Positron Scattering From Small Hydrocarbons

Abstract

Positrons are anti-matter electrons and have applications in testing of fundamental physics and non-invasive imaging. Positrons can also bind with electrons to form positronium atoms, Ps, and can also bind to atoms and molecules. A phenomenon related to positron binding is scattering resonances, where an incident charged particle is temporarily bound to a target molecule. Contrary to electron scattering, there is limited experimental evidence for positron resonances, only the Surko group in UC San Diego have managed to observe Feshbach resonances via the annihilation cross section for positron scattering from hydrocarbons. This PhD aimed to search for more evidence of the Feshbach resonances for molecules and generate new cross sections for these molecules as well.

The resonances observed by the Surko group are vibrational Feshbach resonances (VFR). There is a theoretical framework developed that uses the positron binding energy as a free parameter and assumes that the annihilation escape channel for the VFR decay is small compared to other channels, and it predicts the VFR structure in the annihilation cross section well. However, a corollary of this framework is that the resonance structure can couple to vibrational states lower in energy and allow the positron to escape the resonance structure, known as intramolecular vibrational redistribution (IVR), which is information that the annihilation cross section is unable to provide.

The hydrocarbons chosen for the experiment in this PhD were ethane, ethylene and acetylene, which are all two-carbon molecules with an increasing number of bonds between the two carbon atoms. The measurements in this PhD set out to conduct were primarily the vibrational excitation cross sections to search for evidence of inelastic escape of positron from the VFR structure as well as the effects of IVR.

Despite strong resonance structure in the annihilation cross section associated with the CH stretch mode of all three target molecules, the CH stretch excitation cross section show no enhancement in the cross section for all three target molecules. The CH3 deformation mode cross section for ethane shows no resonance enhancement either, while the CH2 deformation mode for ethylene shows evidence of multimode VFR structure. The CC stretch mode for both ethylene and acetylene are dipole forbidden, and that of ethylene has a very small cross section and it is unclear if it is real, while that of acetylene shows a clear resonance structure that can only be a VFR coupling into this vibrational mode via IVR. The total cross section for positron scattering from ethane, ethylene and acetylene for this energy range was also measured and showed no resonance structure in the cross section. Consequently, it can be interpreted that for these three molecules, the VFR does not decay via elastic escape. This, combined with the limited evidence for inelastic escape is contrary to the assumptions used by the theoretical calculations. While there is evidence for IVR, it remains that the theory needs to be improved upon.

Next, the differential cross section (DCS) of all three target molecules were measured, and are all strongly forward peaked. Such behaviour is typically attributed to molecules with a dipole moment, which ethane, ethylene and acetylene lack, and the strong forward scattering must then be due to the polarizability of the target molecules, which is not accounted for well by the theory. The current grand total cross sections measured were all consistently around 10 angstrom squared larger than the previous measurements, which is attributed to the previous experiments missing a larger portion of the cross section due to the strongly forward peaked DCS. The total elastic, total electronic excitation, positronium and ionisation cross sections are presented as well, where the theory again misses the forward scattering and is smaller than the experimental total elastic cross sections. Show more