Session U3: Interactions Involving Antimatter or Charge Transfer

Efficient Production of Rydberg Positronium

We demonstrated the efficient production of Rydberg Positronium atoms using a two-step incoherent laser excitation process. The two step process occurs first to the 23P state and then to the quantum numbers ranging from 10 to 25. We found a 90{\%} efficiency going from the 23P state to the Rydberg levels and an overall conversion efficiency of 25{\%} of the production of Rydberg atoms. This high efficiency is due to the overlap of the laser bandwidth with the Doppler broadened width of the 1s-2p transition and the suppression of the stimulated emission back to the 2P states, due to the intermixing of the Rydberg state Stark sublevels. By demonstrating the production of long lived Rydberg Ps atoms in a high magnetic field may make it possible to perform gravitational measurements of free falling positronium atoms.   

Towards a hyperspherical description of positronium-positronium scattering

The interest in the scattering properties of the positronium (Ps) has grown with the possibility of creating a Bose-Einstein condensate of matter-antimatter. Such an experimental realization would not only permit an exploration of quantum effects at macroscopic temperatures, but it would also contribute to our understanding of the relationship between the Ps$_2$ molecule and more conventional molecules like H$_2$. A deep understanding of positronium's collisional properties is crucial for cooling techniques and for the realization of a Bose-Einstein condensate. A hyperspherical analysis of this four-body system with two electrons and two positrons allow us not only to benchmark calculations of the Ps-Ps scattering length which have been previously studied (see e.g. Ref. [1]) but also to analyze recombination processes such as Ps+e+e$^+$ $\to$ Ps+Ps or Ps$^-$ + e$^+$, etc... We extend previous implementations of the correlated Gaussian hyperspherical method [2] to describe four-body systems with Coulomb interactions. The results from this hyperspherical analysis are compared with previous studies and correlated Gaussian calculations. \\[4pt] [1] I. A. Ivanov, J. Mitroy, and K. Varga. Phys. Rev. A 65, 022704 (2002)\\[0pt] [2] J. von Stecher and C.H. Greene. Phys. Rev. A 80, 22504(2009)   

Low-Energy Positron Scattering from H$_{2}$

We present low-energy positron scattering measurements from molecular hydrogen. Our measurements were conducted at the high-resolution, low-energy positron beamline at the Australian National University [1]. The energy width of the positron beam was typically 60 meV. We present results for positron scattering from H$_{2}$ from 1 to 200 eV for the total and positronium formation cross sections as well as total elastic and elastic differential cross sections. Comparison will be made with previous results, in particular recent results of the Trento group [2], along with a discussion of the experimental advantages and limitation of present techniques [3]. The greatest discrepancy between the available experimental and theoretical data sets lies at energies below the positronium formation threshold. A plausible explanation for these differences will be presented.\\[4pt] [1] J. P. Sullivan \textit{et al.},\textbf{79}, 113105 (2008)\\[0pt] [2] A. Zecca \textit{et al.}, Phys. Rev. A \textbf{80}, 032702 (2009)\\[0pt] [3] J. P. Sullivan \textit{et al.}, J. Phys. B \textbf{44}, 035201 (2010)   

Low-energy S- and P-wave Positronium-Hydrogen Collisions

Positronium-atom scattering is of experimental interest. We have investigated low-energy positronium-hydrogen scattering, a fundamental four-body Coulomb process. We computed the S- and P-wave phase shifts using a number of variants of the Kohn variational method. For the S-wave, we implemented various techniques to overcome linear dependence problems. Our results compare favorably with earlier Kohn variational calculations [1]. We determined the S-wave scattering length and effective range using a quantum defect theory for the van der Waals interaction [2]. \\[4pt] [1] P. Van Reeth and J. W. Humberston, J. Phys. B {\bf 36}, 1923 (2003), Nucl. Intrum. and Methods Phys. Res. B {\bf 221}, 140 (2004).\\[0pt] [2] Bo Gao, Phys. Rev. A {\bf 58}, 4222 (1998).   

Few Body Quantum Dynamics of high-Z Ions studied at the Future Relativistic HESR Storage Ring

At the FAIR facility for antiprotons and ion research the high energy storage ring HESR, originally conceived for experiments using antiprotons, will be configured to also provide highly-charged heavy ions up to beam energies corresponding to $\gamma $=5. This opens a wealth of opportunities for in-ring atomic physics experiments on few- body quantum dynamics ranging from e.g. dynamics of various e$^{+}$-e$^{-}$ pair creation processes to quasi-photoionisation of inner shells of the highest-Z ions. We will discuss various in-ring spectrometers permitting characterization of the pertaining fundamental processes in a kinematically complete fashion.   

