering length, ${A}_{0}=2.06\ifmmode\pm\else\textpm\fi{}0.10\phantom{\rule{0.16em}{0ex}}{a}_{0}$, which indicates that the positronium-xenon interaction in this energy region is repulsive and suggests that it is dominated by the scattering amplitude of the positron rather than that of the electron. An extrapolation of the analytical result into the experimentally inaccessible energy regions from 80 meV to 1.0 eV indicates that there should not be a Ramsauer-Townsend minimum but rather a peak in the total cross section at an energy of approximately 0.4 eV.

The ab initio method based on the Faddeev-Merkuriev equations is used to calculate cross sections involving the $(\overline{p},{e}^{+},{e}^{\ensuremath{-}})$ three-body system, with an emphasis on antihydrogen formation $(\overline{\mathrm{H}})$ via antiproton $(\overline{p})$ scattering on positronium. This system is studied in the energy range between the ${e}^{\ensuremath{-}}+\overline{\mathrm{H}}(n=2)$ and the ${e}^{\ensuremath{-}}+\overline{\mathrm{H}}(n=3)$ thresholds, where precisely calculated cross sections can be useful for future experiments (GBAR, AEGIS, etc.) aiming to produce antihydrogen atoms. A special treatment is developed to take into account the long-range charge-dipole interaction effect on the wave function. Emphasis is placed on the impact of Feshbach resonances and Gailitis-Damburg oscillations appearing in the vicinity of the $\overline{p}+\mathrm{Ps}(n=2)$ threshold.

We generalize the two-center convergent close-coupling approach of Kadyrov and Bray [Phys. Rev. A 66, 012710 (2002)] for positron scattering on neutral targets to charged targets, and apply it to positron scattering on the singly charged helium ion. Where possible internal consistency checks are used to validate the two-center method, which required a significant redevelopment of the positronium-formation matrix elements, by comparison with a single-center approach. Only the two-center approach explicitly yields positronium formation, and so it also provides a mechanism to calculate positronium scattering on $\ensuremath{\alpha}$ particles. As yet there are no experiments for the calculated processes, but there are some previous theoretical calculations, with which comparison is mixed.

The lithium atom in its ground state can bind positronium (Ps) forming LiPs, an electronically stable system. In this study we use the fixed node diffusion Monte Carlo method to perform a detailed investigation of the internal structure of LiPs, establishing to what extent it could be described by smaller interacting subsystems. To study the internal structure of positronic systems we propose a way to analyze the particle distribution functions: We first order the particle-nucleus distances, from the closest to the farthest. We then bin the ordered distances obtaining, for LiPs, five distribution functions that we call sorted distribution functions. We used them to show that Ps is a quite well-defined entity inside LiPs: The positron is forming positronium not only when it is far away from the nucleus, but also when it is in the same region of space occupied by the $2s$ electrons. Hence, it is not correct to describe LiPs as positronium ``orbiting'' around a lithium atom, as sometimes has been done, since the positron penetrates the electronic distribution and can be found close to the nucleus.

An ``ultrafine'' splitting in positronium between the $L=1$ spin-singlet state and the spin average of the spin-triplet states is shown to arise only at order ${\ensuremath{\alpha}}^{6}$. The QED prediction for $n=2$ states is ${\mathrm{\ensuremath{\Delta}}}_{2,P}=(683/172\phantom{\rule{0.16em}{0ex}}800)m{\ensuremath{\alpha}}^{6}=73.7(2.6)$ kHz. This represents the smallest leading-order QED splitting known. Current experimental efforts could observe this splitting, and its observation can constrain new ultralight interactions, such as axions or ${Z}^{\ensuremath{'}}$.

In order to clarify the physics underlying the observations of the electronlike behavior of positronium (Ps) and its resonant scattering from CO2, we have measured the Ps+N2 total cross section and found it also to exhibit significant structure. Analysis of the resonances reveals that Ps is distorted in the collisions and classical trajectory Monte Carlo calculations indicate that the electron is on average closer to the target than the positron, which may in turn bind resonantly to the ensuing temporary negative ion. This description of the nature of Ps resonances agrees with long-standing theoretical predictions.

We report experiments in which positronium (Ps) atoms are produced in the $2\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$ level by single-photon excitation from the ground state. To accomplish this, Stark-mixed $n=2$ states were optically excited in electric fields. By adiabatically switching off the electric field after laser excitation, some of the mixed states evolved into pure $2\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$ levels, whose presence was detected via the time dependence of their annihilation $\ensuremath{\gamma}$ radiation. The observed $\ensuremath{\approx}4%$ production efficiency relative to that of Rydberg Ps states is consistent with a Monte Carlo simulation that takes into account the rate at which the electric fields were switched off.

We determine the rates and energy and angular distributions of the positronium decays into a photon and a neutrino-antineutrino pair, Ps-->gamma+nu+anti-nu. We find that both positronium spin states have access to this decay channel, contrary to a previously published result. The low-energy tails of the spectra are shown to be sensitive to the binding effects and agree with Low's theorem. Additionally, we find a connection between the behaviour of the soft photon spectrum in both o-Ps-->gamma+nu+anti-nu and o-Ps-->3gamma decays, and the Stark effect., Submitted to Physical Review D

We investigate the consequences of unexpectedly large elastic cross sections for the scattering of low-energy antiprotons from $n\ensuremath{\le}3$ positronium (Ps) on the experimental implementation of antihydrogen formation via Ps-antiproton collisions. The integrated elastic cross sections, obtained using the two-center convergent close-coupling theory, can be up to three orders of magnitude greater than their counterparts for antihydrogen formation. The differential momentum transfer cross sections, which suppress the large cross sections at forward scattering angles, show remarkably rich behavior across all scattering angles. We discuss the implications of these findings for the heating, via momentum transfer, of clouds of trapped antiprotons that are typically used for the creation of antihydrogen.

The antihydrogen formation by charge exchange between cold antiprotons and Rydberg positronium Ps is studied by using the Classical Trajectory Monte Carlo (CTMC) method., Comment: Accepted by Phys. Rev. A

We report experiments in which positronium (Ps) atoms were guided using inhomogeneous electric fields. Ps atoms in Rydberg-Stark states with principal quantum number n=10 and electric dipole moments up to 610 D were prepared via two-color two-photon optical excitation in the presence of a 670 V cm^{-1} electric field. The Ps atoms were created at the entrance of a 0.4 m long electrostatic quadrupole guide, and were detected at the end of the guide via annihilation gamma radiation. When the lasers were tuned to excite low-field-seeking Stark states, a fivefold increase in the number of atoms reaching the end of the guide was observed, whereas no signal was detected when high-field-seeking states were produced. The data are consistent with the calculated geometrical guide acceptance.