Description
The ANU positron group has been investigating metastable excitation of atoms and molecules by electron and positron impact, with a current focus on positron-impact excitation of noble gases$^{[1]}$. Excitation of low-lying metastable states in these systems typically requires an electron spin-flip. In electron collisions, such transitions can proceed via the exchange or spin-orbit interactions. A key feature of positron collisions is the absence of exchange – spin-flips only occur via the spin-orbit interaction. Additionally, an atom’s static Coulomb potential attracts electrons but repulses positrons, reducing the strength of the spin-orbit interaction in positron collisions. Consequently, excitation cross sections for these states are expected to be small or negligible
These measurements provide a proxy for the strength of spin-orbit interactions in the noble gases. As a relativistic effect, spin-orbit coupling scales strongly with atomic number, which can be seen in the increasing separation between the $J = 0$ and $J = 2$ metastable levels in the lowest excited manifold across the noble gas series. Accordingly, the influence of spin-orbit interactions on collision processes is expected to be more pronounced for targets like Xe.
In the experiment, a pulsed, high-resolution positron beam intersects a target gas beam at 90$^{\circ}$. The crossed-beam geometry enables high-efficiency ($>$60%) detection of metastable atoms using a channeltron detector. Time-of-flight measurements allow discrimination against fast reaction products (e.g., photons and Ps*), whilst high transparency retarding grids block charged particles (electrons, positrons, and ions). Thus, the experiment is sensitive to small metastable excitation cross sections.
We will report recent progress on these measurements.
[1] Wymer, L., Kumar, S., Gay, T.J. et al. Excitation of metastable helium atoms by positron impact. Eur. Phys. J. D 80, 12 (2026). https://doi.org/10.1140/epjd/s10053-026-01123-0
| I am the presenting author | Yes |
|---|