Computational investigation of positron scattering from ${\mathrm{C}}_{60}$

The low-energy (0–6-eV) behavior of positrons scattering from gaseous ${\mathrm{C}}_{60}$ molecules is examined through a computational study which employs a nonempirical modeling of all the relevant interaction forces (static, correlation, and polarization potentials). There is uncertainty concerning the most appropriate correlation-polarization potential to use in the study of positron-molecule scattering. Here we have considered two such potentials which are representative of the potentials which have been proposed for this purpose. The coupled quantum scattering equations are solved in the body-fixed frame of reference, and the fixed-nuclei approximation is used to decouple the nuclear motion during the collision. The elastic integral cross sections without positronium formation exhibit strong one-particle resonances in the very-low-energy region up to $\sim 4$ eV. The low-angular-momentum scattering components are shown to be the dominant contributions to trapped state wave functions. The existence of resonances in positron scattering from molecules is unusual. In the system studied here there are two types of resonances. First there are angular-momentum barrier resonances with the positron trapped outside of the ${\mathrm{C}}_{60}$ cage. Second, there are resonances of low angular momentum $(l<~2)$ with the positron trapped inside of the ${\mathrm{C}}_{60}$ cage by the repulsive interaction between the positron and the nuclei. The possibility of the experimental observation of these resonances is also discussed.