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.

Figure 1

Positron excitation of the C-H stretch vibrational mode of (a) ethane, (b) ethylene. Blue points are the experimental data, and the red solid line is the annihilation data from [18], scaled to fit to the vibrational excitation cross section measurements. The dashed vertical line indicates the threshold for the excitation in each molecule.

Figure 2

Positron excitation of the C-C stretch vibrational mode of (a) ethylene and (b) acetylene. Blue points are the experimental data, and the red solid line is the annihilation data from [18], scaled to fit to the vibrational excitation cross section. The dashed vertical line indicates the threshold for the excitation in each molecule.

Figure 3

Positron excitation of the C-${\mathrm{H}}_3$ and C-${\mathrm{H}}_2$ deformation modes of (a) ethane and (b) ethylene, respectively. Blue points are the experimental data, and the red solid line is the annihilation data from [18], scaled to fit to the vibrational excitation cross section. The dashed vertical line indicates the threshold for the excitations in each molecule.

Figure 4

Low energy positron scattering total cross sections for ethane (blue circles), ethylene (red squares) and acetylene (green triangles). Error bars are absolute, and dominated by statistical error. The cross sections miss a significant portion of the forward angle scattering of the positrons, but are still assumed to be sensitive to the presence of any resonance features. A 10 Å offset has been added to the acetylene data to allow for a qualitative comparison between the three measurements.