Antihydrogen formation in low-energy antiproton collisions with excited-state positronium atoms

The convergent close-coupling method is used to obtain cross sections for antihydrogen formation in low-energy antiproton collisions with positronium (Ps) atoms in specified initial excited states with principal quantum numbers n_i ≤?5. The threshold behaviour as a function of the Ps kinetic energy, E, is consistent with the 1/E law expected from threshold theory for all initial states. We find that the increase in the cross sections is muted above n_i =?3 and that here their scaling is roughly consistent with \({n_{i}^{2}}\), rather than the classically expected increase as \({n_{i}^{4}}\).     

Neutralization and excitation in low-energy ion-surface collisions

A model is presented which describes neutralization and excitation occurring in low-energy ion-surface collisions. Important processes that determine experimentally measurable quantities such as the charged fraction ?⁺ and photon yield γ are assumed to be Auger neutralization, electronic excitation and radiationless deexcitation. The model yields ?⁺ and γ in good agreement with experiment data, although log ?⁺ and log γ are found to exhibit nonlinear dependences on the inverse velocity of the incident ion beam.     

Differential scattering and rotational excitation in low-energy positron-hydrogen-molecule collisions

Differential cross sections are calculated for positron-hydrogen-molecule scattering below the positronium formation threshold usingK-matrix elements obtained by the Kohn variational method. The internuclear separation of the hydrogen molecule is fixed at its equilibrium value, 1.4a₀. The trial function used in the Kohn calculation is very flexible. It contains separable and Hylleraas-type basis functions and also basis functions to take into account longrange polarization. The cross sections for rotational excitation are also calculated, using the adiabatic-rotation approximation. Preliminary results are presented in this paper; full details will be published elsewhere.     

The theory of low-energy positron collisions with molecules

This review begins by describing the theory of the collisions of low-energy positrons with molecules. It then goes on to describe in detail the calculations, both phenomenological and ab initio, which have been carried out to date to predict the results of low-energy positron-molecule scattering experiments. The review conludes with a discussion of possible future developments.     

Neutron transfers at low-energy collisions of nonspherical nuclei

A semiclassical model is used to analyze the evolution of the valence-neutron wave function at a collision between a spherical ⁴⁸Ca nucleus and a deformed ²⁴⁸Cm nucleus with near-Coulomb-barrier energies. The probabilities of neutron transfers are determined as a function of minimum internuclear distance.     

Mechanisms of transfer reactions in low-energy collisions with neutron-enriched nuclei

A mechanism of neutron transfer between nuclei in close low-energy collisions is established. It includes three stages: the transformation of the initial one-nuclear state into a two-nucleus state, the transitions of this state to other molecular states, and the transformation of those states into one-nuclear states in which nuclei are separated from one another. An explanation is offered for the fusion of nuclei in the ¹⁸O + ⁵⁸Ni reaction exceeding the cross section, relative to the ¹⁶O + ⁶⁰Ni reaction.     

Deuteron formation in nuclear collisions

Deuteron production cross sections are calculated from an intranuclear cascade model for Ne + U and Ar + Ca at 400 MeV/A. In effect, there are no adjustable parameters in this calculation. The parameters of the cascade code are fixed by sum charged inclusive data. Furthermore, the high momentum deuteron inclusive distribution is found to be insensitive to the details of the deuteron wave function. This latter observation leads to a justification of a generalized coalescence formula involving both momentum and spatial coordinates. Satisfactory agreement is found between our calculations and both the untriggered and high multiplicity triggered inclusive spectra of deuterons with energies ?50 MeV/A.     

Production of slow protonium in vacuum

We descrbe how protonium, the quasi-stable antiproton-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events in the ATHENA experiment, evidence is presented for protonium production with sub-eV kinetic energies in states around n = 70, with low angular momenta. This work provides a new two-body system for studies using laser spectroscopic techniques.