Bound and Quasi-Bound Sates of Positrons with Atoms and Molecules
- Chief Supervisor
- Dr. James Sullivan
- Supervisor
- Professor Stephen Buckman
- Centre Node
- ANU
One of the "big" questions in positron physics is whether positrons bind to atoms and molecules to form a stable or quasi-stable, positively charged complex, analogous to the case of electron binding to form negative ions. Intuitively, the likelihood of such states existing would seem low, as the repulsive potential between the nucleus and the positron means that the positron may never penetrate the atomic charge cloud enough to enable such binding. However, many state-of-the-art theoretical calculations (the bulk of them by Mitroy and colleagues [1]) consistently predict positron binding for a range of atomic species. There are also experiments from the University of California San Diego (Surko and collaborators [2]), on positron annihilation, that are strongly indicative of positron binding to a class of hydrocarbon molecules.
We therefore propose a program to systematically search for positron bound states by conducting measurements of positron scattering at very low energies. These measurements will then be extrapolated to zero energy, using effective-range techniques. The zero energy cross section yields the scattering length, the sign of which is an indicator of the presence or otherwise of bound states.
Another class of quasi-bound states, will also be investigated. These are states that exist for a very short period of time (less than a picosecond) and are often referred to as scattering "resonances." The only observation of these states was made recently, but in annihilation experiments with molecules, rather than scattering measurements [2]. Such states have never been directly observed in other scattering or excitation channels, although there is a tantalising suggestion of their presence in the near-threshold excitation of electronic states of molecules such as N2 [3]. The comprehensive investigation and characterisation of these states would add a significant new tool to the theoretical understanding of the correlated motion of charged particles
The experimental studies for this project will be undertaken at the ANU on the Positron Beamline Facility.
[1] J. Mitroy, M. Bromley and G. Ryzhikh J. Phys. B. 35 R81 (2002)
[2] S.J. Gilbert, L. D. Barnes, J. P. Sullivan and C. M. Surko, "Vibrational-resonance Enhancement of Positron Annihilation in Molecules", Phys. Rev. Letts. 88, 043201 (2002)
[3] J.P. Sullivan, J.P. Marler, S.J. Gilbert, S.J. Buckman and C.M. Surko Phys. Rev. Lett. 87, 073201 (2001)
