Observation of a shape resonance of the positronium negative ion.

Koji Michishio,Ken Wada,Izumi Mochizuki,Akira Yagishita,Tsuneto Kanai,Toshiyuki Azuma,Yasuyuki Nagashima,Susumu Kuma,Toshio Hyodo

doi:10.1038/ncomms11060

Abstract

When an electron binds to its anti-matter counterpart, the positron, it forms the exotic atom positronium (Ps). Ps can further bind to another electron to form the positronium negative ion, Ps− (e−e+e−). Since its constituents are solely point-like particles with the same mass, this system provides an excellent testing ground for the three-body problem in quantum mechanics. While theoretical works on its energy level and dynamics have been performed extensively, experimental investigations of its characteristics have been hampered by the weak ion yield and short annihilation lifetime. Here we report on the laser spectroscopy study of Ps−, using a source of efficiently produced ions, generated from the bombardment of slow positrons onto a Na-coated W surface. A strong shape resonance of 1Po symmetry has been observed near the Ps (n=2) formation threshold. The resonance energy and width measured are in good agreement with the result of three-body calculations.

Highlights

When an electron binds to its anti-matter counterpart, the positron, it forms the exotic atom positronium (Ps)
The three-body problem with a Coulomb interaction has been the focus of attention in fundamental physics for classical mechanics and quantum mechanics, since the Schrodinger equation for a three-body system has not been solved analytically, despite the proposal of a variety of approximation approaches
Since the theoretical simplifications applied to atoms or molecules may often be inadequate, research on Ps À structure and dynamics can provide a stringent testing ground for the quantum mechanical three-body problem

Summary

Result

The Ps À ions formed in this setup were accelerated by the potential a e+ MCP. Neutral Ps atoms formed both by the direct photodetachment process and via the resonances (Fig. 1b) were detected by a micro-channel plate (MCP), of effective diameter 42 mm, while charged particles were removed by the curved magnetic field. In order to reduce the MCP signal due to the stray light, baffles and cylindrical tubes with 5 mm diameter apertures were placed between the target and each window. Owing to the short flight length (o20 mm) of para-Ps atoms, even in the n 1⁄4 2 state, due to self-annihilation, only ortho-Ps atoms were detected by the MCP which was placed at a distance, L, of 0.88 m from the target. RPs was normalized to the average photon flux and the overlapping volume of the laser beam and the Ps À beam estimated from each spatial and temporal profile to ensure proportionality to the photodetachment cross sections

Discussion

Methods