Charge exchange and spectroscopy with isolated highly-charged ions

Compact ion traps can be useful in facilitating the study and manipulation of highly charged ions isolated in a controlled environment. Various ions of interest, including bare nuclei, are produced in the NIST electron beam ion trap (EBIT), extracted through a beamline that selects a single charge/mass species, then captured in a compact permanent magnet Penning trap\footnote{J.N. Tan, S.M. Brewer, and N.D. Guise, at this meeting (poster).} or RF trap. The isolated ions are detected optically or by ejection to a fast time-of-flight microchannel plate detector. In this room-temperature apparatus, demonstrated ion storage lifetimes exceed one second for species including Ne$^{10+}$ and Ar$^{13+}$, sufficiently long to measure certain metastable lifetimes via fluorescence detection,\footnote{S.M. Brewer, N.D. Guise, and J.N. Tan, at this meeting.} and to observe charge-exchange processes between trapped ions and residual background gas. A beam of Rydberg rubidium atoms, under development, may enable production of hydrogenlike ions in circular Rydberg states, via charge exchange with trapped bare nuclei; such one-electron ions are attractive for tests of theory and fundamental metrology.\footnote{U.D. Jentschura, \textit{et al.}, Phys. Rev. Lett. \textbf{100}, 160404 (2008).} Other applications include spectroscopic studies of trapped highly charged ions relevant to atomic physics, astrophysics, and plasmas.   

Vibrationally Resolved NonDissociative Charge Transfer in Collisions between Hydrogen or Deuterium Molecules and Atomic or Molecular Ions

Using a 3-D imaging technique, the vibrational distributions of slow H$_{2}^{+}$ or D$_{2}^{+ }$ produced by charge transfer (CT) between an H$_{2}$ or D$_{2}$ target and various fast ions (H$^{+}$, D$^{+}$, H$_{2}^{+}$, D$_{2}^{+}$, He$^{+}$, and H$_{3}^{+})$ were measured from 10 eV to few keV energies in the laboratory frame. The atomic/molecular ions are extracted from a duoplasmatron ion source, accelerated and decelerated to enter the collision cell hosting neutral molecules from an effusive jet. The CT daughter molecular ions are extracted sideways and accelerated to 2keV before crossing an effusive potassium jet to undergo resonant dissociative CT. The positions and flight time difference of the two resulting particles give access to the vibrational distribution of the CT products. At 50 eV and above, our results on the (H$_{2, }$H$^{+})$ system benchmark state-to-state calculations [1]. At lower energies, deviations from theory suggest that rovibrational modes start to play an important role in the CT dynamics. \\[4pt] [1] L. F. Errea, L. Fernandez, L. M\'{e}ndez, B. Pons, I. Rabad\'{a}n and A. Riera \textit{Phys. Rev. A }\textbf{\textit{75}}\textit{ 032703 (2007)}.   

Observing forbidden radiative decay of highly charged ions in a compact Penning trap

We report observations of radiative decay from metastable states of highly charged ions captured in a newly-developed compact Penning trap. Ions of interest are created in the NIST electron beam ion trap (EBIT), extracted in an ion beamline, and captured in a compact Penning trap built in a novel unitary architecture to facilitate collection of photons emitted by stored ions. As an example, Ar$^{13+}$ ions are captured in one of the two fine structure levels forming the lowest lying states, allowing us to monitor the fluorescence (blue light) of the spin-flipping (M1) decay to the ground state. We present recent results from the newly deployed ion capture apparatus and briefly discuss previous in-EBIT experiments as well as an earlier study involving ion capture in an electrostatic Kingdon ion trap. Our results illustrate the potential of unitary Penning traps for a variety of studies,\footnote{J. N. Tan, \textit{et. al.}, ``Unitary Penning Traps,'' at this meeting (poster).} including experiments to produce hydrogenlike ions for spectroscopic tests of QED.\footnote{N.D. Guise,\textit{ et. al.}, ``Charge Exchange and spectroscopy with isolated highly charged ions,'' at this meeting.}   

Si$^{3+}$ +H collisions: role of rotational couplings

State selective charge exchange cross sections are calculated using both the quantal and the semi classical molecular orbital close coupling approaches in the adiabatic representation. In addition to radial coupling, all angular couplings are also incorporated in our close coupling calculations. The multi-reference single- and double-excitation configuration interaction (MRD-CI) method [1] is employed to describe the adiabatic electronic states of (SiH)$^{3+}$ system. Details of our findings will be reported at the conference.\\[4pt] [1] R. J. Buenker, in \textit{Current Aspects of Quantum Chemistry}, edited by R. Carbo, in Physical and Theoretical Chemistry Vol. 21 (Elsevier, Amsterdam, 1981).