Positron resonances in vibrational excitation

Scattering resonances are relatively rare in positron scattering, with the only observed instances to date being vibrational Feschbach resonances, observed through their effect on the annihilation cross section. In this study, we present observations of resonances in vibrational excitation by positron impact on ethane, ethylene, and acetylene, and explore the dynamics of positron resonance formation and decay. There are far fewer resonances observed in the excitation spectra than in the annihilation channel. Due to the difficulties inherent in measuring relevant scattering processes at the low energies required to observe the resonances, the formation and decay modes of these resonances are still not well understood. Theory suggests that intramolecular vibrational energy redistribution plays a key role in resonance formation and decay, which should lead to enhancement of the vibrational excitation cross sections in the region of the resonance energies. These results demonstrate that further work is required to fully understand these systems.