We have calculated the rates for the processes that result from collisions of positrons with Rydberg atoms in strong magnetic fields. This is the first step in a two-stage charge transfer, which is being explored as a mechanism for the formation of cold anti-hydrogen. Unlike previous theoretical explorations we have also investigated the cases when the energy of the positrons can be comparable to or larger than the binding energy of the atom. We have also examined how big an electric field is needed to destroy the resulting positronium. We find that the charge transfer does not scale as for n = 40 states for positron temperatures above T ∼ 25 K. Also, we find that the positronium atoms are more easily destroyed by electric fields if they emerge from the charge transfer along the magnetic field lines, contrary to what might be expected for the effective electric field. Both results have implications for the formation of anti-hydrogen atoms.
We have calculated the rates for the processes that result from collisions of positrons with Rydberg atoms in strong magnetic fields. This is the first step in a two-stage charge transfer, which is being explored as a mechanism for the formation of cold anti-hydrogen. Unlike previous theoretical explorations we have also investigated the cases when the energy of the positrons can be comparable to or larger than the binding energy of the atom. We have also examined how big an electric field is needed to destroy the resulting positronium. We find that the charge transfer does not scale as for n = 40 states for positron temperatures above T ∼ 25 K. Also, we find that the positronium atoms are more easily destroyed by electric fields if they emerge from the charge transfer along the magnetic field lines, contrary to what might be expected for the effective electric field. Both results have implications for the formation of anti-hydrogen atoms.
We have performed calculations of two successive charge transfers from Rydberg states in a strong magnetic field. In the first charge transfer, a positron interacts with a highly excited atom to form positronium. In the second stage, the positronium interacts with an antiproton to give antihydrogen. For many parameters, our results are in qualitative agreement with previous calculations with no magnetic field. However, we do find that there are important changes which may affect the usefulness of the method for efficient formation of antihydrogen that can be trapped.
We have performed calculations of two successive charge transfers from Rydberg states in a strong magnetic field. In the first charge transfer, a positron interacts with a highly excited atom to form positronium. In the second stage, the positronium interacts with an antiproton to give antihydrogen. For many parameters, our results are in qualitative agreement with previous calculations with no magnetic field. However, we do find that there are important changes which may affect the usefulness of the method for efficient formation of antihydrogen that can be trapped.
Predicted 20 years ago, positron binding to neutral atoms has not yet been observed experimentally. A scheme is proposed to detect positron-atom bound states by colliding Rydberg positronium (Ps) with neutral atoms. Estimates of the charge-transfer reaction cross section are obtained using the first Born approximation for a selection of neutral atom targets and a wide range of incident Ps energies and principal quantum numbers. We also estimate the corresponding Ps ionization cross section. The accuracy of the calculations is tested by comparison with earlier predictions for charge transfer in Ps collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps beam suitable for producing positron-atom bound states and estimate signal rates based on the calculated cross sections and realistic experimental parameters. We conclude that the proposed methodology is capable of producing such states and of testing theoretical predictions of their binding energies.
Abstract We compare classical and quantum predictions of the cross section for anti-hydrogen formation by charge exchange between Rydberg positronium and antiprotons as a function of the positronium energy. We show that these two highly different approaches furnish the same scaling law with the principal quantum number of positronium n Ps and numerical values in agreement within some tens of percent for n Ps ≥ 3 and down to very low collision energy of tens of μ eV. We discuss these results in the context of a recently claimed hypothesis of quantum suppression of the reaction driven by supposedly different scaling law of classical and quantum predictions. Finally we underline how the basic agreement between classical and quantum results enforces the reliability of the classical calculations that, presently, are the only ones able to obtain the cross section for positronium initially in high Rydberg states and in the presence of external electric and magnetic fields.
An efficient method for production of cold antihydrogen $(\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{})$ is proposed. Alkali-metal atoms laser excited to a Rydberg state are charge exchanged with cold trapped positrons, producing Rydberg states of positronium. In a second Rydberg-state charge exchange, the positronium atoms give up their Rydberg positrons to cold trapped antiprotons, producing Rydberg states of $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$. These $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ atoms soon decay down to the ground state, and, because they are cold, could be trapped in a magnetic trap. The efficiency of the process results from the extremely large cross sections for Rydberg charge exchange. Classical trajectory Monte Carlo calculations indicate an $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ instantaneous production rate of 106/s.
The antihydrogen formation by charge exchange between cold antiprotons and Rydberg positronium Ps∗ is studied by using the classical trajectory Monte Carlo method. In the absence of external magnetic field the cross section scaled by the fourth power of the Ps∗ principal quantum number nPs shows a universal behavior as a function of the ratio kv between the velocity of the Ps center of mass and that of the positron in the classical circular orbit. At low velocity, below about ${k}_{v}\ensuremath{\simeq}0.2--0.3$, we show for Rydberg positronium that the cross section increases as 1/kv2 or, in an equivalent way, as 1/EPscm with EPscm being the Ps∗ center-of-mass energy. In this regime the distribution of the principal quantum number of the antihydrogen state is narrow and it shows a peak at about 2nPs while at higher kv values a broad distribution of antihydrogen states is produced. The study of the collision process in the presence of moderate magnetic field (0.5--2 T) shows that there is an experimentally interesting region of kv with the cross section slightly higher than that in the absence of field. However the presence of a magnetic field changes significantly the cross section behavior as a function of kv, especially at low velocities, where reductions of the cross sections and deviations from the 1/kv2 (1/EPscm) are observed. Our calculations show a dependance of the cross section upon the angle between the magnetic field and the flight direction of the incoming Ps∗.